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CITY OF RENTON
THUNDER HILLS SANITARY SEWER INTERCEPTOR STUDY AND ALTERNATIVES ANALYSIS
BASIS OF DESIGN REPORT
TABLE OF CONTENTS
Page No.
Chapter 1 ........................................................................................................................... 1-1
1.1 INTRODUCTION .................................................................................................... 1-1 1.1 PROJECT PURPOSE ............................................................................................ 1-1 1.2 THUNDER HILLS SANITARY SEWER INTERCEPTOR ....................................... 1-1
1.3 TALBOT HILLS SANITARY SEWER BASIN .......................................................... 1-4 1.4 EXISTING ENVIRONMENT ................................................................................... 1-4
1.4.1 Topography................................................................................................. 1-4
1.4.2 Geology ...................................................................................................... 1-5 1.4.3 Streams ...................................................................................................... 1-5
1.4.4 Flood Plains ................................................................................................ 1-6
1.4.5 Wetlands ..................................................................................................... 1-6 1.4.6 Endangered species/habitats ..................................................................... 1-7
1.4.7 Public Health............................................................................................... 1-8
1.4.8 Public Acceptability ..................................................................................... 1-8 1.5 DISCHARGE STANDARDS ................................................................................... 1-8
1.6 SUPPORTING DOCUMENTATION ....................................................................... 1-8 1.7 ACKNOWLEDGMENTS ......................................................................................... 1-9 1.8 REPORT CONTENTS ............................................................................................ 1-9
Chapter 2 ........................................................................................................................... 2-1
2.1 LAND USE ............................................................................................................. 2-1 2.2 DEMOGRAPHICS .................................................................................................. 2-1
2.3 INFLOW & INFILTRATION..................................................................................... 2-3 2.4 FLOW PROJECTIONS .......................................................................................... 2-4
Chapter 3 ........................................................................................................................... 3-1
3.1 ALTERNATIVE ANALYSIS METHODOLOGY ....................................................... 3-1 3.2 ESTABLISH ANALYSIS CRITERIA (STEP 1)........................................................ 3-2
3.3 DEFINE ALTERNATIVES (STEP 2) ....................................................................... 3-2
3.4 COLLECT INFORMATION ON ALTERNATIVES (STEP 3) ................................... 3-3 3.5 FLOW CAPACITY SCREENING (STEP 4) ............................................................ 3-3
3.5.1 Flow Capacity ............................................................................................. 3-3
3.5.2 Flow Screening Results .............................................................................. 3-4 3.6 EVALUATE ALTERNATIVES BASED ON HIGH PRIORITY CRITERIA (STEP 5) 3-6
3.6.1 Updated Alternatives .................................................................................. 3-7
3.6.2 Pipe Risk..................................................................................................... 3-8 3.7 ALTERNATIVE ANALYSIS (STEP 6) ................................................................... 3-11
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3.7.1 RUL Scoring ............................................................................................ 3-11 3.7.2 Access to Manhole Scoring ..................................................................... 3-12
3.7.3 Equipment Access Scoring ...................................................................... 3-12
3.7.4 Constructability Scoring ........................................................................... 3-12 3.7.5 Summary of Alternatives ......................................................................... 3-13
3.8 EVALUATE DETAILED ALTERNATIVES BASED ON LOW AND MEDIUM PRIORITY CRITERIA (STEP 7) .......................................................................... 3-13 3.8.1 Alternative 1 (Existing – Mix) ................................................................... 3-13
3.8.2 Alternative 2 - Divert to Talbot – Mix of Rehabilitation and Parallel Pipes Alternative (Divert – Mix) ..................................................................................... 3-16 3.8.3 Evaluation of Medium and Low Criteria ................................................... 3-18
3.8.4 O&M Considerations .............................................................................. 3-18 3.8.5 Permitting Considerations ....................................................................... 3-19
3.8.6 Environmental Considerations ................................................................. 3-21
3.8.7 Cost ......................................................................................................... 3-21 3.9 SELECTED ALTERNATIVE ................................................................................ 3-23
Chapter 4 ......................................................................................................................... 4-1
4.1 SUMMARY ........................................................................................................... 4-1 4.1.1 Design life ................................................................................................. 4-1
4.1.2 Environmental Impacts .............................................................................. 4-1 4.1.3 Ability to expand ........................................................................................ 4-2 4.1.4 O&M Staffing Needs .................................................................................. 4-2
4.1.5 Project Funding ......................................................................................... 4-3 4.1.6 State Environmental Policy Act .................................................................. 4-3
LIST OF APPENDICES
APPENDIX A PROJECT PLANS
APPENDIX B TM NO. 1 – ALTERNATIVES AND CRITERIA
APPENDIX C TM NO. 2 – FLOW ANALYSIS
APPENDIX D TM NO. 3 – REMAINING USEFUL LIFE (RUL) & REHABILITATION
ANALYSIS
APPENDIX E TM NO. 4 – GEOTECHNICAL INVESTIGATION
APPENDIX F TM NO. 5 – PRELIMINARY EROSION HAZARD EVALUATION
APPENDIX G TM NO. 6 – RISK ANALYSIS
APPENDIX H TM NO. 7 – OPERATIONS AND MAINTENANCE
APPENDIX I TM NO. 8 – SUMMARY OF SENSITIVE AREA IMPACTS
APPENDIX J TM NO. 9 – PERMITTING ASSESSMENT ALTERNATIVES NO. 1 & 2
APPENDIX K TM NO. 10 – COST STUDY
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LIST OF TABLES
Table 1.1 Classification of Streams Along Thunder Hills Creek, Per Renton Municipal
Code, and Projected Impacts to Each Feature and Its Associated Buffer .... 1-6 Table 1.2 Classification of Wetlands Along Thunder Hills Creek, Per Renton Municipal
Code, and Projected Impacts to Each Feature and Its Associated Buffer .... 1-7
Table 2.1 Thunder Hills Sewer Basin Demographic Projections .................................. 2-3
Table 2.2 Talbot Hills Sewer Basin Demographic Projections ...................................... 2-3 Table 2.3 Peak Flow Rate Projections for Thunder Hills Sewer Interceptor and Talbot
Hills Sewer System ...................................................................................... 2-4
Table 3.1 Criteria and Prioritization.............................................................................. 3-2
Table 3.2 Flow Capacity Alternative Screening............................................................ 3-6 Table 3.3 RUL Designation by Stream Stationing ........................................................ 3-8
Table 3.4 General Erosion Potential Designation by Stream Stationing ..................... 3-10
Table 3.5 Pipe Risk Designation by Stream Reach ................................................... 3-10 Table 3.6 Thunder Hills Sewer Interceptor Alternative High Priority
Criteria and Ratings ................................................................................... 3-11 Table 3.7 Ratings for Low and Medium Priority Criteria ............................................. 3-18 Table 3.8 Permit Matrix for Thunder Hills Interceptor Alternatives ............................. 3-20
Table 3.9 Thunder Hills Alternative Cost Comparison ............................................... 3-23 Table 3.10 Thunder Hills Alternative O&M Cost Comparison ...................................... 3-23
Table 4.1 Preliminary Design Infrastructure Summary ................................................. 4-2
LIST OF FIGURES
Figure 1.1 Interceptor and Contributing Sewer Basins .................................................. 1-2
Figure 1.2 Interceptor Pipe Size and Material ............................................................... 1-3
Figure 2.1 Thunder Hills and Talbot Sanitary Sewer Basin Land Use ........................... 2-2
Figure 3.1 Talbot Hills Diversion and Improvements ..................................................... 3-5
Figure 3.2 Alternative 1 Preliminary Site Layout ......................................................... 3-15 Figure 3.3 Alternative 2 Preliminary Site Layout ......................................................... 3-17
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Chapter 1
INTRODUCTION
1.1 INTRODUCTION
The City of Renton (City) owns, operates, and maintains its sanitary sewer system, which
includes gravity sewer, sewage lift stations and forcemains, and manholes. The City’s
wastewater is discharged to King County facilities and is treated by the King County’s South
Treatment Reclamation Plant. The City has an active Repair and Replacement (R&R)
program to maintain it system and protect public safety. The City has identified the Thunder
Hills Sanitary Sewer Interceptor (Interceptor) in its Long-term Wastewater Management
Plan for rehabilitation. The Interceptor has ongoing maintenance challenges due to poor
vehicle access, lack of manholes, stream bank erosion, and segments with limited capacity.
No known overflows have occurred from the Interceptor; however, it is at an increased risk
for operational failures due to the ongoing maintenance challenges and aging
infrastructure.
This project focuses on 3,100 feet of interceptor largely adjacent to or within Thunder Hills
Creek between Grant Avenue South to north of its crossing of I-405, as shown in
Figure 1.1. The goal of the project is to rehabilitate or replace the Interceptor to provide an
additional 80 years of service. Additionally, the project will minimize risk to environmentally
sensitive/critical areas, consider surface water erosion, inflow and infiltration (I/I), and
improve operations and maintenance (O&M) access to the Interceptor.
1.1 PROJECT PURPOSE
The purpose of the Thunder Hills Sanitary Sewer Interceptor Project is to rehabilitate and/or
replace the Interceptor to provide a fully accessible and maintainable line with 80 years of
usable life. The project aims to reduce risk to the environment and public health by
mitigating potential hazards that could cause a sanitary sewer overflow or pipe break.
1.2 THUNDER HILLS SANITARY SEWER INTERCEPTOR
The Interceptor was originally installed in 1965 and serves a portion of the City’s Rolling
Hills neighborhood, southeast of City Hall. This area is predominately single and multi-
family residential areas. The Interceptor is made up of 10-inch, 12-inch, and 18-inch
concrete, asbestos-cement (AC), and ductile-iron (DI) pipe, as shown in Figure 1.2.
Basin03
Basin45
Basin02
Basin11
Basin04
Basin06
Basin07
Basin01
Basin13
BasinESI1003
Basin46
Figure 1.1
Thunder Hills Interceptor Flow Analysis
Thunder Hills Interceptor Sewer
Tributary Area
.
Proposed Diversion to Talbot Hill Sewer
0 370 740185Feet
Legend
Thunder Hills Interceptor Sewer Tributary
Area
Proposed Thunder Hills Tributary Area
Diversion to Talbot Hill Sewer
S 18th St
E a gle Rid g e Dr.
Grant Ave S
I-405
Figure 1.2
Thunder Hills Interceptor Flow Analysis
Existing Thunder Hills Interceptor
.
0 140 28070Feet
Legend
10" DI
10" AC
10" Concrete
12" DI
12" Concrete
18" Concrete
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Beginning at Grant Avenue South, the pipe alignment roughly parallels Thunder Hills Creek
on the east side, and crosses under the creek approximately 1,700 feet northwest of the
street. The pipe is predominately 10-inch AC, DI, and concrete, with short segments of
12-inch DI pipe. Just upstream of the creek crossing, the pipe transitions from 10-inch to
12-inch DI pipe and parallels the creek on the west for approximately 1,200 feet until
reaching I-405; no usable manholes exist in this reach. On the southeast side of I-405, the
Interceptor increases to an 18-inch diameter concrete pipe, and crosses underneath I-405
to Benson Road South. From there, the interceptor flows to the west, joining with drainage
from the Talbot Hills service area and discharging to the King County interceptor sewer
located north of the South Grady Way/East Valley Road intersection.
1.3 TALBOT HILLS SANITARY SEWER BASIN
Talbot Hills Sewer Basin is adjacent to the Thunder Hills Sewer Basin. To reduce flows in
the Interceptor, flows from Thunder Hills may potentially be diverted in the future to Talbot
Hills Sewer Basin directly upstream of the project area. Talbot Hills Basin also collects
sewage from predominately single and multi-family residential areas. The City has
previously identified capacity constraints in the sewer system in the Talbot Hills Basin and
has studied potential improvements to the crossing under I-405. Therefore, any diversion
will consider impacts to the downstream Sewer System.
A connection to divert from Thunder Hills to Talbot Hills Basin requires approximately
980 lineal feet of new 8-inch sewer main with manholes on South 18th Street at the
intersection of Grant Avenue South to the sewer system located southeast of Eagle Ridge
Drive South. The Talbot Hill sewer main discharges northwesterly through approximately
2,400 feet of existing 8-inch diameter sewer (from South 18th Street, near Eagle Ridge
Drive South), before connecting with the larger system at Smithers Avenue South between
South 15th Street and South 14th Street.
1.4 EXISTING ENVIRONMENT
The Interceptor travels along critical areas, including streams, wetlands, high erosion, and
landslide hazard areas. This environment has historically made maintaining the Interceptor
challenging. Additional information on the existing environment is detailed below.
1.4.1 Topography
Topography is a defining feature of the Interceptor, where the Interceptor drops
approximately 270 foot in elevation within the project area. The Interceptor is largely located
within the Thunder Hills Creek valley, which is characterized by strong downward grade and
localized steep vertical sloped sides. Pipe slopes vary from 8.9 percent when crossing the
access road to over 20 percent. Parts of the project area are considered a high severity
erosion hazard area and a moderate severity landslide hazard area. Geotechnical concerns
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will be addressed as part of the design. Upstream of the Interceptor, the Thunder Hills
sewer system collects sewage from the Rolling Hills development and generally at a more
moderate grade.
Talbot Hills Sewer System piping is largely alongside roadway; therefore, while there is a
similar drop in elevation, slopes are more moderate than the Interceptor. Unlike Thunder
Hills Sewer System, the Talbot Hills Sewer System includes an area at the base of the hill,
where the City has experienced maintenance issues due to flat pipe slopes.
1.4.2 Geology
The project area lies within the Puget Lowland. The Geological Map of King County
indicates that the site is located near the contacts between Vashon Glacial Till and Tertiary
Bedrock (Renton Formation). Geotechnical site investigations completed by Stantec
confirmed the geology, identifying shallow (1 to 4 feet below surface) or exposed
sandstone, especially on steep slopes. The geomorphological investigation indicated that
portions of Thunder Creek have eroded into the underlying bedrock. Hand borings along
steep slopes east and west of Thunder Hills Creek, by Stantec, found several inches of top
soil, 1 to 3 feet of loose silty-sand to silt with sand, underlain by hard sandstone. Deeper
borings completed in October 2014 by Stantec and a driller within the Thunder Hills Valley
encountered Colluvium, Alluvium, Fill, Landslide Debris, and Glacial Till. Areas of deep
Glacial Till were found outside the Thunder Hills valley. Several relatively recent shallow
landslides were identified as part of the site investigations.
1.4.3 Streams
Wetland and streams within the project area were delineated in 2014 by The Watershed
Company; finding Thunder Hills Creek and seven tributary streams. The classification,
direct impact to each feature and its associated buffer are presented in Table 1.1. The
delineation, stream classifications, and buffers can be found in Appendix I. Classifications
are based on Renton's 2015 critical area regulations.
Potential impacts to stream and wetland buffers have been summarized based on the
attached project plans (Appendix A) and a site visit meeting on July 2, 2015. The full extent
of all temporary impacts, including construction entrances, staging areas, and other
temporary clearing areas have not been fully documented; however, it is assumed that
temporary impacts will be limited to buffer areas and mitigated following project completion.
Thunder Hills Creek Forest Practices Application Mapping Tool (FPAMT) and the City of
Renton Maps identify Thunder Hills Creek as a Type N non-fish bearing stream.
SalmonScape identifies the piped section of the stream under I-405 as a total fish passage
barrier. Based on limited watershed area upstream of the project area, and the total fish
passage barriers downstream of the project area, the Type N classification for Thunder Hills
Creek within the project area is warranted.
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Table 1.1 Classification of Streams Along Thunder Hills Creek, Per Renton Municipal Code, and Projected Impacts to Each Feature and Its Associated Buffer
Critical Area
Renton
Municipal Code Classification
Standard
Buffer Width (ft) Direct Impact to Feature Impact to Buffer
Thunder Hills Creek Class Np 75 Possible direct impact from
culvert work associated with
Stream G drainage
Entire Project within buffer
Stream B Class Ns 50 None Gravel Road in Buffer
Stream C Class Ns 50 None Gravel Road in Buffer
Stream D Class Ns 50 None Gravel Road in Buffer
Stream E Class Ns 50 None Gravel Road, gravity wall or
rockery, and parallel sewer in buffer
Stream F Class Ns 50 None Gravel Road,
gravity wall or rockery, and parallel sewer in buffer
Stream G Class Ns 50 Repair/replace culvert or
reroute stream; Cut into portion
of stream
Gravel road, slope cut, and
gravity wall or rockery in
buffer
Stream H Class Ns 50 None Gravity road
and parallel sewer in buffer
1.4.4 Flood Plains
There are no delineated flood plains in the project area.
1.4.5 Wetlands
Site Investigations delineated six wetlands in the project area. The classification, direct
impact to each feature and its associated buffer are presented in Table 1.2. Wetlands
classifications are consistent with Ecology's 2014 requirements, where 2014 forms and figures are provided in Appendix I.
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Table 1.2 Classification of Wetlands Along Thunder Hills Creek, Per Renton
Municipal Code, and Projected Impacts to Each Feature and Its Associated Buffer
Critical Area
Renton Municipal Code
Classification (2014 Ecology Rating - wetlands only)
Standard
Buffer Width (ft) Direct Impact to Feature Impact to Buffer
Wetland A Category 3, Low Habitat Function
(4 points)
75 None Gravel Road and gravity wall or
rockery in buffer
Wetland B Category 2,
Moderate Habitat Function
(6 points)
150 None Gravel road,
parallel sewer, slope cut, and
gravity wall or rockery in buffer
Wetland C Category 4, Moderate Habitat
Function (5 points)
50 None Gravel road, paved road, and
parallel sewer in buffer
Wetland D Category 3, Moderate Habitat
Function (5 points)
100 None Gravel road, parallel sewer,
slope cut, and gravity wall or
rockery in buffer
Wetland E Category 3,
Moderate Habitat Function
(5 points)
100 Small portion of
gravel road in wetland
Gravel road in
buffer
Wetland F Category 3,
Moderate Habitat Function
(5 points)
100 Gravel road
through wetland
Gravel road and
parallel sewer in buffer
1.4.6 Endangered species/habitats
A review of online mapping resources and site visit observations by the Watershed
Company (April and May 2014) indicate that no federally threatened or endangered species
are located in the project area. The project area is located in an urban environment, where
available upland habitat is limited to isolated patches of forests that experience regular
disturbance. Listed birds, mammals, and amphibians are therefore not likely to occur in the
vicinity of the project area. As discussed above, Thunder Hills Creek does not support listed
salmonids.
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1.4.7 Public Health
Renton is committed to ensuring its sewer collection system is consistent with the public
health and water quality goals of Washington State. To this end, it has identified the
Thunder Hills Interceptor to be at risk of failure due to age, stream erosion, and other
factors. A pipeline break or manhole overtopping could potentially result in wastewater
entering Thunder Hills Creek and creating a public health hazard. A primary goal of this
project is to mitigate the public health risk from the Interceptor.
1.4.8 Public Acceptability
The City does not expect challenges in obtaining public acceptance for the project. The
Interceptor lies within land largely owned by the Berkshire Apartments, Woodcliffe
Apartments, and Puget Sound Energy. The existing maintenance road/path is believed to
be used by residents as a walking path; however, the area is not a public park and
terminates at the impassable I-405 embankment. Public outreach will be conducted as part
of the project design and construction.
1.5 DISCHARGE STANDARDS
Renton discharges to King County’s system; therefore, discharge standards are set by the
King County’s Department of Natural Resources – Wastewater Treatment Division. The
Thunder Hills Interceptor project is not expected to impact the City’s compliance with these
standards.
1.6 SUPPORTING DOCUMENTATION
Preliminary design drawings for the project are provided in Appendix A. Additionally,
information collected during pre-design is documented in a series of TMs that included:
TM No. 1 – Alternatives and Criteria (Appendix B).
TM No. 2 – Flow Analysis (Appendix C).
TM No. 3 – Remaining Useful Life (RUL) & Rehabilitation Analysis (Appendix D).
TM No. 4 – Geotechnical Investigation (Appendix E).
TM No. 5 – Preliminary Erosion Hazard Evaluation (Appendix F).
TM No. 6 – Risk Analysis (Appendix G).
TM No. 7 – Operations and Maintenance (Appendix H).
TM No. 8 – Summary of Sensitive Area Impacts (Appendix I).
TM No. 9 – Permitting Assessment Alternatives No. 1 & 2 (Appendix J).
TM No. 10 – Cost Study (Appendix K).
The information provided in the TMs supports both the alternative analysis and design.
Therefore, pertinent information from TMs is included in the discussion of the screening and
evaluation of the Alternatives in the following sections.
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1.7 ACKNOWLEDGMENTS
This report is based on the work of a multi-disciplinary team that was key to success of this
challenging project. Acknowledgement of the team members roles are provided below.
Project Manager and Design Lead - Erik Waligorski of Stantec.
Hydraulic modeling - Erik Brodahl and Brian Wolf of Stantec.
Geotechnical Investigation and Erosion Hazard Evaluation - Phil Haberman of Stantec.
Permitting - Rebecca Saur of Stantec.
Project Staff Professional - Stephanie Ard of Stantec.
Alternative Analysis - Lara Kammereck and Dan Reisinger of Carollo Engineers, Inc.
Environmental and Wetlands - Hugh Mortensen and Katy Crandall of the Watershed
Company.
City Staff - Dave Christensen, John Hobson, Rocky Sittner, Stan Job, and Rich Marshall.
1.8 REPORT CONTENTS
The contents of this report are organized in four Chapters. The proceeding Chapter 1
provided an introduction to the project and the surround area. Chapter 2 presents the land
use, demographics, and flow projections for the Interceptor. Chapter 3 details the
comprehensive alternative analysis leading to a recommended alternative. Chapter 4
provides information on the recommended alternative.
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Chapter 2
FLOW PROJECTIONS
Wastewater can be divided into four major components: 1) domestic wastewater,
2) industrial (/commercial/school) wastewater, 3) inflow and 4) infiltration. Domestic and
industrial wastewater loadings are identified based on contributing population and
employment, which are determined based on land use and demographic projections. I/I
rates are estimated based on flow monitoring data and typically are the largest component
of wastewater flows.
2.1 LAND USE
The Thunder Hills Sewer Basin and Talbot Hill Sewer Basin are largely single-family
residential with substantial multi-family residential areas. Limited commercial land use is
also found in both basins. The City’s Comprehensive Plan directs future development
through land use planning, which is shown for the project in Figure 2.1. As shown in the
figure, land use is expected to remain consistent with existing land use.
2.2 DEMOGRAPHICS
The Thunder Hills Interceptor will be designed to convey the Ultimate population and
employment for its tributary area. Ultimate flows were based on the PSRC Land Use
Baseline demographic projections for 2040. Demographic projections were developed on
the mini-basin scale, which are shown in Table 2.1. For Mini-Basin 3, projections were
developed from the PSRC Land Use Baseline projections for the year 2040. As directed by
the City, an additional 25 percent was added to the 2040 population estimates as a factor of
safety. The PSRC projections for Mini-Basin 45 (which is tributary to the Thunder Hills
interceptor) for the year 2040 showed large decreases in single and multi-family residential
population, which is inconsistent with the City’s Comprehensive Plan land use designations
and current development activity for the basin. Because of this, Ultimate populations for the
portions of Mini-Basin 45 tributary to the Thunder Hills Interceptor were computed by
applying the growth percentages calculated for Mini-Basin 3 based on PSRC estimates.
The Talbot Hills Sewer Basin consists of Mini-Basin 1, 2, and 7. Demographic projections
for these mini-basins are shown in Table 2.2. Projections were developed from the PSRC
Land Use Baseline projections for the years 2012 and 2040. As with the Thunder Hills
Sewer Basin, the Ultimate Population estimates added an additional 25 percent to the 2040
population estimates as a factor of safety.
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THUNDER HILLS AND TALBOT SANITARY SEWER BASIN LAND USE
FIGURE 2.1
CITY OF RENTONTHUNDER HILLS01,000 2,000500FeetO
Legend
Major RoadsParcelsThunder Hills Sewer BasinTalbot Hills Sewer BasinComprehensive Land Use DesignationsResidentialResidential Low DensityResidential Single FamilyResidential Medium DensityResidential Multi-FamilyEmployment Area DesignationsEmployment Area - ValleyCommercial DesignationsCommercial NeighborhoodCommercial Corridor
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Table 2.1 Thunder Hills Sewer Basin Demographic Projections
Mini-Basin 3 Mini-Basin 45
2012 Population (PSRC)
Ultimate Population (2040 + 25%)
2012 Population (PSRC)
Ultimate Population (2040 + 25%)
Single Family Residential 1,102 1,660 20 33
Multi-family Residential 1,307 1,836 1,157 1,626
Employment 95 248 NA NA
Notes:
(1) PSRC = Puget Sound Regional Council
Table 2.2 Talbot Hills Sewer Basin Demographic Projections
Mini-Basin 1 Mini-Basin 2 Mini-Basin 7
2012
Population (PSRC)
Ultimate Population
(2040 + 25%)
2012
Population (PSRC)
Ultimate Population
(2040 + 25%)
2012
Population (PSRC)
Ultimate Population
(2040 + 25%)
Single
Family Residential
180 554 81 209 1,232 1,936
Multi-family
Residential
82 420 1,303 1,608 27 31
Employment 22 29 342 713 67 88
Schools 0 0 1,041 1,367 526 822
Notes:
(1) PSRC = Puget Sound Regional Council
2.3 INFLOW & INFILTRATION
I/I parameters for this analysis were based on the previous model calibration, which used
2001/2002 flow monitoring data. This calibration effort included flow monitors at each of the
mini-basins, which provided more detail. For the Ultimate model analysis, I/I parameters
have been increased by 28 percent to simulate increases in I/I due to pipe deterioration
over the planning period. This results in a rate of 4,062 gallons per acre per day (gpad) for
Basin 3 and 602 gpad for Basin 45. Detailed description of the analysis can be found in TM
7 in Appendix C.
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2.4 FLOW PROJECTIONS
Peak flow projections for the Thunder Hills Interceptor and Talbot Hills Sewer System were
developed for the existing (2012) and Ultimate population scenarios. Flow rates are based
on a 20-year flow event, represented by a November 5, 1998 storm, as shown in
Table 2.2. Detailed description of flow projections can be found in TM No. 2 (Appendix
C). Thunder Hills Interceptor and Talbot Hill Sewer System flows rates were provided at the
downstream end of the I-405 crossing. Sanitary flows were based on the population
projections listed in Table 2.1 and the peak projected per capita flow rate is 215 gal/PE/day.
The projected I/I contribution is approximately 4,100 gpad. I/I represents approximately 700
gallons per minute (gpm) of the 1,125 gpm of flows projected for the Interceptor in the
Ultimate scenario.
The projected Ultimate peak flow rate in the Talbot Hills Sewer System is 600 gpm, and the
projected Ultimate peak flow rate for the Thunder Hills diversion is 850 gpm, which would
increase the overall peak flow rates into the Talbot Hill sewer service area to 1,450 gpm.
Table 2.3 Peak Flow Rate Projections for Thunder Hills Sewer Interceptor and
Talbot Hills Sewer System
Without Flow Diversion With Flow Diversion
Sewer
2012
Population (PSRC)
Ultimate
Population (2040 + 25%)
2012
Population (PSRC)
Ultimate
Population (2040 + 25%)
Thunder Hills Interceptor 865 gpm 1,125 gpm 210 gpm 285 gpm
Talbot Hills
Sewer System
460 gpm 600 gpm 1,120 gpm 1,450 gpm
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Chapter 3
ALTERNATIVE ANALYSIS
Multiple infrastructure alternatives can rehabilitate or replace the Thunder Hills Sewer
System to provide an additional 80 years of service. These alternatives minimize risk to
environmentally sensitive/critical areas, consider surface water erosion and I/I issues, and
improve access to the Interceptor. An extensive alternative analysis process was conducted
to identify a preferred alternative, which is described in this Chapter.
3.1 ALTERNATIVE ANALYSIS METHODOLOGY
A systematic process was used to analyze alternatives for the rehabilitation or repair of the
Thunder Hills Interceptor. The process included the following steps:
1) Establish analysis criteria
2) Define alternatives
3) Collect information on alternatives
4) Flow capacity screening
5) Evaluate alternatives based on high priority criteria
6) Select alternatives for detailed evaluation
7) Evaluate detailed alternatives based on low and medium priority criteria
8) Select preferred alternative for design and construction
The following workshops with City staff and Consultants were conducted to aid the
Alternative Analysis process:
Workshop No. 1 - Establish Criteria and Define Alternatives (August 5, 2014) - Step 1.
Workshop No. 2 - Flow Capacity Screening (December 16, 2014) - Step 4.
Workshop No. 3 - High Priority Criteria Screening (February 26, 2015) - Step 5.
Workshop No. 4 - O&M, Permitting, and Environmental Criteria Screening (April 23, 2015) -
Step 7.
Workshop No. 5 - Thunder Hills Permitting Update (November 5, 2015) - Step 7.
A collaborative workshop was held to define the alternatives and establish analysis criteria.
The defined alternatives were screened based on flow capacity, which eliminated
alternatives that were not consistent with available capacity in the Thunder Hills Interceptor.
Information to aid in evaluating the established criteria was collected and documented in a
series of Technical Memoranda (TM). Alternatives were evaluated based on the criteria and
collected information to identify two or three alternatives for more detailed investigation. A
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preferred alternative was selected based on the detailed investigation. Details on each step
are provided in this Chapter.
3.2 ESTABLISH ANALYSIS CRITERIA (STEP 1)
Criteria for analyzing alternatives were identified by the City during Workshop No. 1. Criteria
included considerations for planning, construction, and operation of the Thunder Hills
Interceptor, as shown in Table 3.1. Criteria were divided into high/medium/low prioritization.
The high priority criteria were used to evaluate the defined alternatives (Step 4 of the
Alternative Analysis Process). The medium and low priority criteria were used to select the
preferred alternative (Step 5 of the Alternative Analysis Process).
Flow capacity in the Thunder Hills Interceptor was identified as a fatal flaw to alternatives,
rather than a criterion. Therefore, flow capacity was used as an initial screen of the
alternatives presented above.
Table 3.1 Criteria and Prioritization
HIGH MEDIUM LOW
• Pipe risk = Criticality x RUL • Maintenance of alignment
and equipment needed
• Schedule
• Access to manholes • Cost: Capital and O&M • Permitting: application/
acquisition & schedule
• Equipment access along pipe • Vehicle access along pipe • Environmental mitigation
• RUL • Access risk = slide/erosion
• Constructability • ROW/Easement
• Regulatory risks
3.3 DEFINE ALTERNATIVES (STEP 2)
Project alternatives for the Thunder Hills Interceptor were developed through brainstorming
at Workshop No. 1 with City Staff and Consultants. The discussions identified seven project
alternatives labeled A through G:
A. Do nothing.
B. Rehabilitate existing line in existing location.
C. Construct a new line parallel to existing line.
D. No Thunder Hills line: divert all of Thunder Hills flow by installation of a lift station for
lower flows to Talbot Basin.
E. Divert a portion of flow from Thunder Hills Basin to Talbot Basin then implement
alternative B or C.
F. No Thunder Hills basin flow under I-405; divert before slide area to Talbot Basin (to
Talbot Road) or north to Renton Hill Basin.
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G. Phase Improvements for projected flows at 10, 20, and 50 years.
Details on the Alternatives and evaluation criteria are provided in TM No. 1 (Appendix B).
3.4 COLLECT INFORMATION ON ALTERNATIVES (STEP 3)
The criteria established for the Thunder Hills Interceptor considered the pipe (e.g.,
Remaining Useful Life [RUL]), the environment (i.e., pipe risk, regulatory risks,
environmental mitigation, etc.), project construction (i.e., constructability, cost, etc.), and
maintenance (i.e., access to manholes, equipment access along pipe, etc.). The collection
information is documented in a series of TMs that are listed in Section 1.6. Pertinent
information from TMs is included in the discussion of the screening and evaluation of the
Alternatives in the following sections.
3.5 FLOW CAPACITY SCREENING (STEP 4)
Flow capacity was used to initially screen Interceptor Alternatives to remove those
alternatives that did not meet the City's capacity criteria. The flow capacity of the Thunder
Hills Interceptor was determined for existing and future conditions from Chapter 2 of this
BODR and in detail in TM No. 2 – Flow Analysis. The City’s hydraulic model was used to
evaluate the Thunder Hills and Talbot Hill Sewer System.
3.5.1 Flow Capacity
A hydraulic analysis of flow in the Interceptor shows no surcharging for existing (2012)
or Ultimate flows, except for a short section of 10-inch pipe upstream of MH#5320049
(MH2323). Modeling shows that a reduction in surface roughness expected from replacing
or slip-lining the Interceptor will provide sufficient capacity resolve surcharging upstream of
MH#5320049 (MH2323). With the improved pipe, the Interceptor is expected to have a
capacity of 1,290 gpm, exceeding the projected Ultimate flows. Significantly, the 18-inch
sewer under I-405 was found to have excess capacity with a Q/Qfull of less than 0.50
during Ultimate flows. Q/Qfull is the ratio of the flow (Q) to the flow capacity when the pipe
is full (Qfull), where larger values represent less remaining capacity in the pipe.
Alternative E proposes a pipeline diversion from the Thunder Hills Interceptor to the Talbot
Hill Sewer System, which is shown as the orange pipe in Figure 3.1. The diversion enables
all Thunder Hills Interceptor flow above Grant Ave S to be diverted. With the additional flow,
flow capacity is exceeded in the Talbot Hill Sewer System. Therefore, upgrades to the
Talbot Hill sewer must be made in conjunction with Alternative E. Required upgrades to the
existing pipes along the Talbot Hill sewer diversion route were identified and are shown in
Figure 3.1.
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3.5.2 Flow Screening Results
Alternatives were screened based on the findings of the flow capacity analysis. Screening
was conducted by City Staff as part of Workshop No. 2 – Flow Capacity Screening and
reaffirmed in Workshop No. 3 – High Priority Criteria Screening. Alternatives were either
eliminated or maintained for future evaluations based on their reasonableness considering
the good available capacity in the Thunder Hills Interceptor. The screening results are
shown in Table 3.2.
The City has established a goal to limit the use of lift stations to the extent practical. Given
available flow capacity, Lift Station Alternatives D and F were eliminated, since lift stations
were not required to convey Ultimate flows. The creation of new lift stations, when available
gravity sewer capacity is available, was not consistent with the intent of the City's Lift
Station Elimination Policy that states "Highest priority should be given to elimination of lift
station facilities because of their high degree of vulnerability and high operation and
maintenance costs." The passive diversion of Thunder Hills flow (Alternative E) was
maintained.
Upon further consideration, Alternatives A and G not considered to be stand-alone options
and were eliminated. For Alternative A, a distinction was made that the analysis should
identify needed improvements to the Interceptor, as the existing Interceptor does not meet
the design criteria. For Alternative G, the City may choose to phase the project during
design, where no work would be required in the short-term.
Be
n
s
o
n
D
r
S
B
e
n
so
n
Rd
S
S 15th St
S 18th St
E a gle Rid g e D r.
Grant Ave S
Smithers Ave S
S Puget Dr
I-405
Figure 3.1
Thunder Hills Interceptor Flow Analysis
Talbot Hill Sewer
Recommended Improvements
.
Proposed 8" Diversion Pipe to
Talbot Hill Sewer
0 200 400100Feet
Legend
Recommmmended Improvements
- Upsize to 10" Pipe
Recommmmended Improvements
- Upsize to 12" Pipe
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Table 3.2 Flow Capacity Alternative Screening
Alternative
Number Alternative Name
Maintained or
Eliminated Comments
A Do nothing Eliminated Without
improvements, the
Interceptor does not
achieve the sought
RUL.
B Rehabilitate existing line in existing location Maintained The alternative is a viable option, since
the Interceptor has
adequate capacity.
C Construct a new line
parallel to existing line
Maintained
D No Thunder Hills line: divert all of Thunder Hills
flow by installation of a lift station for lower flows to Talbot Basin
Eliminated There is sufficient
capacity to convey
the ultimate flows
under I-405;
therefore, a lift
station was not
needed.
E Divert a portion of flow
from Thunder Hills Basin to Talbot Basin then
implement Alternative B or C
Maintained Requires
improvements to Talbot Basin Sewer
System.
F No Thunder Hills basin
flow under I-405; divert before slide area to
Talbot Basin (to Talbot
Road) or north to Renton Hill Basin
Eliminated As with Alternative
D, no lift station was needed; therefore,
this alternative was
eliminated.
G Phase Improvements for
projected flows at 10, 20, and 50 years
Eliminated This alternative will
be incorporated into
design, rather than
a stand-alone
alterative.
3.6 EVALUATE ALTERNATIVES BASED ON HIGH PRIORITY
CRITERIA (STEP 5)
The updated Alternatives were evaluated against the high priority criteria. The goal of the
high priority evaluation was to identify two or three Alternatives for more detailed
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investigation. High priority criteria were evaluated to aid in the City’s decision making.
Criteria ratings were based on the collected information and agreed upon by City Staff.
Ratings were as follows:
• “- -” considered unacceptable.
• “-” considered acceptable.
• “+” considered good.
• “+ +” considered very good.
High priority criteria ratings represent the ability of the alternative to address issues related
to a given high priority criteria. A high priority criteria rating of “- -” was considered to
eliminate an alternative from further consideration. A rating of “-” indicated that additional
infrastructure would likely be required to mitigate issues. A rating of “+”and “+ +” indicated
the City considered the alternative could resolve issues related to a given criteria. Ratings
for each criteria were combined to evaluate the overall quality of a given alternative. A
discussion of the alternative evaluation by criteria is provided below.
3.6.1 Updated Alternatives
Three alternatives were maintained based on the flow screening flowing Workshop No. 2 -
Flow Capacity Screening:
• Alternative B - Rehabilitate existing line in existing location,
• Alternative C - Construct a new line parallel to existing line, and
• Alternative E - Divert a portion of flow from Thunder Hills Basin to Talbot Basin then
implement Alternative B or C.
Alternative E diverts flow to the Talbot Hills Sewer System, which can be done both with
rehabilitating (Alternative B) and parallel piping (Alternative C) the Interceptor. Further, a
third option to use a mix of rehabilitation (Alternative B) and parallel pipe (Alternative C)
was proposed. Therefore, Alternatives were reorganized into six new alternatives
combining the decision to divert flows to the Talbot Hills Sewer Basin and three pipe
techniques. To reduce confusion, alternatives were referred to by text identifications as
follows:
1. Existing – Rehab Alternative designates the Existing Alignment – Rehabilitation Only
alternative.
2. Existing – Parallel Alternative designates the Existing Alignment – Parallel Pipes
Only alternative.
3. Existing – Mix Alternative designates the Existing Alignment – Mix of Rehabilitation and Parallel Pipes alternative.
4. Divert – Rehab Alternative designates the Divert to Talbot – Rehabilitation Only alternative.
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5. Divert – Parallel Alternative designates the Divert to Talbot – Parallel Pipes Only alternative.
6. Divert – Mix Alternative designates the Divert to Talbot – Mix of Rehabilitation and
Parallel Pipes alternative.
Where “Existing” refers alternatives using only the existing Thunder Hills system (no
diversion) and “Divert” refers to alternatives that include the Talbot Hills Basin diversion
(Alternative E).
3.6.2 Pipe Risk
Pipe risk considers the potential for the Thunder Hills Interceptor to overflow or fail;
potentially discharging wastewater into Thunder Hills Creek. Pipe risk was defined as:
Pipe Risk = RUL x Criticality
RUL is defined as the Remaining Useful Life of the pipeline. Criticality was considered to be
the consequence of failure based on the flow capacity and environmental factors. Since
there were no flow capacity issues with the remaining alternatives, Criticality was defined by
general erosion potential. Information on each parameter and the pipe risk rating is
presented below.
Remaining Useful Life
RUL was determined using CCTV Inspection recordings from City Staff. Pipe segments
were rated as good (30 to 40 years), moderate (20 to 30 years), or poor (10 to 20 years), as
shown in Table 3.3. RUL was largely based on pipe material, where Asbestos-Cement (AC)
pipes were rated as having moderate RUL and Ductile Iron (DI) pipes were rated as having
good RUL. Pipes in poor condition were generally related to corrosion of cast iron fittings or
separated joints. To aid in calculating pipe risk, a numeric pipe risk designation was
assigned to each pipe segment: reaches with largely good RUL were given a numeric value
of 1, reaches with largely moderate RUL were given a numeric value of 2, and no reaches
were given a poor designation. Detailed information on RUL is shown in TM No. 3
(Appendix D).
Table 3.3 RUL Designation by Stream Stationing
Reach
Location
Length of Good RUL
(feet)
Length of Moderate RUL
(feet)
Length of Poor RUL
(feet)
RUL
Designation
0+00 to 1+00 100 0 0 1
1+00 to 5+50 437 13 0 1
0+00 to 1+00 100 0 0 1
1+00 to 5+50 437 13 0 1
5+50 to 8+30 277 3 0 1
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Table 3.3 RUL Designation by Stream Stationing
Reach Location
Length of
Good RUL (feet)
Length of
Moderate RUL (feet)
Length of
Poor RUL (feet) RUL Designation
8+30 to 11+00 253 17 0 1
11+00 to 12+30 124 0 6 1
12+30 to 18+90 619 32 9 1
18+90 to 26+75 65 684 36 2
26+75 to 28+00 0 125 0 2
Notes:
(1) RUL = Remaining Useful Life. (2) Data from TM No. 3 – Remaining Useful Life (RUL) & Rehabilitation Analysis (Stantec 2015).
Criticality - General Erosion Potential
Criticality of the Interceptor pipe segments varies depending on the reach of Thunder Hills
Creek. General stream bank erosion was considered the most critical factor for individual
pipe segments. Unstable stream banks may result in pipe failure due to undercutting,
exposure, and other risks.
The generalized erosion potential represents the stability of the stream banks over a long
period of time. Generalized erosion potential was evaluated in 20 year increments for the
80 year planning horizon using the same stationing as RUL. Pipe reaches were
characterized as stable, marginal, and unstable, as shown in Table 3.4. Erosion is expected
to continue in Creek over the next 80 years; therefore, the stability of stream reaches
degrade with time. No reach was considered to be unstable at this time; however, three
locations are expected to become unstable in the next 80 years:
A. At the bottom of the hill from Stationing 1+00 to 5+50.
B. Where the pipe crosses the stream from Stationing 11+00 to 12+30.
C. At the top of the hill from Stationing 26+75 to 28+00.
All other locations were considered stable or marginal. Unstable reaches may be addressed
through stream bank armoring/rockery construction and/or relocating the Interceptor further
from the creek. Detailed information on general erosion potential is provided in TM No. 5
(Appendix F).
To aid in calculating pipe risk, a numeric criticality designation was assigned to each reach
at a workshop with City Staff for each reach shown in Table 3.4: stable reaches were given
a numeric value of 1, marginal reaches were given a numeric value of 2, and unstable
reaches were given a numeric value of 3.
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Table 3.4 General Erosion Potential Designation by Stream Stationing
Reach Location
General
Erosion Potential - 20 Years
General
Erosion Potential - 40 Years
General
Erosion Potential - 80 Years Criticality Designation
0+00 to 1+00 Stable Stable Stable 1
1+00 to 5+50 Marginal Unstable Unstable 3
5+50 to 8+30 Stable Stable Stable 1
8+30 to 11+00 Stable Stable Marginal 2
11+00 to 12+30 Stable Marginal Unstable 3
12+30 to 18+90 Stable Stable Stable 1
18+90 to 26+75 Stable Stable Stable 1
26+75 to 28+00 Marginal Unstable Unstable 3
Notes:
(1) Data from TM No. 5 – Preliminary Erosion Hazard Evaluation (Stantec 2015)
Pipe Risk Summary
Pipe risk was calculated by multiplying the RUL ratings and Criticality ratings, as shown in
Table 3.5. The highest pipe risk was for Stationing 26+75 and 28+00, which is an AC pipe
in an unstable section of the Creek. The Divert to Talbot alternatives allows this pipe
segment to be decommissioned throughout the unstable stream reach. There were also two
additional high pipe risk segments in unstable sections of the creek: Stationing 1+00 to
5+50 and 11+30 to 12+30. Additional information on Pipe Risk is provided in TM No. 6
(Appendix G).
Table 3.5 Pipe Risk Designation by Stream Reach
Reach Location RUL Designation Criticality Designation Pipe Risk Designation
0+00 to 1+00 1 1 1
1+00 to 5+50 1 3 3
5+50 to 8+30 1 1 1
8+30 to 11+00 1 2 2
11+00 to 12+30 1 3 3
12+30 to 18+90 1 1 1
18+90 to 26+75 2 1 2
26+75 to 28+00 2 3 6
Notes:
(1) Data from TM No. 6 – Risk Analysis (Stantec 2015).
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3.7 ALTERNATIVE ANALYSIS (STEP 6)
Pipe Risk Scoring
Qualitative ratings were assigned to each of the remaining alternatives by the City based on
the numeric pipe risk designations. Ratings represent the ability of the alternative to resolve
the existing pipe risk. A group of City Staff and Consultants evaluated the likelihood of the
proposed Alternatives in addressing pipe risk and other criteria in the Alternative Analysis
workshop. The resulting ratings are shown in Table 3.6. A“+ +” rating was assigned to the
Divert – Parallel and Divert – Mix alternatives. These alternatives can address all areas of
highest pipe risk and will divert a substantial quantity of flow from the Interceptor, reducing
the potential impact to sensitive areas in the event of a pipe failure or overflow. A “+” rating
was assigned to the Divert – Rehab alternative. In this alternative, the area of highest pipe
risk can be avoided; however, mitigation through bank protection would be required for
other areas of high risk. A “+” rating was assigned to the Existing – Parallel and Existing –
Mix alternatives. The alternatives can avoid areas of high pipe risk by paralleling the
existing pipe, but maintain a higher flow than similar Divert to Talbot alternatives, and
therefore, more risk. A “-” rating was assigned to Existing – Rehab alternative, since all pipe
risk must be mitigated through bank protection.
Table 3.6 Thunder Hills Sewer Interceptor Alternative High Priority Criteria and Ratings
Full Flow Reduced Flow
Criteria
Rehab. (Existing
– Rehab)
Parallel (Existing
– Parallel)
Mix of
Rehab. and
Parallel (Existing –
Mix)
Rehab. (Divert –
Rehab)
Parallel (Divert –
Parallel)
Mix of
Rehab. and
Parallel (Divert –
Mix)
Pipe Risk - + + + + + + +
RUL + + + + + + + + + + + +
Access to MH + + + + + + + + + +
Equipment access - + + + - + + +
Constructability + - - - - - - -
Notes:
(1) “- -” considered unacceptable. (2) “-” considered acceptable. Additional infrastructure may be required to mitigate issues.
(3) “+” considered good. (4) “+ +” considered very good.
3.7.1 RUL Scoring
A major goal of the project was to provide 80 years of RUL for the Thunder Hills Interceptor,
where the existing pipe has an RUL of 40 year or less. RUL will be increased to at least
80 years in all alternatives; therefore all alternatives were given a “+ +” RUL ratings.
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3.7.2 Access to Manhole Scoring
Access to manholes is difficult for some areas of the existing Thunder Hills Interceptor and
the City would like to improve access as part of the project. Increased access may be
obtained by improving the area around existing manholes, relocating manholes, or adding
new manholes. Rehab alternatives will improve access to manholes by adding a new
manhole between Stationing 1+00 and 5+50 to facilitate pipe rehabilitation and to improve
future access to manholes. A “+ +” rating was assigned to both Parallel and Mix
alternatives, since any access to manhole issues can be addressed through new pipe
alignments or manholes. A “+” rating was assigned to both Rehab alternatives, as shown in
Table 3.6.
3.7.3 Equipment Access Scoring
Equipment access for the existing Thunder Hills Interceptor is difficult. Access is limited by
both the ability to get equipment to the Interceptor’s easement and moving equipment
within the easement. Similar to access to manholes, equipment access was rated in
Workshop No. 3 and is shown in Table 3.6. A “+ +” rating was assigned to both Parallel
alternatives, since establishing full access to the entire length of the Interceptor would be
required for construction. A “+” rating was assigned to both Mix alternatives, since
equipment access would likely be improved during construction. A “-” rating was assigned
to both Rehab alternatives, since changes to equipment access would likely be temporary
or minor.
3.7.4 Constructability Scoring
Construction related to the Thunder Hills Interceptor is expected to be difficult for all
alternatives due to the steep terrain and proximity to the creek. However, some alternatives
will likely be easier to construct than others will. Similar to the other criteria, constructability
was evaluated in Workshop No. 3 and is shown in Table 3.6. Constructability considered
both the existing alignment and the improvements to the Talbot Hills sewer, when
applicable.
The group assigned a “+” rating to the Existing – Rehab alternative, since the alternative
has the least construction effort. The group assigned a “-” rating to the Divert – Rehab
alternative due to the extra construction required for the Talbot Hills sewer improvements.
The group expects the Talbot Hills sewer improvements to be challenging and difficult to
coordinate with Thunder Hills improvements. Construction of the Parallel alternatives may
not be able to be constructed; therefore, a “- -” rating was assigned to both related
alternatives. A “-” rating was assigned to the Existing – Mix alternative due to the difficulty in
constructing in the existing alignment. A “-” rating was assigned to the Divert – Mix
alternative due to the difficulty constructing both the Thunder Hills and the Talbot Hills
sewer improvements.
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3.7.5 Summary of Alternatives
City staff evaluated each alternative using the ratings described above in a workshop
setting with the goal of selecting two to three alternatives for detailed analysis. Two
alternatives were selected based on the consistent good and very good ratings:
• Alternative 1 - Existing Alignment – Mix of Rehabilitation and Parallel Pipes
alternative (Existing – Mix)
• Alternative 2 - Divert to Talbot – Mix of Rehabilitation and Parallel Pipes alternative
(Divert – Mix)
The City chose not to investigate the Rehabilitation only alternatives (Existing – Rehab and
Divert – Rehab) further, since they did not fully mitigate risks from erosion. However, the
City clarified that the selected mix alternatives should make use of rehabilitation
technologies when possible. Due to the “- -“ constructability rating, the parallel pipe only
alternatives (Existing – Parallel and Divert – Parallel) were eliminated from further
consideration.
3.8 EVALUATE DETAILED ALTERNATIVES BASED ON LOW AND MEDIUM PRIORITY CRITERIA (STEP 7)
High priority criteria was used to select two alternatives to review evaluate in further detail:
Alternative 1 (Existing – Mix) and Alternative 2 (Divert – Mix). Medium and low priority
criteria will be used to select a final alternative for design and construction. Medium and low
priority criteria include cost, O&M, permitting, environmental, and construction
management. The two alternatives were developed to the approximately fifteen percent
design level to allow detailed analysis of these criteria.
3.8.1 Alternative 1 (Existing – Mix)
Alternative 1 (Existing - Mix) receives the full flow from the Thunder Hills Sewer Basin and
uses the Interceptor's existing alignment. The Interceptor will be renewed through a mix of
rehabilitation and parallel pipes, as shown in Figure 3.2.
The dashed green lines in Figure 3.2 represent rehabilitation in place for the Interceptor
using a CIPP technology. Thicker dashed pink lines represent new sewer pipe. New parallel
pipes remove the Interceptor from within Thunder Hills Creek and place it in the proposed
gravel maintenance road/path shown in dark gray. Rock buttresses and Gabion walls are
used to protect the road/Interceptor from stream and hillside erosion. Details on
geotechnical improvements are provided in TM No. 4 (Appendix E). Storm culverts will be
installed to convey flow from tributaries without eroding the access road and path.
The access road and path for the Interceptor will be improved to support O&M activities,
which addresses both high and medium priority criteria. Enhancements include:
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1) A 12-foot wide access road is planned upstream of the Berkshire Apartments
(Station 8+00).
2) A paved bell will be used to transition to the Puget Sound Energy (PSE) access road
(12-feet wide), allowing an area to turnaround.
3) A turnaround area will be located at Station 8+00, which will require additional
easements.
4) A 7-foot wide path is planned below the Berkshire Apartments, which requires an
easement machine to access.
5) A turnaround area for the Easement machine is located at Station 0+00.
Additionally, Alterative 1 includes new and rehabilitated stream bank protection and
environmental mitigation within the stream buffers.
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PROPOSED LOCATIONS OF
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ALTERNATIVE NO. 1 LEGEND
PROPOSED LOCATION OF
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3.8.2 Alternative 2 - Divert to Talbot – Mix of Rehabilitation and Parallel
Pipes Alternative (Divert – Mix)
Alternative 2 (Divert - Mix) diverts the majority of the Thunder Hills Sewer Basin flows to the
adjacent Talbot Hills sewer basin upstream of the Interceptor. However the Interceptor will
still be required to serve customers contributing directly adjacent to the Interceptor
(Berkshire apartments, etc.). Identical improvements to Alternative 1 are proposed below
approximate Station 23+50, as shown in Figure 3.3, Sheet 2 of 3. The City intends to retire-
in-place the Interceptor above Station 23+50.
Dashed green lines represent rehabilitation in place of the Interceptor using a CIPP
technology. Thicker dashed pink lines represent new sewer pipe. New parallel pipes
remove the Interceptor from within Thunder Hills Creek and place it in the proposed gravel
maintenance road/path shown in dark gray. Rock buttresses and Gabion walls are used to
protect the road/Interceptor from stream and hillside erosion. Details on geotechnical
improvements are provided in TM No. 4 (Appendix E). Storm culverts will be installed to
convey flow from tributaries without eroding the access road and path.
The access road and path for the Interceptor will be improved to support O&M activities.
Roadway improvements are still required through the retired section of pipe to provide
access to the active portion of the Interceptor. Identically to Alternative 1, enhancements
include:
1) A 12-foot wide access road is planned upstream of the Berkshire Apartments
(Station 8+00).
2) A paved bell will be used to transition to the PSE access road (12-foot wide), allowing
an area to turnaround.
3) A turnaround area will be located at Station 8+00, which will require additional
easements.
4) A 7-foot wide path is planned below the Berkshire Apartments, which requires an
easement machine to access.
5) A turnaround area for the Easement machine is located at Station 0+00.
Additionally, Alterative 2 includes new and rehabilitated stream bank protection and
environmental mitigation within the stream buffers.
The Thunder Hills Sewer Basin to Talbot Hills Sewer Basin Diversion would be conveyed
by 980 feet of new 8-inch diameter pipe along South 18th Street, as shown in Figure 3.3.
Approximately 180 feet of existing sewer on South 18th Street will require upsizing to 12-
inch pipe (dashed fluorescent blue lines). The additional flow also requires upsizing
approximately 690 feet of pipe on 108th Ave. The upsized piping under Talbot Road South
and at Smithers Avenue South will resolve anticipated future deficiencies based on Talbot
Hills sewer flows, as well as the additional Thunder Hills sewer flows. Therefore, these
improvements will be needed regardless of the diversion in the future, although the size of
the improvements may be impacted to provide adequate capacity.
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UPSIZE EX SEWER PIPE
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UPSIZE EX SEWER PIPE
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UPSIZE EX SEWER PIPE
REDIRECT EX SEWER PIPE
ALTERNATIVE NO. 2 LEGEND
PROPOSED LOCATION OF
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PROPOSED LOCATION OF
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3.8.3 Evaluation of Medium and Low Criteria
The City and consultant team rated the Alternatives for each criteria as was performed
during Workshop 4 and 5. Workshop 4 focused on O&M, permitting, and environmental
elements. Workshop 5 focused solely on cost. Alternatives were rated with the "+" and "-"
system used for the high priority criteria (Section 3.7). An explanation of each criteria rating
is provided below. The medium and low priority criteria are presented in Table 3.7, along
with the ratings for the Alternatives.
Table 3.7 Ratings for Low and Medium Priority Criteria
Category Criteria
Criteria Priority
Alternative 1
(Existing – Mix) Alternative 2 (Divert – Mix)
O&M Maintenance of alignment and equipment
needed
Medium + +
O&M City Crews
vehicle access
along pipe
Medium + +
O&M Access risk =
slide/erosion
Medium + +
Permitting Regulatory risks Medium - -
Permitting Permitting:
application/
acquisition & schedule
Low - -
Environmental Environmental
mitigation
Low - -
Cost Cost: Capital and
O&M
Medium - -
Cost ROW/Easement Medium - -
CM Project Schedule Low - -
Notes: “- -” considered unacceptable. “-” considered acceptable. Additional infrastructure may be required to mitigate issues.
“+” considered good.
3.8.4 O&M Considerations
Three elements of O&M were considered for the Alternatives Analysis. Detailed information
on O&M aspects of the Inceptor rehabilitation can be found in TM No. 7 (Appendix H). O&M
considerations were addressed through improvements to the access road and path, as well
as the addition of manholes to the Interceptor. The criteria were rated as the following:
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Maintenance of alignment and equipment needed: This criteria considered O&M of the
interceptor. A “+” or good rating was given to both alternatives. Both alternatives require
identical improvements to the access road along the Thunder Hills Interceptor. New Talbot
Basin sewer constructed in Alterative 2 was considered relatively easy to maintain in the
roadway.
City crew vehicle access along pipe: This criteria was more narrowly focused on ability of
vehicular access (vactor truck, easement machine, etc.), such as road width and
turnaround areas. Both alternatives provide equivalent access; therefore, a “+” or good
rating was given to both alternatives.
Access Risk = slide/erosion: The slide/erosion risk is addressed through stream bank
protection and parallel piping of the Interceptor in both the alternatives. Therefore, a “+” or
good rating was given to both alternatives.
3.8.5 Permitting Considerations
Two elements of permitting considerations were used for the Alternatives Analysis. Detailed
information on regulatory risk permitting aspects of the Inceptor rehabilitation can be found
in TM No. 8 (Appendix I) and TM No. 9 (Appendix J), respectively. A matrix of required
permits, timeline for submittal of the permits, and potential submittal fees are provided in
Table 3.8. Similarities between Alternatives 1 and 2 trigger similar permit requirements.
Improvements to the Talbot Sewer System require Alternative 2 to obtain additional right of
way permits with the City of Renton and Washington State Department of Transportation
(WSDOT) for work in Benson Drive South (State Route 515). Consistent with the City's
general practices for utility projects, an unanticipated discovery plan will be used for the
project to appropriately manage archeological resources, if discovered.
US Army Corps of Engineers (USACE) and PSE will likely require the longest timeline with
a 12 to 15 month period for permitting. WSDOT permitting may also be lengthy, but can be
accomplished within the timeframe needed for USACE and PSE.
Regulatory Risks: At the workshop, a "-" rating was assigned for both alternatives. Both
alternatives were considered to have equivalent regulatory risks. Note, the reduced flow in
the Thunder Hills Interceptor in Alternative 2 was previously accounted for under the high
priority "Pipe Risk" criteria.
Permitting: application/ acquisition & schedule: A rating of “-” or "acceptable" was
assigned to both alternatives due to the length of time required for permitting.
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Table 3.8 Permit Matrix for Thunder Hills Interceptor Alternatives
Agency Permit
Timeline
for Submittal Fees Alternative No.
City of Renton Land Use 4 to 6 months Internal City fees 1 & 2
Grading 1 & 2
Variance 1 & 2
Environmental
Review
1 & 2
Building 1 & 2
Right of Way 2
Willams NW WilSOP Permit 4 to 6 months No review fee 1 & 2
Puget Sound
Energy
Consent for Use 12 to 15
months
To be
determined
1 & 2
US Army Corps of
Engineers
Section 404 12 to 18
months
Submittal fee
of $100
1 & 2
EPA Section 7 1 & 2
DOE Section 401 1 & 2
Department of
Ecology
NPDES 6 to 8
weeks
Fees for
Notice of Intent advertisement
King County Water Treatment Division Review 2 to 4 weeks No review fee 1 & 2
Washington State
Department of
Transportation
Limited Access
Right of Way
Permit
6 to 12
weeks
Submittal fee
of $300
1 & 2
Washington State Department of
Fish and Wildlife
Hydraulic Project Approval (HPA) 6 to 12 weeks Submittal fee of $150 1 & 2
Washington State
Department of Archaeological and Historic
Preservation
(DAHP)
DAHP Review 4 to 6
weeks
No review fee 1 & 2
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3.8.6 Environmental Considerations
Environmental considerations were evaluated by The Watershed Company and
documented in TM No. 8 (Appendix I). The Environmental mitigation criteria, presented in
Table 3.7, were rated as:
Environmental Mitigation: A rating of “-” was assigned to both alternatives.
Sensitive Areas that may be impacted by the Interceptor and any proposed construction are
summarized in Section 1. The entire Thunder Hills Interceptor alignment is within stream
and wetland buffers; therefore, buffer impacts are unavoidable. However, the design will
seek to avoid and minimize impacts.
Examples of proposed avoidance measure include:
• Using existing access points to avoid the need for new stream crossings.
• Using a slip-line approach to repair the existing sewer line where it crosses Thunder
Hills Creek.
Examples of minimization measures include:
• Relocating the sewer line under the access road to minimize the footprint of
construction and maintenance impacts.
• Locating the proposed road over the existing unmaintained trail to minimize impacts
to vegetation in the buffer.
• Locating the road as far from the stream as possible, given topographic constraints.
• Using a narrower path (7 feet compared to 12-foot road) for specialized equipment on
the northern portion of the project area.
• Implementation of Temporary Erosion and Sediment Control (TESC) plan and Storm
Water Pollution Prevention Plan (SWPPP).
• Timing construction to avoid the wet season when the most significant sedimentation
and erosion would be anticipated.
Additionally, construction activities are expected to have direct impacts to Wetland F. Based
on the current alternatives, approximately 1,400 square feet of wetland will be impacted.
On-site mitigation does not appear to be possible, therefore, impacts will be mitigation will
be completed through a wetland bank, such as the City's Springbrook Creek Wetland and
Habitat Mitigation Bank. Ratings for environmental mitigation are below.
3.8.7 Cost
Cost considerations for both capital and O&M and Right of Way (ROW)/Easements were
considered in the Alternative Analysis. Planning level costs were developed based on
preliminary design drawings and investigations into permitting and environmental mitigation
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costs. As planning level costs, a 25 percent contingency has been added to all costs shown
in Table 3.7. Additionally, annual costs for operating and maintaining the Alternatives were
estimated and are shown in Table 3.8. A brief summary of key aspects of the cost
estimates are provided below. Detailed cost estimating information is found in TM No. 10
(Appendix K). ROW/Easement and project schedule criteria were also evaluated based on
the cost estimates.
• Engineering design costs were assumed to be 15 percent of the total construction
costs.
• Construction engineering costs were assumed to be 8 percent of the total
construction costs for Alternative No. 1, and 8 percent of the Thunder Hills
Interceptor section of Alternative No. 2.
• Permitting costs were estimated from similar previous projects, and include the
estimated labor costs to prepare and submit the permits.
• Environmental mitigation costs were estimated for indirect (buffer only impacts) and
direct wetland impacts using the Springbrook Creek Mitigation Bank.
• Easement attainment costs were estimated assuming easements would be
obtained for the entire interceptor using land values from the King County Assessor.
• Construction costs were estimated from the findings and preliminary drawings
presented in Appendix B. Unit costs were estimated from recent projects and
additional pricing research.
• Operation and maintenance costs include standard cleaning of the sewer main every
two years, and providing CCTV for the sewer main every seven years.
Alternative No. 1 is more economical to design, construct, and operate than Alternative
No. 2 by approximately $2.3 million, as shown in Table 3.9.
Given the larger scope of Alterative 2, it is expected to require proportionally more design
costs. Construction costs for the Thunder Hills portion of Alternative 2 are approximately
$2,752,000 versus approximately $2,960,000 for Alternative 1. The remaining $2,207,000 in
construction costs are for the Talbot Hills diversion and improvements. Other costs are
anticipated to be comparable between Alternatives.
However, approximately $240,000 of the Alterative 2 improvements to the Talbot Sewer
Basin would be required regardless of whether or not the Thunder Hills Sewer Basin is
diverted. Therefore, considering only improvements specifically necessary due to the
Thunder Hills project, there was an approximate $2.1 million difference.
O&M costs between the Alternatives will be similar, as shown in Table 3.10. Alternative 2
has a net increase in piping, which is anticipated to require approximately $600 more
annually than Alternative 1 in operation and maintenance costs.
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Cost: Capital and O&M: A rating of “-” or acceptable rating was given to both alternatives.
The relatively high costs for both Alternatives did not warrant a "+" or good. While
Alternative 2 was more expensive than Alternative 1, the additional costs were considered
reasonable for the proposed infrastructure.
ROW/Easement: A rating of “-” or acceptable rating to both alternatives, which would
require identical easements.
Table 3.9 Thunder Hills Alternative Cost Comparison
Description
Estimated Initial Cost Alternative 1 (Existing – Mix)
Estimated Initial Cost Alternative 2 (Divert – Mix)
Engineering Design $440,000 $740,000
Construction Engineering $240,000 $220,000
Permitting $40,000 $41,000
Environmental Mitigation $131,000 $131,000
Easement Attainment $170,000 $170,000
Construction $2,960,000 $4,959,000
Total $3,981,000 $6,261, 000
Notes:
(1) Planning level costs including a 25 percent contingency.
Table 3.10 Thunder Hills Alternative O&M Cost Comparison
Description
Estimated Initial Cost Alternative 1 (Existing – Mix)
Estimated Initial Cost Alternative 2 (Divert – Mix)
Thunder Hills Interceptor $1,750 $1,750
Additional sewer main $0 $600
Total $1,750 $2,350
Notes:
(1) Planning level costs
Project Schedule: A rating of “-” or acceptable was assigned to both alternatives. While
Alternative 2 requires Talbot Hills Sewer Improvements, the work may be completed in
parallel with Interceptor construction or potentially prior to Interceptor construction activities;
therefore; no differentiation was made between the Alternatives.
3.9 SELECTED ALTERNATIVE
The detailed selection process narrowed Interceptor rehabilitation to two questions:
• Should the Interceptor be rehabilitated, replaced, or a mixture of both?
• Should the majority of Interceptor flows be diverted to the Talbot Hills basin?
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The City comprehensively considered these questions through the Alternative Analysis
presented in above sections. The evaluation of high priority criteria rating clearly showed
that a mixture of rehabilitation in place and new pipes is the most beneficial method of
rehabilitating the Interceptor.
The diversion of Thunder Hills Basin flows to the Talbot Hills Basin is compared with
Alternative 2 (Divert - Mix) and Alternative 1 (Existing - Mix). There were no differences in
ratings for medium or low priority criteria (O&M, Permitting, Environmental, and Cost).
However, Alternative 2 (Divert - Mix) had a Pipe Risk (high priority criteria) of very good
versus good for Alternative 1 (Existing - Mix). By diverting flows to the Talbot Hills Sewer
Basin, Alternative 2 is able to retire the segment of the Interceptor with the highest pipe risk.
Additionally, Alternative 2 substantially reduces the sewage flows in the Interceptor;
reducing the environmental impact if an overflow or pipe break did occur.
The City selected Alternative 2 as the recommended Alternative for the rehabilitation of the
Interceptor, given available funding, and based on the reduced pipe risk. If funding for
Alternative 2 is not available, Alternative 1 should be implemented. The additional
infrastructure required due to the diversion would increase project costs from $3,981,000
for Alternative 1 to $6,261,000 for Alternative 2. Benefits of Alternative 2 include:
• Creates at least 80-years of RUL for the Interceptor.
• Provides full access to the Interceptor.
• Allows proper maintenance of the Interceptor.
• Reduces risk to the environment and public health by allowing the highest risk pipe
segment to be abandoned.
• Diverts flow from the Interceptor, reducing the impact of a sanitary sewer overflow or
pipe break.
• Reduces risk of stream bank erosion risk through stream bank improvements.
• Reduces access risk from steep slopes through geotechnical improvements.
• Improves local environment while mitigating stream bank buffer impacts.
Major challenges for Alternative 2 include:
• Lengthy permit acquisition.
• Obtaining easements and ROW.
• Increased costs to divert flow to Talbot Hills Sewer Basin.
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Chapter 4
RECOMMENDED ALTERNATIVE
4.1 SUMMARY
The proposed alternative for the rehabilitated Interceptor, Talbot Hills diversion, and Talbot
Hills Sewer System improvements is shown in Appendix A. A summary of preliminary
design infrastructure for the recommended Alterative is provided in Table 4.1.
Improvements are grouped under three Schedules to aid in design and phasing:
• Schedule A - Thunder Hills Improvements
• Schedule B - Talbot Hill Sewer Basin Improvements
• Schedule C - Talbot Hill Sewer Basin Improvements for Thunder Hills Basin Diversion
All infrastructure will be designed using adopted City standards and industry standard
practices. In addition to the summarized infrastructure, the construction activities will
include: mobilization and demobilization; construction survey, staking, and as-builts; site
clearing and grubbing; temporary erosion/sedimentation controls; traffic control; trench
excavation safety systems; 48-inch diam. sanitary sewer manholes; rechanneling existing
manholes, television inspection of sanitary sewers; abandon & fill sanitary sewer main;
connect to existing sewer system; pavement restoration; removal and replacement of
unsuitable foundation material; and miscellaneous site restoration.
The project will also require new easements from Berkshire Apartments, Woodcliffe
Apartments, and PSE.
4.1.1 Design life
Consistent with project goals, all project elements will have an 80-year design life.
Appropriate infrastructure will be provided for the ongoing maintenance of the Interceptor,
including CCTV and cleaning. Geotechnical improvements will be made to protect the
access road and/or sewer main from streambank erosion and steep slopes.
4.1.2 Environmental Impacts
The project incorporates measures to avoid or minimize environmental impacts.
Unavoidable environmental impacts will be appropriately mitigated. Approximately
12,500 square feet of buffer enhancement will likely be needed to mitigate indirect wetland
buffer impacts. Buffer enhancement may include removal of invasive weeds and replanting
of native vegetation. Mitigation of approximately 1,408 square feet of direct wetland impacts
will be mitigated through the purchase of credits at the City of Renton Springbrook Creek
Mitigation Bank or through the King County Mitigation Reserves Program.
October 2016 - FINAL 4-2 pw://Carollo/Documents/Client/WA/Renton/9610A00/Deliverables/BODR.docx
Table 4.1 Preliminary Design Infrastructure Summary
Schedule Description Quantity
Schedule A - Thunder Hills Improvements 12-Inch Diam. HDPE Gravity Sewer Pipe Installed under Access Road
1,040 LF
Cured-In-Place Pipe (CIPP)
Rehab.
1,010 LF
Gravel Access Road & Path 35,000 SF
Storm Culverts 50 LF
Gravity Walls and Rockeries To be
Determined
Schedule B - Talbot Hill Sewer
Basin Improvements
12-Inch PVC Gravity Sewer Pipe
Installed in ROW
110 LF
Pipe Bursting under Talbot (8-Inch
Pipe to 12-Inch Pipe)
170 LF
Schedule C - Talbot Hill Sewer
Basin Improvements for Thunder
Hills Basin Diversion
8-Inch PVC Gravity Sewer Pipe
Installed in ROW
980 LF
10-Inch PVC Gravity Sewer Pipe Installed in ROW 200 LF
12-Inch PVC Gravity Sewer Pipe Installed in ROW 910 LF
Notes:
(1) Based on Preliminary Design.
4.1.3 Ability to expand
The Recommended Alternative has been sized to convey Ultimate (build-out) flows. The
downward sloping topography provides excess flow capacity in the majority of both Thunder
Hills and Talbot Hills sewer mains. Areas of flatter grade will be improved to meet Ultimate
flows, as shown in Sheet 3 of the site plan. Note, flows from the Thunder Hills Sewer Basin
have been accounted for in the separate Talbot Hills I-405 sewer crossing rehabilitation
project.
4.1.4 O&M Staffing Needs
Ongoing operation and maintenance is expected to include standard cleaning of the sewer
main every two years and providing CCTV for the sewer main every seven years. No
additional staff is required due to project improvements.
October 2016 - FINAL 4-3 pw://Carollo/Documents/Client/WA/Renton/9610A00/Deliverables/BODR.docx
4.1.5 Project Funding
The Thunder Hills Sanitary Sewer Interceptor rehabilitation project will be funded through
existing wastewater funds. The project will be included in the City's biennial budget for
public and council review and approval, with design expected to occur in 2016-2017 and
construction beginning in 2017-2018.
4.1.6 State Environmental Policy Act
The City will conduct a State Environmental Policy Act (SEPA) review during design,
including public comment.
Appendix A
PROJECT PLANS
SO
-
D
P
6
I
-
4
0
5
EX
.
8
"
A
C
S
S
STREAM D
WETLAND E
STREAM B
WETLAND F
WET BFR
WET BFR
WET BFR
WET BFR
WET BFR
WET BFR
WET BFR
WET BFR
WET BFR
WET BFR
WET BFR
WET BFR
WET BFR
WE
T
B
F
R
WE
T
B
F
R
WE
T
B
F
R
WET BFR
WET BFRWET BFR
WET BFR
WET BFR
WET BFR
WET BFR
WET BFR
WET BFR
WE
T
B
F
R
WE
T
B
F
R
WET BFR
WET BFR
WET BFR
WET BFR
WET B
F
R
WET B
F
R
WET
B
F
R
WET BFR
WET B
F
R
WET BFR
WET BFR
WET BFR
WET BFR
SO
-
D
P
6
STREAM D
WETLAND E
I
-
4
0
5
EX
.
8
"
A
C
S
S
STREAM B
CONNECT TO
EX SEWER MAIN
MA
T
C
H
L
I
N
E
-
S
T
A
.
3
+
5
0
.
0
0
SE
E
S
H
E
E
T
2
20
0
6
6*70&'4*+..5+06'4%'2614
#.6'40#6+8'5#0#.;+5+5
EX
.
8
"
A
C
S
S
WET BFR
WET BFR
WET BFR
WET BFR
WE
T
B
F
R
WE
T
B
F
R
WE
T
B
F
R
WET BFRWET BFR
EX
.
8
"
A
C
S
S
ADDITIONAL ESMT
WILL BE NECESSARY
FOR TURN-AROUND
REMOVE EX
RETAINING WALL
EDGE OF EX
GRAVEL ROAD
CONNECT TO
EX SEWER MAIN
PROPOSED SSMH - ADDED TO
IMPROVE ACCESSIBILITY TO SEWER
SYSTEM FOR VACTOR TRUCK
MA
T
C
H
L
I
N
E
-
S
T
A
.
3
+
5
0
.
0
0
SE
E
S
H
E
E
T
1
MA
T
C
H
L
I
N
E
-
S
T
A
.
7
+
0
0
.
0
0
SE
E
S
H
E
E
T
3
20
0
6
6*70&'4*+..5+06'4%'2614
#.6'40#6+8'5#0#.;+5+5
X
X
X
PROPOSED SSMH - ADDED
T
O
IMPROVE ACCESSIBILITY T
O
S
E
W
E
R
SYSTEM FOR VACTOR TRU
C
K
MA
T
C
H
L
I
N
E
-
S
T
A
.
7
+
0
0
.
0
0
SE
E
S
H
E
E
T
2
MA
T
C
H
L
I
N
E
-
S
T
A
.
1
0
+
5
0
.
0
0
SE
E
S
H
E
E
T
4
20
0
6
6*70&'4*+..5+06'4%'2614
#.6'40#6+8'5#0#.;+5+5
EX. 10"
D
I
S
S
EX. 12" DI SS
WETLAND D, LOCATION
APPROX. (NOT SURVEYED).
SURVEY MAY BE REQUIRED
FOR PERMITS
WE
T
B
F
R
WE
T
B
F
R
WE
T
B
F
R
WET BFR WET BFR WET BFR
WE
T
B
F
R
WE
T
B
F
R
WE
T
B
F
R
WE
T
B
F
R
WET B
F
R
WET B
F
R
WET B
F
R
WET B
F
R
WETLAND D
WETLAND C
(APPROX.)
SSMH 036
SSMH 046
EX. 10"
D
I
S
S
EX. 12" DI SS
THUNDER HILLS CREEK
OPTIONAL GRAVITY WALL OR
ROCKERY - MAY NOT BE NEEDED
MA
T
C
H
L
I
N
E
-
S
T
A
.
1
0
+
5
0
.
0
0
SE
E
S
H
E
E
T
3
MA
T
C
H
L
I
N
E
-
S
T
A
.
1
4
+
0
0
.
0
0
SE
E
S
H
E
E
T
5
20
0
6
6*70&'4*+..5+06'4%'2614
#.6'40#6+8'5#0#.;+5+5
DP
2
EX. 10
"
D
I
S
S
WETLAND C, LOCATION
APPROX. (NOT SURVEYED).
SURVEY MAY BE
REQUIRED FOR PERMITS WETLAND B
THIS SECTION OF
THUNDERHILLS CREEK
APPROX. (NOT SURVEYED).
SURVEY MAY BE
WET BFR
WET BFR
WET BFR
WET
B
F
R
WET
B
F
R
WET BFR
WET BFR
WET BF
R
WET BFR
WET BF
R
WET
B
F
R
WET
B
F
R
WET
B
F
R
WE
T
B
F
R
WET BFR
WET BFRWE
T
B
F
R
WE
T
B
F
R
WET B
F
R
DP
2
WETLAND C
(APPROX.)
WETLAND B
SSMH 046
EX. 10
"
D
I
S
S
ROCKERY - MAY NOT BE NEEDED
CUT INTO TOE OF
SLOPE TO CREATE
12' WIDE GRAVEL RD
CUT INTO TOE OF
SLOPE TO CREATE
12' WIDE GRAVEL RD
MA
T
C
H
L
I
N
E
-
S
T
A
.
1
4
+
0
0
.
0
0
SE
E
S
H
E
E
T
4
MA
T
C
H
L
I
N
E
-
S
T
A
.
1
7
+
5
0
.
0
0
SE
E
S
H
E
E
T
6
20
0
6
6*70&'4*+..5+06'4%'2614
#.6'40#6+8'5#0#.;+5+5
EX. 10" AC SS
EX. 10" A
C
S
S
EX. 10
"
D
I
S
S
THUNDER HILLS CREEK
EX. 10" AC SS
EX. 10" A
C
S
S
EX. 10
"
D
I
S
S
CUT INTO TOE OF
SLOPE TO CREATE
12' WIDE GRAVEL RD
CUT INTO TOE OF
SLOPE TO CREATE
12' WIDE GRAVEL RD
CUT INTO TOE OF
SLOPE TO CREATE
12' WIDE GRAVEL RD
MA
T
C
H
L
I
N
E
-
S
T
A
.
1
7
+
5
0
.
0
0
SE
E
S
H
E
E
T
5
MA
T
C
H
L
I
N
E
-
S
T
A
.
2
1
+
0
0
.
0
0
SE
E
S
H
E
E
T
7
20
0
6
6*70&'4*+..5+06'4%'2614
#.6'40#6+8'5#0#.;+5+5
EX. 10" AC SS
STREAM F
STREAM G
STREAM F
STREAM G
EX. 10" AC SS
THUNDER HILLS CREEK
CUT INTO TOE OF
SLOPE TO CREATE
12' WIDE GRAVEL RD
CUT INTO TOE OF
SLOPE TO CREATE
12' WIDE GRAVEL RD
CUT INTO TOE OF
SLOPE TO CREATE
12' WIDE GRAVEL RD CUT INTO TOE OF
SLOPE TO CREATE
12' WIDE GRAVEL RD
CUT INT
O
T
O
E
O
F
SLOPE TO
C
R
E
A
T
E
12' WIDE
G
R
A
V
E
L
R
D
MA
T
C
H
L
I
N
E
-
S
T
A
.
2
1
+
0
0
.
0
0
SE
E
S
H
E
E
T
6
MA
T
C
H
L
I
N
E
-
S
T
A
.
2
4
+
5
0
.
0
0
SE
E
S
H
E
E
T
8
20
0
6
6*70&'4*+..5+06'4%'2614
#.6'40#6+8'5#0#.;+5+5
SO-D
P
1
///
///
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///
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///
///
///
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///
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//////
///
///
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///
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////////////
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///
WE
T
B
F
R
WE
T
B
F
R
WE
T
B
F
R
WE
T
B
F
R
WE
T
B
F
R
WE
T
B
F
R
WET
B
F
R
WET
B
F
R
WET
B
F
R
WET
B
F
R
WET
B
F
R
WET
B
F
R
WET
B
F
R
WET
B
F
R
WET B
F
R
WE
T
B
F
R
WET
B
F
R
WE
T
B
F
R
WET B
F
R
WETLAND A
STREAM E
SO-D
P
1
WETLAND A
STREAM E
SS
M
H
0
5
1
SSMH 051A
THUNDER HILLS CREEK
OPTIONAL GRAVITY WALL OR
ROCKERY - MAY NOT BE NEEDED
CUT IN
T
O
T
O
E
O
F
SLOP
E
T
O
C
R
E
A
T
E
12' WI
D
E
G
R
A
V
E
L
R
D
CUT INTO TOE OF
SLOPE TO CREATE
12' WIDE GRAVEL RD
MA
T
C
H
L
I
N
E
-
S
T
A
.
2
4
+
5
0
.
0
0
SE
E
S
H
E
E
T
7
MA
T
C
H
L
I
N
E
-
S
T
A
.
2
8
+
0
0
.
0
0
SE
E
S
H
E
E
T
9
20
0
6
6*70&'4*+..5+06'4%'2614
#.6'40#6+8'5#0#.;+5+5
////////////
///
////////////
///
///
///
///
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///
///
////////////////////////
///
//////////////////
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///
////////////////////////////////////////////////////////////
///
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///
///
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///
///
///
///
///
///////////////////////////////////////////////////////////////
///
///
WET BFR
WET BFR
WET BFR
WET BFR
WET BFR
WET
B
F
R
WET
B
F
R
WET
B
F
R
WET
B
F
R
WET
B
F
R
WET B
F
R
GRANT AVE S
S
1
8
t
h
S
T
SSMH 051
SS
M
H
0
5
1
A
SSMH 052
///
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///
///
RE-PAVE
ACCESS ROAD
M
A
T
C
H
L
I
N
E
-
S
T
A
.
2
8
+
0
0
.
0
0
S
E
E
S
H
E
E
T
8
20
0
6
6*70&'4*+..5+06'4%'2614
#.6'40#6+8'5#0#.;+5+5
MATCH LINE - STA.32+20
SEE SHEET 10
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//////////////////
//////////////////////////////////////////
20
0
6
6*70&'4*+..5+06'4%'2614
#.6'40#6+8'5#0#.;+5+5
MA
T
C
H
L
I
N
E
-
S
T
A
.
3
6
+
9
0
SE
E
S
H
E
E
T
1
1
MA
T
C
H
L
I
N
E
-
S
T
A
.
3
2
+
2
0
SE
E
S
H
E
E
T
9
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EX. SSMH 320
CONNECT NEW SS
PIPE TO EX SSMH
EX. SSMH 168
20
0
6
6*70&'4*+..5+06'4%'2614
#.6'40#6+8'5#0#.;+5+5
MA
T
C
H
L
I
N
E
-
S
T
A
.
4
1
+
5
0
SE
E
S
H
E
E
T
1
2
MA
T
C
H
L
I
N
E
-
S
T
A
.
3
6
+
9
0
SE
E
S
H
E
E
T
1
0
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///
///
///
///
///
///
///
///
///
///
///
///
///
///
///
///
///
///
//////
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///
///
///
///
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///
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///
///
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SSMH 167
20
0
6
6*70&'4*+..5+06'4%'2614
#.6'40#6+8'5#0#.;+5+5
MA
T
C
H
L
I
N
E
-
S
T
A
.
4
7
+
1
0
SE
E
S
H
E
E
T
1
3
MAT
C
H
L
I
N
E
-
S
T
A
.
4
1
+
5
0
SEE
S
H
E
E
T
1
1
///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
SSMH 165
20
0
6
6*70&'4*+..5+06'4%'2614
#.6'40#6+8'5#0#.;+5+5
MA
T
C
H
L
I
N
E
-
S
T
A
.
5
1
+
9
0
SE
E
S
H
E
E
T
1
4
MATCH LINE - STA. 47+10
SEE SHEET 12
/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
////////////////////////////////////////////////
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///
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///
EX. SSMH 160
EX. SSMH 163
EX. SSMH 161
EX. SSMH 159
20
0
6
6*70&'4*+..5+06'4%'2614
#.6'40#6+8'5#0#.;+5+5
MA
T
C
H
L
I
N
E
-
S
T
A
.
4
6
+
7
0
SE
E
S
H
E
E
T
1
5
MA
T
C
H
L
I
N
E
-
S
T
A
.
5
1
+
9
0
SE
E
S
H
E
E
T
1
3
///
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EX. SSMH 156
EX. SSMH 084
20
0
6
6*70&'4*+..5+06'4%'2614
#.6'40#6+8'5#0#.;+5+5
MA
T
C
H
L
I
N
E
-
S
T
A
.
6
1
+
5
0
SE
E
S
H
E
E
T
1
6
MATCH LI
N
E
-
S
T
A
.
5
1
+
9
0
SEE SHEE
T
1
4
20
0
6
6*70&'4*+..5+06'4%'2614
#.6'40#6+8'5#0#.;+5+5
MA
T
C
H
L
I
N
E
-
S
T
A
.
6
1
+
5
0
SE
E
S
H
E
E
T
1
5
Appendix B
TECH MEMO NO. 1 - ALTERNATIVES AND CRITERIA
November 2014 i pw://Carollo/Documents/Client/WA/Renton/9610A00/Deliverables/TM01_DefineAlts-Criteria
CITY OF RENTON
THUNDER HILLS SANITARY SEWER INTERCEPTOR ALTERNATIVES ANALYSIS
TECHNICAL MEMORANDUM
NO. 1
DEFINE ALTERNATIVES & CRITERIA
TABLE OF CONTENTS
Page No.
1.0 INTRODUCTION .................................................................................................... 1-1 1.1 Purpose ...................................................................................................... 1-1
2.0 EXISTING SYSTEM ............................................................................................... 1-3 2.1 Interceptor Location .................................................................................... 1-3
2.2 Environmental and Permitting Conditions ................................................... 1-6
3.0 PROJECT OVERVIEW .......................................................................................... 1-6 3.1 Project Goals .............................................................................................. 1-6
3.2 Project Success .......................................................................................... 1-6
4.0 ALTERNATIVES ANALYSIS .................................................................................. 1-7
LIST OF TABLES
Table 1.1 Elements of Project Success ........................................................................ 1-7
Table 1.2 Criteria and Prioritization ............................................................................... 1-9
LIST OF FIGURES
Figure 1.1 Vicinity Map ................................................................................................... 1-2 Figure 1.2 System Map .................................................................................................. 1-4
Figure 1.3 Vactor Excavation Locations ......................................................................... 1-5
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Technical Memorandum No. 1
DEFINE ALTERNATIVES & CRITERIA
1.0 INTRODUCTION
The Thunder Hills Sanitary Sewer Interceptor serves the Rolling Hills neighborhood in the
City of Renton’s (City) Thunder Hills Basin. The portion of the interceptor included in this
project runs Grant Avenue South at its upstream to Benson Road South at its downstream
end. For the entirety of this portion of the alignment, the interceptor runs adjacent to or
within Thunder Hills Creek, with the exception of the crossing of Interstate 405 (I-405).
The interceptor was originally installed in 1965, just prior to the construction of I-405
through Renton. Due to proximity to Thunder hills Creek, this sewer interceptor has
historically been compromised by erosion. Sections of this sewer were replaced and
portions of the stream bank stabilized with gabion baskets in the early 1980’s. Interim
repairs were done in 2008 and 2009 because of erosion caused by heavy storm events.
Although there is no visible evidence that the interceptor has been damaged in recent
years, continued erosion throughout this stream corridor has destroyed the majority of the
previous stabilization measures.
The purpose of this project is to complete an alternatives analysis for the rehabilitation
and/or replacement of the Thunder Hills Sanitary Sewer Interceptor. The City would like to
reestablish access to the interceptor, determine the existing condition and capacity of the
interceptor, and evaluate options for restoring the creek corridor to ensure the protection of
the interceptor. Figure 1.1 is a vicinity map of the project area.
1.1 Purpose
The goal of the alternatives analysis is to clarify the project goals, establish design criteria,
and evaluate alternative solutions before proceeding with final design and construction.
This Technical Memorandum No. 1 (TM) summarizes the Workshop #1 Alternatives
Development, held on August 5, 2014 at the City’s Central Shop. The objectives of this TM
are to:
Identify project goals and elements of project success
Document suggested alternatives.
Document criteria prioritization.
The best approach for the pipeline will be determined by evaluating key criteria for
proposed alternatives. The feasibility of the proposed alternatives is directly related to the
condition of the interceptor and the hydraulic capacity to meet the design flows. Data will
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VICINITY MAP
FIGURE 1.1
CITY OF RENTON
THUNDER HILLS SANITARY SEWER INTERCEPTOR
Benson Rd. S.
Grant Ave. S.
Pipeline
Apartments
Legend
Pipeline Project Area
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be developed by the project team that will be presented to the City team for consideration.
Future evaluation and analysis will be summarized in future Technical Memorandums.
2.0 EXISTING SYSTEM
The portion of the Thunder Hills Sanitary Sewer Interceptor under the scope of this project
consists of approximately 3,600 lineal feet of 10-inch to 18-inch pipe and varies in depth
from typical depths of 8 to 10 feet to areas where it is believed to be at depths greater than
30 feet. The original pipe was asbestos cement (AC), with repairs over time completed with
cast iron and ductile iron (DI). Therefore, the existing pipe varies from AC to DI and cast
iron pipe.
2.1 Interceptor Location
At this time, the city has been unable to access all portions of the existing interceptor. As
such, the existing condition of this facility and capacity of the pipeline are unknown. Flows
within the interceptor have not been monitored.
Stantec has performed an assessment of the alignment to determine the location, depth,
and material of the pipeline. At the time of Workshop #1, Stantec was able to verify the
following:
The pipe is DI until line crosses the Thunder Hills Creek at the southeast corner of the
Berkshire Apartments complex.
Survey reveals the pipe is AC until it reaches the Bonneville Power (BP) lines near
I-405.
The size of pipe changes from 10 inches to 12 inches in the BP right-of-way near
I-405.
Currently, the depth of the pipeline through the entire alignment is unknown. A CCTV
inspection of the entire length of sewer was conducted and is being evaluated to determine
the existing condition of the sewer.
An attempt to pothole the location of the existing sewer line behind the Berkshire
Apartments was made on August 25th and 26th. Bravo Environmental (Bravo) made three
attempts to locate the line at the locations shown on Figure 1.3. At the location of pothole
no. 1, Bravo excavated down approximately 14 feet and 3 feet wide, but was unable to find
the existing line. At the location of pothole no. 2, excess water infiltration from the adjacent
creek limited the excavation. Excavation at the location of pothole no. 3 was unsuccessful
due a large amount of concrete construction debris placed in the fill area. The City decided
not to attempt any further excavations to locate the sewer line.
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SYSTEM MAP
FIGURE 1.2
CITY OF RENTON
THUNDER HILLS SANITARY SEWER INTERCEPTOR
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VACTOR EXCAVATION LOCATIONS
FIGURE 1.3
CITY OF RENTON
THUNDER HILLS SANITARY SEWER INTERCEPTOR
Approx. Location of Hole 3
Approx. Location of Hole 2
Approx. Location of Hole 1
Apartments
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2.2 Environmental and Permitting Conditions
The Watershed Company performed a Wetland and Stream Delineation Study in April and
May of 2014. Thunder Hills Creek, seven tributary streams, and six wetland were flagged in
the study and classified per Renton Municipal Code and Ecology’s Western Washington
Wetland Rating System (Aug 2004, version 2).
Wetlands rating system from Ecology changes in July 2015. The City will need to modify
forms for the new wetland classifications. The changes to Ecology’s rating system may
require changes to the Renton Municipal Code in order to comply with Ecology’s
classifications. The Watershed Group is completing the wetland delineation report, which
will aid in understanding the potential for erosion to impact the existing interceptor and the
possibility of inflow from Thunder Hills Creek. During the Wetland and Stream Delineation
Study, it was observed that Wetland F is a concern to the project, as there is groundwater
seep that saturates the access road. This will be evaluated further in the future by the team
to identify any additional flow impacts to the pipeline.
3.0 PROJECT OVERVIEW
3.1 Project Goals
The City is looking for an evaluation that identifies a long-term solution with the following
ultimate goals:
Determine an accurate location of the existing interceptor
Ensure that the finished interceptor is sized to convey ultimate flows
Provide a pipe with a “remaining useful life” (RUL) of at least 80 years
Provide proper manhole access points for maintenance and inspection.
Minimize risk to environmentally sensitive/critical areas.
Consider erosion from Thunder Hills Creek and potential inflow.
3.2 Project Success
The City’s goals were used as the basis for defining the elements of project success.
Discussion and evaluation of each goal in Workshop #1 and its relevance to project
success revealed the need for clarity to some goals and criticality weight for each goal.
Table 1.1 presents the changes and additions to the goals and its importance as an
element of project success.
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Table 1.1 Elements of Project Success
Element Change Criticality
Operations staff will have complete access to the interceptor across the entire
alignment.
Strong desire
Manhole access points will be provided
for proper maintenance and CCTV capability.
Absolute
Complete and accurate information
regarding the location of the existing/new
interceptor
Absolute
Interceptor pipe will be sized to accommodate ultimate build-out flows for
basin.
Ultimate + 25% Absolute
Interceptor pipe and access points are
conditioned as new.
Conditioned for RUL of 80
years
Absolute
Thunder Hills Creek will not have an
adverse impact on our access road or interceptor facilities.
And minimizing risk to
environmentally sensitive/critical areas
Absolute
Plan for today and future relating to flash
flows and storms impacting surrounding
facility.
New
Do not adversely impact Berkshire
Apartment complex
New
4.0 ALTERNATIVES ANALYSIS
The alternatives analysis consists of developing a list of alternatives and criteria by which
the feasibility of an alternative will be evaluated. The alternative analysis is a refinement of
alternatives based on key critical criteria, such as the hydraulic capacity of the pipeline.
Then evaluation of the alternatives compared to the next key criteria, such as condition until
only a few alternatives remain. This alternatives refinement process will occur after data is
developed and presented to the City team.
Workshop #1 included a brainstorming session where all team members suggested ideas
for alternatives; none of the suggestions were evaluated nor eliminated at this stage,
everything was written down.
The list of suggested alternatives for the pipeline is summarized below, in no particular
order:
A. Do nothing.
B. Rehabilitate existing line in existing location.
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C. Construct a new line parallel to existing line.
D. No Thunder Hills line: divert all of Thunder Hills flow by installation of a lift station
for lower flows to Talbot Basin.
E. Divert a portion of flow from Thunder Hills Basin to Talbot Basin then implement
alternative B or C.
F. No Thunder Hills basin flow under I-405; divert before slide area to Talbot Basin (to
Talbot Road) or north to Renton Hill Basin.
G. Phase Improvements for projected flows at 10, 20, and 50 years.
If Alternative B, C, E or F then alternatives for the minimization of the stream erosion
include:
Look at stream flows; establish Design Criteria.
Move/”pipe”/control creek to I-405 and implement pipeline alternatives.
“Pipe/ control a portion of creek and implement pipeline alternatives for high flow
bypass.
Install “Walls” between stream and pipe/road.
Install “Walls” between pipe/road and apartments.
If Alternative B, C, E or F then alternatives for the access of the pipeline include:
Regrade between pipe/road and apartments and include into private property of
apartments.
Tightline (either above or below) sewer from apartments to I-405. No manholes,
tightline system will need evaluation of length and across equipment).
Access pipeline from I-405 to NE 35th.
Access road “types”: rock, gravel, elevated grate, timber, (between apartments and
I-405).
Size of road: foot access vs. vehicle (between apartments and I-405).
Using the goals outlined by the City and the Project Elements, the team developed the
criteria and prioritized into High/Medium/Low. Table 1.2 presents the criteria and
prioritization:
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Table 1.2 Criteria and Prioritization
HIGH MEDIUM LOW
Pipe risk = Criticality x
RUL
Maintenance of alignment
and equipment needed
Project Schedule
Access to manholes Cost: Capital and O&M Permitting: application/
acquisition & schedule
Equipment access along
pipe
City Crews vehicle access
along pipe
Environmental mitigation
Condition of
Pipe/Remaining useful life
Access risk = slide/erosion
Constructability ROW/Easement
Regulatory risks
Appendix C
TECH MEMO NO. 2 - FLOW ANALYSIS
CITY OF RENTON
Technical Memorandum No. 2
Flow Analysis
Thunder Hills Sewer Interceptor
February 24, 2015
Prepared By:
Stantec Consulting Services Inc.
11130 NE 33rd Place
Suite 200
Bellevue, WA 98004
425.869.9448
TECHNICAL MEMORANDUM NO. 2
February 24, 2015
Contents
1.0 BACKGROUND ................................................................................................................. 1
2.0 MODEL DEVELOPMENT AND FLOW ANALYSIS ............................................................... 1
3.0 ALTERNATIVES HYDRAULIC ANALYSIS ............................................................................ 3
FIGURES
FIGURE 1: THUNDER HILLS INTERCEPTOR SEWER TRIBUTARY AREA
FIGURE 2: EXISTING THUNDER HILLS INTERCEPTOR SEWER
FIGURE 3: INFILTRATION AND INFLOW ANALYSIS – PEAK FLOWS IN THUNDER HILLS
INTERCEPTOR
FIGURE 4: 2012 ANALYSIS – EXISTING THUNDER HILLS INTERCEPTOR PLAN
FIGURE 5: 2012 ANALYSIS – EXISTING THUNDER HILLS INTERCEPTOR PROFILE (PART A)
FIGURE 6: 2012 ANALYSIS – EXISTING THUNDER HILLS INTERCEPTOR PROFILE (PART B)
FIGURE 7: ULTIMATE ANALYSIS – EXISTING THUNDER HILLS INTERCEPTOR PLAN
FIGURE 8: ULTIMATE ANALYSIS – EXISTING THUNDER HILLS INTERCEPTOR PROFILE (PART A)
FIGURE 9: ULTIMATE ANALYSIS – EXISTING THUNDER HILLS INTERCEPTOR PROFILE (PART B)
FIGURE 10: 2012 ANALYSIS – EXISTING TALBOT HILLS SEWER
FIGURE 11: 2012 ANALYSIS – EXISTING TALBOT HILLS SEWER PROFILE
FIGURE 12: 2012 ANALYSIS – EXISTING TALBOT HILLS SEWER WITH THUNDER HILLS
DIVERSION PLAN
FIGURE 13: 2012 ANALYSIS – EXISTING TALBOT HILLS SEWER WITH THUNDER HILLS
DIVERSION PROFILE
FIGURE 14: ULTIMATE ANALYSIS – EXISTING TALBOT HILLS SEWER PLAN
FIGURE 15: ULTIMATE ANALYSIS – EXISTING TALBOT HILLS SEWER PROFILE
FIGURE 16: ULTIMATE ANALYSIS – EXISTING TALBOT HILLS SEWER WITH THUNDER HILLS
DIVERSION PLAN
FIGURE 17: ULTIMATE ANALYSIS – EXISTING TALBOT HILLS SEWER WITH THUNDER HILLS
DIVERSION PROFILE
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TECHNICAL MEMORANDUM NO. 2
February 24, 2015
FIGURE 18: ULTIMATE ANALYSIS – TALBOT HILLS SEWER IMPROVEMENTS WITH THUNDER HILLS
DIVERSION PLAN
FIGURE 19: ULTIMATE ANALYSIS – TALBOT HILLS SEWER IMPROVEMENTS WITH THUNDER HILLS
DIVERSION PROFILE
FIGURE 20: TALBOT HILL SEWER RECOMMENDED IMPROVEMENTS
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TECHNICAL MEMORANDUM NO. 2
February 24, 2015
1.0 BACKGROUND
The Thunder Hills Sanitary Sewer Interceptor was originally installed in 1965 and serves a
portion of the City’s Rolling Hills neighborhood, southeast of Renton City Hall. The
portion of the Thunder Hills Interceptor from Grant Avenue South to Benson Road South
runs within or adjacent to Thunder Hills Creek. Over the years, erosion created by
Thunder Hills Creek has compromised the maintenance access road in various locations
and has required stream bank stabilization. One example of this occurred in November
of 2009; a storm drain inlet became plugged in the lower reach of the stream, leading
to extensive damage to the access road, where a portion of the existing sewer is
located. To alleviate future erosion issues at this location, an overflow channel,
reinforced with large boulders and rip rap, was constructed later that year to protect
the access road, sewer, and storm drain.
Due to the risk created by the ongoing erosion issues in Thunder Hills Creek, an
alternatives analysis is being conducted to determine the best method for serving this
area in the future. A portion of this analysis is to determine the projected peak flow
rates from the tributary area, and assess the impact of those flows on the existing sewer
as well as any proposed alternatives.
2.0 MODEL DEVELOPMENT AND FLOW ANALYSIS
The Thunder Hills Sewer Interceptor collects sewage discharge from predominately
single and multi-family residential areas southeast of I-405. It conveys these flows
through approximately 3,100 feet of 10-inch and 12-inch concrete, AC, and DI pipe
from the intersection of South 18th Street and Grant Avenue South to I-405. See Figure
1and Figure 2. On the southeast side of I-405, the sewer increases to an 18-inch
diameter concrete pipe, and crosses underneath I-405 to Benson Road South. From
there, the interceptor flows to the west, joining with drainage from the Talbot Hills
service area and discharging to the King County interceptor sewer located north of the
South Grady Way/East Valley Road intersection. Beginning at Grant Avenue South, the
pipe alignment roughly parallels Thunder Hills Creek on the east side, and crosses under
the stream approximately 1,700 feet northwest of the street. The sewer then parallels
the stream on the west. A portion of the sewer is located under an existing gravel
access road, while other portions are located in the stream bank.
The City of Renton’s Mike Urban sewer hydraulic model was updated to include sewer
rim and invert elevations obtained during the topographic survey for this project.
Although much of the horizontal pipe alignment adjacent to Thunder Hills Creek
between Grant Avenue South and I-405 was determined with the use of locating
equipment, attempts to pothole or otherwise determine pipe inverts and locations were
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TECHNICAL MEMORANDUM NO. 2
February 24, 2015
unsuccessful. Consequently, the pipe depths between manholes were estimated and
interpolated. Where the terrain was uniform (under access roads and similar features),
pipe depths were generally assumed to be constant, and consistent with the grade.
Sewer inspection videos collected as part of the project did not show any abrupt grade
changes in the sewer between manholes. This method of estimating sewer depth was
duplicated as much as possible in the model to minimize backwater effects.
In order to determine projected flow rates, Ultimate population projections were
determined. For Mini-Basin 3, projections were developed from the PSRC Land Use
Baseline projections for the year 2040. As directed by the City, an additional 25% was
added to the 2040 population estimates as a factor of safety. The PSRC projections for
Mini-Basin 45 (which is tributary to the Thunder Hills interceptor) for the year 2040 showed
large decreases in single and multi-family residential population, which is inconsistent
with the City’s Comprehensive Plan land use designations and current development
activity for the basin. Because of this, Stantec developed Ultimate populations for the
portions of Mini-Basin 45 tributary to the Thunder Hills Interceptor by applying the growth
percentages calculated for Mini-Basin 3 based on PSRC estimates. The population
projections for the Talbot Hill Sewer tributary mini-basins are listed in Table 1. The mini-
basins that are tributary to the Thunder Hills interceptor are shown on Figure 1.
Table 1: Population Projections for Thunder Hills Sewer Interceptor Tributary Mini-Basins
2012 Population (PSRC)
Ultimate Population (2040+25%)
2012 Population (PSRC)
Ultimate Population (2040+25%)
Mini-Basin 3 (Tributary to the Talbot Hills Area) Mini-Basin 45 (Tributary to Thunder Hills Interceptor)
Single Family Residential 1,012 1,660 20 33
Multi-Family Residential 1,307 1,836 1157 1,626
Employment 95 248 N/A N/A
There were several factors affecting the sewer model hydraulic analysis for this project.
Calibration of the entire City of Renton Sewer Model based on King County’s 2010 flow
monitoring data has not yet been completed. Therefore, the I/I parameters for this
analysis were based on the previous model calibration (which used 2001/2002 flow
monitoring data). The previous calibration effort included flow monitors at each of the
mini-basins, which provided more detail, whereas King County only monitored at the
Model Basin level in 2010. For the Ultimate model analysis, I/I parameters have been
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TECHNICAL MEMORANDUM NO. 2
February 24, 2015
increased by 28% to simulate increases in I/I due to pipe deterioration over the planning
period.
3.0 ALTERNATIVES HYDRAULIC ANALYSIS
To be consistent with previous modelling efforts, the hydraulic analysis was conducted
using the November 5, 1998 storm, which approximately represents the 20-year flow
event for this region of the Renton Sewer Service Area. For current (2012) population
projections and I/I parameters, results of the hydraulic analysis show a peak flow rate of
approximately 680 gpm entering the Thunder Hills Interceptor in MH#5320051(MH2325)
on Grant Avenue South, northeast of South 18th Street. There are approximately seven
tributary sewer connections to the interceptor between MH#5320051(MH2325) and
MH#5320035(MH2309) at I-405. The peak design flow rate at MH#5320035(MH2309) is
approximately 865 gpm.
Results of the Ultimate hydraulic analysis show a peak flow rate of approximately 870
gpm entering the Thunder Hills Interceptor in MH#5320051(MH2325). The peak
projected Ultimate flow rate at MH#5320035(MH2309) is approximately 1,125 gpm.
Approximately 700 gpm of this flow rate is the result of I/I. The estimated tributary area
to the Thunder Hills Interceptor Sewer, as shown in Figure, is 264 acres, and the
projected I/I contribution is approximately 4,200 gpad. The attached Figure 3 shows
the sanitary and total peak flows discharging from MH#5320035(MH2309).
Based on the Population projections listed in Table1, the peak projected per capita flow
rate is 215 gal/PE/day. For both the 2012 and Ultimate analysis, the results show no
surcharging for most of the pipe alignment between MH#5320051(MH2325) and
MH#5320035(MH2309), with Q/Qfull <0.50. Only a short section of 10-inch pipe directly
upstream of MH#5320049(MH2323), approximately between Station 20+40 and 20+65,
projects any surcharging. For the 2012 analysis, this section shows moderate
surcharging (Q/Qfull between 1.0 & 1.2) of approximately 0.1 feet above the pipe
crown. The Ultimate analysis shows more significant surcharging (Q/Qfull>1.2) of
approximately 0.5 feet above the pipe crown. However, if the pipe is steeper than
estimated, this result may be inaccurate. Additionally, a highly conservative roughness
value for this pipe was used in the model, resulting in an overestimation of the level of
surcharging. Even still, the level of projected surcharging here does not appear to pose
a significant risk of impacts to the upstream system or side sewers.
The results show no surcharging for the existing 18-inch sewer which runs under I-405
between MH#5320035(MH2309) and MH#5305033(MH2307), with Q/Qfull <0.50.
Since the actual slopes of the pipes are not known, it is difficult to determine the
accurate capacity of the pipes. However, assuming that the modeled profile is
relatively accurate, the capacity of most of the pipes in the interceptor exceeds 1,300
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TECHNICAL MEMORANDUM NO. 2
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gpm, with the exception of the pipe upstream of MH#5320049(MH2323), which has a
capacity of 1160 gpm. Based on the plan and profile capacity analysis results shown in
the attached Figures 4 through 9, it is unlikely that the interceptor will need to be
upsized to convey projected Ultimate peak flows.
Since the stream bank has a history of erosion, and past events have damaged the
pipe, it may be necessary to line the interceptor to prolong its lifespan. Since the liners
are typically made of plastic resin, lining the interceptor should improve the smoothness
of the interior of the pipe over the existing materials. However, the improved flow
characteristics from the smoothness could be offset by the decreased internal pipe
diameter, depending on the required liner thickness used to reinforce the pipe. An
analysis was conducted to determine the effects of the liner, assuming that a 6mm liner
was added to the 10-inch and 12-inch pipe. This would decrease the diameter in each
pipe by approximately 0.5 inches. Results show a net improvement in flow
characteristics in the pipe, with a minimum flow in the 10-inch pipe upstream of
MH#5320049(MH2323) increasing to 1,290 gpm, and capacity in the rest of the Thunder
Hills Interceptor exceeding 1,500 gpm.
One alternative under consideration is to divert a portion of the flows which are
currently tributary to the Thunder Hills Sewer Interceptor into the Talbot Hill Sewer. This
would be accomplished by installing approximately 980 lineal feet of new 8-inch sewer
main on South 18th Street between MH#5320052(MH2326), which is located at the
intersection of Grant Avenue South, and MH#5320168(MH2680), which is located
southeast of Eagle Ridge Drive South. This system discharges northwesterly through
approximately 2,400 feet of existing 8-inch diameter sewer (from South 18th Street, near
Eagle Ridge Drive South), ending on Smithers Avenue South between South 15th Street
and South 14th Street.
The existing (2012) peak flow rate in this portion of the Talbot Hills sewer system is 460
gpm, and the estimated 2012 peak flows for the diversion are 660 gpm, which would
increase the overall peak flow rates into the Talbot Hill sewer service area to 1,120 gpm,
creating capacity problems in the downstream sewer. With the diversion, modeling
results show major surcharging in the existing 8-inch sewer main in South 18th Street from
MH#5320168(MH2680) to MH#5320167(MH2679), moderate surcharging on Benson
Road South between MH#5320160(MH2672) and MH#5320159(MH2671) and between
MH#5320158 (MH2670) and MH#5320157(MH0083), and major surcharging between
Benson Road and Smithers Avenue South from MH#5319085(MH2482) to
MH#5319084(MH2481). The existing system appears to be very near capacity for a 20-
year flow event, limited in particular by the 8-inch diameter pipe immediately upstream
of the 10-inch diameter pipe recently installed by WSDOT.
The projected Ultimate peak flow rate in this portion of the Talbot Hills sewer system is
600 gpm, and the projected Ultimate peak flows for the diversion are 850 gpm, which
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TECHNICAL MEMORANDUM NO. 2
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would which would increase the overall peak flow rates into the Talbot Hill sewer service
area to 1,450 gpm, and create or exacerbate capacity problems in the existing
downstream sewer, which already would have moderate to major surcharging due to
capacity limitations in a few locations during Ultimate peak flows. In addition an
increase to the capacity problems shown in the 2012 analysis, modeling results show
moderate surcharging on Benson Road South from MH#5320164(MH2676) though
MH#5320160(MH2672), and moderate to major surcharging between Benson Road and
Smithers Avenue South from MH#5319085(MH2482) to the recently installed manhole on
the east side of the Talbot Pond, directly west of MH#5319084(MH2481) and between
MH#5319083(MH2480) and MH#5319082(MH2479). The remainder of the existing 8-inch
sewer along this flow path should be able to accommodate the projected flow rates,
since the pipe grades are much steeper than those with projected capacity problems.
See the attached Figures 10 through 17 for plan and profile views of the capacity
analysis results for the existing downstream sewer through Talbot Hill for existing and
Ultimate conditions. Table 2 contains are summary of the projected flows into the
Thunder Hills Interceptor and the Talbot Hills sewer under each of the scenarios
described above.
Table 2: Peak Flow Rate Projections for Thunder Hills Sewer Interceptor and Talbot Hills Sewer
2012 Population (PSRC)
Ultimate Population (2040+25%)
2012 Population (PSRC)
Ultimate Population (2040+25%)
Existing Sewer With Upstream Flows Diverted to Talbot Hills Sewer
Thunder Hills Interceptor 865 gpm 1,125 gpm 210 gpm 285 gpm
Talbot Hills Sewer 460 gpm 600 gpm 1,120 gpm 1,450 gpm
Possible upgrades to the existing pipes along the Talbot Hill sewer diversion route were
identified and modeled. Approximately 225 LF of existing sewer from
MH#5320168(MH2680) to MH#5320167(MH2679) was upsized to 12-inch diameter pipe,
approximately 730 LF of existing sewer from MH#5320164(MH2676) though
MH#5320159(MH2671) was upsized to 10-inch diameter pipe, and approximately 285 LF
of existing sewer from MH#5319085(MH2482) to the new manhole west of
MH#5319084(MH2481), and from MH#5319083(MH2480) to MH#5319082(MH2479) was
upsized to 12-inch diameter pipe. Results of the analysis show that these upgrades
would have the capacity to convey the peak flows diverted from the Thunder Hills
Sewer Interceptor. See the attached Figures 18 and 19 for plan and profile views of the
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TECHNICAL MEMORANDUM NO. 2
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capacity analysis results for the upsized piping through Talbot Hill. Figure 20 shows the
recommended improvements.
To determine the priority and phasing of the proposed upgrades along the Talbot Hill
sewer diversion route, the results of the analysis were compared to the Ultimate model
analysis results to determine which improvements need to occur before the diversion
sewer is installed, and which can be installed at a later time. Based on this comparison,
we recommend the improvements between MH#5320168(MH2680) and
MH#5320167(MH2679), between MH#5320160(MH 2672) and MH#5320159(MH2671),
and between MH#5319085(MH2482) and the new manhole west of
MH#5319084(MH2481) be made prior to the installation of the diversion sewer.
Although the section between MH#5319083(MH2480) and MH #5319082(MH2479) does
not show capacity problems under existing conditions, we recommend that it be
upsized in conjunction with the proposed Talbot Hill Sewer Relocation. The
improvements between MH#5320164(MH2676) and MH#5320160(MH2672), upsizing
approximately 500 LF of existing sewer to 10-inch pipe, may be delayed. However, that
section of sewer should be monitored, and upsized before it reaches capacity.
Temporary diversions during dry weather could be feasible for construction; however,
large wet weather events could cause issues for diversion, even in the immediate
future.
The modeling analysis results described above for the diversion were also considered in
the in the Talbot Hill Sewer Relocation – SR167 IC/DC Project Alternatives Analysis
Report, dated October 14, 2014. In general, for the alternatives in that study, piping
sizes across I-405 would need to be upsized to accommodate the increased flow rates
from the diversion.
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Appendix D
TECH MEMO NO. 3 - REMAINING USEFUL LIFE (RUL) AND
REHABILITATION ANALYSIS
CITY OF RENTON
Technical Memorandum No. 3
Remaining Useful Life (RUL) & Rehabilitation Analysis
Thunder Hills Sanitary Sewer Interceptor Alternative Analysis
February 13, 2015
Prepared By:
Stantec Consulting Services Inc.
11130 NE 33rd Place
Suite 200
Bellevue, WA 98004
425.869.9448
TECHNICAL MEMORANDUM NO. 3
February 13, 2015
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Contents
1.0 INTRODUCTION ...........................................................................................................1.1
2.0 EXISTING INTERCEPTOR CONDITION ..........................................................................2.1
2.1 MANHOLE 051 TO 051A ................................................................................................. 2.1
2.2 MANHOLE 051A TO 050 ................................................................................................. 2.2
2.3 MANHOLE 050 TO 049 ................................................................................................... 2.2
2.4 MANHOLE 049 TO 046 ................................................................................................... 2.2
2.5 MANHOLE 046 TO 036 ................................................................................................... 2.3
2.6 MANHOLE 036 TO 035 ................................................................................................... 2.3
2.7 MANHOLE 035 TO 034 TO 033....................................................................................... 2.4
3.0 REMAINING USEFUL LIFE ASSESSMENT ........................................................................3.4
4.0 REHABILITATION FEASIBILITY .......................................................................................4.6
4.1 SPOT REPAIR STATION 23+20 ......................................................................................... 4.6
4.2 SPOT REPAIR STATION 22+40 ......................................................................................... 4.6
4.3 SPOT REPAIR STATION 12+20 ......................................................................................... 4.6
4.4 CIPP REHABILITATION ..................................................................................................... 4.7
Appendices
APPENDIX A: FIGURES
APPENDIX B: SEWER MAIN INSPECTION
APPENDIX C: FIELD INVESTIGATION REPORTS
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1.0 INTRODUCTION
This memorandum includes the remaining useful life (RUL) and rehabilitation analysis of the
Thunder Hills Alternative Analysis project. The Alternative Analysis aims to find the best alternative
that will improve the functionality of the Thunder Hills Interceptor. The project focuses on the Thunder Hills Interceptor, particularly the section between Grant Avenue South and Benson Road South. The Interceptor was originally constructed in 1965, and serves a portion of the City’s
Rolling Hills neighborhood southeast of Renton City Hall. Several portions of the existing
interceptor have been replaced over time as additional development (interties) has occurred, or as required due to erosion from Thunder Hills Creek.
The goals of the RUL and rehabilitation analysis section of the Alternative Analysis are to identify
the existing condition of the interceptor, to assign condition rankings from good to moderate to
poor, and to assess the feasibility of rehabilitating the existing line using cured-in-place-pipe (CIPP).
2.0 EXISTING INTERCEPTOR CONDITION
The relevant portion of the Thunder Hills Interceptor runs from Grant Avenue South to Benson
Road South through a small canyon created by Thunder Hills Creek. As stated above, the interceptor was originally installed in 1965 and appears to have been constructed with Asbestos Cement (AC) pipe using cast iron fittings for bends and wyes. Over the years, sections of the
existing AC interceptor have been replaced with ductile iron pipe likely due to either exposure
of the interceptor from erosion caused by Thunder Hills Creek, or due to development in the area and the need to install new interties from perpendicular sewer lines. Figures 1 through 4
contained in “Appendix A – Figures” show the existing interceptor information.
In June of 2014 Stantec contracted with Bravo Environmental NW (Bravo) to perform a field
investigation of the existing Thunder Hills Interceptor and to provide an analysis of the condition of the existing sewer line. The field investigation was performed using PANORAMO digital
scanning which allows for a 360 degree view of the interceptor along its alignment. A summary
of existing conditions is provided below. The detailed condition assessment is contained in
“Appendix B – Sewer Main Inspection”, which is further supported by the field investigation reports generated by Bravo in “Appendix C – Field Investigation Reports”.
2.1 MANHOLE 051 TO 051A
The existing interceptor from Manhole 051, located in Grant Avenue S, to Manhole 051A,
located adjacent to Thunder Hills Creek, is approximately 98 feet long and is constructed with
10-inch diameter concrete pipe. The pipe appears to be from the original construction of the Thunder Hills Interceptor and, for its age, is in generally good condition. There are several
locations along the alignment with grease buildup, as well as several broken lips at the 6 o’clock
position, presumably from years of flow and debris wearing down the concrete. Additionally, there are several pipe joints that have separated either over time, or were installed that way. The separated joints are not severe enough to expose the pipe to the surrounding soil, and were
more likely caused by deflecting the joints to allow for a moderate bend in the existing pipe.
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2.2 MANHOLE 051A TO 050
The existing interceptor from Manhole 051A to 050 parallels Thunder Hills Creek on the northeast side of the creek. The interceptor shifts from being in the area between the existing access road
and the creek to within the access road as it continues from Manhole 051A to Manhole 050.
Starting at Manhole 051A, the existing interceptor is constructed of 10-inch diameter asbestos cement (AC) pipe until it reaches approximately Station 23+20 where the line changes from 10-
inch diameter AC to 12-inch diameter ductile iron (DI). Based on the digital scan of the pipe, it
appears that the last 65 feet of the pipe prior to Manhole 050 was replaced and the existing 10-
inch AC pipe was stubbed into the new 12-inch DI line. The overall length of this section of interceptor is approximately 512 feet.
The interceptor from Manhole 051A to Manhole 050 appears to be part of the original
construction of the Thunder Hills Interceptor in 1965, except for the final portion of ductile iron
line. The pipe is in generally good condition for its age except were cast iron fittings were used. There are three wye fittings in this section of the existing interceptor, with the branch for all wye
fittings pointing in the 12 o’clock position to act as a clean out for the interceptor. In addition to
the cast iron wye fittings there are two other cast iron bends and one cast iron sleeve at the transition from 10-inch AC to 12-inch DI pipe. Each of the cast iron fittings showed severe corrosion from their years of service and exposure to sewage and hydrogen sulfide (H2S). As
seen in the interceptor run above, there are several separated joints that appear to be due to
pipe deflection as the pipe was installed to parallel the creek.
2.3 MANHOLE 050 TO 049
The existing interceptor from Manhole 050 to 049 is a short section of pipe that parallels the northeast side of Thunder Hills Creek. The interceptor is generally located within the existing
access road and appears to be part of the original construction of the interceptor. The line is
constructed of 10-inch diameter AC pipe and is approximately 211 feet long. There is a possibility that the first 13 feet of interceptor out of Manhole 050 is DI pipe that was installed when Manhole 050 was constructed. It is possible that Manhole 050 was constructed after the
original interceptor, which would explain the installation of 65 feet of DI pipe coming into
Manhole 050. Both Manhole 050 and 049 have 8-inch diameter DI sewer lines coming into them from the other side of Thunder Hills Creek.
As with the previous pipe section, the pipe is in generally good condition for its age except were
cast iron fittings were used. There are two wye fittings in this section of the existing interceptor,
with the branch for all wye fittings pointing in the 12 o’clock position. It appears that the branch of both wye fittings are capped and are not used as clean outs. In addition to the cast iron wye
fittings there is one other cast iron sleeve in this section of interceptor. Each of the cast iron
fittings showed severe corrosion from their years of service and exposure to sewage and
hydrogen sulfide (H2S). There are several separated joints that appear to be due to pipe deflection as the pipe was installed to parallel the creek along with some minor root intrusion at
the connection to Manhole 049.
2.4 MANHOLE 049 TO 046
The existing interceptor from Manhole 049 to 046 parallels the northeast side of the creek south
of the existing BPA easement. The interceptor is generally located within the existing access road and follows both the road and creek with several bends in the line, either using actual
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bends or by deflecting the pipe. This section of pipe is approximately 548 feet long and contains both existing asbestos cement (AC) pipe and new ductile iron (DI) pipe. The material transitions
from 10-inch diameter AC pipe to 10-inch diameter DI pipe approximately 147 feet downstream
of Manhole 049. There is heavy root intrusion and debris at the connection with Manhole 049. There are also several separated joints in both the AC and DI portions of the interceptor to allow
it to follow the existing creek. In addition to the separated joints, there are six cast iron fittings,
one wye, three bends, and two sleeves, contained in this section of pipe. Five of the six cast iron
fittings show severe corrosion from H2S, while one of the bends only shows moderate corrosion. Within the DI portion of the interceptor, there are several locations where the concrete mortal
lining (CML) has spalled off of the DI pipe and there is evidence of onset corrosion beginning to
happen. The wye fitting in this section of interceptor is again oriented in the 12 o’clock position as if it was intended to be a clean out, but it appears to be capped.
2.5 MANHOLE 046 TO 036
The existing interceptor from Manhole 046 to 036 is the last section of interceptor that completely
parallels Thunder Hills Creek on the northeast side. This section of interceptor consists of
approximately 182 feet of 10-inch diameter ductile iron (DI) pipe and generally runs within the
existing BPA easement between the existing access road and the creek. There are several separated joints and two bends contained in this portion of the interceptor to allow it to follow
the creek alignment. One of the bends is DI and is in fairly good condition. The second bend
near Manhole 036 is cast iron and is very corroded due to H2S. The overall condition of the existing DI pipe is fairly good with some deposits attached to the pipe wall throughout the length of the pipe.
2.6 MANHOLE 036 TO 035
The existing interceptor from Manhole 036 to 035 is the longest section of interceptor without
access points at almost 1,300 lineal feet. The interceptor also crosses under Thunder Hills Creek at approximately Station 11+80 and continues to follow the creek behind the Berkshire Apartments to the bottom of the ravine before going under I-405. Beginning at Manhole 036,
the existing interceptor is constructed of 12-inch diameter ductile iron (DI) pipe. Although there
are no bend fittings in this section of interceptor, there are many slightly separated joints that are clearly meant to deflect the pipe as if traverses and then follows the creek to the bottom of the
ravine. There are nine connection points to the existing interceptor between Manhole 036 and
035. The connection points are broken into sets of three and occur approximately 80 feet, 500
feet and 915 feet downstream from Manhole 036. Each block of fittings includes a wye fitting pointed in the 12 o’clock direction, and two tee connections, one pointing at 3 o’clock, and
one at 9 o’clock. All of the fittings have evidence of heavy corrosion, even though they appear
to be DI, except for the first wye, which appears to be cast iron. This is likely due to either the
lack of CML lining in the fittings or the CML lining has spalled off. We know that the tee fittings at 9 o’clock that are 500 and 915 feet downstream of Manhole 036 connect to the Berkshire
Apartments. There is no evidence that the tee connections at 80 feet downstream or the
connections at 3 o’clock from the tees at 500 and 915 feet downstream connect to any part of the City of Renton’s sewer system.
The overall condition of the existing interceptor is moderate at best. In addition to the corroded
fittings, there is evidence of heavy scale deposits on the wall of the pipe for the entire length.
There are also several locations where the existing CML lining has spalled off the pipe wall and there is evidence of onset corrosion where H2S can come in contact with the DI pipe.
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2.7 MANHOLE 035 TO 034 TO 033
The existing interceptor from Manhole 035 to 034 and then on to 033 includes the sections of
interceptor that go under I-405 and Benson Road to Manhole 033 behind Sam’s Club. These
sections of interceptor consist of approximately 500 feet of 18-inch diameter ductile iron (DI)
pipe. There is significant CML scaling and corrosion throughout both sections of interceptor likely
due to the amount of flow and the presence of H2S. There also appears to be a sag in the
interceptor between Benson Road and I-405. The water level in the pipe during the inspection
increased from approximately 30% full to 75% full from a point 271 feet downstream of Manhole
035 and continued at that level until roughly 338 feet downstream of Manhole 035. There was
also evidence of gravel debris during one of the inspection runs which could have caused the
rise in water level if it was of sufficient depth to dam the existing pipe.
3.0 REMAINING USEFUL LIFE ASSESSMENT
Remaining Useful Life (RUL) of the existing interceptor is a critical part of determining the “Risk”
associated with the existing pipeline. Risk is defined as the pipes RUL multiplied by its “Criticality”.
The criticality of the pipe is being determined through an analysis of the potential for erosion to
expose or damage the pipe. The Preliminary Erosion Hazard Evaluation is contained in Technical
Memorandum #5 prepared by Stantec. In an attempt to correlate the RUL with the Erosion
Evaluation, the existing interceptor has been broken into eight distinct “reaches”. The RUL for
each reach will be determined based on the frequency of “good”, “moderate”, and “poor”
remaining useful life designations within each reach. This will allow for a simple multiplication of
the RUL and Criticality to come up with useful Risk number for each reach.
For the purposes of this analysis, we have separated the overall alignment into the following
eight reaches:
Reach Location Approximate Gradient (%)
0+00 to 1+00 Not Applicable (Culvert)
1+00 to 5+50 15.1
5+50 to 8+30 4.3
8+30 to 11+00 8.2
11+00 to 12+30 10.4
12+30 to 18+90 5.2
18+90 to 26+75 7.1
26+75 to 28+00 14.4
The remaining useful life for each section of existing interceptor has been designated as either
“Good”, “Moderate”, or “Poor”. Based on the visual condition assessment conducted, and as
identified in Appendix B – Sewer Main Inspection, a pipe segment with a “Good” RUL is
anticipated to have a remaining life expectancy of 30 to 40 years, a segment with a
“Moderate” RUL is anticipated to have a remaining life expectancy of 20 to 30 years, and a
segment with a “Poor” RUL is expected to have a remaining life expectancy of 10 to 20 years.
TECHNICAL MEMORANDUM NO. 3
February 13, 2015
For the purposes of completing the RUL analysis a pipe section with a “Good” RUL was given a numerical value of 1 and it represents a pipe that is constructed of ductile iron (DI) pipe that still
has the majority of its CML lining and limited corrosion spots. Evidence of separated joints and
minor root intrusion did not have an impact on the RUL designations. A section of pipe with a “Moderate” RUL was given a numerical value of 2 and it represents a portion of asbestos
cement (AC) pipe that appears to be in good shape, or a DI pipe that is showing more
significant CML scaling and corrosion. The reason for downgrading an AC pipeline was due to
its inherent fragility when compared to DI pipe. A section of pipe with a “Poor” RUL was given a numerical value of 3 and it represents a portion of pipe that has severe corrosion or a physical
defect in the pipe that will cause flow constraints which would potentially reduce the remaining
lifespan of the pipe. These are locations identified as the most likely to fail and cause damage to the environment which would require emergency repairs. Both Appendix A – Figures and Appendix B – Sewer Main Inspection were color coded based on the RUL designation
determined during the condition assessment. A section of pipe with Good RUL was coded
“Green”, a section of pipe with Moderate RUL was coded “Yellow”, and a section of pipe with Poor RUL was coded “Red”. Table 3-1 summarizes the amount of pipe in each reach with Good, Moderate, and Poor RUL and provides a designated numerical value for the RUL for each reach.
Table 3-1 Remaining Useful Life (RUL) Assessment
Reach Location Length of Good RUL (feet)
Length of Moderate RUL (feet)
Length of Poor RUL (feet) RUL Designation
0+00 to 1+00 100 0 0 1
1+00 to 5+50 437 13 0 1
5+50 to 8+30 277 3 0 1
8+30 to 11+00 253 17 0 1
11+00 to 12+30 124 0 6 1
12+30 to 18+90 619 32 9 1
18+90 to 26+75 65 684 36 2
26+75 to 28+00 0 125 0 2
It should be noted that these reaches do not include the existing pipe from Manhole 035 to 033
because this section of pipe was not analyzed as part of the erosion evaluation due to it being
under I-405. Likewise, a small portion of pipe from Station 28+00 to approximately 28+75 between Manhole 051 and 051A was not susceptible to erosion from Thunder Hills Creek and
was not included in the reaches described above. Both of these portions of pipe would be
given an RUL designation of Moderate, or 2, due to corrosion in the DI pipe from Manhole 035 to
033 and the condition of the concrete pipe from Manhole 051 to 051A. These sections of pipe will be included in the Risk analysis.
Due to the limited amount of Poor RUL pipe, the majority of the Thunder Hills Interceptor falls
under the Good RUL designation. The only reaches that are considered Moderate RUL is due to the presence of existing asbestos cement (AC) pipe.
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4.0 REHABILITATION FEASIBILITY
This section covers the feasibility of rehabilitating the existing Thunder Hills Interceptor using
cured-in-place-pipe (CIPP) technology. The existing Thunder Hills Interceptor is approximately
3,350 feet long from Manhole 051 in Grant Avenue S to Manhole 033 behind Sam’s Club on the west side of I-405. The pipe varies in both material and size over its length and includes pipe ranging from 10-inch to 12-inch to 18-inch diameter, and material ranging from concrete to
asbestos cement (AC) to ductile iron (DI), with spot locations of cast iron at various fittings.
Overall, the pipe in general is in relatively good condition for its age and is an ideal candidate for pipeline rehabilitation using CIPP technology. Over the length of the interceptor there are only three locations that were identified as requiring open cut excavation and repair prior to
being able to rehabilitate the existing interceptor. These locations are discussed in further detail
below.
4.1 SPOT REPAIR STATION 23+20
At approximately Station 23+20 in the interceptor run between Manhole 051A and 050 there is a
severe offset in the pipe. It appears that Manhole 050 was installed at some time after the
original interceptor was constructed and during the installation of the manhole, roughly 65 feet
of 12-inch ductile iron (DI) pipe was installed upstream of Manhole 050. To create a transition from the existing 10-inch AC pipe to the new 12-inch pipe it appears that the 10-inch AC line
was stubbed into the new 12-inch DI pipe thus creating a severe offset in the pipe. Because the
pipe goes from 10-inch to 12-inch in the downstream direction, the flow is not effected by the
offset. That being said, it is very difficult to inspect and maintain this section of line. We recommend that this location be dug up and a proper transition fitting be installed that will allow
for the installation of a CIPP liner. It should also be noted that the CIPP liner for this run of
interceptor will need to be custom made to include the transition for 10-inch to 12-inch pipe.
4.2 SPOT REPAIR STATION 22+40
At approximately Station 22+40 just downstream of Manhole 050 there is an offset joint that is significant enough to disrupt the flow and can potentially cause debris build up. In addition,
there is an existing cast iron wye fitting with a capped clean out adjacent to the offset joint. We
recommend that this location be dug up and repaired prior to installing a CIPP liner.
4.3 SPOT REPAIR STATION 12+20
At approximately Station 12+20 which is roughly 90 feet downstream of Manhole 036, there is a large piece of concrete debris at the location of a tee fitting. While it does not appear that the
existing fitting is active, the debris is large enough that it could end up in the flow line of the
existing interceptor and could potentially dam the pipe causing upstream overflows. The concrete debris is too big to clean out with conventional jetting equipment. Add to that the fact that the next downstream access point is almost 1,200 feet away, and we recommend that
this location be excavated and the piece of debris removed.
\\us1314-f01\workgroup\2002\active\2002003607\analysis\remaining useful life\mem_thunderhills_rul_20150213.docx 6
TECHNICAL MEMORANDUM NO. 3
February 13, 2015
4.4 CIPP REHABILITATION
Save for the three spot repair locations identified above, the existing interceptor is an ideal candidate for CIPP rehabilitation. A large portion of the interceptor has significant grease
buildup and deposits attached to the walls. This condition, along with the minor root intrusion
observed, will not impact the feasibility of installing a CIPP liner. Likewise, the numerous separated joints and bends will not affect the ability to install the liner or prevent the City from
conducting future inspections or maintenance.
The only limiting factor with the existing interceptor that will need to be addressed prior to CIPP
rehabilitation is the distance between Manhole 036 and 035. At almost 1,300 feet, the distance between manholes is more than ideal for the installation of a CIPP liner. The distance is also
greater than the capability that the City of Renton has for future maintenance of the line
anyway. As such, we recommend that a manhole access point be installed in the interceptor
behind the Berkshire apartments to help facilitate the installation of a CIPP liner if rehabilitation is chosen
\\us1314-f01\workgroup\2002\active\2002003607\analysis\remaining useful life\mem_thunderhills_rul_20150213.docx 7
TECHNICAL MEMORANDUM NO. 3
February 13, 2015
This page intentionally left blank.
\\us1314-f01\workgroup\2002\active\2002003607\analysis\remaining useful life\mem_thunderhills_rul_20150213.docx 8
TECHNICAL MEMORANDUM NO. 4
FEBRUARY 12, 2015
APPENDIX A
Remaining Useful Life Figures
TECHNICAL MEMORANDUM No. 4
February 13, 2015
\\us1314-f01\workgroup\2002\active\2002003607\analysis\remaining useful life\mem_thunderhills_rul_20150213.docx
TECHNICAL MEMORANDUM NO. 4
FEBRUARY 12, 2015
APPENDIX B
Sewer Main Inspection
TECHNICAL MEMORANDUM No. 4
February 13, 2015
\\us1314-f01\workgroup\2002\active\2002003607\analysis\remaining useful life\mem_thunderhills_rul_20150213.docx
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e
c
t
i
o
n
US
M
H
D
S
M
H
Le
n
g
t
h
(f
e
e
t
)
US
M
H
de
p
t
h
DS
M
H
de
p
t
h
Pip
e
D
i
a
(i
n
c
h
e
s
)
Pi
p
e
Ma
t
e
r
i
a
l
No
.
o
f
Co
n
n
e
c
t
i
o
n
s
R
e
v
i
e
w
e
r
In
s
p
e
c
t
i
o
n
Da
t
e
In
s
p
e
c
t
i
o
n
Dir
e
c
t
i
o
n
Ob
s
e
r
v
e
d
C
o
n
d
i
t
i
o
n
Di
s
t
a
n
c
e
F
r
o
m
US
M
H
(
f
e
e
t
)
Co
m
m
e
n
t
s
04
9
0
4
6
5
4
8
5
.
7
9
.
9
1
0
A
C
/
D
I
1
E
J
W
6
/
3
/
2
0
1
4
D
S
He
a
v
y
r
o
o
t
i
n
t
r
u
s
i
o
n
a
n
d
d
e
b
r
i
s
0
.
6
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
2
8
.
8
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
4
1
.
9
O
k
De
p
o
s
i
t
s
a
t
t
a
c
e
d
t
o
w
a
l
l
9
t
o
3
5
5
.
6
t
o
3
1
8
O
k
Wy
e
f
i
t
t
i
n
g
1
2
o
'
c
l
o
c
k
,
c
a
s
t
i
r
o
n
,
v
e
r
y
co
r
r
o
d
e
d
,
a
p
p
e
a
r
s
c
a
p
p
e
d
81
.
1
D
o
n
o
t
r
e
i
n
s
t
a
t
e
w
y
e
i
f
l
i
n
e
d
Ca
s
t
i
r
o
n
s
l
e
e
v
e
,
v
e
r
y
c
o
r
r
o
d
e
d
9
5
.
6
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
1
0
2
.
6
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
1
0
9
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
1
1
5
.
6
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
1
2
2
O
k
Ca
s
t
i
r
o
n
s
l
e
e
v
e
,
v
e
r
y
c
o
r
r
o
d
e
d
1
2
8
.
8
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
1
2
9
.
2
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
1
4
2
.
2
O
k
Ma
t
e
r
i
a
l
c
h
a
n
g
e
f
r
o
m
A
C
t
o
D
I
1
4
7
.
4
O
k
Be
n
d
,
c
a
s
t
i
r
o
n
,
v
e
r
y
c
o
r
r
o
d
e
d
2
1
4
.
3
Sl
i
g
h
t
a
l
i
g
n
m
e
n
t
s
h
i
f
t
r
i
g
h
t
a
n
d
do
w
n
,
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
3
2
6
.
2
O
k
Be
n
d
,
c
a
s
t
i
r
o
n
3
3
7
.
7
A
l
i
g
n
m
e
n
t
s
h
i
f
t
l
e
f
t
a
n
d
d
o
w
n
,
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
3
4
5
.
6
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
3
6
3
.
9
O
k
CM
L
s
c
a
l
i
n
g
,
c
o
r
r
o
s
i
o
n
s
t
a
r
t
i
n
g
,
8
o
'
c
l
o
c
k
3
7
7
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
3
8
2
.
2
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
4
0
0
.
5
O
k
CM
L
s
c
a
l
i
n
g
,
c
o
r
r
o
s
i
o
n
s
t
a
r
t
i
n
g
,
8
o
'
c
l
o
c
k
4
1
6
.
4
O
k
Be
n
d
,
c
a
s
t
i
r
o
n
,
v
e
r
y
c
o
r
r
o
d
e
d
4
3
7
.
1
Al
i
g
n
m
e
n
t
s
h
i
f
t
r
i
g
h
t
a
n
d
d
o
w
n
,
Ok
Se
p
a
r
a
t
e
d
j
o
i
n
t
4
3
9
.
7
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
4
7
6
.
1
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
4
9
4
.
2
O
k
CM
L
s
c
a
l
i
n
g
,
c
o
r
r
o
s
i
o
n
s
t
a
r
t
i
n
g
,
4
o
'
c
l
o
c
k
5
1
9
.
3
O
k
CM
L
s
c
a
l
i
n
g
,
c
o
r
r
o
s
i
o
n
s
t
a
r
t
i
n
g
,
4
o
'
c
l
o
c
k
5
2
2
.
6
O
k
En
d
m
a
n
h
o
l
e
5
4
8
.
7
O
k
V:
\
2
0
0
2
\
a
c
t
i
v
e
\
2
0
0
2
0
0
3
6
0
7
\
a
n
a
l
y
s
i
s
\
i
n
s
p
e
c
t
i
o
n
\
V
i
d
e
o
I
n
s
p
e
c
t
i
o
n
A
n
a
l
y
s
i
s
_
O
b
s
e
r
v
e
d
C
o
n
d
i
t
i
o
n
s
.
x
l
s
x
3
o
f
5
Pr
o
j
e
c
t
:
T
h
u
n
d
e
r
H
i
l
l
s
S
a
n
i
t
a
r
y
S
e
w
e
r
I
n
t
e
r
c
e
p
t
o
r
A
l
t
e
r
n
a
t
i
v
e
s
A
n
a
l
y
s
i
s
Pr
o
j
e
c
t
N
o
.
:
2
0
0
2
0
0
3
6
0
7
Se
w
e
r
M
a
i
n
I
n
s
p
e
c
t
i
o
n
US
M
H
D
S
M
H
Le
n
g
t
h
(f
e
e
t
)
US
M
H
de
p
t
h
DS
M
H
de
p
t
h
Pip
e
D
i
a
(i
n
c
h
e
s
)
Pi
p
e
Ma
t
e
r
i
a
l
No
.
o
f
Co
n
n
e
c
t
i
o
n
s
R
e
v
i
e
w
e
r
In
s
p
e
c
t
i
o
n
Da
t
e
In
s
p
e
c
t
i
o
n
Dir
e
c
t
i
o
n
Ob
s
e
r
v
e
d
C
o
n
d
i
t
i
o
n
Di
s
t
a
n
c
e
F
r
o
m
US
M
H
(
f
e
e
t
)
Co
m
m
e
n
t
s
04
6
0
3
6
1
8
2
9
.
9
1
0
.
8
1
0
D
I
0
E
J
W
6
/
4
/
2
0
1
4
D
S
De
p
o
s
i
t
s
a
t
t
a
c
h
e
d
t
o
w
a
l
l
0
t
o
1
8
2
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
3
.
7
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
2
2
.
1
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
4
0
.
3
Ok
.
P
e
n
n
y
l
o
c
a
t
e
d
a
t
4
3
.
2
5
a
t
4
o'c
l
o
c
k
DI
b
e
n
d
10
4
.
3
A
l
i
g
n
m
e
n
t
s
h
i
f
t
r
i
g
h
t
,
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
1
5
0
.
7
O
k
Ca
s
t
i
r
o
n
b
e
n
d
,
v
e
r
y
c
o
r
r
o
d
e
d
1
8
0
.
6
A
l
i
g
n
m
e
n
t
s
h
i
f
t
l
e
f
t
,
O
k
03
6
0
3
5
1
2
9
9
1
0
.
8
1
8
.
8
1
2
D
I
9
E
J
W
6
/
4
/
2
0
1
4
D
S
De
p
o
s
i
t
s
a
t
t
a
c
h
e
d
t
o
w
a
l
l
0
t
o
1
2
9
9
Ok
.
H
e
a
v
y
s
c
a
l
e
d
e
p
o
s
i
t
s
o
n
w
a
l
l
th
r
o
u
g
h
o
u
t
CM
L
s
c
a
l
i
n
g
,
c
o
r
r
o
s
i
o
n
s
t
a
r
t
i
n
g
,
1
2
o
'
c
l
o
c
k
1
3
.
1
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
1
7
.
5
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
5
4
O
k
Wy
e
f
i
t
t
i
n
g
1
2
o
'
c
l
o
c
k
,
c
a
s
t
i
r
o
n
,
v
e
r
y
co
r
r
o
d
e
d
80
O
k
Te
e
f
i
t
t
i
n
g
3
o
'
c
l
o
c
k
,
d
u
c
t
i
l
e
i
r
o
n
8
4
.
2
O
k
Te
e
f
i
t
t
i
n
g
9
o
'
c
l
o
c
k
,
d
u
c
t
i
l
e
i
r
o
n
8
8
.
3
O
k
Co
n
c
r
e
t
e
d
e
b
r
i
s
i
n
t
e
e
b
r
a
n
c
h
,
9
o
'
c
l
o
c
k
8
9
.
1
N
e
e
d
s
t
o
b
e
r
e
m
o
v
e
d
CM
L
s
c
a
l
i
n
g
,
c
o
r
r
o
s
i
o
n
s
t
a
r
t
i
n
g
,
8
o
'
c
l
o
c
k
1
6
4
.
3
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
2
4
2
O
k
CM
L
s
c
a
l
i
n
g
,
c
o
r
r
o
s
i
o
n
s
t
a
r
t
i
n
g
,
4
o
'
c
l
o
c
k
&
7
o
'
c
l
o
c
k
24
6
.
6
t
o
2
5
2
.
2
O
k
Co
r
r
o
s
i
o
n
a
t
j
o
i
n
t
,
4
o
'
c
l
o
c
k
t
o
8
o
'
c
l
o
c
k
2
5
2
.
2
O
k
CM
L
s
c
a
l
i
n
g
,
c
o
r
r
o
s
i
o
n
s
t
a
r
t
i
n
g
8
o
'
c
l
o
c
k
3
2
4
.
2
t
o
3
2
9
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
4
2
3
.
7
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
4
7
8
.
5
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
4
9
6
.
7
O
k
Wy
e
f
i
t
t
i
n
g
1
2
o
'
c
l
o
c
k
,
D
I
,
v
e
r
y
c
o
r
r
o
d
e
d
,
ca
p
p
e
d
50
1
.
1
D
o
n
o
t
r
e
i
n
s
t
a
t
e
w
y
e
i
f
l
i
n
e
d
Te
e
f
i
t
t
i
n
g
3
o
'
c
l
o
c
k
,
d
u
c
t
i
l
e
i
r
o
n
5
0
5
.
5
O
k
Te
e
f
i
t
t
i
n
g
9
o
'
c
l
o
c
k
,
d
u
c
t
i
l
e
i
r
o
n
5
0
9
.
4
O
k
CM
L
s
c
a
l
i
n
g
,
6
o
'
c
l
o
c
k
,
d
i
s
r
u
p
t
i
n
g
f
l
o
w
7
4
9
.
4
O
k
Se
p
a
r
a
t
e
d
j
o
i
n
t
7
6
6
.
7
O
k
V:
\
2
0
0
2
\
a
c
t
i
v
e
\
2
0
0
2
0
0
3
6
0
7
\
a
n
a
l
y
s
i
s
\
i
n
s
p
e
c
t
i
o
n
\
V
i
d
e
o
I
n
s
p
e
c
t
i
o
n
A
n
a
l
y
s
i
s
_
O
b
s
e
r
v
e
d
C
o
n
d
i
t
i
o
n
s
.
x
l
s
x
4
o
f
5
Pr
o
j
e
c
t
:
T
h
u
n
d
e
r
H
i
l
l
s
S
a
n
i
t
a
r
y
S
e
w
e
r
I
n
t
e
r
c
e
p
t
o
r
A
l
t
e
r
n
a
t
i
v
e
s
A
n
a
l
y
s
i
s
Pr
o
j
e
c
t
N
o
.
:
2
0
0
2
0
0
3
6
0
7
Se
w
e
r
M
a
i
n
I
n
s
p
e
c
t
i
o
n
US
M
H
D
S
M
H
Le
n
g
t
h
(f
e
e
t
)
US
M
H
de
p
t
h
DS
M
H
de
p
t
h
Pip
e
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TECHNICAL MEMORANDUM No. 4
February 13, 2015
APPENDIX C
Field Investigation Reports
\\us1314-f01\workgroup\2002\active\2002003607\analysis\remaining useful life\mem_thunderhills_rul_20150213.docx
TECHNICAL MEMORANDUM No. 4
February 13, 2015
\\us1314-f01\workgroup\2002\active\2002003607\analysis\remaining useful life\mem_thunderhills_rul_20150213.docx
VideoCount CD Code Fr To RemarksImRefIn1%JntIn2
0.0 ST Start of Survey
0.0 AMH Manhole 5320-051
0.0 MWL Water Level 0
0.0 MWM Water Mark 15
0.0 S01 SRI Surface Roughness Increased 07 04
5.8 DAGS Deposits Attached Grease 02 03 film/residue of grease5
6.5 B Broken 06
9.2 S02 DAGS Deposits Attached Grease 12 03 film/residue of grease5
31.6 F02 DAGS Deposits Attached Grease 12 035
34.0 S03 DAGS Deposits Attached Grease 01 025
36.6 MWM Water Mark 25
57.5 F03 DAGS Deposits Attached Grease 01 025
60.9 S04 DAGS Deposits Attached Grease 01 035
78.7 MWM Water Mark 40
93.8 F04 DAGS Deposits Attached Grease 01 035
97.8 F01 SRI Surface Roughness Increased 07 04 surface damage aggregate proj...
97.8 AMH Manhole 5320_051A
97.8 FH End of Survey
Total Length Surveyed97.8 M
Scores Pipe Ratings Index
Pipe Ratings Index
1.1
1.7
Peak
Peak
4
2
Mean Pipe
Mean Pipe
0.2
0.4
Pipe Rating
Pipe Rating
23
36
Structural:
O&M:
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/03
Setup 1 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control De-Watered using
Jetter
Location Light Highway
CatPurpose
Use Sanitary
Shape Circular
Material Concrete Pipe (non-reinforced)
Lining
Total length 97.8 M
Height 10 Width 10 mm
Joint length M
Start 5320_051 Rim to invert M
Finish 5320_051A Rim to invert M
Direction Down
Preclean J
Year rehabilitated
Additional info
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Tabular Report of PSR 5320-051 X Stantecfor
Certificate #U-513-18121
Time 11:07
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 97.8
Pressure
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
0 M
97.8 M
Pi
p
e
F
l
o
w
Su
r
v
e
y
D
i
r
Start of Survey
Manhole [5320-051]
Water Level 000%
Water Mark 015%
Surface Roughness Increased 07 to 04 o'clock ST: 1 [S01]
0.0 M
Deposits Attached Grease 02 to 03 o'clock 005% OM: 2 [film/residue of grease] 5.8 M
Broken 06 o'clock ST: 3 6.5 M
Deposits Attached Grease 12 to 03 o'clock 005% OM: 2 [film/residue of grease] [S02] 9.2 M
Deposits Attached Grease 12 to 03 o'clock 005% OM: 2 [F02] 31.6 M
Deposits Attached Grease 01 to 02 o'clock 005% OM: 2 [S03] 34.0 M
Water Mark 025% 36.6 M
Deposits Attached Grease 01 to 02 o'clock 005% OM: 2 [F03] 57.5 M
Deposits Attached Grease 01 to 03 o'clock 005% OM: 2 [S04] 60.9 M
Water Mark 040% 78.7 M
Deposits Attached Grease 01 to 03 o'clock 005% OM: 2 [F04] 93.8 M
Surface Roughness Increased 07 to 04 o'clock ST: 1 [surface damage aggregate projecting in MH] [F01]
Manhole [5320_051A]
End of Survey
97.8 M
Direction
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/03
Setup 1 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control De-Watered using
Jetter
Location Light Highway
CatPurpose
Use Sanitary
Shape Circular
Material Concrete Pipe (non-reinforced)
Lining
Total length 97.8 M
Height 10 Width 10 mm
Joint length M
Start 5320_051 Rim to invert M
Finish 5320_051A Rim to invert M
Downstream
Preclean J
Year rehabilitated
Additional info
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Pipe Graphic Report of PSR 5320-051 X Stantecfor
Certificate #U-513-18121
Time 11:07
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 97.80
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
VideoCount CD Code Fr To RemarksImRefIn1%JntIn2
0.0 AMH Manhole 5320_051A
0.0 MWL Water Level 0
2.8 S01 SRI Surface Roughness Increased 06 04
2.8 MWM Water Mark 15
6.4 S02 DSZ Deposits Settled Other 06 debris film at invert5
11.0 F01 SRI Surface Roughness Increased 06 04
11.0 S03 DAGS Deposits Attached Grease 12 12 film/residue of grease5
12.3 F02 DSZ Deposits Settled Other 06 debris film at invert5
32.0 LLD Alignment Left Down 20
58.6 MWM Water Mark 30
110.7 MMC Material change metallic
110.7 SCP Surface Corrosion Metal Pipe 07 04
112.6 MMC Material change concrete
321.1 MMC Material change metallic
322.6 MMC Material change concrete
335.0 F03 DAGS Deposits Attached Grease 12 12 film/residue of grease5
335.1 MSA Abandoned Survey Bend
Total Length Surveyed335.1 M
Scores Pipe Ratings Index
Pipe Ratings Index
0
0
Peak
Peak
0
0
Mean Pipe
Mean Pipe
0
0
Pipe Rating
Pipe Rating
0
0
Structural:
O&M:
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/02
Setup 2 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control De-Watered using
Jetter
Location Easement/Right of Way
CatPurpose
Use Sanitary
Shape Circular
Material Asbestos Cement
Lining
Total length M
Height 10 Width 10 mm
Joint length M
Start 5320_051A Rim to invert M
Finish 5320_050 Rim to invert M
Direction Down
Preclean N
Year rehabilitated
Additional info
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Tabular Report of PSR 5320_051A X Stantecfor
Certificate #U-513-18121
Time 12:23
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 335.1
Pressure
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
0 M
335.1 M
Pi
p
e
F
l
o
w
Su
r
v
e
y
D
i
r
Manhole [5320_051A]
Water Level 0% 0.0 M
Surface Roughness Increased 06 to 04 o'clock [S01]
Water Mark 015% 2.8 M
Deposits Settled Other 06 o'clock 005% [debris film at invert] [S02] 6.4 M
Surface Roughness Increased 06 to 04 o'clock [F01]
Deposits Attached Grease 12 to 12 o'clock 005% [film/residue of grease] [S03] 11.0 M
Deposits Settled Other 06 o'clock 005% [debris film at invert] [F02] 12.3 M
Alignment Left Down 020% 32.0 M
Water Mark 030% 58.6 M
Material change [metallic]
Surface Corrosion Metal Pipe 07 to 04 o'clock 110.7 M
Material change [concrete] 112.6 M
Material change [metallic] 321.1 M
Material change [concrete] 322.6 M
Deposits Attached Grease 12 to 12 o'clock 005% [film/residue of grease] [F03] 335.0 M
335.1 M
Direction
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/02
Setup 2 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control De-Watered using
Jetter
Location Easement/Right of Way
CatPurpose
Use Sanitary
Shape Circular
Material Asbestos Cement
Lining
Total length M
Height 10 Width 10 mm
Joint length M
Start 5320_051A Rim to invert M
Finish 5320_050 Rim to invert M
Downstream
Preclean N
Year rehabilitated
Additional info
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Pipe Graphic Report of PSR 5320_051A X Stantecfor
Certificate #U-513-18121
Time 12:23
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 335.10
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
VideoCount CD Code Fr To RemarksImRefIn1%JntIn2
0.0 ST Start of Survey
0.0 AMH Manhole 5320-050
0.0 MWL Water Level 0
2.5 MWM Water Mark 30
2.5 MWM Water Mark 60
2.5 S01 DAZ Deposits Attached Other 12 03 pipe wall coated with materia...5
16.3 RFB Roots Fine Barrel 12
16.4 DAGS Deposits Attached Grease 08 095
16.4 DAGS Deposits Attached Grease 03 045
17.3 RFJ Roots Fine Joint 10 02J
34.5 DAGS Deposits Attached Grease 08 095
34.5 DAGS Deposits Attached Grease 03 045
35.8 RFJ Roots Fine Joint 10 02J
60.8 DAGS Deposits Attached Grease 03 045
64.1 JOL Joint Offset Large possible soil ingress at 4 o ...
64.8 MSC Shape or Size Change 10.000 appears to be 10" pipe
64.8 F01 DAZ Deposits Attached Other 12 03 pipe wall coated with materia...5
64.8 MSA Abandoned Survey cannot negotiate offset/size ...
Total Length Surveyed64.8 M
Scores Pipe Ratings Index
Pipe Ratings Index
2
1.9
Peak
Peak
2
6
Mean Pipe
Mean Pipe
0
0.7
Pipe Rating
Pipe Rating
2
44
Structural:
O&M:
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/03
Setup 3 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control De-Watered using
Jetter
Location Easement/Right of Way
CatPurpose
Use Sanitary
Shape Circular
Material Asbestos Cement
Lining
Total length M
Height 10 Width 10 mm
Joint length M
Start 5320-051A Rim to invert M
Finish 5320-050 Rim to invert M
Direction Up
Preclean N
Year rehabilitated
Additional info
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Tabular Report of PSR 5320-051A X Stantecfor
Certificate #U-513-18121
Time 10:05
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 64.8
Pressure
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
0 M
64.8 M
Pi
p
e
F
l
o
w
Su
r
v
e
y
D
i
r
Start of Survey
Manhole [5320-050]
Water Level 000%
0.0 M
Water Mark 030%
Water Mark 060%
Deposits Attached Other 12 to 03 o'clock 005% OM: 2 [pipe wall coated with material, evidence of H2S] [S01]
2.5 M
Roots Fine Barrel 12 o'clock OM: 2 16.3 M
16.4 M
Roots Fine Joint 10 to 02 o'clock OM: 1 17.3 M
Deposits Attached Grease 08 to 09 o'clock 005% OM: 2
Deposits Attached Grease 03 to 04 o'clock 005% OM: 2 34.5 M
Roots Fine Joint 10 to 02 o'clock OM: 1 35.8 M
Deposits Attached Grease 03 to 04 o'clock 005% OM: 2 60.8 M
Joint Offset Large ST: 2 [possible soil ingress at 4 o clock] 64.1 M
64.8 M
Direction
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/03
Setup 3 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control De-Watered using
Jetter
Location Easement/Right of Way
CatPurpose
Use Sanitary
Shape Circular
Material Asbestos Cement
Lining
Total length M
Height 10 Width 10 mm
Joint length M
Start 5320-051A Rim to invert M
Finish 5320-050 Rim to invert M
Upstream
Preclean N
Year rehabilitated
Additional info
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Pipe Graphic Report of PSR 5320-051A X Stantecfor
Certificate #U-513-18121
Time 10:05
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 64.80
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
VideoCount CD Code Fr To RemarksImRefIn1%JntIn2
0.0 ST Start of Survey
0.0 AMH Manhole 5320-050
0.0 MWL Water Level 15
512.2 AMH Manhole 5320-051A
512.2 FH End of Survey
Total Length Surveyed512.2 M
Scores Pipe Ratings Index
Pipe Ratings Index
0
0
Peak
Peak
0
0
Mean Pipe
Mean Pipe
0
0
Pipe Rating
Pipe Rating
0
0
Structural:
O&M:
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/05
Setup 18 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control Not Controlled
Location
CatPurpose
Use
Shape Circular
Material Ductile Iron Pipe
Lining
Total length 512.2 M
Height 10 Width mm
Joint length M
Start 5320-050 Rim to invert M
Finish 5320-051A Rim to invert M
Direction Up
Preclean N
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Tabular Report of PSR 5320-051A D Stantecfor
Certificate #U-513-18121
Time 12:58
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 512.2
Pressure
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
VideoCount CD Code Fr To RemarksImRefIn1%JntIn2
0.0 ST Start of Survey
0.0 AMH Manhole 5320-050
0.0 MWL Water Level 15
512.0 AMH Manhole 5320-051A
512.0 FH End of Survey
Total Length Surveyed512.0 M
Scores Pipe Ratings Index
Pipe Ratings Index
0
0
Peak
Peak
0
0
Mean Pipe
Mean Pipe
0
0
Pipe Rating
Pipe Rating
0
0
Structural:
O&M:
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/05
Setup 19 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control Not Controlled
Location
CatPurpose
Use
Shape Circular
Material Ductile Iron Pipe
Lining
Total length 512.0 M
Height 10 Width mm
Joint length M
Start 5320-050 Rim to invert M
Finish 5320-051A Rim to invert M
Direction Down
Preclean N
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Tabular Report of PSR 5320-050 E Stantecfor
Certificate #U-513-18121
Time 13:39
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 512.0
Pressure
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
0 M
512 M
Pi
p
e
F
l
o
w
Su
r
v
e
y
D
i
r
Start of Survey
Manhole [5320-050]
Water Level 15%
0.0 M
Manhole [5320-051A]
End of Survey 512.0 M
Direction
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/05
Setup 19 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control Not Controlled
Location
CatPurpose
Use
Shape Circular
Material Ductile Iron Pipe
Lining
Total length 512.0 M
Height 10 Width mm
Joint length M
Start 5320-050 Rim to invert M
Finish 5320-051A Rim to invert M
Downstream
Preclean N
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Pipe Graphic Report of PSR 5320-050 E Stantecfor
Certificate #U-513-18121
Time 13:39
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 512.00
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
VideoCount CD Code Fr To RemarksImRefIn1%JntIn2
0.0 ST Start of Survey
0.0 AMH Manhole 5320-050A
0.0 MWL Water Level 0
2.5 S01 SAP Surface Aggregate Projecting 08 04
2.5 MWM Water Mark 25
5.1 TFC Tap Factory Capped 8.000 12
13.7 JOM Joint Offset Medium
16.1 MMC Material change METALLIC
17.7 MMC Material change AC
17.9 S02 DAGS Deposits Attached Grease 09 035
27.6 F02 DAGS Deposits Attached Grease 09 035
39.5 S03 DSGV Deposits Settled Gravel 05 0710
46.6 MSA Abandoned Survey traction
46.6 F01 SAP Surface Aggregate Projecting 08 04
46.6 F03 DSGV Deposits Settled Gravel 05 0710
Total Length Surveyed46.6 M
Scores Pipe Ratings Index
Pipe Ratings Index
0
0
Peak
Peak
0
0
Mean Pipe
Mean Pipe
0
0
Pipe Rating
Pipe Rating
0
0
Structural:
O&M:
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/03
Setup 5 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control De-Watered using
Jetter
Location Easement/Right of Way
CatPurpose
Use Sanitary
Shape Circular
Material Ductile Iron Pipe
Lining
Total length M
Height 10 Width 10 mm
Joint length M
Start 5320-050A Rim to invert M
Finish 5320-049A Rim to invert M
Direction Down
Preclean N
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Tabular Report of PSR 5320-050A X Stantecfor
Certificate #U-513-18121
Time 11:53
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 46.6
Pressure
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
0 M
46.6 M
Pi
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Start of Survey
Manhole [5320-050A]
Water Level 0%
0.0 M
Surface Aggregate Projecting 08 to 04 o'clock [S01]
Water Mark 025% 2.5 M
Tap Factory Capped 8.000 12 o'clock 5.1 M
Joint Offset Medium 13.7 M
Material change [METALLIC] 16.1 M
Material change [AC] 17.7 M
17.9 M
Deposits Attached Grease 09 to 03 o'clock 005% [F02] 27.6 M
Deposits Settled Gravel 05 to 07 o'clock 010% [S03] 39.5 M
Abandoned Survey [traction]
Surface Aggregate Projecting 08 to 04 o'clock [F01]
Deposits Settled Gravel 05 to 07 o'clock 010% [F03]
46.6 M
Direction
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/03
Setup 5 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control De-Watered using
Jetter
Location Easement/Right of Way
CatPurpose
Use Sanitary
Shape Circular
Material Ductile Iron Pipe
Lining
Total length M
Height 10 Width 10 mm
Joint length M
Start 5320-050A Rim to invert M
Finish 5320-049A Rim to invert M
Downstream
Preclean N
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Pipe Graphic Report of PSR 5320-050A X Stantecfor
Certificate #U-513-18121
Time 11:53
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 46.60
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
VideoCount CD Code Fr To RemarksImRefIn1%JntIn2
0.0 ST Start of Survey
0.0 AMH Manhole 5320-050
0.0 MWL Water Level 10
0.9 MWM Water Mark 15
3.0 S01 SAV Surface Aggregate Visible 12 12
13.2 JSM Joint Separated Medium flow impeded at JSM
15.3 MMC Material change metallic
16.1 TFC Tap Factory Capped 8.000 12
17.3 MMC Material change AC
20.3 MWM Water Mark 50
67.8 LD Alignment Down 15
73.2 MWM Water Mark 15
80.3 S02 SSS Surface Spalling 03
107.5 F02 SSS Surface Spalling 03
107.8 SZ Surface Other 09 pipe material chipped at join...
172.8 MMC Material change metallic
172.8 SCP Surface Corrosion Metal Pipe 12 12
173.3 TFC Tap Factory Capped 8.000 12
174.4 MMC Material change AC
187.3 MMC Material change metallic
187.4 SCP Surface Corrosion Metal Pipe 12 12
188.3 MMC Material change AC
189.6 F01 SAV Surface Aggregate Visible 12 12
207.9 RFB Roots Fine Barrel 09
208.2 RFB Roots Fine Barrel 10
210.8 AMH Manhole 5320-049
210.8 FH End of Survey
Total Length Surveyed210.8 M
Scores Pipe Ratings Index
Pipe Ratings Index
0
0
Peak
Peak
0
0
Mean Pipe
Mean Pipe
0
0
Pipe Rating
Pipe Rating
0
0
Structural:
O&M:
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/03
Setup 7 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control De-Watered using
Jetter
Location
CatPurpose
Use
Shape Circular
Material Asbestos Cement
Lining
Total length 210.8 M
Height 10 Width mm
Joint length M
Start 5320-050 Rim to invert M
Finish 5320-049 Rim to invert M
Direction Down
Preclean N
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Tabular Report of PSR 5320-050 Y Stantecfor
Certificate #U-513-18121
Time 15:01
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 210.8
Pressure
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
0 M
172.8 M
Pi
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Start of Survey
Manhole [5320-050]
Water Level 10%
0.0 M
0.9 M
Surface Aggregate Visible 12 to 12 o'clock [S01] 3.0 M
Joint Separated Medium [flow impeded at JSM] 13.2 M
Material change [metallic] 15.3 M
Tap Factory Capped 8.000 12 o'clock 16.1 M
Material change [AC] 17.3 M
Water Mark 050% 20.3 M
Alignment Down 015% 67.8 M
Water Mark 015% 73.2 M
Surface Spalling 03 o'clock [S02] 80.3 M
Surface Spalling 03 o'clock [F02] 107.5 M
107.8 M
Material change [metallic]
Surface Corrosion Metal Pipe 12 to 12 o'clock 172.8 M
Direction
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/03
Setup 7 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control De-Watered using
Jetter
Location
CatPurpose
Use
Shape Circular
Material Asbestos Cement
Lining
Total length 210.8 M
Height 10 Width mm
Joint length M
Start 5320-050 Rim to invert M
Finish 5320-049 Rim to invert M
Downstream
Preclean N
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Pipe Graphic Report of PSR 5320-050 Y Stantecfor
Certificate #U-513-18121
Time 15:01
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 210.80
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
172.8 M
210.8 M
Pi
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Tap Factory Capped 8.000 12 o'clock 173.3 M
Material change [AC] 174.4 M
Material change [metallic] 187.3 M
187.4 M
Material change [AC] 188.3 M
Surface Aggregate Visible 12 to 12 o'clock [F01] 189.6 M
Roots Fine Barrel 09 o'clock 207.9 M
Roots Fine Barrel 10 o'clock 208.2 M
Manhole [5320-049]
End of Survey 210.8 M
Direction
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/03
Setup 7 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control De-Watered using
Jetter
Location
CatPurpose
Use
Shape Circular
Material Asbestos Cement
Lining
Total length 210.8 M
Height 10 Width mm
Joint length M
Start 5320-050 Rim to invert M
Finish 5320-049 Rim to invert M
Downstream
Preclean N
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Pipe Graphic Report of PSR 5320-050 Y Stantecfor
Certificate #U-513-18121
Time 15:01
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 210.80
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
VideoCount CD Code Fr To RemarksImRefIn1%JntIn2
0.0 ST Start of Survey
0.0 AMH Manhole 5320-049
0.0 MWL Water Level 15
0.6 MSA Abandoned Survey traction/root ball (removed b...
Total Length Surveyed0.6 M
Scores Pipe Ratings Index
Pipe Ratings Index
0
0
Peak
Peak
0
0
Mean Pipe
Mean Pipe
0
0
Pipe Rating
Pipe Rating
0
0
Structural:
O&M:
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/03
Setup 8 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control De-Watered using
Jetter
Location
CatPurpose
Use
Shape Circular
Material Ductile Iron Pipe
Lining
Total length M
Height 10 Width mm
Joint length M
Start 5320-049 Rim to invert M
Finish 5320-046 Rim to invert M
Direction Down
Preclean N
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Tabular Report of PSR 5320-049 Y Stantecfor
Certificate #U-513-18121
Time 15:09
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 0.6
Pressure
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
0 M
0.6 M
Pi
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Start of Survey
Manhole [5320-049]
Water Level 15%
0.0 M
0.6 M
Direction
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/03
Setup 8 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control De-Watered using
Jetter
Location
CatPurpose
Use
Shape Circular
Material Ductile Iron Pipe
Lining
Total length M
Height 10 Width mm
Joint length M
Start 5320-049 Rim to invert M
Finish 5320-046 Rim to invert M
Downstream
Preclean N
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Pipe Graphic Report of PSR 5320-049 Y Stantecfor
Certificate #U-513-18121
Time 15:09
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 00.60
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
VideoCount CD Code Fr To RemarksImRefIn1%JntIn2
0.0 ST Start of Survey
0.0 AMH Manhole 5320-049
0.0 MWL Water Level 10
0.0 MWM Water Mark 20
55.6 S01 DAZ Deposits Attached Other 09 035
80.0 F01 DAZ Deposits Attached Other 09 035
80.2 MMC Material change metallic
81.1 TFC Tap Factory Capped 8.000 12
82.1 MMC Material change AC
82.5 S02 DAZ Deposits Attached Other 09 035
95.1 RTJ Roots Tap Joint 08J10
95.6 MMC Material change metallic
95.7 SCP Surface Corrosion Metal Pipe 08 04
95.7 MMC Material change AC
102.9 LLD Alignment Left Down 20
128.3 MMC Material change metallic
128.7 MMC Material change AC
129.5 DAGS Deposits Attached Grease 085
129.8 F02 DAZ Deposits Attached Other 09 035
131.0 LLD Alignment Left Down 15
132.2 S03 DAZ Deposits Attached Other 10 055
146.8 F03 DAZ Deposits Attached Other 10 055
147.0 S04 DAZ Deposits Attached Other 08 0410
213.7 F04 DAZ Deposits Attached Other 08 0410
214.1 MMC Material change metallic
215.3 RFJ Roots Fine Joint 12J
215.3 LRD Alignment Right Down 15
215.4 MMC Material change AC
218.9 LD Alignment Down 15
222.0 S05 DAZ Deposits Attached Other 08 045
252.8 LRD Alignment Right Down 10
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/03
Setup 9 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control
Location
CatPurpose
Use
Shape Circular
Material Ductile Iron Pipe
Lining
Total length 548.7 M
Height 10 Width mm
Joint length M
Start 5320-049 Rim to invert M
Finish 5320-046 Rim to invert M
Direction Down
Preclean J
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Tabular Report of PSR 5320-049 Z Stantecfor
Certificate #U-513-18121
Time 15:27
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 548.7
Pressure
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/03
Setup 9 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control
Location
CatPurpose
Use
Shape Circular
Material Ductile Iron Pipe
Lining
Total length 548.7 M
Height 10 Width mm
Joint length M
Start 5320-049 Rim to invert M
Finish 5320-046 Rim to invert M
Direction Down
Preclean J
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Tabular Report of PSR 5320-049 Z Stantecfor
Certificate #U-513-18121
Time 15:27
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 548.7
Pressure
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
VideoCount CD Code Fr To RemarksImRefIn1%JntIn2
271.6 RFJ Roots Fine Joint
12J
315.2 S06 SRI Surface Roughness Increased
12 12
318.8 F05 DAZ Deposits Attached Other
08 045
436.6 MMC Material change
metallic
436.6 LRD Alignment Right Down
15
438.2 MMC Material change
AC
441.0 LD Alignment Down
10
548.0 F06 SRI Surface Roughness Increased
12 12
548.7 AMH Manhole
5320-046
548.7 FH End of Survey
Total Length Surveyed548.7 M
Scores Pipe Ratings Index
Pipe Ratings Index
0
0
Peak
Peak
0
0
Mean Pipe
Mean Pipe
0
0
Pipe Rating
Pipe Rating
0
0
Structural:
O&M:
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
0 M
128.7 M
Pi
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Start of Survey
Manhole [5320-049]
Water Level 10%
Water Mark 020%
0.0 M
Deposits Attached Other 09 to 03 o'clock 005% [S01] 55.6 M
Deposits Attached Other 09 to 03 o'clock 005% [F01] 80.0 M
80.2 M
Tap Factory Capped 8.000 12 o'clock 81.1 M
Material change [AC] 82.1 M
82.5 M
Roots Tap Joint 08 o'clock 010% 95.1 M
95.6 M
95.7 M
Alignment Left Down 020% 102.9 M
Material change [metallic] 128.3 M
128.7 M
Direction
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/03
Setup 9 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control
Location
CatPurpose
Use
Shape Circular
Material Ductile Iron Pipe
Lining
Total length 548.7 M
Height 10 Width mm
Joint length M
Start 5320-049 Rim to invert M
Finish 5320-046 Rim to invert M
Downstream
Preclean J
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Pipe Graphic Report of PSR 5320-049 Z Stantecfor
Certificate #U-513-18121
Time 15:27
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 548.70
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
128.7 M
315.2 M
Pi
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Deposits Attached Grease 08 o'clock 005% 129.5 M
129.8 M
Alignment Left Down 015% 131.0 M
Deposits Attached Other 10 to 05 o'clock 005% [S03] 132.2 M
Deposits Attached Other 10 to 05 o'clock 005% [F03] 146.8 M
Deposits Attached Other 08 to 04 o'clock 010% [S04] 147.0 M
Deposits Attached Other 08 to 04 o'clock 010% [F04] 213.7 M
Material change [metallic] 214.1 M
Roots Fine Joint 12 o'clock
Alignment Right Down 015% 215.3 M
215.4 M
Alignment Down 015% 218.9 M
Deposits Attached Other 08 to 04 o'clock 005% [S05] 222.0 M
Alignment Right Down 010% 252.8 M
Roots Fine Joint 12 o'clock 271.6 M
Surface Roughness Increased 12 to 12 o'clock [S06] 315.2 M
Direction
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/03
Setup 9 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control
Location
CatPurpose
Use
Shape Circular
Material Ductile Iron Pipe
Lining
Total length 548.7 M
Height 10 Width mm
Joint length M
Start 5320-049 Rim to invert M
Finish 5320-046 Rim to invert M
Downstream
Preclean J
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Pipe Graphic Report of PSR 5320-049 Z Stantecfor
Certificate #U-513-18121
Time 15:27
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 548.70
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
315.2 M
548.7 M
Pi
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Deposits Attached Other 08 to 04 o'clock 005% [F05] 318.8 M
Material change [metallic]
Alignment Right Down 015% 436.6 M
Material change [AC] 438.2 M
Alignment Down 010% 441.0 M
Surface Roughness Increased 12 to 12 o'clock [F06] 548.0 M
548.7 M
Direction
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/03
Setup 9 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control
Location
CatPurpose
Use
Shape Circular
Material Ductile Iron Pipe
Lining
Total length 548.7 M
Height 10 Width mm
Joint length M
Start 5320-049 Rim to invert M
Finish 5320-046 Rim to invert M
Downstream
Preclean J
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Pipe Graphic Report of PSR 5320-049 Z Stantecfor
Certificate #U-513-18121
Time 15:27
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 548.70
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
VideoCount CD Code Fr To RemarksImRefIn1%JntIn2
0.0 ST Start of Survey
0.0 AMH Manhole 5320-046
0.0 MWL Water Level 15
0.0 S01 SRI Surface Roughness Increased 12 12
0.0 S02 DAZ Deposits Attached Other 08 045
0.3 LRD Alignment Right Down 10
20.7 LD Alignment Down 10
39.5 JSM Joint Separated Medium flow imbeded as JSM
103.4 LRD Alignment Right Down 15
180.0 MMC Material change metallic
180.8 LLD Alignment Left Down 10
181.8 AMH Manhole 5320-036
181.8 FH End of Survey
181.8 MMC Material change AC
181.8 F01 SRI Surface Roughness Increased 12 12
181.8 F02 DAZ Deposits Attached Other 08 045
Total Length Surveyed181.8 M
Scores Pipe Ratings Index
Pipe Ratings Index
0
0
Peak
Peak
0
0
Mean Pipe
Mean Pipe
0
0
Pipe Rating
Pipe Rating
0
0
Structural:
O&M:
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/04
Setup 10 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control
Location
CatPurpose
Use
Shape Circular
Material Ductile Iron Pipe
Lining
Total length 181.8 M
Height 10 Width mm
Joint length M
Start 5320-046 Rim to invert M
Finish 5320-036 Rim to invert M
Direction Down
Preclean J
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Tabular Report of PSR 5320-046 Z Stantecfor
Certificate #U-513-18121
Time 10:37
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 181.8
Pressure
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
0 M
181.8 M
Pi
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Start of Survey
Manhole [5320-046]
Water Level 15%
Surface Roughness Increased 12 to 12 o'clock [S01]
Deposits Attached Other 08 to 04 o'clock 005% [S02]
0.0 M
0.3 M
Alignment Down 010% 20.7 M
Joint Separated Medium [flow imbeded as JSM] 39.5 M
Alignment Right Down 015% 103.4 M
Material change [metallic] 180.0 M
180.8 M
Manhole [5320-036]
End of Survey
Material change [AC]
Surface Roughness Increased 12 to 12 o'clock [F01]
Deposits Attached Other 08 to 04 o'clock 005% [F02]
181.8 M
Direction
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/04
Setup 10 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control
Location
CatPurpose
Use
Shape Circular
Material Ductile Iron Pipe
Lining
Total length 181.8 M
Height 10 Width mm
Joint length M
Start 5320-046 Rim to invert M
Finish 5320-036 Rim to invert M
Downstream
Preclean J
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Pipe Graphic Report of PSR 5320-046 Z Stantecfor
Certificate #U-513-18121
Time 10:37
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 181.80
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
VideoCount CD Code Fr To RemarksImRefIn1%JntIn2
0.0 ST Start of Survey
0.0 AMH Manhole 5320-036
0.0 MWL Water Level 15
3.7 S01 SRI Surface Roughness Increased 12 12
79.5 MMC Material change metallic
80.0 TFC Tap Factory Capped 12.000 12
81.8 MMC Material change AC
84.5 TFA Tap Factory Active 8.000 02
88.3 TFA Tap Factory Active 8.000 09
88.3 OBM Obstacle Pipe Material 09 upstream broken of hole in pi...25
251.2 DAE Deposits Attached Encrustation 02 11 flow impeded at DAE5
501.1 MMC Material change metallic
503.0 TFC Tap Factory Capped 8.000 12
503.5 MMC Material change AC
506.0 TFA Tap Factory Active 8.000 02
510.0 TFA Tap Factory Active 8.000 09
765.0 JOM Joint Offset Medium flow impeded at JOM
877.5 LRD Alignment Right Down 10
917.0 TFC Tap Factory Capped 8.000 12
921.0 TF Tap Factory 8.000 03 unclear if active or abandone...
924.6 TFA Tap Factory Active 8.000 10
1298.0 F01 SRI Surface Roughness Increased 12 12
1298.0 IG Infil Gusher 12 12 infiltration in MH and DAE in...
1298.9 AMH Manhole 5320-196
1298.9 FH End of Survey
Total Length Surveyed1,298.9 M
Scores Pipe Ratings Index
Pipe Ratings Index
0
0
Peak
Peak
0
0
Mean Pipe
Mean Pipe
0
0
Pipe Rating
Pipe Rating
0
0
Structural:
O&M:
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/04
Setup 12 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control
Location
CatPurpose
Use
Shape Circular
Material Ductile Iron Pipe
Lining
Total length 1298.9 M
Height 12 Width mm
Joint length M
Start 5320-036 Rim to invert M
Finish 5320-196 Rim to invert M
Direction Down
Preclean J
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Tabular Report of PSR 5320-036 C Stantecfor
Certificate #U-513-18121
Time 13:14
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 1298.9
Pressure
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
0 M
765 M
Pi
p
e
F
l
o
w
Su
r
v
e
y
D
i
r
Start of Survey
Manhole [5320-036]
Water Level 15%
0.0 M
Surface Roughness Increased 12 to 12 o'clock [S01] 3.7 M
Material change [metallic] 79.5 M
Tap Factory Capped 12.000 12 o'clock 80.0 M
Material change [AC] 81.8 M
Tap Factory Active 8.000 02 o'clock 84.5 M
Tap Factory Active 8.000 09 o'clock
Obstacle Pipe Material 09 o'clock 025% [upstream broken of hole in pipe not found - unclear if occurred at installation or later] 88.3 M
Deposits Attached Encrustation 02 to 11 o'clock 005% [flow impeded at DAE] 251.2 M
Material change [metallic] 501.1 M
Tap Factory Capped 8.000 12 o'clock 503.0 M
503.5 M
Tap Factory Active 8.000 02 o'clock 506.0 M
Tap Factory Active 8.000 09 o'clock 510.0 M
Joint Offset Medium [flow impeded at JOM] 765.0 M
Direction
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/04
Setup 12 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control
Location
CatPurpose
Use
Shape Circular
Material Ductile Iron Pipe
Lining
Total length 1298.9 M
Height 12 Width mm
Joint length M
Start 5320-036 Rim to invert M
Finish 5320-196 Rim to invert M
Downstream
Preclean J
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Pipe Graphic Report of PSR 5320-036 C Stantecfor
Certificate #U-513-18121
Time 13:14
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 1298.90
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
765 M
1298.9 M
Pi
p
e
F
l
o
w
Su
r
v
e
y
D
i
r
Alignment Right Down 010% 877.5 M
Tap Factory Capped 8.000 12 o'clock 917.0 M
Tap Factory 8.000 03 o'clock [unclear if active or abandoned] 921.0 M
Tap Factory Active 8.000 10 o'clock 924.6 M
Surface Roughness Increased 12 to 12 o'clock [F01]
Infil Gusher 12 to 12 o'clock [infiltration in MH and DAE in channel] 1,298.0 M
1,298.9 M
Direction
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/04
Setup 12 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control
Location
CatPurpose
Use
Shape Circular
Material Ductile Iron Pipe
Lining
Total length 1298.9 M
Height 12 Width mm
Joint length M
Start 5320-036 Rim to invert M
Finish 5320-196 Rim to invert M
Downstream
Preclean J
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Pipe Graphic Report of PSR 5320-036 C Stantecfor
Certificate #U-513-18121
Time 13:14
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 1298.90
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
VideoCount CD Code Fr To RemarksImRefIn1%JntIn2
0.0 ST Start of Survey
0.0 AMH Manhole 5320-034
0.0 MWL Water Level 10
3.2 S01 SSS Surface Spalling 08 10
3.2 S02 SSS Surface Spalling 02 04
3.7 MWM Water Mark 30
29.7 MWL Water Level 30
35.5 S03 DAZ Deposits Attached Other 12 025
44.9 F03 DAZ Deposits Attached Other 12 025
48.5 MWL Water Level 75
71.5 S04 DAE Deposits Attached Encrustation 08 045
78.8 F04 DAE Deposits Attached Encrustation 08 045
99.2 MWM Water Mark 60
111.5 MWL Water Level 15
222.4 MWL Water Level 20
357.0 F02 SSS Surface Spalling 02 04
357.0 F01 SSS Surface Spalling 08 10
357.5 DAE Deposits Attached Encrustation 08 11 from infiltration in MH (gush...20
357.6 AMH Manhole 5320-035
357.6 FH End of Survey
Total Length Surveyed357.6 M
Scores Pipe Ratings Index
Pipe Ratings Index
0
0
Peak
Peak
0
0
Mean Pipe
Mean Pipe
0
0
Pipe Rating
Pipe Rating
0
0
Structural:
O&M:
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/07
Setup 21 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control Not Controlled
Location Easement/Right of Way
CatPurposeMaintenance Related
Use
Shape Circular
Material Asbestos Cement
Lining
Total length 357.6 M
Height 18 Width mm
Joint length M
Start 5320-034 Rim to invert M
Finish 5320-035 Rim to invert M
Direction Up
Preclean N
Year rehabilitated
Additional info
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Tabular Report of PSR 5320-035 D Stantecfor
Certificate #U-513-18121
Time 1:35
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 357.6
Pressure
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
0 M
357 M
Pi
p
e
F
l
o
w
Su
r
v
e
y
D
i
r
Start of Survey
Manhole [5320-034]
Water Level 10%
0.0 M
Surface Spalling 08 to 10 o'clock [S01]
Surface Spalling 02 to 04 o'clock [S02] 3.2 M
3.7 M
Water Level 030% 29.7 M
Deposits Attached Other 12 to 02 o'clock 005% [S03] 35.5 M
Deposits Attached Other 12 to 02 o'clock 005% [F03] 44.9 M
Water Level 075% 48.5 M
Deposits Attached Encrustation 08 to 04 o'clock 005% [S04] 71.5 M
Deposits Attached Encrustation 08 to 04 o'clock 005% [F04] 78.8 M
Water Mark 060% 99.2 M
Water Level 015% 111.5 M
Water Level 020% 222.4 M
Surface Spalling 02 to 04 o'clock [F02]
Surface Spalling 08 to 10 o'clock [F01] 357.0 M
Direction
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/07
Setup 21 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control Not Controlled
Location Easement/Right of Way
CatPurposeMaintenance Related
Use
Shape Circular
Material Asbestos Cement
Lining
Total length 357.6 M
Height 18 Width mm
Joint length M
Start 5320-034 Rim to invert M
Finish 5320-035 Rim to invert M
Upstream
Preclean N
Year rehabilitated
Additional info
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Pipe Graphic Report of PSR 5320-035 D Stantecfor
Certificate #U-513-18121
Time 1:35
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 357.60
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
357 M
357.6 M
Pi
p
e
F
l
o
w
Su
r
v
e
y
D
i
r
357.5 M
357.6 M
Direction
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/07
Setup 21 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control Not Controlled
Location Easement/Right of Way
CatPurposeMaintenance Related
Use
Shape Circular
Material Asbestos Cement
Lining
Total length 357.6 M
Height 18 Width mm
Joint length M
Start 5320-034 Rim to invert M
Finish 5320-035 Rim to invert M
Upstream
Preclean N
Year rehabilitated
Additional info
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Pipe Graphic Report of PSR 5320-035 D Stantecfor
Certificate #U-513-18121
Time 1:35
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 357.60
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
VideoCount CD Code Fr To RemarksImRefIn1%JntIn2
0.0 ST Start of Survey pipe segment already PACP cod...
0.0 AMH Manhole 5320-034 X
0.0 MWL Water Level 10
358.5 AMH Manhole 5320-035 X
358.5 FH End of Survey
Total Length Surveyed358.5 M
Scores Pipe Ratings Index
Pipe Ratings Index
0
0
Peak
Peak
0
0
Mean Pipe
Mean Pipe
0
0
Pipe Rating
Pipe Rating
0
0
Structural:
O&M:
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/07
Setup 22 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control
Location Easement/Right of Way
CatPurposeMaintenance Related
Use
Shape Circular
Material Asbestos Cement
Lining
Total length 358.5 M
Height 18 Width mm
Joint length M
Start 5320-034 X Rim to invert M
Finish 5320-035 X Rim to invert M
Direction Up
Preclean N
Year rehabilitated
Additional info
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Tabular Report of PSR 5320-035 X D Stantecfor
Certificate #U-513-18121
Time 1:57
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 358.5
Pressure
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
0 M
358.5 M
Pi
p
e
F
l
o
w
Su
r
v
e
y
D
i
r
Start of Survey [pipe segment already PACP coded]
Manhole [5320-034 X]
Water Level 10%
0.0 M
Manhole [5320-035 X]
End of Survey 358.5 M
Direction
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/07
Setup 22 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control
Location Easement/Right of Way
CatPurposeMaintenance Related
Use
Shape Circular
Material Asbestos Cement
Lining
Total length 358.5 M
Height 18 Width mm
Joint length M
Start 5320-034 X Rim to invert M
Finish 5320-035 X Rim to invert M
Upstream
Preclean N
Year rehabilitated
Additional info
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Pipe Graphic Report of PSR 5320-035 X D Stantecfor
Certificate #U-513-18121
Time 1:57
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 358.50
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
VideoCount CD Code Fr To RemarksImRefIn1%JntIn2
0.0 ST Start of Survey
0.0 AMH Manhole 5320-033
0.0 MWL Water Level 10
3.0 MWM Water Mark 20
3.0 S01 DSF Deposits Settled Fine 04 0815
126.7 DAGS Deposits Attached Grease 07 0810
130.0 DAE Deposits Attached Encrustation 12 1215
131.0 F01 DSF Deposits Settled Fine 04 0815
131.1 AMH Manhole 5320-034
131.1 FH End of Survey MH is underneath I-405
Total Length Surveyed131.1 M
Scores Pipe Ratings Index
Pipe Ratings Index
0
2.9
Peak
Peak
0
6
Mean Pipe
Mean Pipe
0
0.6
Pipe Rating
Pipe Rating
0
83
Structural:
O&M:
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/07
Setup 20 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control Not Controlled
Location Easement/Right of Way
CatPurposeMaintenance Related
Use
Shape Circular
Material Asbestos Cement
Lining
Total length 131.1 M
Height 18 Width mm
Joint length M
Start 5320-033 Rim to invert M
Finish 5320-034 Rim to invert M
Direction Up
Preclean N
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Tabular Report of PSR 5320-034 D Stantecfor
Certificate #U-513-18121
Time 1:11
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 131.1
Pressure
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
0 M
131.1 M
Pi
p
e
F
l
o
w
Su
r
v
e
y
D
i
r
Start of Survey
Manhole [5320-033]
Water Level 10%
0.0 M
Water Mark 020%
Deposits Settled Fine 04 to 08 o'clock 015% OM: 3 [S01] 3.0 M
Deposits Attached Grease 07 to 08 o'clock 010% OM: 2 126.7 M
Deposits Attached Encrustation 12 to 12 o'clock 015% OM: 3 130.0 M
Deposits Settled Fine 04 to 08 o'clock 015% OM: 3 [F01] 131.0 M
131.1 M
Direction
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/07
Setup 20 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control Not Controlled
Location Easement/Right of Way
CatPurposeMaintenance Related
Use
Shape Circular
Material Asbestos Cement
Lining
Total length 131.1 M
Height 18 Width mm
Joint length M
Start 5320-033 Rim to invert M
Finish 5320-034 Rim to invert M
Upstream
Preclean N
Year rehabilitated
Additional info Second Run bigger tires
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Pipe Graphic Report of PSR 5320-034 D Stantecfor
Certificate #U-513-18121
Time 1:11
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 131.10
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
0 M
137.6 M
Pi
p
e
F
l
o
w
Su
r
v
e
y
D
i
r
Start of Survey [pipe segment already PACP coded]
Manhole [5320-033 X]
Water Level 5%
0.0 M
Manhole [5320-034 Y]
End of Survey 137.6 M
Direction
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/07
Setup 23 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control
Location Easement/Right of Way
CatPurposeMaintenance Related
Use
Shape Circular
Material Asbestos Cement
Lining
Total length 137.6 M
Height 18 Width mm
Joint length M
Start 5320-033 X Rim to invert M
Finish 5320-034 Y Rim to invert M
Upstream
Preclean N
Year rehabilitated
Additional info
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Pipe Graphic Report of PSR 5320-034 Y D Stantecfor
Certificate #U-513-18121
Time 2:43
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 137.60
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
0 M
137.6 M
Pi
p
e
F
l
o
w
Su
r
v
e
y
D
i
r
Start of Survey [pipe segment already PACP coded]
Manhole [5320-033 X]
Water Level 5%
0.0 M
Manhole [5320-034 Y]
End of Survey 137.6 M
Direction
2014/06/02Year Cleaned
P/O #
Media No
Date 2014/06/07
Setup 23 Surveyor Dominick
Drainage Thunder Hills
Street Grant Ave S
Weather Dry
City Renton
Flow control
Location Easement/Right of Way
CatPurposeMaintenance Related
Use
Shape Circular
Material Asbestos Cement
Lining
Total length 137.6 M
Height 18 Width mm
Joint length M
Start 5320-033 X Rim to invert M
Finish 5320-034 Y Rim to invert M
Upstream
Preclean N
Year rehabilitated
Additional info
Further location details
Structural
Miscellaneous
O&M
Hydraulic
Constructional
System Owner City of Renton
Year laid
Pipe Graphic Report of PSR 5320-034 Y D Stantecfor
Certificate #U-513-18121
Time 2:43
Survey Customer Stantec Engineering
Grade to invert Rim to grade
Grade to invert Rim to grade
Length Surveyed 137.60
Northing Easting Elevation
Coordinate System GPS Accuracy
Project Renton Thunder Hills Work Order
Bravo Environmental NW
Phone: 425-424-9000
Fax: 425-424-9002
Appendix E
TECH MEMO NO. 4 - GEOTECHNICAL INVESTIGATION
CITY OF RENTON
Technical Memorandum No. 4
Geotechnical Investigation
Thunder Hills Sewer Interceptor
February 13, 2015
Prepared By:
Phil A. Haberman, P.G., P.E.G.
Senior Engineering Geologist
Gopal A. Singam, P.E.
Senior Geotechnical Engineer
Stantec Consulting Services Inc.
11130 NE 33rd Place
Suite 200
Bellevue, WA 98004
425.869.9448
TECHNICAL MEMORANDUM NO. 4
February 13, 2015
rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx i
Contents
1.0 INTRODUCTION ............................................................................................................. 1
2.0 PROJECT DESCRIPTION ................................................................................................. 1
3.0 SITE DESCRIPTION .......................................................................................................... 1
4.0 SUBSURFACE DATA ........................................................................................................ 3
5.0 SOIL AND GROUNDWATER CONDITIONS ..................................................................... 3
6.0 GEOLOGIC HAZARDS ................................................................................................... 5
7.0 DISCUSSION .................................................................................................................. 8
8.0 RECOMMENDATIONS .................................................................................................... 8
9.0 CONSTRUCTION FIELD REVIEWS.................................................................................. 15
10.0 CLOSURE ...................................................................................................................... 16
Appendices
APPENDIX A: STATEMENT OF GENERAL CONDITIONS
APPENDIX B: FIGURES
Vicinity Map (Figure 1)
Site Plans (Figures 2 & 3)
Cross Sections (Figures 4-9)
Gravity Wall Schematics (Figures 10 & 11)
APPENDIX C: BORING LOGS
TECHNICAL MEMORANDUM NO. 4
February 13, 2015
rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx ii
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TECHNICAL MEMORANDUM No. 4
February 13, 2015
rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx 1
1.0 Introduction
In accordance with authorization by the City of Renton, Stantec has completed a
geotechnical investigation for the Thunder Hills Creek Sewer Alternatives Analysis
located in Renton, Washington. Figure 1 shows the project area.
The purpose of our investigation was to evaluate the geologic conditions in the most
critical geologic hazard areas of the existing sewer line and to provide preliminary
geotechnical recommendations and opinions for protection of the existing or new
sewer line, manholes, and access roadway.
The scope of work for the study consisted of multiple levels of field investigations and
document reviews followed by cross section development, slope stability analyses, and
engineering analyses to prepare this report. Preliminary recommendations presented
herein pertain to various geotechnical aspects of the proposed project and provide an
overview of the geologic conditions and hazards.
2.0 Project Description
The proposed alternatives include replacement of the existing sewer line that extends
along Thunder Hills Creek from approximately Grant Avenue South to Interstate 405 (I-
405), diverting flow from this basin into sewer infrastructure west of Talbot Road South, or
rehabilitating (lining) the existing line along Thunder Hills Creek.
For the alternatives located in the Thunder Hills Creek basin, construction may include
adding sanitary sewer manholes where none currently exist, creating/rehabilitating an
access roadway to the area near I-405 for maintenance equipment; and potentially
constructing retaining structures to support the existing access roadway and sewer line.
3.0 Site Description
The Thunder Hills Creek project area is located between I-405 and Grant Avenue South,
just east of the Berkshire Apartment Home development (Figure 1). For the purposes of
this study, we narrowed the site area to include the most geologically hazardous areas
within the project area. This area is located from I-405 upstream for approximately 600
feet (Figures 2 and 3). For our study, we also consider the site to include all of the steep
slopes located east and west of Thunder Hills Creek (approximately 150-200 feet wide)
in this area.
Localized steep slope areas, drainage zones, and areas where the existing sewer line
crosses Thunder Hills Creek along the alignment south of the main site area were also
reviewed. Conclusions and opinions regarding these areas, along with geotechnical
recommendations pertaining to slope stability and access roadway re-construction are
included.
TECHNICAL MEMORANDUM No. 4
February 13, 2015
rs v:\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx 2
The site consists of the existing sewer interceptor alignment through the Thunder Hills
Creek valley along with a gravel improved access roadway located adjacent to the
stream for much of the alignment. The access roadway has been damaged
significantly by soil movement and erosion near Station 5+00 (Figure 3). North of this
location, the roadway is improved (partially) with quarry rock north to a flat area near I-
405.
Attempts have been made to locate the existing sewer pipe. The pipe type varies
along the alignment and the pipe depth is known only at existing manholes and has
been interpolated between manholes. Near Station 11+80, the sewer line extends
beneath Thunder Hills Creek at an estimated depth of 7 feet.
Thunder Hills Creek appears to flow continuously throughout the year and there are
numerous intermittent, seasonal streams that join the creek. The site plans show the
locations of various streams that join Thunder Hills Creek from the east, as well as
wetland areas located in and around the creek. Minor springs and seeps seasonally
add volume to the creek throughout the valley. There are numerous residential yard
drains that add stormwater to the stream. Near Station 5+30, a relatively large creek
joins Thunder Hills Creek from the southeast, approximately doubling the volume of
runoff.
South of Station 5+50, variable amounts of sediment are present in the stream channel.
North of this area, the stream has incised into the underlying sandstone (Renton
Formation). Four to 12 inch sized quarry rock is present in the stream bed and banks in
many areas. Larger quarry rock, generally ½ to 4 man sized basalt, has been used to
stabilize the stream banks and/or to prevent ongoing stream erosion. Specifically in the
vicinity of Station 5+00, large quarry rock has been used to fill the stream channel.
Rock filled gabion walls, generally 4 to 6 feet in height, are located between the access
roadway/path and Thunder Hills Creek north of Station 4+00. For the most part, the
gabion baskets have deteriorated significantly and in places the walls are somewhat
overturned. The gabion walls appear to have limited functionality as retaining
structures for the roadway and sewer line.
The slopes extending downward into the Thunder Hills Creek valley between Stations
0+25 and 5+40 are very steep, with magnitudes of 100 to 150 percent. There are
localized slope areas that are near vertical (200 percent magnitude) to overturned due
to excavation, sloughing, and/or landslide activity. Several rockeries are located along
the east and west sides of the access roadway between Stations 3+80 and 5+20. The
rockeries are comprised of 1 to 2 man sized basalt and are up to 7 feet in height. The
rockeries are loosely constructed.
There is evidence that shallow landslide activity occurs periodically along portions of
the slope west of the access roadway. Several large, but shallow, landslides have
occurred within the last several years north of Station 2+50. The slides appear to consist
of the upper colluvium (1 to 4 feet thick) sliding off of the underlying sandstone. The
slides extend upslope between 10 and 50 feet and are up to 70 feet wide.
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The site is bordered to the north by I-405, to the west by the Berkshire Apartment Homes,
to the east by undeveloped land (easements) and single family residences, and to the
south by Thunder Hills Creek, easements, and residential developments.
4.0 Subsurface Data
4.1.1 Site Investigation Program
The geotechnical field investigation program was completed on October 17th and 20th,
2014 and included drilling and sampling four hollow stem auger borings drilled by a
Stantec subcontractor using a limited access drill rig. The borings were located at or
near pre-determined locations and extended approximately 5 to 25 feet below the
existing site grades.
The soils encountered were logged in the field during the exploration and are
described in accordance with the Unified Soil Classification System (USCS). Disturbed
soil samples were obtained by using a 140 pound hammer free falling a vertical
distance of 30 inches for the borings.
The summation of hammer-blows required to drive the sampler the final 12-inches of an
18-inch sample length is defined as the Standard Penetration Resistance, or N-value for
a 140 pound hammer and 2 inch outside diameter split spoon sampler.
The uncorrected blow count is presented graphically on the boring logs in Appendix C.
The resistance, or “N” value, provides a measure of the relative density of granular soils
and the consistency of cohesive soils. Our report discussions regarding soil density as
well as engineering parameters are based on the N values.
A Stantec field representative directed the drilling program, collected disturbed soil
samples from split spoon sampler tubes, classified the encountered soils, kept a detailed
log of each auger hole, and observed and recorded pertinent site features.
The results of the drilling and sampling are presented on the boring logs enclosed in
Appendix C.
We also reviewed six boring logs from a geotechnical investigation conducted by Soil
and Environmental Engineers, Inc. (S&EE) in 2011. This report was conducted to develop
solutions to retain/protect the existing sewer line in the lower portion of the Thunder Hills
Creek valley.
5.0 Soil and Groundwater Conditions
5.1.1 Area Geology
The site lies within the Puget Lowland. The lowland is part of a regional north-south
trending trough that extends from southwestern British Columbia to near Eugene,
Oregon. North of Olympia, Washington, this lowland is glacially carved, with a
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depositional and erosional history including at least four separate glacial
advances/retreats.
The Puget Lowland is bounded to the west by the Olympic Mountains and to the east
by the Cascade Range. The lowland is filled with glacial and nonglacial sediments
consisting of interbedded gravel, sand, silt, till, and peat lenses.
The Geologic Map of King County, indicates that the site is located near the contacts
between Vashon Glacial Till and Tertiary Bedrock.
Vashon Glacial Till is typically characterized by an unsorted, nonstratified mixture of
clay, silt, sand, gravel, cobbles and boulders in variable quantities. These materials are
typically dense and relatively impermeable. The poor sorting reflects the mixing of the
materials as these sediments were overridden and incorporated by the glacial ice.
Tertiary Bedrock in this area consists of the Renton Formation. The Renton Formation
includes feldspathic fine to medium grained sandstone with beds of coal,
carbonaceous siltstone, and claystone. Tertiary Bedrock locally outcrops south of I-90
and the Seattle Fault Zone due to uplift associated with seismic activity.
5.1.2 Soil Conditions
Details of the encountered soil conditions are presented on the boring logs in Appendix
C. The detailed soil description on these logs should be referred to in preference to the
generalized descriptions below.
Boring B-1
In Boring B-1, we encountered approximately 6 inches of topsoil and vegetation
underlain by approximately 5 feet of medium dense to dense, silty-fine to
medium grained sand with variable amounts of gravel and debris (Fill). This layer
was underlain by stiff to very stiff silt with variable amounts of sand and woody
debris (Fill). The silt layer was underlain by stiff silt with variable amounts of sand,
gravel, and trace amounts of woody debris (Highly Weathered Renton
Formation). These materials were underlain by medium dense, silty-sand with
clasts of weathered sandstone (Weathered Renton Formation), which continued
to the termination depth of the boring.
Borings B-2 and B-3
In Borings B-2 and B-3, we encountered approximately 10 to 12 inches of
vegetation and topsoil underlain by approximately 5 feet of medium dense, silty-
fine to medium grained sand with variable amounts of gravel (Fill). This layer was
underlain by dense to very dense, silty-fine to medium grained sand with variable
amounts of gravel (Glacial Till), which continued to the termination depths of
these borings.
Boring B-4
In Boring B-4, we encountered approximately 8 inches of angular rock underlain
by approximately 4 feet of loose to medium dense, silty-fine to medium grained
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sand with variable amounts of gravel (Fill). This layer was underlain by hard
sandstone (Renton Formation), which continued to the termination depth of the
boring (refusal).
Hand Borings
All of the hand borings, generally located on steep slopes east and west of
Thunder Hills Creek, encountered several inches of topsoil and vegetation
underlain by 1 to 3 feet of loose, silty-sand to silt with sand (Colluvium derived
from Renton Formation). These materials were underlain by hard sandstone
(Slightly Weathered and Weathered Renton Formation).
Previous Borings and Hand Borings
The borings and hand borings conducted in May, 2010 by S & EE encountered
similar materials at similar depths and densities as those in our investigation. We
have utilized some of this data to complete our slope stability analyses and
gravity wall design in the northern portion of the site, where their explorations
were focused.
Groundwater
At the time of our investigation, groundwater was encountered in Boring B-4 at
approximately 4.5 feet below the existing site grade. Groundwater was not
encountered in any of the other explorations at the date of our investigation.
We anticipate that groundwater in the Thunder Hills Creek valley is primarily
influenced by area streams and surface water runoff/infiltrating surface waters.
There are areas of the site near I-405 where surface water and groundwater is at
the same level (ground surface) and areas where groundwater is not
encountered below stream depths due to stream channel confinement within
the Renton Formation sandstone (Station 5+50 to 6+50 area).
We anticipate low to moderate seasonal seepage from the valley/channel
sidewalls contributing to the overall volume in Thunder Hills Creek. A majority of
volume contribution to the stream is from drainage conveyance from nearby
residential developments and tributary streams.
Water levels at the time of the field investigation may be different from those
encountered during the construction phase of the project.
6.0 Geologic Hazards
6.1.1 Landslide Hazard & Preliminary Slope Stability Analyses
Typically, slopes with magnitudes greater than about 40 percent and vertical relief of at
least 10 feet can be classified as geologically hazardous (steep slope/landslide
hazards). A majority of the slopes that extend into the Thunder Hills Creek valley meet
these criteria.
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During our field assessment, we traversed the subject property, where accessible. As
we conducted the traverses, we looked for any signs that would indicate past slope
failures or features indicating possible future instability.
We observed several relatively recent landslides in the area north of Station 2+50,
primarily along the steep slopes along the west side of Thunder Hills Creek. These slides
appear to be relatively shallow, consisting of colluvium sliding off of the underlying
sandstone. We did not observe evidence of deep-seated landslide activity within the
valley.
It is our opinion that the contributing causes for landslide activity in the north portion of
Thunder Hills Creek valley include excavations for access roadway construction, surface
water and spring/seep activity along the slopes, and the presence of loose colluvium
over relatively impermeable, hard sandstone at steep inclinations.
The commercially available slope stability computer program Slope/W was used to
preliminarily evaluate the local stability of the lower portion of the existing slopes west of
Thunder Hills Creek extending east toward the creek at each of six cross sections
(Figures 4 through 9). The slope stability was analyzed under static and seismic (pseudo-
static method) conditions for existing conditions and with a proposed gravity retaining
wall in place between the access roadway and creek.
The computer program calculates factors of safety for potential slope failures and
generates the potential failure planes. This software calculates the slope stability under
seismic conditions using pseudo-static methods. The stability of the described
configuration was analyzed by comparing observed factors of safety to minimum
values as set by standard geotechnical practice. A factor of safety of 1.0 is considered
equilibrium and less than 1.0 is considered failure. A typical minimum factor of safety for
global stability is 1.3 to 1.5 for static conditions and 1.1 to 1.2 for seismic conditions. In
accordance with typical engineering standards, we used a horizontal peak ground
acceleration of 0.2g.
Based on the presence of loose fill and colluvium over hard sandstone and relatively
recent slide activity, we would expect that the current factors of safety are
approximately at equilibrium or a 1.0 factor of safety for much of the upper portions of
the area slopes. Slope stability analyses are not warranted for the upper portions of the
slopes extending into the Thunder Hills Creek valley as their relative stability can be
visually assessed.
Factors that influence the relative factors of safety along the slope areas include
surface water, vegetation and root systems, colluvium/fill density and thickness, and
slope magnitude. We anticipate lower factors of safety and higher probability of
landslide activity to occur from approximately November to May when precipitation is
highest.
We conducted slope stability analyses for the lower portion of the slope west of the
access roadway extending east through the creek. Our initial analyses indicate that
adequate factors of safety currently exist in the area between the access roadway and
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Thunder Hills Creek under static and seismic conditions. It is our opinion that the sewer
line is more susceptible to movement caused by long-term stream erosion rather than
shallow landslide activity in the area.
6.1.2 Erosion Hazard
The Natural Resources Conservation Services (NRCS) maps for King County indicate that
the project area and directly adjacent side slopes are underlain by Alderwood and
Kitsap soils (very steep), Alderwood gravelly sandy loam (8 to 30 percent slopes) and
Beausite gravelly sandy loam (15 to 30 percent slopes). Since the project is located
within an actively incising stream environment adjacent to very steep slope areas, all
soils should be considered to have “Severe” to “Very Severe” erosion potential.
It is our opinion that soil erosion potential at this project site, if grading activities are
proposed, can be reduced through surface water runoff control and local removal of
problem soil areas (discussed in Section 8.1). Typically erosion of exposed soils will be
most noticeable during periods of rainfall and may be controlled by the use of normal
temporary erosion control measures, such as silt fences, hay bales, mulching, control
ditches and diversion trenches. The typical wet weather season, with regard to site
grading, is from October 31st to April 1st. Erosion control measures should be in place
before the onset of wet weather.
While erosion of the sandstone that underlies the site between Stations 0+00 and 5+50
will occur at a low to very low rate over the lifespan of the sewer line (80 years), large
storm events and long term erosion of the sandstone could erode the existing gabion
walls and slope between the stream and sewer line. To reduce adverse effects of soil
and slope erosion caused by Thunder Hills Creek, permanent erosion prevention systems
should be constructed. Large rock buttressing or gravity walls embedded adequately
into the unweathered sandstone should provide adequate protection for the sewer line
and access roadway.
6.1.3 Seismic Hazard
We encountered generally medium dense to very dense soils and locally soft rock at
the project site. The overall subsurface profile corresponds to a Site Class D as defined
by Chapter 20 of ASCE 7 (Table 20.3-1) and referenced in Table 1613.3.2 of the 2012
International Building Code (2012 IBC). A Site Class D applies to an overall profile
consisting of medium dense/stiff to very dense/hard materials within the upper 100 feet.
Areas of the site, including areas directly underlain by soft bedrock (sandstone), would
be considered as a Site Class C, soft rock profile. We do not anticipate the need to
utilize seismic parameters from this profile as part of the currently proposed project and
therefore they have not been included.
We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website
(seismic calculator) to obtain values for SS, S1, Fa, and Fv. The USGS website includes
the most updated published data on seismic conditions. The site specific seismic design
parameters and adjusted maximum spectral response acceleration parameters are as
follows:
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PGA (Peak Ground Acceleration, in percent of g)
32.24 (10% Probability of Exceedence in 50 years)
62.52 (2% Probability of Exceedence in 50 years)
SS 141.10% of g
S1 48.30% of g
Additional seismic considerations include liquefaction potential and amplification of
ground motions by soft/loose soil deposits. The liquefaction potential is highest for loose
sand with a high groundwater table. The dense to very dense, glacially consolidated
materials and bedrock that underlie the site have a very low potential for liquefaction.
7.0 Discussion
7.1.1 General
It may not be economically feasible to construct preventative structures to eliminate
shallow landslide activity which originates higher up the steep slopes on the
proposed/existing access roadway or stream. At a minimum, we recommend
performing remedial excavation work to reduce the likelihood and adverse effects of
shallow colluvial slides on the proposed/existing access path along with select hazard
tree removal and drainage improvements below the slope.
Gravity retaining walls or rock buttresses should be constructed between the access
roadway and stream to prevent erosion/undercutting of the roadway and sewer line by
Thunder Hills Creek over the design lifespan of the sewer line (approximately 80 years).
It is our opinion that rock buttresses or gravity walls will be the most economical method
to prevent lateral soil movement and significant stream erosion in the area between
the existing (or proposed/new) sewer line and Thunder Hills Creek.
We anticipate that significant grading may be necessary in the vicinity of Station 4+70
to 5+40 to create roadway grades suitable for the maintenance equipment. In this
area, additional excavation work may be necessary to repair sloughed soils and erosion
channels. Also, gravity walls in this area may be required to support the access
roadway. Depending on grading configurations, these walls may be over 8 feet in
height. Once the final grading plans and road width/locations have been determined,
we can provide specific wall construction recommendations.
8.0 Recommendations
8.1 Site Preparation
In general, site preparation should consist of vegetation and topsoil removal from
proposed excavation/improvement areas. Based on observations from the site
investigation program and site reconnaissance work, it is anticipated that the stripping
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depth will generally be less than 12 inches where topsoil and vegetation are present.
The excavated material is not suitable as structural fill but could be used as fill material
in non-settlement sensitive areas such as landscaping. In these non-settlement sensitive
areas, the fill should be placed in maximum 12 inch thick lifts that should be
compacted to at least 90 percent of the modified proctor (ASTM D 1557 Test Method)
maximum dry density.
As needed, leaning trees and other trees designated as hazard trees located in critical
areas, may be removed during site preparation. It may be useful to leave root systems
in place depending on the location of the hazard trees. We can provide
recommendations on which trees are suitable for full removal or partial removal upon
request.
The existing roadway alignment is underlain by approximately 4 to 14 feet of fill. The fill
consists of silty-sand and silt with variable amounts of sand. Areas of woody debris and
other fill debris occur locally.
These materials are generally considered suitable for use as structural fill provided they
are within 3 percent of the optimum moisture content. It should be noted that these
materials are typically suitable for structural fill only during the summer months and are
highly moisture sensitive due to their fines content.
Imported structural fill should consist of a sand and gravel mixture with a maximum grain
size of 3 inches and less than 5 percent fines (material passing the U.S. Standard No. 200
Sieve). Structural fill should be placed in maximum lift thicknesses of 12 inches and
should be compacted to a minimum of 95 percent of the modified proctor maximum
dry density, as determined by the ASTM D 1557 test method.
8.2 Temporary Excavations
Based on our understanding of the project, grading associated with access road and
retaining wall construction could include significant cuts and/or fills. Preliminarily,
temporary excavations should be sloped no steeper than 1.5H:1V (Horizontal:Vertical)
in medium dense fill soils and 1H:1V in medium dense to dense native soils. Locally,
steeper cuts may be feasible; however, Stantec should be on site to provide specific
recommendations at that time.
If an excavation is subject to heavy vibration or surcharge loads, we recommend that
the excavation be sloped no steeper than 2H:1V and 1.5H:1V, respectively as above,
where room permits. If groundwater is encountered, lower declinations may be
required.
In general, excavations in slightly weathered sandstone may be stable up to a vertical
condition; however, we do not anticipate the need to excavate into sandstone other
than removing loose colluvium from existing slopes along the west side of the proposed
access roadway. In these areas, we recommend scraping the loose materials and any
loose sandstone that readily comes free from the rock faces only. Again, Stantec
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should be on site to observe the conditions during construction and provide location-
specific recommendations.
All temporary cuts should be in accordance with the Washington Administrative Code
(WAC) Part N, Excavation, Trenching, and Shoring. The temporary slopes should be
visually inspected daily by a qualified person during construction activities and the
inspections should be documented in daily reports. The contractor is responsible for
maintaining the stability of the temporary cut slopes and reducing slope erosion during
construction.
The temporary cut slopes should be covered with visqueen to help reduce erosion
during wet weather, and the slopes should be closely monitored until the permanent
retaining systems or slope configurations are complete. Materials should not be stored
or equipment operated within 10 feet of the top of any temporary cut slope.
Soil conditions may not be completely known from the geotechnical investigation. In
the case of temporary cuts, the existing soil conditions may not be completely revealed
until the excavation work exposes the soil. Typically, as excavation work progresses, the
maximum inclination of the temporary slopes will need to be re-evaluated by the
geotechnical engineer so that supplemental recommendations can be made. Soil
and groundwater conditions can be highly variable. Scheduling for soil work will need
to be adjustable, to deal with unanticipated conditions, so that the project can
proceed and required deadlines can be met.
If any variations or undesirable conditions are encountered during construction, Stantec
should be notified so that supplemental recommendations can be made. If room
constraints or groundwater conditions do not permit temporary slopes to be cut to the
maximum angles allowed by the WAC, temporary shoring systems may be required.
The contractor should be responsible for developing temporary shoring systems, if
needed. We recommend that Stantec and the project structural engineer review
temporary shoring designs prior to installation, to verify the suitability of the proposed
systems.
8.3 Erosion and Sediment Control
Erosion and sediment control (ESC) is used to reduce the transportation of eroded
sediment to wetlands, streams, lakes, drainage systems, and adjacent properties.
Erosion and sediment control measures should be implemented and these measures
should be in general accordance with local regulations. At a minimum, the following
basic recommendations should be incorporated into the design of the erosion and
sediment control features for the site:
Schedule the soil, foundation, utility, and other work requiring excavation
or the disturbance of the site soils, to take place during the dry season
(generally June through September). However, provided precautions are
taken using Best Management Practices (BMP’s), certain grading activities
can be completed during the wet season in areas of the site upstream
from the I-405 to AB-1 area (generally October through April).
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All site work should be completed and stabilized as quickly as possible.
Additional perimeter erosion and sediment control features may be
required to reduce the possibility of sediment entering the surface water.
This may include additional silt fences, silt fences with a higher Apparent
Opening Size (AOS), construction of a berm, or other filtration systems.
Any runoff generated by dewatering discharge should be treated
through construction of a sediment trap if there is sufficient space. If
space is limited, other filtration methods will need to be incorporated.
Specifically for this project, site grading should only be performed during the summer
months (late June through mid-September) when the creek is at it lower levels and
surface waters will be less prevalent. Additional erosion control measures will likely be
required between the proposed roadway and Thunder Hills Creek during construction
due to the presence of wetlands.
There are areas where loose colluvium overlies hard sandstone at steep angles
between Station 0+50 and 4+00. Depending on the roadway grading plans, access
roadway setback from the toe of the slope, and project/maintenance requirements,
some soil removal may be necessary. The removal of soils and trees from site slopes
should be observed by the geotechnical engineer as slope stability above these
locations could be adversely affected. Replacement of loose soils with quarry rock
may be warranted. In general, we recommend removal of trees and colluvium only
where necessary.
8.4 Retaining Walls
As needed, primarily in the area between Stations 1+00 to 5+50 and locally south of the
site area extending toward Grant Avenue South, gravity retaining walls or rock
buttresses may be utilized to support the access roadway and prevent lateral pipe
movements and/or erosion/undercutting of the sewer line. However, any type of
retaining structure will only be effective if they are embedded into the underlying
sandstone. In other words, fill material or soil deposits that are used to support retaining
structures could be easily eroded by the stream during storm events when flows are
high in volume and velocity, potentially causing the structures to fail (as observed with
the current gabion walls).
Based on the existing site grades, we anticipate that the gravity walls in the lower site
area will range from 4 to 8 feet in height, with a typical height of 6 feet. Tiered systems
may be utilized if there is adequate lateral space. The new gravity walls may also be
constructed between the existing gabion walls and access roadway if there is
adequate space.
There are numerous types of retaining structures that could be utilized in these areas.
We can provide alternative options upon request. We anticipate that Ultra block or
ecology block walls will be the most economical for the proposed construction.
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Gravity Retaining Walls
For 2.5 foot by 2.5 foot by 5 foot long Ultra blocks, we recommend a minimum of
1.5 feet of embedment in dense to very dense native soils or hard sandstone. A
rock breaker attachment to a trackhoe may be necessary to achieve
embedment grades. The wall keyway should be cut to create a 8-10V:1H
(Vertical to Horizontal) batter for the walls with level backfill conditions. A
maximum 4-inch thick gravel base (leveling course) should be placed and
compacted prior to block placement (1-1/4 to 1-1/2 inch minus crushed rock).
For an exposed wall height of 6 feet, the lowest level of blocks (1st row) should
be placed perpendicular to the face of the wall. The two rows above this level
should be placed parallel to the wall direction and interlocking. Figures 10 and
11 shows the layout and generalized wall profiles. The keyway, drainage system,
backfill, and block placement should be verified by the geotechnical engineer.
A minimum 1.5 feet thick drainage/backfill zone should extend behind the back
of the wall down to the base of the excavation. Mirafi N140 filter fabric (or
equivalent) should be placed between the excavated area of roadway and
drainage/backfill zone and extend over the drainage zone. This area should be
backfilled with clean angular rock, 2 to 8 inches in size. A minimum 4-inch
diameter perforated PVC pipe (Schedule 40) should be installed behind the wall
at its base. At least 6 inches of clean washed rock (1 to 2 inches in size) should
surround the pipe and the pipe should be sloped to drain from the behind the
walls utilizing perpendicular drains placed below the wall approximately every 25
feet along the length of the wall. Additional drainage systems could be installed
across the access roadway and connected to the wall drains if desired.
Stantec should be on site during all wall construction activities, including keyway
excavation work, drainage placement, block and quarry rock placement, and
slope grading.
Rock Buttresses
In lieu of constructing gravity walls, large quarry rock could be used to stabilize
eroding areas and replace failing gabion walls. The usefulness of the existing
gabion walls over time is limited. Since their construction is unknown, we
recommend removing the gabion walls prior to rock buttress placement. Much
of the angular rock backfill may be utilized between large rocks.
We recommend that quarry rock used as buttress material be 4-man sized or
larger for applications between Stations 4+50 and 5+50. North of Station 4+50,
smaller rock may be utilized. Once the location of the access roadway (width,
elevation, and setback) has been determined, we can provide specific buttress
recommendations.
In general, buttressing should consist of the following:
Loose soil removal and temporary excavation creation (generally
between 1H:1V and 2H:1V)
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Excavation of a keyway at the toe of the buttress (at least 18 inches into
underlying dense soils or weathered sandstone)
Placement of large angular basalt with interstitial quarry rock (as void fill)
Other Gravity Retaining Walls
Shorter gravity walls or rockeries (where soil conditions are suitable), on the order
of 8 feet or less, are necessary along the upslope side (east of the access
roadway south of this site area. During our field assessments, we observed
undermined cuts and local near-vertical excavations along the upslope (east)
side of the access roadway at several locations and we understand that this
roadway may be widened in the near future. General locations of necessary
gravity walls are as follows:
Location Anticipated Height
15+80 to 18+10 Up to 8 feet
19+55 to 20+65 4 feet or under
21+30 to 23+25 4 feet
24+10 to 25+60 Up to 6 feet
Additionally, short gravity walls or rockeries may be needed between Stations
12+70 and 13+10 and 18+10 and 19+00 depending on the final grading plans.
We can provide final wall recommendations during the next phase of the
project.
8.5 Utilities
Sewer line trenches should be excavated according to accepted engineering
practices following OSHA (Occupational Safety and Health Administration) standards,
by a contractor experienced in such work. The contractor is responsible for the safety
of open trenches. Traffic and vibration adjacent to trench walls should be reduced;
cyclic wetting and drying of excavation side slopes should be avoided. Depending
upon the location and depth of some utility trenches, groundwater flow into open
excavations could be experienced, especially during or shortly following periods of
precipitation.
In general, silty and gravelly fill soils, as well as fine-grained native soils were
encountered at shallow depths in the explorations at this site. At this site, these soils
have variable density and minimal cohesion and will have a tendency to cave or
slough in excavations. Shoring or sloping back trench sidewalls is required within these
soils. If sewer line excavations extend deep enough to encounter sandstone (not
anticipated), rock chipping/breaking equipment would likely be required to allow for
excavation. Excavations in sandstone should be adequately safe to remain vertical for
a significant amount of time.
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All utility trench backfill should consist of imported structural fill. Certain on site soils may
be suitable for use as backfill in landscaping areas during the summer months; however,
we should evaluate these soils at that time to determine their moisture levels.
The upper 5 feet of utility trench backfill placed in pavement areas should be
compacted to at least 95 percent of the maximum dry density based on ASTM Test
Method D1557. Below 5 feet and in landscaping areas, utility trench backfill in
pavement areas should be compacted to at least 90 percent of the maximum dry
density based on ASTM Test Method D1557. Pipe bedding should be in accordance
with the pipe manufacturer's recommendations.
The contractor is responsible for removing all water-sensitive soils from the trenches
regardless of the backfill location and compaction requirements. Depending on the
depth and location of the proposed utilities, we anticipate the need to re-compact
existing fill soils below the utility structures and pipes. The contractor should use
appropriate equipment and methods to avoid damage to the utilities and/or structures
during fill placement and compaction procedures.
8.6 Utility Protection
Between Stations 1+00 and 5+50, the proposed gravity retaining wall(s) or rock
buttresses will be effective in protecting the sewer line from stream erosion. South of
Station 5+50, the Thunder Hills Creek valley has variably incised sides (to near vertical)
and the channel is filled with sediments (to variable depths). Erosion hazard potential
and recommendations related to sewer interceptor protection south of Station 5+50
can be found in our Preliminary Erosion Hazard Evaluation report dated February 13,
2015.
The existing sewer line crosses Thunder Hills Creek near Station 11+80 at an estimated
depth of 7 feet. For sewer line locations that are at or above bottom of stream channel
elevations, additional backfill erosion measures may be necessary, depending on the
alternative that is chosen. Once an alignment and profile have been developed, we
can finalize our recommendations; however, the following options may be considered
as part of erosion protection for the sewer line:
Angular rock backfill placement (2 to 4 inch sized quarry rock)
Lean mixed concrete placement around sewer lines
Localized ecology/Ultra block wall placement along sewer lines
In general, Thunder Hills Creek could adversely affect the sewer line through severe
erosion and undercutting in localized areas. Our observations and analyses indicate
that there are areas where groundwater is not present below stream levels in the
vicinity of the existing sewer line, indicating that the channel is confined and not likely
to create issues for the sewer line.
For the alternative that includes lining of the existing sewer line, we do not anticipate
the need to add specific erosion protection other than gravity walls/buttresses between
TECHNICAL MEMORANDUM No. 4
February 13, 2015
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the access roadway and stream from Stations 0+00 to 5+50 (this site area). Analysis of
stream erosion effects on the existing sewer line south of 5+50 can be found in our
Preliminary Erosion Hazard Evaluation report.
If a new sewer line is proposed, there is a potential need for utility protection in areas
where the sewer line crosses Thunder Hills Creek, or a combination of being located
within 10 feet laterally of Thunder Hills Creek and at a depth of 3 feet below stream level
or deeper. We should review the preliminary plans in order to provide comments
regarding utility erosion protection recommendations.
8.7 Groundwater Influence on Construction
At the time of our investigation, groundwater was encountered in Boring B-4 at
approximately 4.5 feet below the existing site grade. Groundwater was not
encountered in any of the other explorations at the date and time of our investigation.
We anticipate that groundwater in the Thunder Hills Creek valley is primarily influenced
by area streams and surface water runoff/infiltrating surface waters. There are areas of
the site near I-405 where surface water and groundwater is at the same level (ground
surface) and areas where groundwater is not encountered below stream depths due to
stream channel confinement within the Renton Formation sandstone (Station 5+50 to
6+50 area).
We would expect groundwater to be locally encountered at sewer line depths south of
Station 7+00 and north of Station 5+00. If groundwater is encountered in trenches south
of Station 7+00, we would expect it to be found at or near the elevations of Thunder Hills
Creek. There may be areas where groundwater is not encountered due to channel
confinement, similar to our observations in the area of Station 6+00. When groundwater
is encountered, we would expect moderate to heavy seepage.
North of Station 5+00, we would anticipate light to moderate seepage from trench
sidewalls extending from the ground surface down to the level of the sandstone (upper
4 to 5 feet).
If temporary dewatering systems to remove groundwater are used, their design should
be the responsibility of the contractor. We should review any dewatering design prior
to their use on site.
9.0 Construction Field Reviews
Stantec should be retained to provide part time field review during construction in order
to verify that the soil conditions encountered are consistent with our design assumptions
and that the intent of our recommendations is being met. This will require field and
engineering review to:
Observe all aspects of gravity wall, rock buttress, and access roadway
construction
Monitor temporary excavations, slope stability, and grading activities
TECHNICAL MEMORANDUM No. 4
February 13, 2015
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Density testing to verify compaction of structural fills
Geotechnical design services should also be anticipated during the subsequent final
design phase to support the structural design and address specific issues arising during
this phase. Field and engineering review services will also be required during the
construction phase in order to provide a Final Letter for the project.
10.0 Closure
This report was prepared for the exclusive use of the City of Renton and their appointed
consultants. Any use of this report or the material contained herein by third parties, or
for other than the intended purpose, should first be approved in writing by Stantec.
The recommendations contained in this report are based on limited data from provided
test holes, and preliminary proposed construction. Additional exploration work is
warranted and necessary to provide engineering parameters and recommendations
for sewer line placement.
Use of this report is subject to the Statement of General Conditions provided in
Appendix A. It is the responsibility of the City of Renton who is identified as “the Client”
within the Statement of General Conditions, and its agents to review the conditions and
to notify Stantec should any of these not be satisfied.
TECHNICAL MEMORANDUM NO. 4
February 13, 2015
APPENDIX A
Statement of General Conditions
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TECHNICAL MEMORANDUM No. 4
February 13, 2015
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TECHNICAL MEMORANDUM NO. 4
February 13, 2015
Statement of General Conditions
USE OF THIS REPORT: This report has been prepared for the sole benefit of the Client or its agent
and may not be used by any third party without the express written consent of Stantec
Consulting Services, Inc. and the Client. Any use which a third party makes of this report is the responsibility of such third party.
BASIS OF THE REPORT: The information, opinions, and/or recommendations made in this report
are in accordance with Stantec Consulting Services, Inc.’s present understanding of the site
specific project as described by the Client. The applicability of these is restricted to the site conditions encountered at the time of the investigation or study. If the proposed site specific
project differs or is modified from what is described in this report or if the site conditions are
altered, this report is no longer valid unless Stantec Consulting Services, Inc. is requested by the
Client to review and revise the report to reflect the differing or modified project specifics and/or the altered site conditions.
STANDARD OF CARE: Preparation of this report, and all associated work, was carried out in
accordance with the normally accepted standard of care in the state of execution for the
specific professional service provided to the Client. No other warranty is made.
INTERPRETATION OF SITE CONDITIONS: Soil, rock, or other material descriptions, and statements
regarding their condition, made in this report are based on site conditions encountered by
Stantec Consulting Services, Inc. at the time of the work and at the specific testing and/or sampling locations. Classifications and statements of condition have been made in accordance with normally accepted practices which are judgmental in nature; no specific description
should be considered exact, but rather reflective of the anticipated material behavior.
Extrapolation of in situ conditions can only be made to some limited extent beyond the sampling or test points. The extent depends on variability of the soil, rock and groundwater conditions as influenced by geological processes, construction activity, and site use.
VARYING OR UNEXPECTED CONDITIONS: Should any site or subsurface conditions be
encountered that are different from those described in this report or encountered at the test locations, Stantec Consulting Services, Inc. must be notified immediately to assess if the varying
or unexpected conditions are substantial and if reassessments of the report conclusions or
recommendations are required. Stantec Consulting Services, Inc. will not be responsible to any
party for damages incurred as a result of failing to notify Stantec Consulting Services, Inc. that differing site or sub-surface conditions are present upon becoming aware of such conditions.
PLANNING, DESIGN, OR CONSTRUCTION: Development or design plans and specifications should
be reviewed by Stantec Consulting Services, Inc., sufficiently ahead of initiating the next project
stage (property acquisition, tender, construction, etc), to confirm that this report completely addresses the elaborated project specifics and that the contents of this report have been
properly interpreted. Specialty quality assurance services (field observations and testing) during
construction are a necessary part of the evaluation of sub-subsurface conditions and site preparation works. Site work relating to the recommendations included in this report should only be carried out in the presence of a qualified geotechnical engineer; Stantec Consulting
Services, Inc. cannot be responsible for site work carried out without being present.
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TECHNICAL MEMORANDUM No. 4
February 13, 2015
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TECHNICAL MEMORANDUM No. 4
February 13, 2015
APPENDIX B: FIGURES
Vicinity Map (Figure 1)
Site Plans (Figures 2 & 3)
Cross Sections (Figures 4 – 9)
Gravity Wall Schematics (Figures 10 & 11)
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TECHNICAL MEMORANDUM NO. 4
February 13, 2015
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SITE
N
VICINITY MAP
FIGURE 1
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
Project
Location
Renton
WASHINGTON
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
SITE PLAN
FIGURE 2
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
PB-1
PB-2
PB-3
3015015
(In Feet)
1 inch = 30 feet
Approximate Graphic Scale
A B
C
A’
B’
C’
B-4
B-3
Exposed
Sandstone
Exposed
Sandstone
Exposed
Sandstone
Exposed Sandstone
& Near Vertical Slopes
Older Slide
Older Slide
Approximate Thunder Hills Creek Channel
Approximate Boring Location
Approximate Boring Location (P&EE)
Large Area of Exposed Sandstone
B-1
PB-1
Cross Section
A
A’
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
N
Wetland E
I-405
1+00 4+00
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
SITE PLAN
FIGURE 3
3015015
(In Feet)
1 inch = 30 feet
Approximate Graphic Scale
PB-6
PB-5
PB-4PB-3
B-1
B-2
D
D’
E
E’
F
F’
Exposed Sandstone
“New” Stream
Joining THC
Exposed
Sandstone
Approximate Thunder Hills Creek Channel
Approximate Boring Location
Approximate Boring Location (P&EE)
Large Area of Exposed Sandstone
B-1
PB-1
Cross Section
A
A’
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
N
6+00
7+004+00
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
CROSS SECTION
A - A’
FIGURE 4
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
10505
(In Feet)
1 inch = 10 feet
Approximate Graphic Scale
A
80
90
100
Elevation
(Feet)
110
120
130
C Colluvium (Primarily Loose Silty Sand (SM))
F Fill/Debris (Loose to Medium Dense Silty Sand (SM) and Silt with Sand (ML))
T Vashon Glacial Till (Generally Medium Dense to Dense Silty Sand with Gravel (SM))
W Highly Weathered Renton Formation (Generally Medium Dense Silty Sand (SM) to Stiff Sandy Silt (ML))
R Renton Formation (Slightly Weathered Sandstone)
C
C
RR
F
T
Approximate Sewer Line Location
A’
Observed/Interpreted Surface/Stream Water Elevation
Interpreted Ground Surface
Ground Surface (Surveyed)
Approximate Geologic Contact
Approximate Location of Existing Gabion Wall
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
CROSS SECTION
B - B’
FIGURE 5
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
10505
(In Feet)
1 inch = 10 feet
Approximate Graphic Scale
80
90
100
Elevation
(Feet)
B’
110
120
130
140
C Colluvium (Primarily Loose Silty Sand (SM))
F Fill/Debris (Loose to Medium Dense Silty Sand (SM) and Silt with Sand (ML))
T Vashon Glacial Till (Generally Medium Dense to Dense Silty Sand with Gravel (SM))
W Highly Weathered Renton Formation (Generally Medium Dense Silty Sand (SM) to Stiff Sandy Silt (ML))
R Renton Formation (Slightly Weathered Sandstone)
R
C/W
C
T
R
F
Approximate Sewer Line Location
Observed/Interpreted Surface/Stream Water Elevation
Interpreted Ground Surface
Ground Surface (Surveyed)
Approximate Geologic Contact
Approximate Location of Existing Gabion Wall
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
CROSS SECTION
C - C’
FIGURE 6
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
10505
(In Feet)
1 inch = 10 feet
Approximate Graphic Scale
100
Elevation
(Feet)
110
120
130
140
C C’
C Colluvium (Primarily Loose Silty Sand (SM))
F Fill/Debris (Loose to Medium Dense Silty Sand (SM) and Silt with Sand (ML))
T Vashon Glacial Till (Generally Medium Dense to Dense Silty Sand with Gravel (SM))
W Highly Weathered Renton Formation (Generally Medium Dense Silty Sand (SM) to Stiff Sandy Silt (ML))
R Renton Formation (Slightly Weathered Sandstone)
T
R
C
F
C
R
T
Approximate Sewer Line Location Observed/Interpreted Surface/Stream Water Elevation
Interpreted Ground Surface
Ground Surface (Surveyed)
Approximate Geologic Contact
Approximate Location of Existing Gabion Wall
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
CROSS SECTION
D - D’
FIGURE 7
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
10505
(In Feet)
1 inch = 10 feet
Approximate Graphic Scale
Elevation
(Feet)
110
120
130
140
150
D’
C Colluvium (Primarily Loose Silty Sand (SM))
F Fill/Debris (Loose to Medium Dense Silty Sand (SM) and Silt with Sand (ML))
T Vashon Glacial Till (Generally Medium Dense to Dense Silty Sand with Gravel (SM))
W Highly Weathered Renton Formation (Generally Medium Dense Silty Sand (SM) to Stiff Sandy Silt (ML))
R Renton Formation (Slightly Weathered Sandstone)
T
R
C
F
R
C
T
Approximate Sewer Line Location Observed/Interpreted Surface/Stream Water Elevation
Interpreted Ground Surface
Ground Surface (Surveyed)
Approximate Geologic Contact
Approximate Location of Existing Gabion Wall
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
CROSS SECTION
E - E’
FIGURE 8
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
10505
(In Feet)
1 inch = 10 feet
Approximate Graphic Scale
130
140
Elevation
(Feet)
150
E’
C Colluvium (Primarily Loose Silty Sand (SM))
F Fill/Debris (Loose to Medium Dense Silty Sand (SM) and Silt with Sand (ML))
T Vashon Glacial Till (Generally Medium Dense to Dense Silty Sand with Gravel (SM))
W Highly Weathered Renton Formation (Generally Medium Dense Silty Sand (SM) to Stiff Sandy Silt (ML))
R Renton Formation (Slightly Weathered Sandstone)
T
F
W
R
R
W
T
C/W
Approximate Sewer Line Location Observed/Interpreted Surface/Stream Water Elevation
Interpreted Ground Surface
Ground Surface (Surveyed)
Approximate Geologic Contact
F
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
CROSS SECTION
F - F’
FIGURE 9
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
Elevation
(Feet)
140
150
160
F’
C Colluvium (Primarily Loose Silty Sand (SM))
F Fill/Debris (Loose to Medium Dense Silty Sand (SM) and Silt with Sand (ML))
T Vashon Glacial Till (Generally Medium Dense to Dense Silty Sand with Gravel (SM))
W Highly Weathered Renton Formation (Generally Medium Dense Silty Sand (SM) to Stiff Sandy Silt (ML))
R Renton Formation (Slightly Weathered Sandstone)
T
W
R
F
W
T
R
Approximate Sewer Line Location Observed/Interpreted Surface/Stream Water Elevation
Interpreted Ground Surface
Ground Surface (Surveyed)
Approximate Geologic Contact
10505
(In Feet)
1 inch = 10 feet
Approximate Graphic Scale
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
GRAVITY WALL
SCHEMATIC
FIGURE 10
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
Min. 4 Inch Diameter Perforated PVC Pipe
(Schedule 40, Sloped to Drain
Mirafi 140N Filter Fabric or Equivalent
1 to 2 Inch Diameter Washed Rock
2 to 4 Inch Angular Quarry Rock
1-1/4 Inch Crushed Rock Base
(4 Inch Thickness)
1 Min.
8 Max.
1.5 Ft. Min.
Not to Scale
6 Inches
Minimum
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
NOTES:
For use with 2.5’x2.5’x5’ interlocking concrete blocks
Intended for use between Thunder Hills Creek and Sewer Line/Access Roadway
Rock base to be compacted to at least 95% of the
modified proctor (ASTM D1557 Test Method)
Stantec to verify keyway, drainage, backfill, soil conditions,
and block installation during construction
Medium Dense Road Fill
(Silty Sand/Rock)
Max. 2H:1V Slope
2’ Max. Ht.
Sandstone
Thunder Hills
Creek
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
GRAVITY WALL
SCHEMATIC
FIGURE 11
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
Min. 4 Inch Diameter Perforated PVC Pipe
(Schedule 40, Sloped to Drain
Mirafi 140N Filter Fabric or Equivalent
1 to 2 Inch Diameter Washed Rock
2 to 4 Inch Angular Quarry Rock
Medium Dense Road Fill
(Silty Sand/Rock)
Sandstone
1-1/4 Inch Crushed Rock Base
(4 Inch Thickness)
1 Min.
8 Max.
NOTES:
For use with 2.5’x2.5’x5’ interlocking concrete blocks
Intended for use between Thunder Hills Creek and Sewer Line/Access Roadway
Rock base to be compacted to at least 95% of the
modified proctor (ASTM D1557 Test Method)
Stantec to verify keyway, drainage, backfill, soil conditions,
and block installation during construction
Thunder Hills
Creek 1.5 Ft. Min.
Not to Scale
6 Inches
Minimum
Max. 2H:1V Slope
2’ Max. Ht.
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
TECHNICAL MEMORANDUM NO. 4
February 13, 2015
APPENDIX C
Boring Logs
rs \\us1314-f01\workgroup\2002\active\2002003607\civil\design\report\mem_th geotech investig_20150210.docx
Vegetation/Topsoil
SM; Medium dense to dense, silty-sand with variable amounts of gravel
and debris, dark yellowish brown to grayish brown, moist to very moist.(Fill)
ML; Stiff to very stiff, silt with variable amounts of sand, trace gravel,trace debris, trace woody debris, grayish brown to olive gray, moist to
very moist. (Fill)
ML; Stiff, silt with variable amounts of sand, trace gravel, trace woody
debris, olive gray, moist. (Highly Weathered Renton Formation)
SM; Medium dense, silty-sand, tan to yellow clasts of highly weatheredsandstone, moist. (Renton Formation - Slightly Weathered)
Borehole terminated at 25 feet.
SM
ML
ML
SM
76
4
21617
3
33
711
12
2
43
346
235
547
INITIAL DTW (ft):Not Encountered
WELL CASING DIA. (in):---
STARTED
B-1
EXCAVATION COMPANY:CN
EQUIPMENT:Limited Access DEPTH (ft):25.0
BORING NO.:
SAMPLING EQUIPMENT:Split Spoon CHECKED BY:GS
Time
&
Depth
(feet)
5
10
15
20
25
PROJECT NUMBER:2002003607
PROJECT:Thunder Hills Interceptor
STATIC DTW (ft):Not Encountered
LONG:LAT:COMPLETED:10/17/14 10/17/14
LOGGED BY:PH
GROUND ELEV (ft): TOC ELEV (ft):
LOCATION:Renton, WA
Description
NORTHING (ft): EASTING (ft):
PAGE 1 OF 1
USCS
Graphic
Log
METHOD:HSA SIZE:6
WELL DEPTH (ft):---
EXCAVATION / INSTALLATION:
GEOFORM
304
THUNDERHILLS.GPJ
STANTEC
ENVIRO
TEMPLATE
010509.GDT
2/12/15
Measured
Recov.
(feet)
Depth
(feet)
5
10
15
20
25
Blow
Count
Sample
Headspace
PID
(units)TimeSample ID
Topsoil/Vegetation
SM; Medium dense, silty-sand with variable amounts of gravel, trace
debris, yellowish brown, moist. (Fill)
SM; Dense, silty-sand with variable amounts of gravel, sandstoneremnants at 8.5-9 feet, yellowish brown to grayish brown, moist. (GlacialTill)
Borehole terminated at 9 feet.
SM
SM
239
101511
51419
161722
INITIAL DTW (ft):Not Encountered
WELL CASING DIA. (in):---
STARTED
B-2
EXCAVATION COMPANY:CN
EQUIPMENT:Limited Access DEPTH (ft):9.0
BORING NO.:
SAMPLING EQUIPMENT:Split Spoon CHECKED BY:GS
Time
&
Depth
(feet)
5
10
PROJECT NUMBER:2002003607
PROJECT:Thunder Hills Interceptor
STATIC DTW (ft):Not Encountered
LONG:LAT:COMPLETED:10/20/14 10/20/14
LOGGED BY:PH
GROUND ELEV (ft): TOC ELEV (ft):
LOCATION:Renton, WA
Description
NORTHING (ft): EASTING (ft):
PAGE 1 OF 1
USCS
Graphic
Log
METHOD:HSA SIZE:6
WELL DEPTH (ft):---
EXCAVATION / INSTALLATION:
GEOFORM
304
THUNDERHILLS.GPJ
STANTEC
ENVIRO
TEMPLATE
010509.GDT
2/12/15
Measured
Recov.
(feet)
Depth
(feet)
5
10
Blow
Count
Sample
Headspace
PID
(units)TimeSample ID
Topsoil/Vegetation
SM; Medium dense, silty-sand with variable amounts of gravel, trace
debris, yellowish brown, moist. (Fill)
SM; Dense, silty-sand with variable amounts of gravel, yellowish brownto grayish brown, moist. (Glacial Till)
Borehole terminated at 9 feet.
SM
SM
235
101110
81415
182022
INITIAL DTW (ft):Not Encountered
WELL CASING DIA. (in):---
STARTED
B-3
EXCAVATION COMPANY:CN
EQUIPMENT:Limited Access DEPTH (ft):9.0
BORING NO.:
SAMPLING EQUIPMENT:Split Spoon CHECKED BY:GS
Time
&
Depth
(feet)
5
10
PROJECT NUMBER:2002003607
PROJECT:Thunder Hills Interceptor
STATIC DTW (ft):Not Encountered
LONG:LAT:COMPLETED:10/20/14 10/20/14
LOGGED BY:PH
GROUND ELEV (ft): TOC ELEV (ft):
LOCATION:Renton, WA
Description
NORTHING (ft): EASTING (ft):
PAGE 1 OF 1
USCS
Graphic
Log
METHOD:HSA SIZE:6
WELL DEPTH (ft):---
EXCAVATION / INSTALLATION:
GEOFORM
304
THUNDERHILLS.GPJ
STANTEC
ENVIRO
TEMPLATE
010509.GDT
2/12/15
Measured
Recov.
(feet)
Depth
(feet)
5
10
Blow
Count
Sample
Headspace
PID
(units)TimeSample ID
Quarry rock
SM; Loose to medium dense, silty-sand with variable amounts of gravel,trace debris, yellowish brown, moist to wet. (Fill)
SM; Dense to hard, slightly weathered sandstone, yellowish brown to
tan, moist. (Renton Formation)
Borehole terminated at 6 feet.
SM
SM
25
4
812
11
50
INITIAL DTW (ft):Not Encountered
WELL CASING DIA. (in):---
STARTED
B-4
EXCAVATION COMPANY:CN
EQUIPMENT:Limited Access DEPTH (ft):5.0
BORING NO.:
SAMPLING EQUIPMENT:Split Spoon CHECKED BY:GS
Time
&
Depth
(feet)
5
PROJECT NUMBER:2002003607
PROJECT:Thunder Hills Interceptor
STATIC DTW (ft):Not Encountered
LONG:LAT:COMPLETED:10/20/14 10/20/14
LOGGED BY:PH
GROUND ELEV (ft): TOC ELEV (ft):
LOCATION:Renton, WA
Description
NORTHING (ft): EASTING (ft):
PAGE 1 OF 1
USCS
Graphic
Log
METHOD:HSA SIZE:6
WELL DEPTH (ft):---
EXCAVATION / INSTALLATION:
GEOFORM
304
THUNDERHILLS.GPJ
STANTEC
ENVIRO
TEMPLATE
010509.GDT
2/12/15
Measured
Recov.
(feet)
Depth
(feet)
5
Blow
Count
Sample
Headspace
PID
(units)TimeSample ID
Appendix F
TECH MEMO NO. 5 - PRELIMINARY EROSION
HAZARD EVALUATION
CITY OF RENTON
Technical Memorandum No. 5
Preliminary Erosion Hazard Evaluation
Thunder Hills Sewer Interceptor
February 13, 2015
Prepared By:
Phil A. Haberman, P.G., P.E.G.
Senior Engineering Geologist
Stantec Consulting Services Inc.
11130 NE 33rd Place
Suite 200
Bellevue, WA 98004
425.869.9448
TECHNICAL MEMORANDUM NO. 5
February 24, 2015
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Contents
1.0 PROJECT DESCRIPTION ................................................................................................. 1
2.0 EXISTING CONDITIONS ................................................................................................. 1
3.0 REACH LOCATIONS ....................................................................................................... 2
4.0 SITE GEOLOGY .............................................................................................................. 2
5.0 AREA HYDROLOGY ....................................................................................................... 3
6.0 SITE HYDROLOGY OBSERVATIONS ............................................................................... 4
7.0 EROSION POTENTIAL ..................................................................................................... 4
8.0 BANK STABILITY ............................................................................................................. 5
9.0 EROSION HAZARD ESTIMATION .................................................................................... 6
10.0 EROSION POTENTIAL AT CRITICAL CROSS SECTION LOCATIONS ............................... 7
11.0 GENERALIZED EROSION POTENTIAL AT REACH LOCATIONS ....................................... 8
12.0 CONCLUSIONS AND RECOMMENDATIONS ................................................................. 9
13.0 ROCK BUTTRESS/ROCKERY............................................................................................ 9
14.0 FURTHER STUDY .............................................................................................................. 9
15.0 CLOSURE ...................................................................................................................... 10
Appendices
APPENDIX A – VICINITY MAP
APPENDIX B – SITE PLANS (FIGURES 2-7)
APPENDIX C – CROSS SECTIONS (FIGURES 8-11)
APPENDIX D – BOOTH & HENSHAW:
RATES OF CHANNEL EROSION IN SMALL URBAN STREAMS
TECHNICAL MEMORANDUM NO. 5
February 24, 2015
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TECHNICAL MEMORANDUM NO. 5
February 24, 2015
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1.0 Project Description
Proposed utility work in this area could include replacement or rehabilitation of the
existing sewer interceptor that extends along Thunder Hills Creek from approximately
Grant Avenue South to Interstate 405 (I-405) and/or diverting flow from this basin into
sewer infrastructure west of Talbot Road South.
For the alternatives located in the Thunder Hills Creek basin, construction may include
adding sanitary sewer manholes where none currently exist, creating/rehabilitating an
access roadway to the area near I-405 for maintenance equipment; and potentially,
constructing retaining structures to support the access roadway and sewer main.
2.0 Existing Conditions
The Thunder Hills Creek project area is located between I-405 and Grant Avenue South,
just east of the Berkshire Apartment Home development (Figure 1). For the purposes of
this evaluation, the site area includes the sewer alignment, access roadways along the
alignment, Thunder Hills Creek, and adjacent areas potentially affected by stream
erosion.
The site consists of the existing sewer line alignment through the valley along with a
gravel improved access roadway located adjacent to the stream for much of the
alignment. The access roadway has been damaged significantly by soil movement
and erosion near Station 5+00. North of this location, the roadway is improved
(partially) with quarry rock north to a flat area near I-405. There is a gap where there is
no access roadway where the sewer line crosses beneath Thunder Hills Creek (Station
11+80).
Thunder Hills Creek appears to flow continuously throughout the year and there are
numerous intermittent, seasonal streams that join the creek. The site plans show the
locations of various streams that join Thunder Hills Creek from the east, as well as
wetland areas located in and around the creek. Minor springs and seeps seasonally
add volume to the creek throughout the valley. There are numerous residential yard
drains that add stormwater to the stream. Near Station 5+30, a relatively large creek
joins Thunder Hills Creek from the southeast, approximately doubling the volume of
runoff.
South of Station 5+50, variable amounts of sediment are present in the stream channel
indicating some level of deposition. There is also evidence of periodic flooding and
heavy erosion locally in the northern portion of the alignment. North of Station 5+50, the
stream has incised into the underlying sandstone (Renton Formation).
Four to 12-inch sized quarry rock is present in the stream bed and banks in many areas
along the entire alignment. Larger quarry rock, generally ½ to 4 man sized basalt, has
been used to stabilize the stream banks and/or to prevent ongoing stream erosion.
TECHNICAL MEMORANDUM NO. 5
February 24, 2015
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Specifically in the vicinity of Station 5+00, large quarry rock has been used to fill the
stream channel. Smaller rockeries, gabion walls, and areas of smaller ballast rock are
present locally along portions of the stream banks from Grant Avenue to I-405.
The site is bordered to the north by I-405, to the west by the Berkshire Apartment Homes,
to the east by undeveloped land (easements) and single family residences, and to the
south by Grant Avenue South, easements, and residential developments.
3.0 Reach Locations
For the purposes of this report, we have separated the overall alignment into eight
reaches of varying length and gradient. The bank and stream characteristics are very
similar south of Station 12+50; however, further study, including setting up survey
monitoring points for a period of years, would be necessary to adequately determine
channel erosion and deposition over time.
Reach Location Approximate Gradient (%)
0+00 to 1+00 Not Applicable (Culvert)
1+00 to 5+50 15.1
5+50 to 8+30 4.3
8+30 to 11+00 8.2
11+00 to 12+30 10.4
12+30 to 18+90 5.2
18+90 to 26+75 7.1
26+75 to 28+00 14.4
4.0 Site Geology
The Geologic Map of King County, indicates that the site is located near the contacts
between Vashon Glacial Till and Tertiary Bedrock.
Based on our site explorations and observations, the Thunder Hills Creek valley is
underlain by the following materials:
Colluvium
Alluvium
Fill
Landslide Debris
Glacial Till
Renton Formation (primarily sandstone)
TECHNICAL MEMORANDUM NO. 5
February 24, 2015
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Colluvium includes highly weathered glacial till and sandstone that overlies more dense
materials along sloped areas above the valley. In general, these materials consist of
loose mixtures of silt, sand, and clay and are up to 5 feet in thickness.
Alluvium includes soil materials deposited by stream action within the base level of the
valley. These materials consist of loose mixtures of silt, sand, and gravel, and are up to 4
feet in thickness (where present).
In general, fill is present along the existing access roadways within the valley. Deeper
areas of fill are present below the Berkshire Apartment buildings extending to Thunder
Hills Creek. Our borings indicate that the fill consists of silty-sand with variable amounts
of gravel to sandy silt. Local areas of organic debris and construction materials were
observed within the fill. The fill is loose to medium dense and up to 15 feet in thickness
locally.
Landslide debris is present near the toe of steep slopes within the north portion of the
valley. Landslide debris generally consists of very loose to loose mixtures of silt, sand,
and clay, and are up to several feet in thickness, where observed. In general, landslide
debris consists of colluvium, or highly weathered native soils.
Vashon Glacial Till is typically characterized by an unsorted, nonstratified mixture of
clay, silt, sand, gravel, cobbles and boulders in variable quantities. These materials are
typically dense and relatively impermeable. The poor sorting reflects the mixing of the
materials as these sediments were overridden and incorporated by the glacial ice. In
general, weathered glacial till is yellowish brown and medium dense to dense.
Unweathered glacial till is dense to very dense and typically blue-gray or olive gray in
color. Vashon Glacial Till is present throughout the site area south of Station 5+50.
Tertiary Bedrock in this area consists of the Renton Formation. The Renton Formation
includes feldspathic fine to medium grained sandstone with beds of coal,
carbonaceous siltstone, and claystone. Tertiary Bedrock locally outcrops south of I-90
and the Seattle Fault Zone due to uplift associated with seismic activity. Variable
thicknesses of highly weathered bedrock (soil) are present locally within the valley and
consist of medium stiff to stiff/medium dense mixtures of sandy silt and silty sand.
5.0 Area Hydrology
Groundwater in the Thunder Hills Creek valley is primarily influenced by area streams
and surface water runoff/infiltrating surface waters. The overall area that contributes
surface water to Thunder Hills Creek extends from the site area to the south toward
Puget Drive Southeast, to the east to Beacon Way South, and to the west to the ridge
slightly east of Benson Road South.
The approximate area of this basin is 230 acres and it is about 50 percent developed. A
majority of the development consists of single family residential plats constructed
between the 1950’s and 1980’s. Newer construction, including apartment building
developments, is located directly adjacent to Thunder Hills Creek in the site vicinity.
TECHNICAL MEMORANDUM NO. 5
February 24, 2015
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We expect that surface water runoff flows through available stormwater infrastructure
and/or ditches, where present. We expect that stormwater that infiltrates into the
subsurface materials becomes perched between the fill and weathered glacial
deposits and the underlying dense to very dense glacial till and/or sandstone. The very
dense till and sandstone are nearly impermeable and shallow perched groundwater
will slowly infiltrate vertically through fractures and joints, and/or migrate laterally.
Perched groundwater that migrates laterally will either daylight along a slope as a seep
or spring, or continue below grade toward area rivers.
6.0 Site Hydrology Observations
There are areas of the site near I-405 where surface water and groundwater is at the
same level (ground surface) and areas where groundwater is not encountered below
stream depths due to stream channel confinement within the Renton Formation
sandstone (Stations 5+50 to 6+50).
We observed numerous 4-inch diameter (or larger) drain pipes extending into or
towards Thunder Hills Creek north of Grant Avenue South.
We anticipate low to moderate seasonal seepage from the valley/channel sidewalls
contributing to the overall volume in Thunder Hills Creek. A majority of volume in
Thunder Hills Creek originates south and upland from the site. Surface waters appear to
contribute a majority of the stream volume.
Some streams that join Thunder Hills Creek are identified on the site plans. During our
site reconnaissance work, we observed minor flows at Stream B (near Station 0+90) and
Stream D (near Station 2+60), both of which are located on the east side of Thunder Hills
Creek. A significant stream joins Thunder Hills Creek from the east at approximately
Station 5+25. We did not observe significant surface water contributions at any other
stream or pipe locations along the alignment during our site visits, nor did we observe
evidence of significant erosion from areas below these pipes, indicating minimal flows
occur at these locations.
7.0 Erosion Potential
The Natural Resources Conservation Services (NRCS) maps for King County indicate that
the project area and directly adjacent side slopes are underlain by Alderwood and
Kitsap soils (very steep), Alderwood gravelly sandy loam (8 to 30 percent slopes) and
Beausite gravelly sandy loam (15 to 30 percent slopes).
Based on our observations and explorations, it is our opinion that the valley area is
underlain by Beausite gravelly sandy loam from Station 0+00 to 5+50 and Alderwood
gravelly sandy loam from Station 5+50 to Grant Avenue South. Kitsap soils are present in
the north portion of site along steep slopes.
The parent material for Beausite soils includes residuum of sandstone (Renton
Formation). The parent material for Alderwood soils include glacial drift (till).
TECHNICAL MEMORANDUM NO. 5
February 24, 2015
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Since the project is located within an actively incising stream environmental adjacent
to very steep slope areas, all soils should be considered to have “Severe” to “Very
Severe” erosion potential with regard to grading activities and earthwork (soil
exposure). In an undisturbed state, unweathered glacial till and slightly weathered
Renton Formation (sandstone) has very low to low rates of erosion.
8.0 Bank Stability
A bank stability evaluation system, originally proposed by Henshaw & Booth (2000) for
evaluation of urban watersheds in the Northwest, was adapted for the purposes of
identifying areas of increased erosion during the site assessment. The degree (or class)
of bank stability was used as a basis for determining mitigation/stabilization options.
For example, a site with a higher degree of instability may require an engineered bank
stability structure, while a stable reach may not require any treatment for erosion. Table
1 provides a summary of the physical features typically used to classify the banks at the
site. This protocol was adapted to separately describe portions of the bank where the
banks are naturally-occurring or where armoring or manmade structures influence bank
stability.
Table 1
Class
IV Stable
Vegetation (other than grasses) to ordinary high water line
No raw or undercut banks (some erosion on outside of meander bends OK with deposition on
inside bank)
No recently‐exposed roots
No recent tree falls
If bank armoring is present it does not appear to contribute to overall stability
IV.A Stable with Bank Armoring
Bank armoring is present and appears to be the primary source of bank stability
Bank armoring appears stable and/or was observed in conjunction with the above features
III Slightly Unstable
Vegetation to ordinary high water line in most places
Some scalloping of banks
Minor erosion and/or bank undercutting
Recently exposed tree roots rare but present
If bank armoring is present it does not appear to contribute to overall stability
III.A Slightly Unstable with Bank Armoring
Bank armoring is present and appears to be the primary source of bank stability
Bank armoring appears to be degrading slightly or is located such that it only partially stabilizes
the bank II Moderately Unstable
TECHNICAL MEMORANDUM NO. 5
February 24, 2015
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Vegetation to waterline sparse (mainly scoured or stripped by lateral
erosion) Bank held mainly by hard points (trees, boulders) and eroded
back elsewhere Extensive erosion and bank undercutting
Recently exposed tree roots and fine root hairs common
If bank armoring is present it does not appear to contribute to overall stability
II.A Moderately Unstable with Bank Armoring
Bank armoring is present and appears to be the primary source of bank stability
Bank armoring is degrading extensively or is located such that it only stabilizes the bank somewhat
I Completely Unstable
No vegetation (other than grasses) at ordinary high water line
Banks held only by hard points
Severe erosion of both banks (straight runs) or outside bank with no deposition on inside bank
(bends) Recently exposed tree roots common
Tree falls and/or severely undercut trees common
If bank armoring is present it does not appear to contribute to overall stability
Note: Adapted from Henshaw and Booth (2000).
Preliminarily, we classify the overall site as having classes III, IV, and IV.A , showing signs
of active erosion and loss of bank materials. Minimizing excessive erosion with treatment
actions would include implementing one of the stabilization options described in this
report and the geotechnical report, such as toe protection or slope re-grading (if
feasible).
9.0 Erosion Hazard Estimation
The erosion factor (whole soil) is 0.15 for Alderwood soils and 0.10 for Beausite soils. The
erosion factor (K) indicates the susceptibility of a soil to sheet and rill erosion by water
and is one of six factors used in the Revised Universal Soil Loss Equation (RUSLE) to
predict the annual rate of soil loss by water erosion. Values of K range from 0.02 to 0.69
with the lower values indicating soils more resistant to erosion.
It is our opinion that estimating the rate of erosion using this equation for this site area is
not practical or effective. Determining several of the other factors used in the equation
is difficult if not impossible due to the variable slope conditions within the valley,
variable vegetation patterns, and variations in soil types with depth. Also, this equation
and method is used primarily to determine soil loss over a large uniform area, such as
grazing or farm land.
A scientific study conducted in 2001 on small urban streams in King County provides
estimated rates of channel erosion more applicable to the site area. This study can be
found as an attachment to this report.
This study found that the rates of channel change did not correlate very closely with
development intensity, especially in areas where developments have been established
for a longer period of time.
TECHNICAL MEMORANDUM NO. 5
February 24, 2015
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The study found that the type of geologic material that underlies the stream strongly
influenced the rate of erosion. The study concluded that granular soil deposits eroded
more readily than the more cohesive silt-clay deposits, which showed low or very low
rates of erosion.
The Thunder Hills Creek area is underlain by glacial till and sandstone. While sandstone
is generally granular, it contains silt particles and is also well indurated and locally
cemented. Both soil units that underlie the site area are cohesive and consistent with
those indicated in the study to have a low to very low rate of erosion. Their respective K
values confirm a low erosion potential when undisturbed.
From this study, our site observations and drilled borings, we anticipate that the rate of
vertical channel erosion is approximately 0.5 inches per year (on average). The rate of
bank erosion varies widely with channel shape and soil characteristics.
Other factors will influence the erosion rate in any given stream. These include gradient
changes, additional stormwater volumes, large storm events, ballast placement or
other mitigation work, and local variations in soil composition/density. Furthermore,
some areas of any given stream will experience soil deposition and not incision. In the
site area, we observed the following general aspects of the Thunder Hills Creek
channel.
Location Observation
Station 1+00 to 5+40 Incision
Station 5+40 to 8+30 Deposition
Station 8+30 to 27+50 Primarily Incision, Minor Local Deposition
10.0 Erosion Potential at Critical Cross Section Locations
We created eight cross sections approximately perpendicular to the stream and sewer
line direction to analyze the potential effect of stream erosion on bank stability (Figures
8-11). Site plans showing the location of the cross sections can be found in Figures 2
through 7.
For this evaluation, we chose the most critical situations based on proximity of the sewer
line to the stream, sewer line depth relative to stream elevation, soil conditions, and
topography between the sewer line and stream.
We utilized these cross sections, along with estimated soil characteristics and
groundwater levels, to assess the potential long term effect of stream erosion on the
existing sewer line. We estimated soil erosion at a rate of 0.5 inches per year while
considering stream level fluctuations and stream bank erosion (channel widening). We
then analyzed the slope stability to determine where potential failure planes may occur
with regard to the location of the sewer line (as necessary). We used an 80 year
effective lifespan for the sewer line and assumed a current stable condition for the
sewer line at all locations.
TECHNICAL MEMORANDUM NO. 5
February 24, 2015
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The overall stability of the soils supporting the sewer line at the eight cross section
locations, following 20, 40, and 80 years of stream erosion at approximately 0.5
inches/year is as follows:
Location 20 Years 40 Years 80 Years
9+03 Stable Stable Marginal
9+70 Stable Stable Marginal
11+85 Stable Marginal Unstable
17+47 Stable Stable Stable
20+70 Stable Stable Stable
24+00 Stable Stable Stable
25+85 Stable Stable Stable
27+50 Marginal Unstable Unstable
Additionally, six cross sections were created as part of our Geotechnical Study for the
City of Renton dated February 13, 2015. This study was primarily focused on the portion
of the site north of Station 6+00 and considered both landslide and erosion hazards
within that area. Erosion and landslide hazard mitigation recommendations can be
found in that report.
It should be noted that these results are estimates only based on assumed soil
conditions. Also, the schematic cross sections indicate possible levels of erosion
assuming no maintenance or mitigation of the access roadway and stream banks over
time. We expect, and recommend, periodic observation and documentation of
erosion (at least annually) and implementation of rock armoring and other slope
stabilization as needed.
11.0 Generalized Erosion Potential at Reach Locations
In terms of overall sections of the sewer line alignment, the stability of the soils
supporting the sewer line following 20, 40, and 80 years of stream erosion at
approximately 0.5 inches/year are as follows:
Section/Reach 20 Years 40 Years 80 Years
0+00 to 1+00 Stable Stable Stable
1+00 to 5+50 Marginal Unstable Unstable
5+50 to 8+30 Stable Stable Stable
8+30 to 11+00 Stable Stable Marginal
11+00 to 12+30 Stable Marginal Unstable
TECHNICAL MEMORANDUM NO. 5
February 24, 2015
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12+30 to 18+90 Stable Stable Stable
18+90 to 26+75 Stable Stable Stable
26+75 to 28+00 Marginal Unstable Unstable
12.0 Conclusions and Recommendations
Our preliminary erosion analyses indicate that some mitigation work will be required to
achieve the desired 80 year life span for the sewer line extending along Thunder Hills
Creek.
Specifically, the areas between Station 1+00 and 5+50 and near Station 27+50 require
bank stabilization in the near future. The section of stream between Station 11+00 and
12+30 may require mitigation within approximately 40 years. Prior to the 80 year lifespan
of the sewer line, approximately 975 feet of the alignment will likely require some level
of bank stabilization and erosion protection (Station 1+00 to 5+50, 8+30 to 12+30, and
26+75 to 28+00).
It should be noted that our estimation assumes only stream incision and that there will
be no periods of cyclical deposition and erosion or mitigation efforts/maintenance of
existing slopes/banks. Other than near Station 27+50, erosion prevention and
maintenance of existing slopes and stream banks may reduce the effect of stream
erosion during the useful life span of the sewer line and access roadways.
The sewer line near Station 27+50 should be stabilized from the effects of soil erosion
during project construction. We anticipate that the most cost effective means of sewer
line protection and erosion prevention is slope armoring/rockery construction in this
area. Note: gravity wall recommendations as part of bank stabilization are included in
the Stantec Geotechnical Report dated February 13, 2015.
13.0 Rock Buttress/Rockery
We recommend using 1 to 4 man sized rocks and embedding the base rocks into the
dense to very dense glacial till at least 2 feet. This may mean excavation through a
variable thickness of weathered glacial till and/or other soil materials before achieving
the dense till. We can investigate the area using hand borings prior to construction to
estimate the excavation depths required. Alternative recommendations may be made
at that time if soft soils are present to greater depths.
14.0 Further Study
Additional geomorphologic analyses, including further bank stability analyses, reach
delineation, and a surveyed study of channel migration, erosion, and deposition should
be conducted over a period of 5 to 10 years. This type of geomorphologic study would
more adequately determine the extent, limits, and effects of stream erosion within the
TECHNICAL MEMORANDUM NO. 5
February 24, 2015
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site area and on sewer line stability, as well as provide suitable bank stabilization options
as needed.
15.0 Closure
This report was prepared for the exclusive use of the City of Renton and their appointed
consultants. Any use of this report or the material contained herein by third parties, or
for other than the intended purpose, should first be approved in writing by Stantec.
The general recommendations contained in this report are based on site observations
and boring explorations, along with the preliminarily proposed construction.
TECHNICAL MEMORANDUM NO. 4
February 24, 2015
APPENDIX A
Vicinity Map
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TECHNICAL MEMORANDUM No. 4
February 24, 2015
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SITE
N
VICINITY MAP
FIGURE 1
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
Project
Location
Renton
WASHINGTON
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
TECHNICAL MEMORANDUM NO. 4
February 24, 2015
APPENDIX B
Site Plans (Figures 2-7)
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TECHNICAL MEMORANDUM No. 4
February 24, 2015
v:\2002\active\2002003607\civil\design\report\mem_th erosion study_20150224.docx
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
SITE PLAN
FIGURE 2
Cross Section
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
9+03 Section
9+70 Section
N
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
SITE PLAN
FIGURE 3
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
11+85 Section
Cross Section
Potential Location of
Gravity Wall or Rockery N
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
Cross Section
17+47 Section
SITE PLAN
FIGURE 4
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
Potential Location of
Gravity Wall or Rockery N
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
SITE PLAN
FIGURE 5
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
Potential Location of
Gravity Wall or Rockery N
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
SITE PLAN
FIGURE 6
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
24+00 Section
Cross Section
Potential Location of
Gravity Wall or Rockery
N
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
27+50 Section
25+85 Section
Cross Section SITE PLAN
FIGURE 7
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
Potential Location of
Gravity Wall or Rockery
TECHNICAL MEMORANDUM No. 4
February 24, 2015
APPENDIX C
Cross Sections (Figures 8-11)
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TECHNICAL MEMORANDUM No. 4
February 24, 2015
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Cross Sections
Figure 8
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
9+03
155’
165’
.
9+70
157’
167’
.
A
B
A
B
T
T
10505
(In Feet)
1 inch = 10 feet
Approximate Graphic Scale
Note: All contacts and elevations are approximate
Schematic of possible erosion only
Elevation
Elevation
.
A Generally loose to medium dense soils (SM-ML)
B Generally medium dense to very dense soils (SM)
T Thunder Hills Creek.Approximate Sewer Location
Potential Area Eroded in 40 year span
Potential Area Eroded in 80 year span
..
Cross Sections
Figure 9
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
11+85
170
180
.
17+47
204’
214’
.
A
A
B
B
T
T
Building Rockery
A
10505
(In Feet)
1 inch = 10 feet
Approximate Graphic Scale
Note: All contacts and elevations are approximate
Schematic of possible erosion only
Elevation
.
.
A Generally loose to medium dense soils (SM-ML)
B Generally medium dense to very dense soils (SM)
T Thunder Hills Creek.Approximate Sewer Location
Potential Area Eroded in 40 year span
Potential Area Eroded in 80 year span
..
Cross Sections
Figure 10
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
20+70
227’
237’
.
.251’
261’
24+00
A
A
B
B
T
T
10505
(In Feet)
1 inch = 10 feet
Approximate Graphic Scale
Note: All contacts and elevations are approximate
Schematic of possible erosion only
Elevation
.
.
A Generally loose to medium dense soils (SM-ML)
B Generally medium dense to very dense soils (SM)
T Thunder Hills Creek.Approximate Sewer Location
Potential Area Eroded in 40 year span
Potential Area Eroded in 80 year span
..
Cross Sections
Figure 11
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
256’.
266’
25+85
.
27+50
280’
290’
A
A
B
B
T
T
A Generally loose to medium dense soils (SM-ML)
B Generally medium dense to very dense soils (SM)
T Thunder Hills Creek.Approximate Sewer Location
10505
(In Feet)
1 inch = 10 feet
Approximate Graphic Scale
Note: All contacts and elevations are approximate
Schematic of possible erosion only
Elevation
Elevation
Potential Area Eroded in 20 year span
.
.
Potential Area Eroded in 40 year span
Potential Area Eroded in 80 year span
..
TECHNICAL MEMORANDUM No. 4
February 24, 2015
APPENDIX D
Booth & Henshaw: Rates of Channel
Erosion in Small Urban Streams
v:\2002\active\2002003607\civil\design\report\mem_th erosion study_20150224.docx
TECHNICAL MEMORANDUM No. 4
February 24, 2015
v:\2002\active\2002003607\civil\design\report\mem_th erosion study_20150224.docx
1
Rates of Channel Erosion in Small Urban Streams1
By Derek B. Booth and Patricia C. Henshaw
Center for Urban Water Resources Management
Department of Civil Engineering
University of Washington
ABSTRACT
We address four objectives, focused on urban and urbanizing watersheds that drain forested (or once-forested)
landscapes in humid regions: to document rates of channel change, to evaluate the relationship between development
intensity and the rate of channel change, to evaluate geologic and topographic controls on channel change, and to
determine if predevelopment conditions can be used to predict erosion-susceptible reaches. We used an 11-year data set
covering 21 urban and suburban channels in western Washington, draining watersheds from 0.1 to 20 km2, a range that
covers both seasonal and perennial channels that generally respond readily and rapidly to watershed disturbance. The
results indicate:
1. Rates of vertical channel change vary from below the range of measurement error (<20 mm vertical change
between visits) to about 1 m (width-averaged) per year. The median rate for this sample population was 60 mm per
year.
2. Within these lightly to moderately urbanized watersheds, rates of channel change did not correlate with
development intensity.
3. The nature of the geologic substrate strongly influenced whether or not significant channel change occurred, but no
unconsolidated substrate appears immune to change given sufficiently severe watershed disturbance. Erosion rates
are independent of channel gradient within the range investigated (0.013-0.52).
4. Channels with the greatest susceptibility to rapid vertical change share the following characteristics, which can be
used to predict and so reduce the consequences of future urban development on natural stream systems:
Erosion-susceptible geologic substrate;
Moderate to high gradient;
Absence of natural or artificial grade controls;
Predevelopment runoff inputs predominantly via subsurface discharge, likely to be converted to surface (point)
discharge in the post-development condition.
INTRODUCTION
Changes to channel morphology are among the most common and readily visible effects of urban development on
natural stream systems in humid environments. The actions of deforestation, channelization, and paving of the uplands
can produce tremendous changes in the delivery of water and sediment into the channel network. In channel reaches that
are alluvial, subsequent responses can be rapid, dramatic, and readily documented. Channels widen, deepen, and in
extreme cases may incise many meters below the original level of their beds. Alternatively, they may fill with sediment
derived from farther upstream and braid into multiple rivulets threading between gravel bars. In either case, they become
aesthetic eyesores and biological invalids; natural populations of benthic invertebrates and fish are decimated, to be
replaced by limited numbers and taxa of disturbance-tolerant species.
This chapter reviews the current understanding of the behavior and physical changes reported from stream channels in
urban and urbanizing watersheds. Our focus is on those draining forested, or once-forested, landscapes in humid regions
because the subsequent channel changes there appear to be most dramatic and detrimental to physical, biological, and
aesthetic attributes. The historic data on channel changes is supplemented by our own 11-year data set on urban and
suburban channels in western Washington, draining watersheds from 0.1 to 20 km2, a range that covers both seasonal and
perennial channels that generally respond readily and rapidly to watershed disturbance.
1 D.B. Booth and P.C. Henshaw, 2001, Rates of channel erosion in small urban streams: chapter in M. Wigmosta and S. Burges,
eds., Land Use and Watersheds: Human Influence on Hydrology and Geomorphology in Urban and Forest Areas: AGU
Monograph Series, Water Science and Application Volume 2, pp. 17–38.
2
Previous Studies
Although urban-induced channel changes are widely recognized, their magnitudes, rates, and controls are largely
matters of sparse data and anecdotal information. As a result, we have only limited understanding of the physical
determinants of channel change and even less predictive ability of the likely consequences of urban development on
downstream channels. Although a variety of papers have explored the phenomenon of channel change in urban
environments, they do not establish a consistent framework because they have analyzed the process from multiple
perspectives:
Watershed hydrology, where the most detailed analysis is conducted on the hydrologic changes brought by urbanization,
and any channel response is shown or presumed to follow those changes directly;
Sediment loading, where the delivery of sediment into the channel from an urban or urbanizing watershed is analyzed
most completely, and the observed channel response is explained in whole or part through the trends of that sediment
delivery over time; or
Conceptual models of channel erosion and sedimentation, where a sequence of channel responses, initiated by
watershed change, follows a predictable path somewhat independent of the details of the initiating upstream activity.
Watershed Hydrology. Changes in the hydrologic response of an urban watershed, notably the increase in stream-flow
discharges, are demonstrably the clearest single determinant of urban channel change. Even where the channel is
physically isolated from both physical disturbance and new inputs of coarse sediment, the occurrence and magnitude of
increased discharges generally are mirrored by observed increases in channel dimensions. Previous studies that present
such relationships include Hammer [1972], Hollis and Luckett [1976], Morisawa and LaFlure [1982], Neller [1988],
Whitlow and Gregory [1989], Moscrip and Montgomery [1997], and Booth and Jackson [1997]. Yet this relationship,
although common and intuitive, is not universal. A few studies note a reduction in channel width or depth with increases
in watershed urbanization and, presumably, the discharge that accompanies it [e.g., Leopold, 1973; Nanson and Young,
1981; Ebisemiju, 1989a; Odemerho, 1992].
Sediment Loading. Delivery of sediment into the channel network is a common consequence of urban development with
potentially significant expression in the channel form. The broad relationship between stages of watershed development
and resulting sediment loads have been presented in studies such as Wolman [1967], Graf [1975], and Douglas [1985;
Table 1, below]. Increased sediment loads, generated at particular stages in the forest-agriculture-urban sequence of North
American land development, exert an opposing tendency on the channel to that of increasing discharge and probably
explain much of the channel narrowing or shallowing that is sometimes measured.
TABLE 1. Conceptual relationship between stages of development, sediment yield, and channel stability [from Douglas, 1985].
“Stage” Land Use Lag time Sediment Yield Channel Stability
A Natural forest or
grassland 100 years low Relatively stable with some
bank erosion
B Heavily grazed 80 low to moderate Somewhat less stable
C Cropping 75 moderate to heavy Some aggradation and
increased bank erosion
D
Abandonment of
cropping;
permanent grass
85 low to moderate Increased stability
E Urban construction 40 very heavy Rapid aggradation and some
bank erosion (can braid)
F Stabilization 25 moderate Degradation and severe
bank erosion
G Stable urban 15 low to moderate Relatively stable
Conceptual Models of Channel Erosion and Sedimentation. Previous efforts to integrate the generally similar, but
locally disparate, observations of channel change [see Park 1997] into a unified model generally articulate a sequence of
anticipated changes over time. For example, Douglas [1985] suggested a specific pattern of watershed development and
channel response (Table 1).
3
Simon [1989] evaluated the consequences of channelization and described a predictable evolutionary sequence of
undercutting, bank failure, channel widening, and restabilization that closely resembles that of urbanization. Arnold et al.
[1982] also recognized the interplay of spatial factors, notably upstream stream erosion and downstream deposition, that
can result in multiple “responses” along the same channel, a theme of complex spatial and temporal response echoed by
Gregory et al. [1992] and Park [1997].
Lessons from Previous Work
In any given locality, observed correlations between channel size, rate of channel change, and watershed characteristics
are likely to be fortuitous or at least non-universal. In general, the dimensions of channels in an urban stream network will
tend to follow the overall pattern of discharge across that network—larger flows beget larger channels. Consequently, a
naïve prediction of channel change based on the magnitude of anticipated hydrologic change [Booth, 1991] is also
probably justified as a first-order estimate (e.g., Figure 1 as an example from the Pacific Northwest). Yet details of the
channel, the watershed, and the timing and location of the measurement itself may overwhelm this presumption of
channel-discharge equilibrium for the following reasons:
Location of the measurement station in the channel network: Is the measurement located in a “transport” reach,
where water and sediment are passed downstream with little channel adjustment, or a “response” reach, where
channel form readily adjusts to changing conditions? Not every channel responds to increasing sediment load or
water discharge in the same way [e.g., Montgomery and Buffington, 1997]. Local channel gradient and the pattern of
gradient changes across a channel network are particularly important factors, but they are rarely reported or
incorporated into case-study analyses.
Location of urban development relative to the channel network: This includes the obvious factors that headwater
development will affect more of the channel network than one that drains into the stream farther downstream, and that
the influence of a particular area of disturbance will be proportionally greater on progressively smaller catchments.
Similarly, developments that concentrate urban effects in only a few areas tend to have less impact on the channel
network as a whole than equivalent development spread across the watershed [Ebisemiju, 1989b]. In addition, flow
increases introduced at one point in the channel network may be far more effective at eroding sediment than at
another, because of the spatial variability of watershed soils and the distribution of alluvial and bedrock (or other non-
alluvial) reaches.
Interplay of the timing of watershed development, large storms, and stream-channel observations: Many of the
“relationships” advocated in the literature between channel form and the magnitude and age of the watershed
development are probably artifacts of a particular combination of unique temporal or geomorphic factors [Henshaw,
1999]. “Stable” stream channels may simply reflect a lack of recent rainfall [e.g., Bird, 1982]. They are expected in
mature systems where fluvial equilibrium has truly been reestablished (as anticipated for example by Hammer 1972,
Neller 1988, Ebisemiju 1989b]; but alternatively, they may simply be the product of flushing all mobile sediment
from the system to produce a relatively static, non-alluvial channel, where change may still occur [e.g. Tinkler and
Parish, 1998] but at rates generally slower than reported elsewhere. If equilibrium can be achieved in a disturbed
fluvial system, it will depend not only on the at-a-station fluvial processes but also on factors outside of, and perhaps
wholly unaccounted by, fluvial conditions in the immediate channel reach. These factors include adjacent hillslope
stability, which may have a dramatically longer time scale for stabilization than the fluvial system; and channel
stability farther upstream, particularly the absence of large upstream sediment sources.
Most previous studies have reported examples of particularly dramatic channel changes. These sites are commonly
erosional, because this response is generally more rapid and more localized than deposition, and because the occurrence of
channel erosion (particularly downcutting, commonly the first such fluvial response) can initiate adjacent hillslope failures
that mobilize substantially more sediment over a wider area than the original fluvial process. Thus they call attention to
themselves from researchers and the public alike. That attention is entirely appropriate—such changes are among the most
serious environmental disruptions for both human and biological use of streams in the urban environment. However, such
a level of attention introduces a bias into our assessment of what constitutes “urban channel changes.”
4
Figure 1. Bankfull channel widths, segregated by percent effective impervious area (EIA; see Dinicola, 1990)
contributing to the measurement point. A discrimination at 6 percent EIA was chosen because it approximates
the rural-to-suburban transitional land use in this region. From Booth and Jackson [1997].
Channel Types and Classification
Principles and Limitations. Geomorphologists and biologists have been organizing and categorizing the myriad array of
stream channels for about a century. The purpose of such an organization is fundamental: if a channel of interest can be
placed in a group, and the properties of that group are already known, then the properties of the new channel will also be
known with little additional work [Kondolf, 1995]. Those “properties” depend on the organizational scheme, but they
include such attributes as the channel’s response to environmental change (such as increased sediment load or placement
of an artificial habitat-enhancement structure) or its importance in supporting stream biota [Mosley, 1987]. Intrinsic
differences between channels will strongly influence channel response to urban development.
Yet the influence of a classification scheme can be detrimental, by suggesting an overly simplistic range of channel
conditions that obscures critical differences between channels that are ostensibly “the same.” It may also impart a false
understanding if the classification method is taken outside of where it was developed to where the dominant landscape
processes, or range of landscape conditions, are significantly different: channels may be “classified” but the predictive
power of that classification will be low or misleading.
Two examples, both relevant to urban stream channels of the Pacific Northwest, illustrate this problem. The
classification method of Rosgen (e.g., 1994 and prior informal publications), applied widely throughout the United States,
does not include the influence of large logs and other woody debris on channel processes, reflecting the non-forested
environment in which this method was first developed. A forested stream may be “classified” by this method but the
nature of its response to human disturbance may be poorly predicted. In contrast, the classification of Montgomery and
Buffington [1997] was established explicitly to address the channels found in forested watersheds of the Pacific
Northwest, where such large woody debris (LWD) is ubiquitous and its influence can be dominant. Yet this method was
developed in mountain drainage basins sharing a typical downstream progression from steep headwater catchments
underlain by bedrock to gentler, larger watershed areas in broad alluvial valleys. This orderly sequence may not be
matched in a lowland setting—the smallest watersheds of urbanizing Puget Sound can be quite flat, with steeper reaches
located some distance farther downstream. Sediment-delivery processes and sources of channel roughness are very
different in lowland urban channels than in nearby mountainous channels, and so this classification system also may not
fully predict the response of a particular urban stream.
Criteria. Despite these caveats, different channel “types” display different intrinsic channel behaviors and have
different responses to watershed disturbance. No framework has been fully developed for our environment of interest, but
we are using the conceptual approach of Montgomery and Buffington [1997] because of its orientation on channel-forming
processes, its development in the same climatic region as the present study, and its explicit recognition of the influence of
LWD and other such obstructions on channel morphology. By their terminology, most of our channels are either “plane-
bed” or “forced pool-riffle” channels—relatively flat-bottomed channels lacking well-defined bedforms and instead
displaying long, and commonly channel-wide, reaches of uniform riffles or glides which can aggrade or degrade rapidly in
response to changing water and sediment fluxes. Development of a more heterogeneous morphology depends on the
presence of immobile material, most commonly LWD. By restricting our evaluation to such channels, we may be limiting
the potential utility of our work. Yet the vast majority of the small, responsive, urban streams in our region fit these
categories, and this selectivity helps avoid the risk of transferring results inappropriately.
5
A Conceptual Framework to Assess Channel Change in Urban Watersheds
Past studies and repeated observation suggest a “typical” scenario for channel change in an urbanizing lowland
watershed. Recognizing that this scenario does not encompass the full range of potential watershed conditions or stream-
channel responses, it nonetheless characterizes the most common “problems” of urban channel change and highlights those
settings where an unexpected response suggests the presence of atypical channel or watershed conditions.
Consider a watershed of some tens of hectares up to several square kilometers, where development has blanketed the
upper watershed and so the first-order channel(s) are the most fully affected of any in the channel network. In most cases,
channel expansion of at least several times the original cross sectional area accompanies the progression from rural to
suburban to urban land uses. Whether or not the response of the channel to these flow increases is “orderly” (i.e. channel-
size increases in approximate proportion to discharge increases in the sense of Booth, 1990) or “catastrophic” (i.e. rapid
incision) is largely independent of the magnitude of the watershed disturbance (see below). Even low levels of land-cover
changes, if accompanied by an efficient collection system (e.g., road ditches) can produce significant increases in
headwater channel discharges, which in turn will initiate increased in-channel erosion and sediment transport.
Because such land-use changes typically occur over a period of many years or decades, they tend to produce continuous
changes in the downstream channel subject only to the variability of seasonal runoff. Any tendency towards “equilibrium,”
either dynamic or static, is completely obscured during this period. Sparse long-term data suggest that true equilibrium
may be possible in watersheds with constant land use, over a years-to-decades time lag [Henshaw, 1999], but actually
observing such a condition depends on achieving stable hillslope conditions as well, which may take many times longer.
With these complications, it is not surprising that “reequilibration” may be more useful as a theoretical construct than as a
widely observed condition.
The sediment released by this scenario of headwater flow increases may or may not accumulate as it passes through the
downstream channel network. The potential input of additional urban-flow-induced sediment from other lateral tributaries
will combine to influence whether sediment, eroded from upstream reaches, can remain in active transport or will
accumulate in noteworthy volumes. Curiously, the vagaries of human infrastructure, particularly small roadway culverts
that were sized and installed during an earlier pre-headwater-development era when only lower discharges of water (and
tremendously lower discharges of sediment) occurred, appear to be one of the strongest single determinants of whether the
urban channel change is perceived to be mainly a problem of “erosion” or one of “deposition” (Figure 2).
Figure 2. Deposition of stream-channel sediment, eroded from upslope reaches of tributary 0143G (station C8).
6
FIELD INVESTIGATIONS
No single study can cover all settings in which urban-induced channel change is observed. Yet even a geographically
limited set of new data can increase our understanding and prediction of this threat to aquatic-system integrity. This study
was initiated to provide some of that new data, focused on a part of western Washington state where (and beginning at a
time when) the rate of new urban development was accelerating to historically unprecedented rates. It also began when the
social and political desire to alleviate the worst environmental consequences of that development far exceeded the concrete
knowledge necessary to achieve that goal.
Starting in 1986, 35 stations along an equal number of independent streams were established to monitor long-term
channel changes in urbanizing watersheds. The purpose of this effort was four-fold:
1. To document erosion and deposition rates in a variety of physiographic settings;
2. To test the hypothesis that urban development consistently increases the rate of channel change, and that higher
levels of urban development are correlated with faster rates of channel change;
3. To test the hypothesis that certain geologic and/or topographic settings are particularly susceptible to urban-
induced channel changes; and
4. To improve identification of the most susceptible sites before development, and thus before degradation, has
begun.
Methods
Study sites. The choice of channel reaches for monitoring began in early 1986, following a particularly large storm in
January that resulted in many instances of channel modification and property damage from high discharges. These first
sites were chosen because of known stream-channel erosion, reported downstream problems, or knowledge of impending
development that might prove problematic. Over the next several years, a number of additional sites were identified and
some unsuitable sites were relocated or abandoned, mainly due to unrepresentative channel conditions but also because of
subsequent obliteration by development activity. A range of channel conditions, particularly slope, degree of upstream
development, and geographic location, were covered by the final set of 21 selected sites (see Figures 3 and 4 and Table 2).
Previous observations had suggested that channel changes were particularly rapid downstream of recent urban
development in small headwater catchments and in channels traversing hillslope deposits of a specific regionally common
geologic deposit, so these characteristics were emphasized in the initial site selection.
7
TABLE 2. Stream erosion station characteristics.
Station
Station Name
Drainage
Area
(km2)
Local
Slope
%
%
EIA1
Channel
Width
(m)
Substrate
Type
Length
of
Record
(yrs)
C1 McAleer Creek 18 2 22 9.8 silt-clay 7
C2 McAleer
tributary 0.6 2 20 4.1 silt-clay 11
C3 Holmes Point
trib. 0.5 5 16 2.4 sand 4
C4 Juanita Point
trib. 0.4 4 12 1.7 sand 11
C6 Skookum
tributary 1.7 2 5 2.5 sand 9
C7 Timberline trib.
0143F 0.2 14 1 2.3 sand 7
C8 Timberline trib.
0143G 0.1 48 15 2.3 sand 11
C9 upper 0164A 0.5 3 16 4.9 sand 7
C11 Ginger Creek 1.8 5 22 7.9 sand 5
C12 mid-Madsen
Creek 6.5 3.5 10 6.6 sand 10
C13 Hollyw'd Hills
trib. 4 2 7 4.6 sand 7
G2 Garrison Creek 2.6 5 10 1.8 sand 2
G3 Mill Creek 4 2.8 19 4.3 sand 3
G5 Cobble Creek 0.7 5.5 11 4.8 silt-clay 11
PS3 Easter Lake
outlet 0.6 1.3 40 3.7 sand 11
PS4 Olympic View
Park 1.5 2.1 17 2.4 sand 11
PS7 Boeing Creek 5 2 20 8 sand 2
S2 Chasm Creek
mainstem 0.2 5 3 2.4 sand 4
S3 Pepper Creek 1 5 5 6.6 sand 7
S4 Lk. Alice Estates
trib. 0.1 52 10 2.6 sand 11
S5 Joule short plat
trib. 0.1 5 5 2.4 sand 4
1EIA = Effective impervious area
Most of the sites were, broadly speaking, alluvial channels [Leopold et al., 1964]: carved by running water into the very
sediment carried by that flow in the past, and that presumably could be carried by that flow in the future. These "self-
formed" channels are free to adjust their shape in response to subsequent changes in flow and thus were anticipated to
respond most sensitively to future development. However, as the channel changes in response to increased flows (and
particularly if it begins to incise) the underlying hillslope deposit becomes more dominant as the channel-bounding
sediment and the alluvial “character” of the channel can be reduced. In contrast, a channel formed in alluvial sediment but
also choked with immovable roughness elements, such as logs, is not strictly “alluvial.” Yet if those logs are removed, or
if progressive bed erosion strands those logs above the elevation of the flow, the channel will become more
characteristically alluvial over time.
The sample population was chosen to explore the influence of the underlying geology by emphasizing sites located on a
particularly erodible substrate. Most of the stations have as their underlying substrate a thick and widespread sandy
deposit with local concentrations of pebble to cobble gravel, laid down by glacial outwash streams during the last advance
of the continental ice sheet (regionally named the “Vashon” by Armstrong and others [1965] and spanning an interval of
8
about 17,000-13,000 years ago). This emphasis was established to quantify rates in what previously had been observed
locally to be the most erosion-susceptible deposit. A moderate number of sites with other substrates were also included to
test this hypothesis more precisely.
Figure 3. Location of stream measurement stations listed in Table 2.
Seattle
C2
C1
C3
C4
C6
C13
C8
C7
C9
C12
C11
S3
S2
S5S4
PS3
PS4
PS2
PS1
G2
G3
G5
10 mi
20 km
122 15’
o 122 00’
o
P
u
g
e
t So
und
Ta co m a
Green Ri v er
C
e
d
ar River
Sno
q
u
a
l
m
i
e
River
47 45’
o
47 30’
o
47 15’
o
1
10
100
0.1 1 10 100
Drainage Area (square km)
Lo
c
a
l
C
h
a
n
n
e
l
G
r
a
d
i
e
n
t
(
p
e
r
c
e
n
t
)
.
High Urban (EIA > 20%)
Medium Urban (20% > EIA > 6%)
Low Urban (6% > EIA)
Figure 4. Summary attributes of measurement stations listed in Table 2.
9
The work was initially sponsored by the jurisdiction of King County, Washington, so all sites were located within its
boundaries. By virtue of its location and size, however, this 5000-square-kilometer area cuts a remarkably diverse and
representative swath across the Puget Lowland and Cascade Range of western Washington; it also spans a range of land
uses from forested wilderness through agriculture, suburbia, and intensely urban. In the rapidly developing suburban
fringe targeted by this study, annual precipitation averages about 1000 mm, falling primarily from October to April as
large frontal storms of several days’ duration. Unlike much of the rest of the continent, short but intense storms are rare.
The 100-year 6-hour rainfall intensity in this region is only about 5 mm/hr [Miller et al., 1973], whereas the largest stream
discharges are generally associated with moderate-intensity rainfall following a period of extended wintertime
precipitation or snowmelt.
Measurement Techniques. Cross sections were measured using two procedures, modified from that suggested in Dunne
and Leopold [1978]. A specific location along a relatively straight and uniform part of the channel, qualitatively judged to
be “representative” of the reach in question, was selected. Two endpoints were established to define a line approximately
perpendicular to the channel. Where available, streamside trees were used and the precise endpoint was marked with a
12d galvanized nail driven nearly flush with the trunk. Where trees were unavailable, 0.6-meter-long pieces of reinforcing
bar (“1/2-inch rebar”) were driven into the ground to provide a suitable monument. We rarely had much difficulty in
relocating such markers, even after four years’ absence, with sufficiently detailed notes.
In the early years of this study, a 50-m steel tape was stretched and held at constant tension between endpoints to
provide both horizontal and vertical reference. The vertical distance between the tape and the ground surface was recorded
at 0.3-m intervals, together with additional intermediate measurements at marked breaks in slope. At each station, the
vertical angle and the parabolic sag of the tape were measured and used to correct the raw data. This method had the
advantage of speed and minimal field equipment but had some inherent inaccuracies. In the last year of the study we used
an automatic level and surveyor’s rod to determine channel depths, relying on the stretched tape only to specify horizontal
distance. By direct comparison of these two measurement methods, we could determine the precision of the early method;
its error was consistently less than 0.02 m, at or below the degree of inescapable measurement imprecision imposed by
ground irregularities and sediment clasts on the channel bed.
In the early years of the project, measurements at most sites were made annually in the summer, the season when
qualitative observations suggest that little or no channel changes occur from about June until October. After collecting
data in the summer of 1990 from changes during the large storms of January 1990, the primary objectives of the study had
been achieved and measurement frequency was reduced, with visits to most sites only in 1993 and 1997.
Results
Overview. Rates of erosion and deposition vary by over two orders of magnitude (see Figure 5). In this population, the
minimum amount of the annual width-averaged vertical channel change was below the level of measurement error (about
20 mm); the maximum was about 1 m per year. Over the 11-year period, 80 percent of all measurements show an annual
width-averaged vertical change (erosion or deposition) of less than 0.2 meters, with the median of all measurements at 60
mm/year.
-3
-2
-1
0
1
2
3
4
0246810
CHANNEL WIDTH (m)
Ch
a
n
n
e
l
E
r
o
s
i
o
n
(
s
q
u
a
r
e
m
)
.
0.06 m incision
0.06 m aggradation
Figure 5. Results of all measurements, expressed as the average change in cross-section area per year between visits. Dotted lines plot
the median vertical change of 0.06 m.
10
The most consistent pattern is the correlation of rainfall with channel change. This outcome is qualitatively intuitive,
although the nature of this relationship is more complex than might be first anticipated. For example, 1990 channel
changes (i.e. occurring between the 1989 and 1990 measurements) are the largest, by a significant degree, at nearly all
sites (Figure 6). Although the 1990 rainfall intensities are also the largest in the period as well (Figure 7), they do not
exceed other “large” years (1991 and 1996) by nearly as much as the erosion/deposition measurements would suggest.
0
10
20
30
40
50
60
70
%
o
f
St
a
t
i
o
n
s
.
1987-
1988
1988-
1989
1989-
1990
1990-
1993
1993-
1997
0-0.06 m
0.06-0.25 m
> 0.25 m
Figure 6. Distribution of stations below, above, and greatly above the median vertical change of 0.06 m/year. Channel changes are
calculated across the full period between measurements and are not annual averages. Note that the distribution of stations in the 1989-
1990 interval is nearly identical to that of 1993-1997, even thought the earlier period is only one-quarter as long.
0
10
20
30
40
50
60
70
80
90
1988 1989 1990 1991 1992 1993 1994 1995 1996
Water Year
Ma
x
i
m
u
m
2
4
-
H
o
u
r
R
a
i
n
f
a
l
l
(
m
m
)
.
Figure 7. Maximum 24-hour rainfall for each year of the interval 1988-1996. The chosen gauge (“Maplewood”) was centrally located
for the study sites (near station C11 on Figure 3).
11
Most noteworthy of this data set, however, is the overall absence of general relationships between measured channel
changes and simple, physical parameters of the stream or of the watershed, such as slope (Figure 8) or imperviousness
(Figure 9). This condition bodes poorly for the kinds of simple predictive methods favored by local governmental
jurisdictions in the prediction and avoidance of environmental impacts. Only the role of geologic materials shows any
consistency, with cohesive silt-clay substrates generally permitting only low rates of channel adjustment.
The poor correlation between effective impervious area (EIA) and channel change is quite robust. It is displayed by
both the station averages for the period of record (Figure 9) and the single-year (1989-1990) data (Figure 10). We
therefore reject the first of our initial hypotheses, that urban development consistently increases the rate of channel change,
and that higher levels of urban development are correlated with faster rates of channel change.
Evidence against this hypothesis is particularly clear at the following stations, discussed below in greater detail:
Moderate to high development, moderate to minimal changes:
Easter Lake outlet (40% EIA, moderate change)
Olympic View Park (17% EIA, very little change)
McAleer Tributary (20% EIA, very little change)
Little development, large changes:
Pepper Creek (3% EIA, very large change)
Although hydrologic processes may impose a general tendency for increased urbanization to yield greater channel change,
the expression of that change is completely swamped by the vagaries of local conditions.
Our other initial hypothesis, the association of particular topographic or geologic conditions with rate of change, finds
much more consistent support from the data. Granular hillslope deposits, normally mantled by alluvium but accessible to
streamflow in an incising environment, were anticipated to display the greatest changes for a given degree of upstream
urbanization; indeed, a majority of sites were chosen on the basis of this very attribute. The type of deposit does appear to
exert a significant influence on channel-change rates; the most common alternative, cohesive silt-clay deposits,
consistently showed low or very low rates of change.
0.01
0.10
1.00
110100
Channel Slope (%)
In
c
i
s
i
o
n
o
r
A
g
g
r
a
d
a
t
i
o
n
,
1
9
8
9
-
1
9
9
0
(
m
)
.
Sandy Deposits
Silt-clay Deposits
Figure 8. Demonstration of the poor correlation between local channel slope and the magnitude of one year’s width-averaged vertical
change. A more consistent pattern is suggested by the relatively low change shown at each of the three stations underlain by cohesive
(silt and clay) hillslope deposits.
12
0.0
0.1
0.2
0.3
0.4
0.5
0 1020304050
Effective Impervious Area Percentage in Watershed
Av
e
r
a
g
e
A
n
n
u
a
l
I
n
c
i
s
i
o
n
o
r
D
e
p
o
s
i
t
i
o
n
.
Sandy Deposits
Silt-clay Deposits
(m
e
t
e
r
s
)
Figure 9. Demonstration of the poor correlation between contributing impervious area and the magnitude of the annual width-averaged
vertical change, averaged over the full duration of each station’s measurement history.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0 1020304050
Effective Impervious Area Percentage in Watershed
A
v
e
r
a
g
e
I
n
c
i
s
i
o
n
o
r
D
e
p
o
s
i
t
i
o
n
.
(
m
e
t
e
r
s
)
Sandy deposits
Silt-clay deposits
Pepper Ck.
Easter Lk. Outlet
McAleer trib.
Olympic View Pk.
(0143G)
(0143F)
Figure 10. The equivalent parameters as for Figure 9 but considering only the measurement interval with the greatest change, 1989-1990.
There are no appreciable differences between the pattern expressed by either the single-year or the time-averaged results. Labeled
points are discussed in the following section.
13
Specific Site Conditions (see Figure 10)
Olympic View Park. This channel is located in a lightly developed parkland (Figure 11), established around the long-
protected riparian corridor of the stream. The surrounding watershed has been almost fully developed for several decades,
primarily with single-family residences. The ravine that contains the channel and associated park is excavated into
deposits of the sandy Vashon advance outwash. Incision has clearly been part of the channel’s past history; several
hundred meters downstream of the measured cross section, large gabion baskets stabilize what must have been a major
knickpoint in the 1970’s. Yet the current decade of measurements is noteworthy in its near-negligible change from one
year to the next (Figure 12), although the channel morphology is distinctly unappealing from either a biological or an
aesthetic standpoint. It is relatively uniform, slightly sinuous, with virtually no heterogeneity or variability in size, shape,
or roughness. Much of this uniformity is surely the result of close human contact—foot traffic up and down the channel
(commonly dry in the summertime) is frequent, and any sticks or twigs would be promptly “cleaned up.” Yet even where
encroaching riparian shrubs limit the immediate access of people, conditions are essentially unchanged.
Figure 11. View of the channel of Olympic View Park (Station PS4) in 1986, looking upstream. Line across the channel (below the scale
bar) is the tape measure used to determine horizontal distance at the cross section location.
0
0.5
1
1.5
2
2.5
3
3.5
4
1234567
Distance from Left Bank Datum (m)
El
e
v
a
t
i
o
n
(
m
)
.
1997
1993
1990
1989
1987
Figure 12. All cross section measurement of Station PS4, showing only minor changes from 1987 to 1993 (the total width-averaged
erosion during this interval is 8 cm) and from 1993 to 1997 (total average erosion 7 cm).
14
Easter Lake Outlet. This channel is also in a well-established part of King County, with a high level of urban
development in the contributing watershed but with most of it predating 1970. About 80 percent of the runoff from that
watershed drains through Easter Lake, which occupies about 10 percent of the contributing surface area of the watershed
and which provides significant hydraulic control of discharges. Continuous hydrologic modeling of this lake [King
County, 1990] displays a marked reduction in the unit-area discharges here relative to other catchments without lakes in
the immediate region.
The channel-measurement station lies within a reasonably well-protected riparian buffer, located entirely on private
property and generally not accessed by nearby residents because of topography (Figure 13). The channel is incised about
1.5 m into a narrow upland terrace, probably the old floodplain, set within a broader valley. A complex of logs and a large
tree root, about 20 m downstream of the section, have formed the lip of a 1-m knickpoint and probably have inhibited
more dramatic degradation.
3 m
Figure 13. View of the channel of the Easter Lake outlet (Station PS3) in 1986, looking upstream.
15
This site displays the interplay of fluvial and hillslope processes and it also demonstrates why cross-sectional
measurements can be a very incomplete characterization of stream-channel conditions. Net change at this site has not been
dramatic, but bed scour has clearly oscillated with bank failure on a multi-year scale (Figure 14). For example, the steep
high right bank remaining after the 1986 storms degraded over the next several years, with progressive bank collapse and
channel widening contributing sediment to an aggrading channel bed. Renewed scour during the large flows of January
1990 lowered the bed by nearly 0.5 meters in the active part of the channel, and additional sediment was flushed out
through at least 1993. The 1993-1997 interval was a period of substantial channel widening; but in contrast to many other
stations during this time, channel deepening did not occur, and in fact aggradation was substantial.
Changes in channel cross section were accompanied by a progressive simplification of in-stream morphological features.
Over the 11 years of observations this stream has become more like a drainage ditch, with a marked lack of heterogeneity
in either sediment or bedforms. This has not required any direct human intervention; simplification of the channel, with
attendant loss of aesthetic and biological benefits, has occurred only through the indirect effects of watershed disturbance.
0
0.5
1
1.5
2
2.5
3
3.5
4
2345678
Distance from Left Bank Datum (m)
El
e
v
a
t
i
o
n
(
m
)
.
1997
1993
1990
1989
1988
1987
1986
Figure 14. All cross section measurement of Station PS3, showing episodic deepening in 1990 and 1993 but aggradation both before and
after. Widening has been progressive but particularly during the interval 1986-1988.
16
McAleer Tributary. The third example of a relatively highly impervious, minimally changing channel is a small stream
which, for a 200-m-reach, is surrounded by a surprisingly intact riparian buffer and wetland system. It is isolated from
most human traffic by private property and distance from the adjacent county road. This part of the valley has been eroded
into resistant silt and clay deposits, which are nowhere visible in the stream bed itself but do lie within a few decimeters of
the ground surface and which contribute to the generally silty cohesive banks.
Systematic change at this station over the past decade has been minimal (Figure 15). The channel has aggraded slightly,
and the channel banks have become more rounded and have retreated a few tenths of a meter. The clearest interval of
change was between 1990 and 1993, when two threads of the flow evolved into a more broadly flowing single channel.
Some textural changes in the bed sediment have been noticed over the years, with areas of gravel riffle in one visit
becoming patches of silty sand in the next, but no long-term trends in these changes are evident.
Pepper Creek. In contrast to the modest changes observed at the previous stations, this site has shown tremendously
variable conditions (Figure 16). It collects runoff from a watershed in the very earliest stages of urbanization; the major
hydrologic changes have been related to channelization and road-ditch interception of shallow subsurface flow [Burges et
al., 1989], whereas the total fraction of contributing imperviousness is still quite low. The channel is extremely well
protected from direct human intrusion, lying several hundred (very brushy) meters from the nearest structure or public road
and entirely on private property. It is eroded into sandy valley-bottom deposits, delivered by episodic landslides from the
surrounding hillsides and locally reworked by past fluvial action.
The likely magnitude of channel changes was first suggested by extensive deposition on the downstream alluvial fan of
the stream, beginning in about 1980 and coincident with the first extensive road construction and forest removal in recent
history. Following first measurements in 1986, two episodes of significant erosion were evident: 1986-1987 and 1989-
1990. Channel readjustment, but little net erosion/deposition, occurred in 1987-1988. Near-static conditions persisted
during the low-rainfall year of 1989. Substantial erosion continued following 1993, but landsliding off the hillside above
the right bank, probably in 1996, completely obliterated the measurement station.
0
0.5
1
1.5
2
2.5
3
3.5
4
0.5 1.5 2.5 3.5 4.5 5.5 6.5
Distance from Left Bank Datum (m)
El
e
v
a
t
i
o
n
(
m
)
.
1997
1993
1990
1989
1987
Figure 15. All cross section measurement of Station C2, one of the three sites underlain by cohesive deposits and showing almost no
change 1987-1997.
0
0.5
1
1.5
2
2.5
3
3.5
4
02468101214
Distance from Left Bank Datum (m)
El
e
v
a
t
i
o
n
(
m
)
.
1993
1990
1989
1988
1987
1986
Figure 16. All cross section measurement of Station S3, showing active change in every measurement interval. Vertical exaggeration 2:1
(note expanded horizontal scale).
17
Timberline Tributaries. In a topographic and geologic setting remarkably similar to that of Pepper Creek, these smaller
tributaries drain adjacent areas generally subjected to intensive urban development. A primary difference between them,
however, is that the upland storm-drain system bypasses one (0143F) and discharges into the other (0143G). The stability
of 0143F (Figure 17) throughout the measurement period demonstrates that there is nothing inherently unstable about these
channels, even where gradients are steep and deposits are erodible. Curiously, the discharge into 0143G was constructed
with some advance awareness of the susceptibility of the downstream channel to erosion: rather than simply releasing the
runoff from the end of a pipe, the discharges are first attenuated in a detention pond and then released at the head of a
swale through a 20-m-long level dispersion pipe. Despite these efforts, first documented in Booth [1989], channel incision
of more than one meter occurred in 1989-1990 (Figure 18). In contrast, the station without such flows remained virtually
unchanged throughout the period of its measurement.
The respective (in)stabilities of these two channels emphasize a fundamental point about streams in urbanizing
environments. Prior to watershed disturbance, “stability” (whether static or dynamic) is the norm. Although gradients are
steep and substrate is easily erodible, woody debris and other in-channel roughness elements maintain an overall balance
with the tractive stress of the steeply flowing water and the delivery of sediment from farther up in the watershed. If urban
development alters any element in this balance, however, the relative stability of the entire system can be lost with rapid
and sometimes catastrophic results.
0
0.5
1
1.5
2
2.5
3
3.5
4
-1012345
Distance from Left Bank Datum (m)
El
e
v
a
t
i
o
n
(
m
)
.
1993
1990
1989
1987
Figure 17. All cross section measurement of Station C9, the Timberline tributary that does not receive any appreciable runoff from the
stormwater system of the upslope development.
0
0.5
1
1.5
2
2.5
3
3.5
4
0123456
Distance from Left Bank Datum (m)
El
e
v
a
t
i
o
n
(
m
)
.
1997
1993
1990
1989
1987
Figure 18. All cross section measurement of Station C8, the Timberline tributary that does receive significant stormwater runoff from the
upslope development and which responded with abrupt downcutting in 1989-1990, and further incision by 1997,.
18
Rates of Channel Restabilization
In the final days of 1996, a major storm in the Seattle metropolitan area resulted in the failures of a road embankment
and adjacent berm of a regional stormwater detention facility. Massive quantities of water and sediment were flushed
down the North Fork and main stem of Boeing Creek, a stream draining several hundred hectares of primarily residential
land use, and filled the valley of that channel as deep as 2 m with deposited sediment. Following this deposition, the
channel of Boeing Creek began to reincise immediately, presenting an unexpected opportunity to document the
establishment of a channel where watershed land cover was essentially stable (and highly developed) and the sediment to
be eroded was very easily transportable by even modest stream discharges. Seven cross sections were established in the
300-m reach between the failed detention pond and Hidden Lake, an artificial pond that marks the downstream end of the
alluvial channel on the main stem of Boeing Creek. The normal protocol of annual measurements was replaced by
monthly, and in some instances weekly, field visits.
The recovery of the channel of Boeing Creek was anticipated to take a period of some years and to be controlled in part
by the reestablishment of floodplain vegetation to help bind and stabilize the very sandy fill that was deposited. Instead,
reestablishment of an apparently stable, “equilibrium” channel was very rapid and occurred at most cross sections in a
matter of a few months (Figures 19 and 20). The subsequent year’s measurements demonstrated that this stability was not
an artifact of the termination of rainfall in spring 1997; the 1998 channel, after another winter’s high discharges, remained
largely unchanged.
0 5 10 15 20
Distance from Left Bank Datum (m)
Ele
v
a
t
i
o
n
(
m
)
.
9/98
11/97
4/22/97
4/4/97
3/97
2/23/97
2/2/97
1/97
0
1
2
3
4
Figure 19. All measurements at Cross Section 4 of Boeing Creek, spanning 20 months since shortly after the site was obliterated by
failure of the upstream detention pond embankment. Within weeks the channel achieved a “metastable” form that persisted for about
two months (2/97-3/97); following several rainstorms in late March and early April 1997, channel form has remained nearly stable.
Vertical exaggeration 2.5:1.
10 m
Figure 20. View of the channel of Boeing Creek (Station PS7) at Cross Section 4 on September 9, 1998 (the last measurement plotted in
Figure 19), looking downstream.
19
Discussion
The factors anticipated to influence the annual rate of channel change are generally well represented by the results.
They include:
Abundant rainfall,
Easily erodible substrate, and Presence or absence of watershed urbanization.
These factors resist simple quantification, however, because of the tremendous variability imposed by the multiplicity of
local geologic conditions, channel type, downstream grade controls (natural or artificial), the location of a chosen site in
the context of the upstream channel network, and the variety of development ages and styles (e.g. residential density, or
sewered vs. unsewered). Hammer [1972] recognized the last of these complications, and he developed a complex
regression equation to express the observed relationship between different development types and channel dimensions.
Yet these results are not readily transferred anywhere else, and the ever-changing patterns and styles of development
render only the most general conclusions of lasting value.
The population of sample sites was neither varied enough nor large enough to allow a systematic evaluation of every
relevant condition. However, several useful observations can be drawn:
1. The average annual rate of change can increase in a single channel by as much as 2 orders of magnitude
between dry and wet years (e.g., the winter of 1989-1990, in this sample population); more typically, the greatest
interannual change is about 5-fold. Because the study years include some of the largest lowland storms in recent
memory as well as several quite unexceptional years, this variability is probably representative of most long-term
conditions. In virtually all cases, the rate of channel change returns to nearly equivalent pre-event levels within
one measurement interval (typically one or two years).
2. The previously recognized characteristics of erosion-susceptible channels are broadly correct—moderate to
steep slopes, susceptible geologic materials, and significant (and recent) upstream development. The unique
factors of any given catchment, however, can greatly influence these predictions. No unconsolidated substrate
appears immune from change, given sufficiently severe watershed disturbance. The streams draining large basins
are more resistant than those draining small ones. Steep slopes in and of themselves are not critical, but they may
increase the magnitude of the response to disturbance.
3. The age of the upstream development appears to be quite significant (as first recognized by Hammer, 1972)
but the reason for this influence is enigmatic [see Henshaw, 1999]. In general, channels draining established
neighborhoods [C1, C2, PS1, and PS4] show low rates of change. Possible explanations include (1)
reequilibration of channel dimensions and sediment size with the increased (but now stable) flow regime; (2)
removal of all erodible sediment from the channel perimeter, leaving non-erosive bed and banks; (3) cementation
of channel sediments, a ubiquitous condition at these sites; or (4) reestablishment of bank vegetation following
initial disruption of the channel by increased flows. Each of these explanations applies to certain sites, although
(1) and (2) appear to be the most significant in a majority of cases. The reestablishment of equilibrium, however,
does not necessarily coincide with a reestablishment of overall stream function or habitat quality: the channel
capable of resisting the frequent, flashy discharges that roll out of an urban catchment is generally inhospitable to
most aquatic organisms.
4. Results are most unpredictable in the smallest basins (those of a few tens of hectares). In these basins, even a
relatively small amount of development can have significant downstream effects if flow concentration occurs as a
result of ditches or road crossings [S4, S5]. These effects, however, are not well represented by traditional
methods of characterizing urbanization, such as impervious-area or disturbed-area percentages.
5. Any potential influence of channel slope is not well displayed by this data set (see Figure 8). All channels
here have slopes of at least 1.3 percent, so true low-slope channels (also correlating, typically, with larger
channels) are not represented. Within this data set, the only apparent relationship is that the very largest changes
(>0.3 m/yr) appear to require a steep slope (4 percent or greater). The experiences from this study also suggest several cautionary notes for future long-term channel monitoring. Most
importantly, single-site measurements do not reliably characterize the overall status of the channel. Conditions both
upstream and downstream of the selected site can be very different from those at the measured station. For example,
delivery of sediment from an unstable upstream source can completely obscure the local behavior of the measured section.
The headward migration of a downstream knickpoint, one obvious (and common) manifestation of an unstable stream
reach [Booth, 1990], can cause dramatic changes in the bed elevation after a long period of apparent channel stability as
measured at a single monitoring site.
Furthermore, the “stability” of a channel, as measured by the absence of change at single cross sections, does not
necessarily equate with other desirable conditions, such as high-quality aquatic habitat. The converse statement, however,
is generally correct: instability does correlate well with low habitat quality. Therefore, evaluating only channel stability
20
does not provide unequivocal information on habitat conditions; if that information is needed, additional measurements are
required.
SUMMARY
This study was begun with four objectives: to document rates of channel change, to evaluate the relationship between
development intensity and the rate of channel change, to evaluate geologic and topographic controls on channel change,
and to determine if predevelopment conditions could be used to predict susceptible reaches. Our results indicate the
following conclusions:
1. Rates of vertical channel change vary from below the range of measurement error (<20 mm vertical change
between visits) to about 1 m (width-averaged) per year. The median rate for this sample population was 60 mm per
year.
2. Within these lightly to moderately urbanized watersheds, rates of channel change did not correlate with
development intensity.
3. The nature of the geologic substrate strongly influenced whether or not significant channel change occurred.
Other likely influences included local downstream grade control, riparian vegetation, and the age of the upstream
development. Gradient was not a significant factor across the range of channels measured (0.013-0.52), but the
importance of local grade controls suggest that low-gradient channels may show lower rates of change for a given
level of disturbance and geologic susceptibility.
4. Channels with the greatest susceptibility share the following characteristics, which could be used to reduce the
consequences of future urban development on natural stream systems:
Erosion-susceptible geologic substrate
Moderate to high gradient
Absence of natural or artificial grade controls
Water inputs via predominantly subsurface discharge, likely to be converted to surface (point) discharge in
the post-development condition.
MANAGEMENT IMPLICATIONS
These results imply several consequences for watershed management. First, urban development is an obvious force in
channel change, yet not all channels respond equivalently. The locations of potential susceptibility can be determined well
in advance, at least in the Puget Lowlands of Washington, based on geologic conditions. Finally, channel changes, if and
when they do occur, can happen so rapidly after development begins that remediation, to be effective, must occur prior to
development.
The results of this study also suggest that channel changes are very responsive to varying rainfall. This source of
variability is obviously beyond the ability of surface-water or land-development agencies to control, yet its effects can be
as significant as those of urban development. The most extreme effects of high rainfall are felt in the urbanized channels,
and so one result of large storms is to amplify the differential response of developed and undeveloped watersheds. This
imposes a challenging task for watershed managers: during low-rainfall years, any “warning” of impending channel-
erosion disaster is muted, along with the public's concern for such issues. When a large storm arrives, however, the
magnitude of channel change in urbanizing watersheds can cause significant damage, and its consequences invariably
surprise almost everyone.
Acknowledgements. We are indebted to the associates and assistants who have helped with project design, site selection,
and field assistance over the years, particularly Laura Casey, Kelin Whipple, Adelaide Johnson, John Buffington, Fred
Bentler, Arny Stonkus, Karen Comings, and Marit Larson. Joan Blainey was instrumental in establishing the initial sites
and making the first-year measurements on Boeing Creek. Partial support for the successful conclusion of this project was
provided by the U. S. Environmental Protection Agency’s Waters and Watersheds Program Grant no. R825284-01-0, and
the Center for Urban Water Resources Management at the University of Washington. The manuscript was greatly
improved from careful reviews by Ellen Wohl, Jim Pizzutto, and Kelin Whipple.
21
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southeastern Australia, Journal of Hydrology, 52, 239-255, 1981.
Neller, R. J., A comparison of channel erosion in small urban and rural catchments, Armidale, New South Wales, Earth Surface
Processes and Landforms, 13, 1-7, 1988.
Odemerho, F. O., Limited downstream response of stream channel size to urbanization in a humid tropical basin, Professional
Geographer, 44, 332-339, 1992.
Park, C. C., Channel cross-sectional change, in Changing River Channels, edited by A. Gurnell and G. Petts, John Wiley and Sons,
Chichester, 117-145, 1997.
Rosgen, D. L., A classification of natural rivers, Catena, 22, 169-199, 1994.
Simon, A., A model of channel response in disturbed alluvial channels, Earth Surface Processes and Landforms, 14, 11-26, 1989.
Tinkler, K. J., and Parish, J., Recent adjustments to the long profile of Cooksville Creek, and urbanized bedrock channel in Mississauga,
Ontario, in Rivers Over Rock: Fluvial Processes in Bedrock Channels, edited by K. J. Tinkler and E. E. Wohl, American Geophysical
22
Union, Monograph 107, Washington, DC, 167-187, 1998.
Whitlow, J. R., and Gregory, K. J., Changes in urban stream channels in Zimbabwe, Regulated Rivers: Research and Management, 4,
27-42, 1989.
Wolman, M. G., A cycle of sedimentation and erosion in urban river channels, Geografiska Annaler, 49A, 385-395, 1967.
Appendix G
TECH MEMO NO. 6 - RISK ANALYSIS
CITY OF RENTON
Technical Memorandum No. 6
Risk Analysis
Thunder Hills Sanitary Sewer Interceptor Alternative
Analysis
February 24, 2015
Prepared By:
Stantec Consulting Services Inc.
11130 NE 33rd Place
Suite 200
Bellevue, WA 98004
425.869.9448
TECHNICAL MEMORANDUM NO. 6
February 24, 2015
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Contents
1.0 INTRODUCTION ............................................................................................................. 1
2.0 REMAINING USEFUL LIFE (RUL) ASSESSMENT ................................................................ 1
3.0 CRITICALITY ASSESSMENT ............................................................................................. 2
4.0 RISK ANALYSIS ............................................................................................................... 3
TECHNICAL MEMORANDUM NO. 6
February 24, 2015
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TECHNICAL MEMORANDUM No. 6
February 24, 2015
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1.0 INTRODUCTION
This memorandum includes the risk analysis of the Thunder Hills Alternative Analysis project. The
Alternative Analysis aims to find the best alternative that will improve the functionality of the
Thunder Hills Interceptor, particularly the section between Grant Avenue South and Benson
Road South. The Interceptor was originally constructed in 1965, and serves a portion of the City’s
Rolling Hills neighborhood southeast of Renton City Hall. Several portions of the existing
interceptor have been replaced over time as additional development (interties) has occurred,
or as required due to erosion from Thunder Hills Creek.
The goal of the risk analysis section of the Alternative Analysis is to quantify the risk associated
with various reaches along the exiting alignment. Risk is defined as the existing pipes Remaining
Useful Life (RUL) multiplied by the pipes Criticality. Each of these factors were determined and
quantified as part of separate technical memorandums. Technical Memorandum No. 3
assessed the existing pipes RUL over identified reaches, while Technical Memorandum No. 5
assessed the criticality, or the potential for erosion to expose or damage the pipe.
For the purposes of comparing the existing interceptors RUL and Criticality, the interceptor was
separated into eight reaches based on either similar geologic conditions or pipe material
changes which would affect either the RUL or criticality for each reach. The eight reaches were
separated as follows:
Reach Location Approximate Gradient (%)
0+00 to 1+00 Not Applicable (Culvert)
1+00 to 5+50 15.1
5+50 to 8+30 4.3
8+30 to 11+00 8.2
11+00 to 12+30 10.4
12+30 to 18+90 5.2
18+90 to 26+75 7.1
26+75 to 28+00 14.4
2.0 REMAINING USEFUL LIFE (RUL) ASSESSMENT
As part of the analysis conducted for Technical Memorandum No. 3, the remaining useful life for
each reach of existing interceptor has been designated as either “Good”, “Moderate”, or
“Poor”. For the purposes of completing the RUL analysis a pipe section with a “Good” RUL was
given a numerical value of 1 and it represents a pipe that is constructed of ductile iron (DI) pipe
that still has the majority of its CML lining and limited corrosion spots. A section of pipe with a
“Moderate” RUL was given a numerical value of 2 and it represents a portion of asbestos
cement (AC) pipe that appears to be in good shape, or a DI pipe that is showing more
significant CML scaling and corrosion. A section of pipe with a “Poor” RUL was given a
numerical value of 3 and it represents a portion of pipe that has severe corrosion or a physical
TECHNICAL MEMORANDUM No. 6
February 24, 2015
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defect in the pipe that will cause flow constraints which would potentially reduce the remaining
lifespan of the pipe. These are locations identified as the most likely to fail and cause damage
to the environment which would require emergency repairs. Table 2-1 summarizes the amount
of pipe in each reach with Good, Moderate, and Poor RUL and provides a designated
numerical value for the RUL for each reach.
Table 2-1 Remaining Useful Life (RUL) Assessment
Reach Location
Length of
Good RUL
(feet)
Length of
Moderate
RUL (feet)
Length of
Poor RUL
(feet)
RUL
Designation
0+00 to 1+00 100 0 0 1
1+00 to 5+50 437 13 0 1
5+50 to 8+30 277 3 0 1
8+30 to 11+00 253 17 0 1
11+00 to 12+30 124 0 6 1
12+30 to 18+90 619 32 9 1
18+90 to 26+75 65 684 36 2
26+75 to 28+00 0 125 0 2
It should be noted that these reaches do not include the existing pipe from Manhole 035 to 033
because this section of pipe was not analyzed as part of the erosion evaluation due to it being
under I-405. Likewise, a small portion of pipe from Station 28+00 to approximately 28+75
between Manhole 051 and 051A was not susceptible to erosion from Thunder Hills Creek and
was not included in the reaches described above. Both of these portions of pipe would be
given an RUL designation of Moderate, or 2, due to corrosion in the DI pipe from Manhole 035 to
033 and the condition of the concrete pipe from Manhole 051 to 051A. These sections of pipe
will be included in the Risk analysis.
Due to the limited amount of Poor RUL pipe, the majority of the Thunder Hills Interceptor falls
under the Good RUL designation. The only reaches that are considered Moderate RUL is due to
the presence of existing asbestos cement (AC) pipe.
3.0 CRITICALITY ASSESSMENT
The analysis conducted as part of Technical Memorandum No. 5 identified the potential for
erosion to occur across the length of the existing interceptors. The analysis estimated the erosion
potential at critical cross section locations and extrapolated that information into generalized
erosion potentials for each of the reaches identified in Section 1.0 of this memorandum. The
potential for erosion at each reach was designated as “Stable”, “Marginal” or “Unstable”. Table
3-1 summarizes the erosion analysis designations for each reach with respect to 20, 40 and 80
years of stream erosion.
TECHNICAL MEMORANDUM No. 6
February 24, 2015
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Table 3-1 Generalized Erosion Potential at Reach Locations
Reach Location 20 Years 40 Years 80 Years
0+00 to 1+00 Stable Stable Stable
1+00 to 5+50 Marginal Unstable Unstable
5+50 to 8+30 Stable Stable Stable
8+30 to 11+00 Stable Stable Marginal
11+00 to 12+30 Stable Marginal Unstable
12+30 to 18+90 Stable Stable Stable
18+90 to 26+75 Stable Stable Stable
26+75 to 28+00 Marginal Unstable Unstable
One of the overriding goals of the Thunder Hills analysis has been to finish with a new interceptor
that has at least an 80 year design life. As such, the erosion potential for the reaches at 80 years
was used to determine the criticality value for each reach. Using the simple designation of
Stable equaling 1, Marginal equaling 2, and Unstable equaling 3, Table 3-2 summarizes the
criticality for each of the designated reaches:
Table 3-2 Criticality at Reach Locations
Reach Location
80 Year
Erosion
Potential
Criticality
Designation
0+00 to 1+00 Stable 1
1+00 to 5+50 Unstable 3
5+50 to 8+30 Stable 1
8+30 to 11+00 Marginal 2
11+00 to 12+30 Unstable 3
12+30 to 18+90 Stable 1
18+90 to 26+75 Stable 1
26+75 to 28+00 Unstable 3
4.0 RISK ANALYSIS
The risk of a reach of interceptor is defined as the reach’s RUL multiplied by the reach’s criticality.
Based on the analyses conducted for Technical Memorandum No. 3 and Technical
Memorandum No. 5, the RUL and Criticality has been given designations from 1 to 3 as
described above. Table 4-1 provides the Risk Analysis for each designated reach based on the
multiplication of the two numbers:
TECHNICAL MEMORANDUM No. 6
February 24, 2015
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Table 4-1 Risk Analysis
Reach Location RUL
Designation
Criticality
Designation
Risk
Designation
0+00 to 1+00 1 1 1
1+00 to 5+50 1 3 3
5+50 to 8+30 1 1 1
8+30 to 11+00 1 2 2
11+00 to 12+30 1 3 3
12+30 to 18+90 1 1 1
18+90 to 26+75 2 1 2
26+75 to 28+00 2 3 6
The risk analysis designation shown above will be used, in part, by the City to determine the level
of comfort with the existing interceptor remaining in its current location whether it is rehabilitated
or not, based on the potential risk to the pipeline at that location.
Appendix H
TECH MEMO NO. 7 - OPERATIONS AND MAINTENANCE
CITY OF RENTON
Technical Memorandum No. 7
Operation & Maintenance Study
Thunder Hills Sanitary Sewer Interceptor Alternative
Analysis
November 13, 2015
Prepared By:
Stantec Consulting Services Inc.
11130 NE 33rd Place
Suite 200
Bellevue, WA 98004
425.869.9448
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Contents
1.0 INTRODUCTION ...........................................................................................................1.1
2.0 PROJECT DESCRIPTION ...............................................................................................2.1
3.0 EXISTING CONDITIONS ...............................................................................................3.2
4.0 EXISTING OPERATION AND MAINTENANCE EQUIPMENT ...........................................4.4
5.0 PRELIMINARY RECOMMENDATIONS ...........................................................................5.5
5.1 ADDITIONAL MANHOLE ................................................................................................. 5.5
5.2 RELOCATE SEWER MAIN ................................................................................................ 5.6
5.3 ACCESS ROAD IMPROVEMENTS ................................................................................... 5.6
5.4 POTENTIAL OPERATION AND MAINTENANCE EQUIPMENT ........................................ 5.7
6.0 CONCLUSIONS ............................................................................................................6.8
APPENDIX A – PROPOSED OPERATION & MAINTENANCE IMPROVEMENTS .........................6.2
THUNDER HILLS SEWER INTERCEPTOR ALTERNATIVE ANALYSIS .............................................. 6.2
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1.0 INTRODUCTION
Stantec has completed an operation and maintenance investigation for the Thunder Hills Creek
Sewer Alternatives Analysis located in Renton, Washington. The purpose of the investigation was
to evaluate the necessary operation and maintenance improvements and to provide
preliminary recommendations on how to meet these needs.
The scope of work for the study consisted of several field investigations, meeting with City staff,
product research, and categorizing existing sewer equipment. This investigation also references
the findings presented in Stantec’s Technical Memorandum (TM) No. 4 “Geotechnical
Investigation” and Technical Memorandum No. 5 “Preliminary Erosion Hazard Evaluation.”
This memorandum aims to find the best alternative to improve the functionality of the Thunder
Hills Interceptor. Specifically, it reviews several structural improvements to the project area:
adding sanitary sewer manholes to improve accessibility; re-locating sewer mains further from
the creek; creating/rehabilitating an access roadway to the area near I-405 for maintenance
equipment; and constructing retaining structures to support the existing access roadway and
creek bed.
2.0 Project Description
The Thunder Hills Interceptor was originally constructed in 1965, and serves a portion of the City’s
Rolling Hills neighborhood southeast of Renton City Hall. The portion of the interceptor between
Grant Avenue South and I-405 runs within or adjacent to Thunder Hills Creek. Over the years,
erosion created by Thunder Hills Creek has compromised the existing sewer interceptor in various
locations and has required stream bank stabilization.
Accessibility is also an issue along this stretch of the interceptor. At some point a narrow gravel
access road was constructed through the canyon. It runs from Grant Avenue South to the
Berkshire Apartments, approximately 2,300 feet. As with the interceptor, erosion from the
Thunder Hills Creek along with vegetation growth has made the road too narrow in some areas
for the vactor truck.
The City is looking to make improvements to the Thunder Hills Canyon to protect the interceptor
and access road from erosion, and to improve the sewer main’s long term viability. The City is
currently reviewing two alternatives to improve the sewer main’s long term viability. Alternative
No. 1 is to improve the existing Thunder Hills interceptor so that it can support the existing and
ultimate flows. Alternative No. 2 is to divert the upstream flows to the Talbot Hills Sewer, and only
improve the section of the interceptor that will remain in active use.
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3.0 EXISTING CONDITIONS
The Thunder Hills Interceptor runs from I-405 to Grant Avenue South through a small canyon
created by Thunder Hills Creek. The canyon is lined by residential and undeveloped land. The
Berkshire Apartment Homes are to the west of the canyon, and undeveloped land (easements)
and single family residences are to the east of the canyon. The canyon is vegetated with both
native and invasive species of deciduous trees, grasses, and brush. Small areas of designated
wetlands are sprinkled throughout the canyon.
Sewer, storm, and power utilities run through the canyon. The Thunder Hills Inceptor (sewer) has
various pipe types along the alignment. The sewer depth is known at existing manholes, and has
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been interpolated between manholes. In the exploration phase of this project, potholing of the
existing sewer main was attempted to better understand the depth of the existing sewer main.
Even with potholing, the depth of the sewer main could not be determined in some areas. Near
Station 11+80, the sewer line extends beneath Thunder Hills Creek at an estimated depth of 7
feet.
Storm runoff drains to the Thunder Hills Creek from the surrounding areas. This runoff is conveyed
to the creek through both constructed storm drains and natural seasonal streams. The Thunder
Hills Creek appears to flow continuously throughout the year.
Major power lines for the area cross the canyon around Station 15+00.
The existing access road is approximately 2,300 feet long, running south from Berkshire
Apartments (Station 5+50) to Grant Avenue South (Station 28+50). Soil movement and erosion
have destroyed the access roadway around Station 5+00. From this section to approximately
Station 0+25, slopes are very steep with magnitudes of 1:1 to 1:1.5. North of this location, the
roadway is improved (partially) with quarry rock north to a flat area near I-405.
South of Station 5+50, large sections of the roadway are overgrown with grass covering the road
and side vegetation narrowing the lane width. In other sections the Creek bed and erosion
have cut away at the access road. The road width currently ranges from 7 feet to 13 feet.
The access road has two access points: one at Grant Avenue South for the portion of the road
that parallels the eastern side of the creek from Station 12+00 to Station 28+50, and the other at
the Berkshire Apartments for the portion of the road that parallels the western side of the creek
from Station 5+50 to Station 12+00. The access road itself does not cross the creek, so vehicles
must enter and exit from the same access point.
Rock filled gabion walls are located between the access roadway/path and Thunder Hills Creek
north of Station 4+00. These gabion walls are generally 4 to 6 feet in height. The Geotechnical
Investigation noted that these gabion baskets have deteriorated significantly and in places the
walls are somewhat overturned. The gabion walls appear to have limited functionality as
retaining structures for the roadway and sewer line.
Rock buttresses armor the creek bed between Station 1+65 to 1+80, Station 2+45 to 3+40, and
Station 4+65 to 5+65. These rock buttresses prevent the stream from wandering, protecting the
sewer main and nearby properties.
The Geotechnical Investigation also found evidence of shallow landslide activities that have
occurred periodically along portions of the slope west of the access roadway. Several large,
but shallow, landslides have occurred within the last several years north of Station 2+50. The
slides appear to consist of the upper colluvium sliding off the underlying sandstone. The slides
extend upslope between 10 and 50 feet and are up to 70 feet wide.
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4.0 EXISTING OPERATION AND MAINTENANCE EQUIPMENT
Manholes are the points of access for operation and maintenance of the sewer main. From the
manholes, various pieces of equipment can be sent into the existing sewer main, and used to
inspect and maintain it. In order to operate and maintain the sewer main, the City must have
equipment that can access the manholes and that have the reach to inspect and maintain the
sewer main between manholes.
The City’s sewer department currently has one Vactor truck with a 1-inch rodder hose, one
closed circuit television (CCTV), and one recently purchased easement machine. In addition,
the City is looking into ordering a new, smaller Vactor truck. Table 4.1 summarizes the
specifications of these pieces of operation and maintenance equipment.
Table 4.1 Existing Operation and Maintenance Equipment
Equipment
Hose
Size
(inch)
Reach
(feet)
Vehicle
Length
(feet)
Vehicle
Width
(feet)
Vehicle
Height
(feet)
Vehicle Turning
Radius
(feet)
Vactor Truck with Rodder Hose 1” 600’ 40.0’ 10.5’ <13’ 57.5’
Mainline Truck with CCTV N/A 1000’ 28.0’ 8.0’ 13’ 38’
New Easement Machine
w/Extendable Tracks 1” 600’ 6.83’ 3.0’ 5.33’ Unknown
The vehicle dimensions listed in Table 4.1 create the minimum requirements for the access road
for the interceptor in order to accommodate proper inspection and maintenance. The current
Vactor truck is the widest and longest, and therefore, has the largest turning radius. Because of
this, the current Vactor will set the access road’s width and turning radius limits. The mainline
truck used for the CCTV has antennas that makes it the tallest vehicle, and therefore, will set the
vertical clearance for the access road. In order to comfortably drive these vehicles on the
access road, the access road should be at least 12 feet wide with a minimum turning radius of
60 feet and a vertical clearance of at least 13.5 feet.
Building an access road of this size is unrealistic between Station 0+00 and Station 5+00 because
of the existing topography. The most northern manhole in the Thunder Hills Interceptor, SSMH
2309, is located at approximately Station 0+00, and it cannot be accessed by the Vactor truck.
In order to access this manhole and other difficult to reach features, the City has recently
purchased an easement machine. The easement machine is a narrow, durable machine that
can access areas that a Vactor truck cannot. A 1-inch rodder hose with a 600 feet reach is
attached to the easement machine, allowing it to perform similar tasks to the Vactor truck. The
construction of a small foot path, approximately 7 feet wide, will allow an easement machine to
reach SSMH 2309.
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The hose and reach dimensions listed in Table 4.1 provide the recommended requirements for
manhole spacing to allow proper operation and maintenance of the main. The rodder hoses on
both the Vactor truck and the easement machine are 1-inch diameter with 600 feet of reach.
The CCTV has a reach of 1000 feet. Therefore, the manholes should be approximately 600 feet
apart in order to allow the rodder hoses to reach and property maintain the entire sewer main
between the manholes.
5.0 PRELIMINARY RECOMMENDATIONS
The goal of this memorandum is to suggest improvements to the Thunder Hill Canyon that would
improve the City’s ability to operate and maintain the interceptor. Specifically, the City needs
better access to the interceptor in order to properly operate and maintain it. In order to do this,
the City needs to add an additional manhole south of I-405, relocate some sections of sewer
main away from the creek, and make significant improvements to the existing gravel access
road. Plans showing these suggested improvements for Alternative No. 1 and No. 2 are included
in Appendix A.
5.1 ADDITIONAL MANHOLE
The most northern manhole in the Thunder Hills Interceptor, SSMH 2309, is located at
approximately Station 0+00 (shown on Sheet 1). Because of the state of the access road from
Station 0+00 to 5+00, SSMH 2309 is difficult to access for inspection and maintenance. The
nearest existing points of access for the system are the upstream and downstream manholes,
SSMH 2310 and SSMH 2308 respectively. The upstream manhole, SSMH 2310, is located at
approximately Station 13+00, 1,300 feet south from SSMH 2309. The downstream manhole, SSMH
2308, located across I-405, is approximately 520 feet north of SSMH 2309. Because of these
circumstances, a section of main between SSMH 2309 and SSMH 2310 is nearly impossible to
maintain.
A new manhole needs to be added around Station 6+00 to provide an additional access point
for inspecting and maintaining the sewer main. This new manhole will create a spacing of
approximately 600 feet to SSMH 2309 and approximately 700 feet to SSMH 2310.
New manholes are proposed in several other places throughout the canyon. These new
manholes are primarily to allow connection points for the relocated sewer main, but they also
provide additional points of access for inspection and maintenance of the sewer main. These
new manholes are shown on the plans at approximately Station 4+25, Station 8+00, Station
11+25, and Station 15+50. Alternative No. 1 adds two additional manholes at Station 24+10 and
Station 27+75.
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5.2 RELOCATE SEWER MAIN
Throughout the Canyon, sections of the interceptor will be abandoned and relocated because
they are too close to Thunder Hills Creek. This relocation will protect both the stream from
contamination and the sewer main from exposure and damage. The relocated pipe will be
HDPE, so the pipe can more easily match the curves of the Canyon.
Where the existing sewer main is to remain, it will be rehabbed with a lining of Cured-In-Place
Pipe (CIPP). CIPP liners are made of non-woven polyester felt with an exterior polyurethane
coating. The felt is impregnated with a catalyzed thermosetting resin mixture. To install, the liner
is threaded through the pipe, and then inflated with either air pressure or water until it presses
against the existing pipe. Once fully inflated, steam or hot water is circulated within the liner to
start the thermosetting resin to cure or harder. Once cured, the CIPP liner provides structural
support for the existing pipe. This structure support will extend the life time of the existing sewer
main. The proposed locations of the CIPP rehab between Station 0+00 to 3+30, Station 11+20 to
13+05, Station 15+50 to 20+40. Alternative No. 1 adds additional CIPP rehab from Station 20+40
to 24+10 and Station 27+80 to 28+75 (see attached plan sheets). For more information about the
CIPP rehabilitation, refer to Technical Memorandum No. 3 “Remaining Useful Life (RUL) &
Rehabilitation Analysis.”
As discussed in the previous section, additional manholes will be added to the interceptor at
approximately Station 4+25, Station 8+00, Station 11+25, Station 15+50. Alternative No. 1 also
adds manholes at Station 24+10 and Station 27+75. These manholes area primarily for
connection to the existing system, but also provide additional access points for inspection and
maintenance.
5.3 ACCESS ROAD IMPROVEMENTS
The existing gravel access road needs to be modified to allow easy access for the sewer
department’s Vactor truck. As previously mentioned, the Vactor truck is 10.5 feet wide (side
mirror to side mirror), 40.5-feet long, and has a minimum turning radius of 57.5 feet. To meet
these needs, the access road should have a width of 12 feet and a minimum radius of 60 feet. A
13.5 feet vertical clearance should also be provided to allow the mainline truck with the CCTV to
drive the access road with damage to its antennas.
The majority of the existing access road between Station 5+50 and 30+75 is already
approximately 12 feet wide (see plan sheets 5 through 8). Only minor widening will be needed,
but the entire access road will need to be resurfaced. The existing access road is asphalt
between Station 29+00 and Station 30+75; this section will need to be repaved. The rest of the
existing access road is gravel, which will also need to be resurfaced.
The majority of the access road can be widened to 12 feet without cutting into the toe of the
slope, but the plans identify the areas where it is needed. The plans also show where retaining
walls or rockeries are suggested to protect the access road from erosion. Refer to the
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recommendations made in Technical Memorandum No. 4 “Geotechnical Investigations” for
additional information about the potential slope stabilization methods.
To improve the ingress and egress of the Vactor truck along the access road, a turnaround
point, wide enough for a three point turn in the Vactor truck is proposed between Station 6+00
and Station 7+00, and an additional access point is proposed around Station 15+00. Both the
turn-around and the additional access point would require additional easements. The turn-
around would require the relocation and extension of an existing retaining wall. Appendix A,
sheets 2 through 9, show the proposed access road with these dimensions.
A narrow gravel path would be constructed from Station 0+00 to Station 5+50. This gravel path
should allow the easement machine to easily access SSMH 2309. The easement machine will be
4 feet wide. Sheets 1 and 2 of Appendix A currently show a 7 feet wide gravel path. In order to
construct this path, heavy vegetation will need to be cut back and the canyon wall would need
to be stabilized. Between Station 1+50 and 3+50, this 7-foot wide path will cross a wetland, and
mitigation efforts will be needed.
5.4 POTENTIAL OPERATION AND MAINTENANCE EQUIPMENT
The City is currently looking into purchasing a smaller Vactor truck to allow easier access to the
interceptor. The smaller Vactor truck would cost approximately $150,000 plus the cost of freight,
warranty, and surcharges. Designing the access road for this smaller Vactor truck would lead to
some construction savings by allowing a narrower access road and creating a smaller
turnaround area. A conservative estimate of the total cost savings accrued by designing the
access road for the smaller Vactor truck is approximately $50,000, about a third of the total
purchase price of the Vactor truck. However, leaving the access road designed for the larger
Vactor truck will allow the City to use the larger truck to maintain the sewer system in the event
the smaller Vactor is down. That being said, having the smaller vactor truck would still be
advantageous for the continual maintenance and operation of the interceptor.
Table 5.1 shows the dimensions of this potential smaller Vactor truck.
Table 5.1 Potential Operation and Maintenance Equipment
Equipment
Hose
Size
(inch)
Reach
(feet)
Vehicle
Length
(feet)
Vehicle
Width
(feet)
Vehicle
Height
(feet)
Vehicle
Turning
Radius
(feet)
Potential Smaller Vactor Truck N/A N/A 17.0’ 8.5’ 12.0’ 35.4’
The current access road design provides a 12-foot access road for the larger 10.5-foot wide (side
mirror to side mirror) Vactor truck. This leaves less than a foot of clearance on either side of the
road. While this is enough clearance to be drive-able, it would not be comfortable, especially
where the creek is directly alongside the road. The smaller 8.5-foot wide Vactor truck would
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leave just under 2 feet of clearance on either side of the 12-foot access road, making it a lot
more comfortable to drive and making maintenance of the interceptor easier to perform.
Accessing the lower half of the interceptor will also be significantly easier with the smaller Vactor
truck. Entering the access road from the Berkshire Apartments is currently difficult with the
existing Vactor truck; the truck has to perform a several point turn to enter the access road due
to the existing limited turning radius. Due to existing parking space requirements for the Berkshire
Apartment complex, there is not enough room at this entrance to provide a simple single point
turn for the larger Vactor truck. However, the smaller Vactor truck has a significantly smaller
turning radius, which will make it significantly easier for the smaller Vactor truck to enter the
access road.
The smaller Vactor truck will also reduce the amount of road maintenance in order to keep the
access road accessible. The lighter weight and smaller turning radius of the smaller vactor truck
will cause less wear and tear on the gravel access road than the existing Vactor truck.
We suggest purchasing the smaller Vactor truck, but continuing to design the access road
improvements for the larger Vactor truck. The smaller Vactor truck would significantly improve
the accessibility of the interceptor, making maintenance easier to perform.
6.0 CONCLUSIONS
The Thunder Hill Interceptor, constructed in 1965, needs improvements to allow the City to
continue to operate and maintain it for the long term. The City’s existing equipment has
difficulty accessing the interceptor because of the poor access road and limited manhole
access points to the system. Erosion caused by the steep canyon walls and the Thunder Hills
Creek also threatens the long term viability of the sewer interceptor.
This TM suggests adding additional sewer manholes, relocating the sewer main, and improving
the access road. In particular this TM suggests the following improvements to the Thunder Hills
Interceptor:
Add a new manhole at approximately Station 6+00 to give the Vactor truck access to
the northern portion of the interceptor.
Relocate the sewer main away from Thunder Hills Creek by creating parallel HDPE sewer
mains from Station 3+30 to 11+20 and Station 13+05 to 15+50. Alternative No. 1 adds an
additional sectionof parallel pipe fromStation 24+10 to 27+80.
Rehabilitate the existing sewer main that is to remain active by lining it with CIPP from
Station 0+00 to 3+30, Station 11+20 to 13+05, and Station 15+50 to 20+40. Alternative No.
1 adds two additional sections of CIPP rehab between Station 20+40 to 24+10 and
Station 27+80 to 28+75.
as \\us1314-f01\workgroup\2002\active\2002003607\analysis\operation_maintenance\mem_thunderhills_o&m_final_20151105.docx 9
Add new manholes for connection between the parallel main and the existing system at
approximately Station 4+25, Station 8+00, Station 11+25, and Station 15+50. Alternative
No. 1 adds two addition manholes at Station 24+10 and Station 27+75.
Resurface and widen the existing gravel access road between Station 5+50 and 29+00.
Repave and widen the existing paved entrance to the access road between Station
29+00 and 30+75.
Create a 7-foot wide gravel path between Station 0+00 and 5+50.
Create a turnaround between Station 6+00 and 7+00.
Create an additional access point to the access road around Station 15+00.
Build gravity walls or rockeries where necessary to protect the access road and/or sewer
main.
Purchase the smaller vactor truck to improve the ease of access to the interceptor.
These improvements to the Thunder Hills Interceptor, related access road, and the City’s
operation and maintenance equipment will allow the City to properly operate and maintain the
interceptor for years to come.
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as \\us1314-f01\workgroup\2002\active\2002003607\analysis\operation_maintenance\mem_thunderhills_o&m_final_20151105.docx
Appendix A – Proposed Operation & Maintenance
Improvements
THUNDER HILLS SEWER INTERCEPTOR ALTERNATIVE ANALYSIS
By Stantec Consulting Services Inc.
Dated November 4, 2015
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CIPP EXISTING THUNDER HILLS
SEWER INTERCEPTOR
PROPOSED LOCATIONS OF
GRAVITY WALL OR ROCKERY
PROPOSED LOCATIONS OF
PARALLEL SEWER PIPE
ALTERNATIVE NO. 1 LEGEND
PROPOSED LOCATION OF
IMPROVED GRAVEL ROAD
PROPOSED LOCATION OF
ROCK BUTTRESSES
PROPOSED LOCATIONS OF
IMPROVED PAVED ROAD
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PROPOSED LOCATIONS OF
PARALLEL SEWER PIPE
ALTERNATIVE NO. 1 LEGEND
PROPOSED LOCATION OF
IMPROVED GRAVEL ROAD
PROPOSED LOCATION OF
ROCK BUTTRESSES
PROPOSED LOCATIONS OF
IMPROVED PAVED ROAD
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PROPOSED NEW SEWER PIPE
UPSIZE EX SEWER PIPE
REDIRECT EX SEWER PIPE
ALTERNATIVE NO. 2 LEGEND
PROPOSED LOCATION OF
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PROPOSED LOCATIONS OF
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PROPOSED NEW SEWER PIPE
UPSIZE EX SEWER PIPE
REDIRECT EX SEWER PIPE
ALTERNATIVE NO. 2 LEGEND
PROPOSED LOCATION OF
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MATCH LINE - SHEET 3 OF 3
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EXISTING SANITARY SEWER
CIPP EXISTING THUNDER HILLS
SEWER INTERCEPTOR
PROPOSED LOCATIONS OF
GRAVITY WALL OR ROCKERY
PROPOSED NEW SEWER PIPE
UPSIZE EX SEWER PIPE
REDIRECT EX SEWER PIPE
ALTERNATIVE NO. 2 LEGEND
PROPOSED LOCATION OF
IMPROVED GRAVEL ROAD
PROPOSED LOCATION OF
ROCK BUTTRESSES
PROPOSED LOCATIONS OF
IMPROVED PAVED ROAD
Appendix I
TECH MEMO NO. 8 - SUMMARY OF SENSITIVE
AREA IMPACTS
CITY OF RENTON
Technical Memorandum No. 8
Alternative 1
Summary of Sensitive Area Impacts
Thunder Hills Sanitary Sewer Interceptor Alternative
Analysis
October 21, 2015
Prepared By:
Hugh Mortensen , Sarah Sandstrom, and Katy Crandall
750 6th Street South
Kirkland, WA 98033
425-822-5242
The Watershed Company Project No. 131113
Contents
1.0 INTRODUCTION ......................................................................................................................1
2.0 SENSITIVE AREAS ....................................................................................................................1
2.1 WETLANDS .................................................................................................................................... 1
2.2 STREAMS ....................................................................................................................................... 1
2.3 OTHER CRITICAL AREAS ................................................................................................................. 2
3.0 POTENTIAL IMPACTS ...............................................................................................................2
3.1 AVOIDANCE AND MINIMIZATION MEASURES .............................................................................. 4
3.2 ENDANGERED SPECIES AND HABITAT ........................................................................................... 5
3.3 IMPLICATIONS OF IMPACTS TO BUFFERS ..................................................................................... 5
3.4 IMPLICATIONS OF DIRECT IMPACTS TO STREAMS ........................................................................ 6
3.5 IMPLICATIONS OF DIRECT IMPACTS TO WETLANDS ..................................................................... 7
ALTERNATIVE NO. 1
Thunder Hills Sanitary Sewer Interceptor
Summary of Sensitive Area Impacts
The Watershed Company
October 21, 2015
1
1.0 INTRODUCTION
The City of Renton seeks to repair and relocate the Thunder Hills Sanitary Sewer Interceptor and
reestablish maintenance access. The purpose of this memorandum is to document anticipated impacts
to wetlands, streams, and habitats and related implications for design and permitting. A separate
memorandum, Technical Memorandum No. 9, prepared by Stantec, discusses the permit needs and
required permit application materials. The findings discussed in this memorandum are based on the
attached plans, dated August 21, 2015. The findings of this report are subject to verification and
agreement by the appropriate local, state and federal regulatory authorities.
2.0 SENSITIVE AREAS
Wetlands and streams within the project study area were delineated and preliminarily documented in a
delineation memorandum dated May 12, 2014. That memorandum identified Thunder Hills Creek,
seven tributary streams, and six wetlands in the study area. In 2015, after the delineation memo was
completed, the City of Renton updated its critical areas regulations, resulting in changes to stream
classifications and wetland and stream buffers. Updated information relating to stream and wetland
classification and applicable standard buffer widths are presented below.
2.1 WETLANDS
The delineation memorandum initially rated wetlands using the Washington State Department of
Ecology’s (Ecology) Western Washington Wetland Rating System (2004), which was in effect at the time
and accepted by local, state and federal permitting agencies. Ecology’s rating system was updated in
2014 and all wetlands were re-rated to be consistent with local, state and federal requirements (2014
forms and figures attached). Updated wetland classifications, based on the 2014 rating system, and
buffers based on the City of Renton’s updated critical area regulations are reported in Table 1.
2.2 STREAMS
For the preparation of this memorandum, additional investigation of stream typing incorporated
information from the Forest Practices Application Mapping Tool (FPAMT)
(https://fortress.wa.gov/dnr/protectiongis/fpamt/index.html), Salmonscape
(http://apps.wdfw.wa.gov/salmonscape/map.html), and the City of Renton Maps Application
(http://rp.rentonwa.gov/SilverlightPublic/Viewer.html?Viewer=COR-Maps).
The stream typing reported in the May 12, 2014 memorandum is generally consistent with the findings
from the review of mapping, except that the city’s stream classification system has changed from Class
1-5 to Class S, F, Ns, and Np, consistent with the Washington Department of Natural Resources (WDNR)
permanent stream typing criteria. Thunder Hills Creek was previously described as a Class 3, non-
salmonid bearing, perennial stream. The FPAMT and the City of Renton Maps identify Thunder Hills
Creek as a Type N non-fish bearing stream. Salmonscape identifies the piped section of stream under
Interstate 405 as a total fish passage barrier. Whereas the City of Renton Maps classify Thunder Hills
TECHNICAL MEMORANDUM NO. 8
ALTERNATIVE NO. 1
Thunder Hills Sanitary Sewer Interceptor
Summary of Sensitive Area Impacts
The Watershed Company
October 21, 2015
Creek as perennial, Salmonscape identifies the upper two-thirds of the project area as seasonal.
Downstream from Interstate 405, the City of Renton Maps and field observations indicate that Thunder
Hills Creek meets Rolling Hills Creek, which runs southwest through an open channel for approximately
1,000 feet before being piped through the stormwater infrastructure under the Renton Village Shopping
Center. Eventually, Rolling Hills Creek drains into the Black River and from there into the Duwamish
River. Based on the limited watershed area upstream of the project area, and the total fish passage
barriers downstream of the project area, the Type N classification for Thunder Hills Creek within the
project area is warranted. Based on field observations, Thunder Hills Creek is assumed to be perennial
in years of normal rainfall within the project area.
Stream C and Stream H were delineated in the field; however, these streams were not located by the
survey. These streams are on the opposite bank of Thunder Hills Creek from the proposed sewer work;
therefore, they do not affect the conclusion of this memorandum. Nevertheless, in the interest of
complete documentation the general location of these streams should be noted in project plans and
permit application documents.
2.3 OTHER CRITICAL AREAS
In addition to wetlands and streams, the project is located at the base of a ravine, and is mapped by the
City of Renton as a high severity erosion hazard area and a moderate severity landslide hazard area.
Geologically hazardous area standards will apply; a geotechnical report, and potentially an independent
review, will be required. Independent review can be administratively waived by the City Administrator.
No priority habitats or species have been mapped or field identified within the project area, therefore,
additional local standards for habitat conservation areas are not expected to apply. Floodplains are also
not mapped within the project area.
3.0 POTENTIAL IMPACTS
Potential permanent impacts to sensitive areas, including wetlands and streams have been generally
summarized in Table 1, and are based upon the attached project plans (Sewer Plan and Profile
Alternative No. 1) provided by Stantec and dated August 21, 2015 and a site visit meeting on July 2,
2015. The full extent of all temporary impacts, including construction entrances, staging areas, and
other temporary clearing areas have not been fully documented. For the purposes of this memorandum,
it is assumed that all temporary impacts will be limited to buffer areas and appropriately mitigated
following project completion. According to the preliminary site plans, the project will impact the buffers
of all wetland and stream critical areas identified in the study area. Additionally, direct impacts will
occur to portions of Thunder Hills Creek, Stream G, and Wetlands E and F. Stream F could also be
modified if Stream G is rerouted.
ALTERNATIVE NO. 1
Thunder Hills Sanitary Sewer Interceptor
Summary of Sensitive Area Impacts
The Watershed Company
October 21, 2015
3
Table 1. Classifications of streams and wetlands along Thunder Hills Creek, per Renton Municipal
Code, and projected impacts to each feature and its associated buffer.
Critical Area Renton Municipal
Code Classification
(2014 Ecology rating –
wetlands only)
Standard
Buffer
Width (ft)
Direct Impact to
Feature
Impact to Buffer
Thunder Hills
Creek Class Np 75
Possible direct impact
from culvert work
associated with Stream
G drainage
Entire project within
buffer
Stream B Class Ns 50
None Gravel road in buffer Stream C Class Ns 50
Stream D Class Ns 50
Stream E Class Ns 50 None Gravel road, gravity
wall or rockery, and
parallel sewer in buffer Stream F Class Ns 50
Possible modifications
to banks if Stream G is
rerouted
Stream G Class Ns 50
Repair/replace culvert
or reroute stream; Cut
into portion of stream
Gravel road, slope cut,
and gravity wall or
rockery in buffer
Stream H Class Ns 50 None Gravel road and
parallel sewer in buffer
Wetland A
Category 3, Low
Habitat Function (4
points)
75 None
Gravel road and gravity
wall or rockery in
buffer
Wetland B
Category 3,
Moderate Habitat
Function (6 points)
100 None
Gravel road, parallel
sewer, slope cut, and
gravity wall or rockery
in buffer
Wetland C
Category 4,
Moderate Habitat
Function (5 points)
50 None
Gravel road, paved
road, and parallel
sewer in buffer
TECHNICAL MEMORANDUM NO. 8
ALTERNATIVE NO. 1
Thunder Hills Sanitary Sewer Interceptor
Summary of Sensitive Area Impacts
The Watershed Company
October 21, 2015
Critical Area Renton Municipal
Code Classification
(2014 Ecology rating –
wetlands only)
Standard
Buffer
Width (ft)
Direct Impact to
Feature
Impact to Buffer
Wetland D
Category 3,
Moderate Habitat
Function (5 points)
100 None
Gravel road, parallel
sewer, slope cut, and
gravity wall or rockery
in buffer
Wetland E
Category 3,
Moderate Habitat
Function (5 points)
100 Small portion of gravel
road in wetland Gravel road in buffer
Wetland F
Category 3,
Moderate Habitat
Function (5 points)
100 Gravel road through
wetland
Gravel road and
parallel sewer in buffer
3.1 AVOIDANCE AND MINIMIZATION MEASURES
Permit applications will need to document unavoidable critical area impacts. This includes temporary
construction-related impacts, as well as permanent impacts from the project. As part of that
documentation, all available avoidance and minimization measures must be discussed and addressed.
Examples of proposed avoidance measures currently planned for this project include:
• Using existing access points to avoid the need for new stream crossings.
• Using a slip-line approach to repair the existing sewer line where it crosses Thunder Hills Creek.
Examples of proposed minimization measures include:
• Relocating the sewer line under the access road to minimize the footprint of construction and
maintenance impacts.
• Locating the proposed road over the existing unmaintained trail to minimize impacts to
vegetation in the buffer.
• Locating the road as far from the stream as possible, given topographic constraints.
• Using a narrower width path (7 feet compared to 12-foot road) for specialized equipment on the
northern portion of the project area.
ALTERNATIVE NO. 1
Thunder Hills Sanitary Sewer Interceptor
Summary of Sensitive Area Impacts
The Watershed Company
October 21, 2015
5
• Implementation of a Temporary Erosion and Sediment Control plan.
• Timing construction to avoid the wet season when sedimentation and erosion would be
anticipated.
A mitigation plan will also be necessary to demonstrate that all unavoidable impacts to functions and
values will be appropriately compensated or replaced consistent with local, state, and federal guidance.
3.2 ENDANGERED SPECIES AND HABITAT
A review of online mapping resources and site visit observations indicate that no federally threatened or
endangered species are located in the project area. The project area is in an urban environment, where
available upland habitat is limited to isolated patches of forests that experience regular disturbance.
Listed birds, mammals, and amphibians are not likely to occur in the vicinity of the project area because
they are generally restricted to less developed areas with large areas of contiguous habitat. Similarly, as
discussed above, Thunder Hills Creek does not support listed salmonid fish, and the nearest salmon-
bearing stream is the Black River, 1.8 miles downstream of the project extent.
Because the project area is well-removed from the likely occurrence of a federally listed species, a
biological evaluation will not likely be required, but a no-effect letter is recommended to describe the
project area relative to the location of listed species. The no-effect letter would be submitted as an
attachment to the JARPA when applying for federal permits. This letter provides the Corps with
necessary documentation to evaluate and confirm that their consultation with National Marine Fisheries
Service and U.S. Fish and Wildlife Service is not necessary.
3.3 IMPLICATIONS OF IMPACTS TO BUFFERS
As described above and in Table 1, the entire project would be located within stream and wetland
buffers designated by the City of Renton. Renton Municipal Code (RMC) 4-3-050.C.3 exempts repair and
replacement of existing utilities within an existing improved public road right-of-way or easement from
critical area standards. Based on the plans, we assume that the proposed project would not qualify for
this exemption, as it would extend beyond the existing improved area and include instream work.
Renton Municipal Code (RMC) 4.3-050.J.2.b. also provides criteria for administrative approval of new
utilities in streams, lakes, and buffers with an approved critical area study, provided:
i. Fish and wildlife habitat areas are avoided to the maximum extent possible; and
ii. The utility is designed consistent with one or more of the following methods:
(a) Installation shall be accomplished by boring beneath the scour depth and hyporheic zone of
the water body and channel migration zone; or
(b) The utilities shall cross at an angle greater than sixty (60) degrees to the centerline of the
channel in streams or perpendicular to the channel centerline; or
(c) Crossings shall be contained within the footprint of an existing road or utility crossing; and
TECHNICAL MEMORANDUM NO. 8
ALTERNATIVE NO. 1
Thunder Hills Sanitary Sewer Interceptor
Summary of Sensitive Area Impacts
The Watershed Company
October 21, 2015
iii. New utility routes avoid paralleling the stream or following a down-valley course near the
channel; and
iv. The utility installation shall not increase or decrease the natural rate of shore migration or
channel migration; and
v. Seasonal work windows are determined and made a condition of approval; and
vi. Mitigation criteria are met.
Although the above standards are intended for new utilities, replacement of an existing utility line is
expected to qualify for the same administrative approval process.
The City of Renton allows for buffer averaging or reduction, provided that the selected approach meets
the city’s minimum width standards and maintains or improves ecological functions. Since the entire
project falls within the buffer of Thunder Creek (as well as numerous additional, overlapping wetland
and stream buffers) and since the project area is mainly on private property, there is likely no
opportunity for buffer expansion under an averaging plan. Therefore, buffer impacts are best mitigated
through enhancement of existing buffers (removing weeds, planting native plants, monitoring and
maintaining vegetation until established). There is ample opportunity for such enhancement
throughout much of the project site. The specific approach to buffer enhancement will need to be
discussed as a part of the critical areas report.
3.4 IMPLICATIONS OF DIRECT IMPACTS TO STREAMS
Direct impacts are those that alter the physical stream bed or banks. Such impacts would require
approval by the City of Renton, WDFW, and the Corps.
The applicant should anticipate that WDFW will require that any stream culverts modified as a part of
the project to be appropriately sized to accommodate transport of high flows (100-year event) and
debris. There is additional potential that WDFW will require that all culverts within the project area are
updated to pass high flows. Fish passage standards should not apply given the permanent status of the
complete fish passage barrier downstream (under Interstate 405).
There is the potential that a short segment of Stream G will be relocated as part of the proposed plan
since it appears Stream G and Stream F were joined at some time the in recent past. RMC 4-3-050.J.2.e.
allows administrative approval for stream relocation for expansion of public road or other public facility
improvements where no feasible alternative exists, provided the following:
(a) Buffer widths shall be based upon the new stream location.
(b) The provided buffer between the facility and the relocated stream shall be enhanced or
improved to provide appropriate functions given the class and condition of the stream.
ALTERNATIVE NO. 1
Thunder Hills Sanitary Sewer Interceptor
Summary of Sensitive Area Impacts
The Watershed Company
October 21, 2015
7
(c) Applicable mitigation criteria must be met.
(d) Proper notification to the City must be made.
As noted above, the project area does not include fish-bearing streams, and the downstream barrier
under Interstate 405 is very unlikely to be corrected. Nevertheless, it should be noted that the project
area is within the designated as Usual and Accustomed (U&A) fishing grounds of the Muckleshoot Tribe.
As such, the Muckleshoot Tribe may review and comment on permits through the public review period.
The Corps heavily weighs comments and recommendations from the tribes, particularly as they relate to
stream habitat and fish passage. Early and frequent coordination with Muckleshoot Tribe during the
permitting phase of this project is advised.
3.5 IMPLICATIONS OF DIRECT IMPACTS TO WETLANDS
Direct wetland impacts are those that change the physical shape or configuration of the wetland itself.
Based on the preliminary project plans, it is estimated that the project will directly impact (eliminate)
approximately 2,000 square feet of wetland. Mitigation will need to be provided for direct wetland
impacts. Frequently, wetland mitigation takes the form of replacement wetland areas rehabilitated,
constructed, or enhanced on-site. However, given the steep nature of the project site and the lack of
public ownership, on-site mitigation does not appear feasible. Additionally, the Corps and Ecology have
established a preference for use of mitigation banks or in lieu fee programs over on-site, permittee-
responsible mitigation. This is in contrast to the city’s preference for mitigation, which prioritizes re-
establishing, rehabilitating, creating, enhancing, and preserving wetlands on-site over mitigation banks
or in-lieu fee programs. The City of Renton has an approved wetland mitigation bank on Springbrook
Creek and King County operates an in lieu fee program through its Mitigation Reserves Program. The
use of either site would be permitted by local, state, and federal standards. The credits required from
either the Springbrook Creek Mitigation Bank or the Mitigation Reserves Program would be determined
by the mitigation banking agreement and/or certification process. Based on the project plans, it is
estimated that if the applicant were to use the Springbrook Creek Mitigation Bank, approximately 0.04
credits would be needed.
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ADDITIONAL ESMT
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FOR TURN-AROUND
TO BE
REMOVED
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#.6'40#6+8'5#0#.;+5+5
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EX. 10"
D
I
S
S
EX. 12" DI SS
WETLAND D, LOCATION
APPROX. (NOT SURVEYED).
SURVEY MAY BE REQUIRED
FOR PERMITS
WE
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B
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WET B
F
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WET B
F
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WET B
F
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EX. 10"
D
I
S
S
EX. 12" DI SS
THUNDER
HILLS CREEK
OPTIONAL GRAVITY WALL OR
ROCKERY - MAY NOT BE NEEDED
ADD CULVERT & DIRECT
SWALE TO THUNDER
HILLS CREEK
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#.6'40#6+8'5#0#.;+5+5
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DP
2
DP
2
EX. 10
"
D
I
S
S
WETLAND C, LOCATION
APPROX. (NOT SURVEYED).
SURVEY MAY BE
REQUIRED FOR PERMITS WETLAND B
THIS SECTION OF
THUNDERHILLS CREEK
APPROX. (NOT SURVEYED).
SURVEY MAY BE
WET BFR
WET BFR
WET BFR
WET
B
F
R
WET
B
F
R
WET BFR
WET BFR
WET BF
R
WET BFR
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WET BFR
WET BFRWE
T
B
F
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WE
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B
F
R
WET B
F
R
EX. 10
"
D
I
S
S
OPTIONAL GRAVITY WALL OR
ROCKERY - MAY NOT BE NEEDED
CUT INTO TOE OF
SLOPE TO CREATE
12' WIDE GRAVEL RD
CUT INTO TOE OF
SLOPE TO CREATE
12' WIDE GRAVEL RD
MA
T
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1
4
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#.6'40#6+8'5#0#.;+5+5
WET BFR
EX. 10" AC SS
EX. 10" A
C
S
S
EX. 10
"
D
I
S
S
THUNDER HILLS CREEK
EX. 10" AC SS
EX. 10" A
C
S
S
THUNDER HILLS CREE
K
EX. 10
"
D
I
S
S
CUT INTO TOE OF
SLOPE TO CREATE
12' WIDE GRAVEL RD
CUT INTO TOE OF
SLOPE TO CREATE
12' WIDE GRAVEL RD
CUT INTO TOE OF
SLOPE TO CREATE
12' WIDE GRAVEL RD
MA
T
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L
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A
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1
7
+
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5
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7
20
0
6
6*70&'4*+..5+06'4%'2614
#.6'40#6+8'5#0#.;+5+5
WET BFR
EX. 10" AC SS
STREAM F
STREAM G
EX. 10" AC SS
THUNDER HILLS CREEK
CUT INTO TOE OF
SLOPE TO CREATE
12' WIDE GRAVEL RD
CUT INTO TOE OF
SLOPE TO CREATE
12' WIDE GRAVEL RD
CUT INTO TOE OF
SLOPE TO CREATE
12' WIDE GRAVEL RD
CUT INTO TOE OF
SLOPE TO CREATE
12' WIDE GRAVEL RD
CUT INT
O
T
O
E
O
F
SLOPE TO
C
R
E
A
T
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12' WIDE
G
R
A
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8
20
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6*70&'4*+..5+06'4%'2614
#.6'40#6+8'5#0#.;+5+5
WET BFR
SO-D
P
1
SO-D
P
1
///
///
///
///
///
///
///
///
///
///
///
///
///
//////
///
///
///
///
///
///
///
///
///
///
///
///
////////////
///
///
///
///
///
///
///
///
WE
T
B
F
R
WE
T
B
F
R
WE
T
B
F
R
WE
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B
F
R
WE
T
B
F
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WE
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B
F
R
WET
B
F
R
WET
B
F
R
WET
B
F
R
WET
B
F
R
WET
B
F
R
WET
B
F
R
WET
B
F
R
WET
B
F
R
WET B
F
R
WE
T
B
F
R
WET
B
F
R
WE
T
B
F
R
WET B
F
R
WETLAND A
STREAM E
THUNDER HILLS CREEK
CUT IN
T
O
T
O
E
O
F
SLOP
E
T
O
C
R
E
A
T
E
12' WI
D
E
G
R
A
V
E
L
R
D
CUT INTO TOE OF
SLOPE TO CREATE
12' WIDE GRAVEL RD
RE
-
P
A
V
E
&
W
I
D
E
N
OPTIONAL GRAVITY WALL OR
ROCKERY - MAY NOT BE NEEDED
MA
T
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2
4
+
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9
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6*70&'4*+..5+06'4%'2614
#.6'40#6+8'5#0#.;+5+5
WET BFR
////////////
///
////////////
///
///
///
///
///
///
///
////////////////////////
///
//////////////////
///
///
///
///
///
///
///
///
///
///
///
///
///
///
///
///
///
///
///
///
///
///
///
///
///
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///
SS
SS
SS
SS
SS
S
S
SS
///
///
///
///
///
///
///
////////////////////////////////////////////////////////////
///
///
///
///
///
///
///
///
///
///
///
///
///
///
///////////////////////////////////////////////////////////////
///
///
///
///
WET BFR
WET BFR
WET BFR
WET BFR
WET BFR
WET
B
F
R
WET
B
F
R
WET
B
F
R
WET
B
F
R
WET B
F
R
WET B
F
R
GRANT AVE S
S
1
8
t
h
S
T
///
///
///
///
///
///
///
RE-PAVE & WIDEN
ACCESS ROAD
M
A
T
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L
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-
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A
.
2
8
+
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Appendix J
TECH MEMO NO. 9 - PERMITTING ASSESSMENT
ALTERNATIVE NOS. 1 AND 2
CITY OF RENTON
Technical Memorandum No. 9
Permitting Assessment Alternatives No. 1 & 2
Thunder Hills Sanitary Sewer Interceptor Alternative Analysis
October 19, 2015
Prepared By:
Rebecca Saur
Stantec Consulting Services Inc.
11130 NE 33rd Place
Suite 200
Bellevue, WA 98004
425.869.9448
ALTERNATIVES NO. 1 & 2 Thunder Hills Sanitary Sewer Interceptor
Permitting Assessment
October 19, 2015
Contents
1.0 INTRODUCTION ............................................................................................................. 1
2.0 PERMIT REQUIREMENT SUMMARY ................................................................................. 1
2.1 CITY OF RENTON PERMITS ................................................................................................. 2
2.2 WILLIAMS NORTHWEST WILSOP ENCROACHMENT/FOREIGN LINE CROSSING
PERMIT ................................................................................................................................ 2
2.3 PUGET SOUND ENERGY CONSENT FOR USE ................................................................... 3
2.4 USARMY CORPS OF ENGINEERS (USACE) SECTION 404 PERMIT .................................. 3
2.5 NATIONAL POLLUTANT DISCHAGE ELIMINATION SYSTEM (NPDES) PERMIT................. 3
2.6 KCWTD APPROVAL ........................................................................................................... 3
2.7 HYDRAULIC PERMIT APPROVAL ....................................................................................... 4
2.8 NATIONAL HISTORIC PRESERVATION ACT SECTION 106 ............................................... 4
2.9 PERMIT MATRIX – ALTERNATIVES NO. 1 AND NO. 2 ....................................................... 5
3.0 ADDITIONAL STUDIES .................................................................................................... 6
4.0 CONCLUSION ................................................................................................................ 6
rs v:\2002\active\2002003607\analysis\permitting\mem_thunder_permit analysis alt 1-2_final.docx i
ALTERNATIVES NO. 1 & 2 THUNDER HILLS SANITARY SEWER INTERCEPTOR
PERMITTING ASSESSMENT
October 19, 2015
1.0 INTRODUCTION
This memorandum evaluates permitting requirements for design Alternatives No. 1 and 2 as
assessed during the Thunder Hills Sanitary Sewer Interceptor Alternative Analysis. The goals of the project are to provide a comprehensive assessment of improvement alternatives to the existing
sewer, accounting for ultimate flows, provide maintenance access to the entire project
alignment, and to protect the integrity of the sewer and access improvements from
environmental impacts and vice versa.
Alternative No. 1 proposes to improve the existing access from the portion of the interceptor
south of Interstate 405 to roughly Station 6+00. This portion of the access improvements will be
narrow, with only enough width to allow the City’s “easement” machine in this area to perform
the maintenance. From Station 6+00 south, proposed improvements include adding a turnaround at Station 6+50, building a new access road across PSE property at Station 15+00,
and rebuilding the entrance road at Station 30+00. Along the alignment, a new parallel sewer is
planned to be installed from approximately Station 3+28 to 11+20 and 13+08 to 15+44; the remaining portion of the existing sewer will be rehabilitated using CIPP technology. The existing rock buttresses will protect improvements north of Station 6+00.
Alternative No. 2 proposes to divert upstream flows from the Thunder Hills Interceptor at Grant
Avenue South. Alternative No. 2 also includes the same improvements as Alternative No. 1 from the interceptor south of Interstate 405, along the current alignment up to Station 24+00, but with
no additional sewer improvements proposed south of this station along Thunder Hills Creek.
Alternative No. 2 additionally proposes sewer main upgrades within the Talbot Hill Sewer Basin,
including upsizing a section of sewer within Benson Road South (between the north and south extents of Eagle Ridge Drive South) to 10-inch main, adding an 8-inch diversion pipe in South 18th
Street from Grant Avenue South to Eagle Lane South, upsizing main to 12-inch pipe from Eagle
Lane South to Eagle Ridge Drive, and upsizing sewer to 12-inch main in the vicinity of Benson
Drive South and Smithers Avenue South (between South 15th Street and Interstate 405). Preliminary plans for Alternative No. 2 are under development.
2.0 PERMIT REQUIREMENT SUMMARY
A route analysis has been completed for the project, taking into consideration environmental,
cost, and sewer system impacts of each alternative. The permitting is typical for this type of project with a primary concern of maintaining the integrity of the sensitive areas along the
project route. Project permits will be based on preliminary design plans dated August 21, 2015,
and are identified and discussed herein. For purposes of presenting a comprehensive permit
summary, both alternatives have been considered in the following discussion. A matrix detailing each alternative and associated permits, timelines, submittal fees, and submittal requirements is
included at the end of this section. The permitting assumptions and estimate herein were based
on the best available information at the time and are subject to update/modification.
Other independent sensitive area reviews are being conducted and preliminary assessment memorandums will be issued concurrently.
rs v:\2002\active\2002003607\analysis\permitting\mem_thunder_permit analysis alt 1-2_final.docx 1
ALTERNATIVES NO. 1 & 2 THUNDER HILLS SANITARY SEWER INTERCEPTOR
PERMITTING ASSESSMENT
October 19, 2015
2.1 CITY OF RENTON PERMITS
The project lays in its entirety within the City of Renton. A pre-application meeting with the City’s
Planning Department is recommended for this project due to the amount of sensitive areas along the project route.
A Land Use Permit Application (LUP) will be required which will be processed by the Planning
Department. There are other City permits which will be processed with the LUP, including the
following:
• Variance – The burden of proof in justifying proposed improvements lies with the permit applicant. A project narrative will be developed discussing project alternatives and
benefits of the chosen alternative, adverse environmental impacts avoided, adverse
environmental impacts mitigated, and requesting approval of construction parameters as well as permanent improvements within critical areas and their buffers.
• Grading – Approval of the proposed project grading plans, including review of
consultant reports, restoration/mitigation plans, site issues, etc. It is presumed any tree
removal will not trigger any additional permit requirements or reviews by the City.
• Environmental Review – This review includes processing of the project SEPA. The
preliminary plans show construction of permanent improvements within wetlands and
streams adjacent to the project route. A USACE 404 permit will be required due to this
work. The potential of dual lead agencies and NEPA are further discussed under Item No. 2.4 below. It is anticipated that the Determination for this project would be a
Mitigated DNS, where the City provides mitigation for specific project elements
impacting the environment. The City will review any sensitive area reports and mitigation
plans under this permit.
Additional City of Renton permits which may be required for this project include building permits
for the rockeries or gravity walls along the southern portion of the project, and right of way
permits for any work within City of Renton Right of Way.
This assessment does not take into consideration any permit exemptions the project owner, the City of Renton Public Works Department, may have with regard to obtaining permits from other
departments within the City.
It is anticipated that City of Renton permits will take 3 – 5 months.
2.2 WILLIAMS NORTHWEST WILSOP ENCROACHMENT/FOREIGN LINE CROSSING PERMIT
As the proposed project alignment crosses over Williams NW pipelines, Williams will need to review and approve the project design drawings. Williams will review project plans and set forth
guidelines the contractor will be required to adhere to in an effort to protect their pipeline
facilities long term as well as during construction, to be included in the City’s bid specification documents. A separate permit will be signed directly between the contractor and Williams prior
to constructing the pipeline crossing.
For purposes of this assessment, it is assumed that Williams NW is the pipeline owner. This has yet
to be verified, however.
The estimated timeline for Williams NW to approve the project plans is approximately 4 – 6
weeks.
rs v:\2002\active\2002003607\analysis\permitting\mem_thunder_permit analysis alt 1-2_final.docx 2
ALTERNATIVES NO. 1 & 2 THUNDER HILLS SANITARY SEWER INTERCEPTOR
PERMITTING ASSESSMENT
October 19, 2015
2.3 PUGET SOUND ENERGY CONSENT FOR USE
As the proposed project crosses through Puget Sound Energy (PSE) owned land, a PSE consent
for land use agreement is required. A PSE LUA in essence is an easement from PSE granting the City of Renton use of their property under a set of restrictions set forth in an agreement to protect PSE’s use and ability to maintain their property/facilities.
Recent experience with PSE LUA’s has taken 12 – 15 months for review and final execution of the
agreement, although the timeframe for this project may be less.
2.4 US ARMY CORPS OF ENGINEERS (USACE) SECTION 404 PERMIT
The proposed project plans show construction within and installation of permanent improvements within wetlands and streams adjacent to the project route. A USACE permit is
required prior to discharging dredged or fill material into the waters of the United States,
including special aquatic sites such as wetlands. Examples of activities which may trigger this permit are grading or clearing, placement of rip rap, or road fill within a wetland or stream.
Close communication with the USACE is prudent for this project. The project may be burdened
with additional requirements as a federal agency, the USACE, could be considered a second
lead agency for the project Environmental Checklist, as well as trigger requirements to follow federal NEPA regulations in addition to state requirements.
Once the project alignment and final alternative has been selected, prompt coordination with
the USACE is recommended to determine the level of USACE permit requirements the local
office would expect for this project. An ESA Section 7 Consultation could also be triggered by a 404 permit review, where the US Fish & Wildlife Service and National Marine Fisheries Service
would conduct a review of the project for impacts to endangered species found in the project
area. A Section 401 Water Quality Certification will also be required by the WA State
Department of Ecology (DOE), certifying that DOE anticipates the project will comply with state water quality discharge and aquatic resource protection standards.
USACE and associated permitting typically take 12 – 18 months for issuance.
2.5 NATIONAL POLLUTANT DISCHAGE ELIMINATION SYSTEM (NPDES) PERMIT
An NPDES permit is issued by DOE for construction projects disturbing an acre or more. The
permit process includes a public notice and comment period as part of the permit review process. Development of a project Stormwater Pollution Prevention Plan (SWPPP) is required for
this permit. The approved permit is typically transferred to the contractor to manage the SWPPP
during construction and permit close out with DOE.
It is estimated that an NPDES permit would be approved in 6 to 8 weeks.
2.6 KCWTD APPROVAL
King County Wastewater Treatment Division (KCWTD) reviews any proposed plans for sewer
improvement projects within King County.
KCWTD approvals typically take 2 – 4 weeks for approval.
rs v:\2002\active\2002003607\analysis\permitting\mem_thunder_permit analysis alt 1-2_final.docx 3
ALTERNATIVES NO. 1 & 2 THUNDER HILLS SANITARY SEWER INTERCEPTOR
PERMITTING ASSESSMENT
October 19, 2015
2.7 LIMITED ACCESS RIGHT OF WAY PERMIT
The Alternative No. 2 concept involves a sewer crossing of State Route 515 (aka Benson Drive
South). As such, a limited access right of way permit will be required from the Washington State Department of Transportation (WSDOT). The right of way permit from WSDOT could potentially take up to a year to obtain, however, for this type of work it is anticipated to take closer to six
months. The permit application process involves completing an application and submittal along
with project design plans, fees, and a performance bond to WSDOT for their review and
approval of the crossing design. WSDOT typically has a set of conditions the City’s contractor
will be required to follow during construction within their right of way.
2.8 HYDRAULIC PERMIT APPROVAL
The Washington State Department of Fish & Wildlife (DFW) administers Hydraulic Permit Approvals
(HPA). Any construction projects or activities in or near state waters are required to obtain permit coverage under an HPA. The HPA permit is required for the DFW review of the
improvements under Thunder Hills Creek as well as the work associated with culverting Stream G.
The application is submitted via an online portal which converts the data into a Joint Aquatic
Resources Permit Application form (which can also be utilized for other relevant permit applications for the project).
It is anticipated that a project HPA approval would be issued 4 – 6 weeks from submittal.
2.9 NATIONAL HISTORIC PRESERVATION ACT SECTION 106
Federal permitting for the project will also trigger a cultural resource review by the Washington
State Department of Archaeological and Historic Preservation (DAHP). This permit involves preparing a cultural resource inventory and evaluation report, submitting to DAHP for review,
monitoring during construction (if required by DAHP’s review of the initial report), and submitting
a monitoring report to DAHP if any field work is required during construction.
DAHP approvals are dependent on meaningful cultural information found on or near the project site. Provided the site has minimal cultural relevance, it is anticipated DAHP review of the
cultural resource inventory report would take approximately 4 – 6 weeks.
rs v:\2002\active\2002003607\analysis\permitting\mem_thunder_permit analysis alt 1-2_final.docx 4
ALTERNATIVES NO. 1 & 2 THUNDER HILLS SANITARY SEWER INTERCEPTOR
PERMITTING ASSESSMENT
October 19, 2015
2.10 PERMIT MATRIX – ALTERNATIVES NO. 1 AND NO. 2
*Alternatives No. 1 and No. 2 have the same basic components which in turn trigger similar permit requirements. While Alternative No. 2 shares the same design elements as Alternative No.
1, it has a greater amount of work in the right of way. Generally, both alternatives have the
same permit requirements, other than the additional right of way permits for Alternative No. 2.
There is also the potential for additional coordination with WSDOT for the right of way use permit
for work in State Route 515 with Alternative No. 2.
Agency Permit Timeline
from Submittal
Fees Alternative
No.*
City of Renton • Land Use
• Grading
• Variance
• Environmental Review
• Building
• Right of Way
4 – 6 Months Internal City Fees 1 & 2
2
Williams NW WilSOP Permit 4 – 6 Weeks No Review Fee 1 & 2
Puget Sound Energy Consent for Use 12 – 15
Months
To Be
Determined
1 & 2
US Army Corps of
Engineers
• Section 404
• ESA Section 7
• DOE Section 401
12 – 18
Months
Submittal Fee =
$100
1 & 2
Department of Ecology NPDES 6 – 8 Weeks Fees Relating
to Notice of
Intent Advertisement
1 & 2
King County Water Treatment
Division Review
2 – 4 Weeks No Review Fee 1 & 2
Washington State Department of
Transportation
Limited Access Right of Way
Permit
6 – 12 Months Submittal Fee = $300 2
WA State Department of
Fish & Wildlife
HPA 4 – 6 Weeks Submittal Fee =
$150
1 & 2
Washington State Department of
Archaeological and
Historic Preservation
DAHP Review 4 – 6 Weeks No Review Fee 1 & 2
rs v:\2002\active\2002003607\analysis\permitting\mem_thunder_permit analysis alt 1-2_final.docx 5
ALTERNATIVES NO. 1 & 2 THUNDER HILLS SANITARY SEWER INTERCEPTOR
PERMITTING ASSESSMENT
October 19, 2015
3.0 ADDITIONAL STUDIES
It is anticipated that the following consultant reports will be necessary to submit for permits on
the Alternative No. 1project:
• Biological/Habitat Assessment
• Wetland & Stream Delineation
• Geotechnical Evaluation
• Cultural Resource Inventory and Evaluation
4.0 CONCLUSION
This permitting assessment of Alternatives No. 1 and 2 is based on preliminary project plans.
Additional, more definitive information such as a determination of the quality of aquatic areas
surrounding the project site, redirection of/culverting of streams along the project route, and/or the installation of new/improving the existing rock buttresses/gabion walls within Thunder Hills
Creek or its bank could impact permit requirements.
A comprehensive list of permits and requirements will progress with the development of project
design plans and consultant reports. Advancement of the project design will allow for project impacts to be discussed and planned for with other agencies, to help streamline the permitting
process.
rs v:\2002\active\2002003607\analysis\permitting\mem_thunder_permit analysis alt 1-2_final.docx 6
Appendix K
TECH MEMO NO. 10 - COST STUDY
CITY OF RENTON
Technical Memorandum No. 10
Cost Study
Thunder Hills Sanitary Sewer Interceptor Alternative
Analysis
March 15, 2016
Prepared By:
Stantec Consulting Services Inc.
11130 NE 33rd Place
Suite 200
Bellevue, WA 98004
425.869.9448
as v:\2002\active\2002003607\analysis\cost\mem_thunderhills_cost_final_20160315.docx
Contents
1.0 INTRODUCTION ...........................................................................................................1.1
2.0 COST COMPARISION ..................................................................................................2.1
3.0 CONCLUSION ..............................................................................................................3.3
APPENDIX A – PERMITTING COST ...............................................................................................
APPENDIX B – ENVIRONMENTAL MITIGATION COST .................................................................
APPENDIX C – EASEMENT ATTAINMENT COSTS .........................................................................
APPENDIX D – CONSTRUCTION COSTS ......................................................................................
APPENDIX E – OPERATION AND MAINTENANCE COSTS ...........................................................
as v:\2002\active\2002003607\analysis\cost\mem_thunderhills_cost_final_20160315.docx
TECHNICAL MEMORANDUM NO. 10
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1.0 INTRODUCTION
Stantec has completed an investigation of the costs associated with improvements to the
Thunder Hills Creek Sanitary Sewer Interceptor located in Renton, Washington. Based on the
findings of previous technical memorandums, the City has narrowed the analysis to two
remaining alternatives. Alternative No. 1 is to improve the existing Thunder Hills interceptor and
access road so that it can support the existing and ultimate flows and can be adequately
maintained by City Staff. Alternative No. 2 is to divert the upstream flows to the Talbot Hills Sewer
Basin, and only improve the section of the interceptor that will remain in active use. Alternative
2 will also require improvements be made to portions of the existing sewers in the Talbot Hill
Sewer Basin.
This memorandum reviews the engineering, permitting, environmental mitigation, easement
attainment, construction, and operation and maintenance costs for the two remaining
alternatives. It draws upon the findings presented in the previous Thunder Hills Sanitary Sewer
Interceptor Alternative Analysis Technical Memorandums (TMs) for the assumptions made for
each cost. The goal of the cost comparison of the two remaining alternatives will be used to
help the City make its final decision on the best alternative for its needs.
2.0 COST COMPARISION
Table 2.1 and Table 2.2 show a summary of the costs associated with both alternatives. The
assumptions used for these costs are provided after the tables, and a more detailed breakdown
on the costs is included in the appendices. Because the following costs are for planning level
purposes, each of the costs listed include a 25% contingency.
Table 2.1 Cost Comparison
Description Estimated Initial Costs
Alternative No. 1 Alternative No. 2
Engineering Design $370,000 $740,000
Construction Engineering $200,000 $220,000
Permitting $40,000 $41,000
Environmental Mitigation $131,250 $131,250
Easement Attainment $170,000 $170,000
Construction $2,442,000 $4,959,000
Total $3,353,250 $6,261,250
TECHNICAL MEMORANDUM NO. 10
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Table 2.2 Annual Cost Comparison (Operation & Maintenance Costs)
Description Estimated Annual Costs
Alternative No. 1 Alternative No. 2
Thunder Hills Interceptor $1,750 $1,750
Additional sewer main $0 $600
Total $1,750 $2,350
Engineering design costs were assumed to be 15% of the total construction costs.
Construction engineering costs were assumed to be 8% of the total construction costs for
Alternative No. 1, and 8% of the Thunder Hills Interceptor section of Alternative No. 2. This cost
assumes that a trenchless specialist will observe the CIPP installation, and a geologist will observe
the gravity walls being installed. A City inspector will observe all open cut installation.
Permitting costs were estimated based on the findings from Technical Memorandum No. 9,
“Permitting Assessment Alternatives No. 1 & 2,” dated October 19, 2015. The permitting costs
were estimated from similar previous projects, and include the estimated labor costs to prepare
and submit the permits. Exact costs required for permitting are impossible to determine until the
reviewing agencies have seen the applications. For more information about the permitting
costs, reference Appendix A.
Environmental mitigation costs were estimated based on findings from Technical Memorandum
No. 8 “Summary of Sensitive Area Impacts,” dated October 21, 2015, and were prepared by The
Watershed Company. The mitigation costs were broken down into indirect (buffer only impacts)
and direct wetland impacts. There are two options for offsetting direct wetland impacts: the
Springbrook Creek Mitigation Bank and the King County Mitigation Reserves Program. The costs
shown in Table 2.1 use the Springbrook Creek Mitigation Bank estimate. For more information on
the environmental mitigation costs, refer to the “Wetland Mitigation Cost Estimate” technical
memorandum, prepared by The Watershed Company and included in Appendix B.
Easement attainment costs were estimated with Stantec’s standard easement value estimator.
The costs assume easements would be obtained for the entire interceptor. Land value was
taken from the King County Assessor. Additional information about the easement cost estimates
is included in Appendix C.
Construction costs were estimated from the findings and preliminary drawings presented in
Technical Memorandum No. 7, “Operation and Maintenance Study,” dated November 13, 2015.
The costs include the features shown in the preliminary plans. Unit costs were estimated from
recent projects and additional pricing research. A more detailed breakdown of preliminary
construction costs is included in Appendix D. In this breakdown, Alternative No. 2 is separated
into three schedules. Schedule A includes the improvements to the Thunder Hills Interceptor
TECHNICAL MEMORANDUM NO. 10
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($2,752,000), Schedule B includes the necessary improvements to the Talbot Sewer Basin
regardless of whether or not the Thunder Hills Basin is diverted to the Talbot Sewer Basin
($239,000), and Schedule C includes the additional improvements to the Talbot Sewer Basin if
the Thunder Hills Basin is diverted to the Talbot Sewer Basin ($1,968,000). Separating Alternative
No. 2’s costs into these three schedules shows that only $1,968,000 (or the cost of Schedule C) of
Alternative No. 2’s estimated $4,959,000 construction cost can be avoided by choosing
Alternative No. 1.
Operation and maintenance costs (shown in Table 2.2) include standard cleaning of the sewer
main every two years, and providing CCTV for the sewer main every seven years. The costs
were estimated based on the number of hours, personnel, and equipment needed to complete
these tasks. The costs estimate how much each alternative would increase the annual
operation and maintenance costs, by reviewing the cost to maintain the Thunder Hills
Interceptor as well as the additional linear feet of sewer pipe added to the system for the Talbot
Hills Sewer Basin. For more information, see Appendix E.
3.0 CONCLUSION
Alternative No. 1 is more economical to design and construct, but Alterative No. 2 better
mitigates the risks associated with Thunder Hills Creek. Alternative No. 1 would save just over $2.0
million of the initial design, permitting, and construction costs (accounting for the necessary
improvements to the Talbot Sewer Basin regardless of whether or not the Thunder Hills Sewer
Basin is diverted). It would also save approximately $600 annually in general operation and
maintenance costs.
However, Alternative No. 1 leaves a major sewer main adjacent to the Thunder Hills Creek,
which is an environmental risk to the City. If Alternative No. 1 is implemented, a failure of the
Thunder Hills Interceptor could lead to a greater sewage spill in an environmentally sensitive
area than if Alternative No. 2 is implemented. A spill in an environmentally sensitive area would
likely cause negative press for the City and cost the City significant money in clean up and
mitigation efforts. This risk would be mitigated and significantly reduced through the
improvements that would be implemented in Alternative No. 2.
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Appendix A – Permitting Costs
By Stantec Consulting Services Inc..
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PLANNING LEVEL COST ESTIMATE - PERMIT COSTS
Stantec Consulting Services Inc.
11130 NE 33rd Pl, Suite 200
Bellevue, WA 98004
OWNER:City of Renton
PROJECT: Thunder Hills Alternative Analysis Report
Agency Permit
Timeline from
Submittal Fees Labor Hours***
• Land Use
• Grading
• Variance
• Environmental Review
• Building
• Right of Way
Williams NW WilSOP Permit 4 – 6 Weeks No Review Fee 6
Puget Sound Energy Consent for Use
12 – 15
Months To Be Determined 22
• Section 404
• ESA Section 7
• DOE Section 401
Department of Ecology NPDES 6 – 8 Weeks
Fees Relating to
Notice of Intent
Advertisement =
apx $800
10
King County Water Treatment Division Review 2 – 4 Weeks No Review Fee 2
Submittal Fee =
$300
Traffic Control
Plans = apx. $500
WA State Department of Fish & Wildlife HPA 4 – 6 Weeks
Submittal Fee =
$150 10
No Review Fee
Subconsultant
apx $6,000 (not
including
construction
monitoring)
TOTALS $7,850 $22,680
$30,530
Contingency 25.00% $7,632.50
$38,163
Carried to Cost Estimate $40,000
***Assumes an average labor rate of $135.00
Labor budget based on an average rate of $135/hr
excludes TWC scope/budget items
City of Renton 4 – 6 Months Internal City Fees 48
TOTAL
US Army Corps of Engineers 12 – 18
Months
Submittal Fee =
$100 56*
**Not a franchise agreement
Washington State Department of
Transportation
Limited Access Right of Way
Permit 6 – 12 Months 6**
Washington State Department of
Archaeological and Historic Preservation DAHP Review 4 – 6 Weeks 8
Subtotal
*Assumes permit coordination work divided between Stantec and TWC;
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Appendix B – Environmental Mitigation Cost
By The Watershed Company
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T E C H N I C A L M E M O R A N D U M
Date: December 18, 2015
To: Stephanie Ard, Stantec
From: Hugh Mortensen
Project
Number: 131113
Project Name: Thunder Hills Sanitary Sewer Interceptor
Subject: Wetland Mitigation Cost Estimates
This estimate covers costs associated with mitigating for impacts arising from
improvements to the Thunder Hills Sanitary Sewer Interceptor system, including
sewer line slip-lining, new sewer line and maintenance access upgrades. Costs
outlined below are broken into costs to mitigate on-site, indirect (stream buffer
and/or wetland buffer only) impacts and two options for offsetting direct
wetland impacts. Assumptions used in the estimates are provided following
each section.
Indirect (buffer only) impacts.
12,500 SF at $2.50 - $3.00 per square foot, $51,300 - $67,500
Assumptions:
1. Project plans provided by Stantec, 8/21/15. Survey does not show all existing
disturbance so some existing areas added in addition to survey.
2. Work within already disturbed buffer areas, such as gravel roads, parking
areas, etc., was not considered a project impact and would not require buffer
mitigation. Only disturbance of existing, vegetated buffer areas was
considered an impact requiring mitigation.
3. Buffer enhancement involving removal of invasive weeds and replanting
native vegetation is suitable to off-set buffer impacts at a 1:1 ratio.
4. Per-square foot mitigation cost ranges are based on past project experience.
Higher accuracy costs should be calculated based on future mitigation plans
for specific buffer areas.
5. Costs include 5-year performance monitoring and maintenance
(weeding/plant replacement).
6. Costs do not include draft or final mitigation plans, permitting applications
or special studies.
The Watershed Company
Technical Memorandum – Thunder Hills Mitigation Costs
December 18, 2015
Page 2
Direct wetland impact
Option 1: Springbrook Creek Mitigation Bank: $22,700 - $37, 500
Assumptions:
1. Project plans provided by Stantec, 8/21/15. Plans show elimination of
Wetland F, which is 1,408 SF.
2. Credit costs based on 12/11/15 email from Ronald Straka, City of Renton
Surface Water Utility Engineering Manager, of $825,000 - $1,500,000 per
category II credit. This calculates to a range of $701,250 - $1,275,000 per
category III credit (85% of Category II credit costs).
3. Estimate does not include costs to prepare the credit release documents or
any local, state or federal permit documentation.
Option 2: King County Mitigation Reserves Program: $175,000.
Assumptions:
1. Project plans provided by Stantec, 8/21/15. Plans show elimination of
Wetland F, which is 1,408 SF.
2. Credit costs based on 12/15/15 phone call with Meagan McNeil, King County
Water & Land Resources Division. Costs quoted are $47,000 per credit, plus
a land fee currently estimated at $1.50 per square foot of impact. Land fee is
subject to change.
3. Credit need based on the attached “Debit” Worksheet (Downloaded 12/15/15
http://www.ecy.wa.gov/programs/sea/wetlands/mitigation/creditdebit/debits
.xlsx), which relies on scores from the 2014 wetland rating form for Wetland F
(the impacted wetland), prepared by The Watershed Company July 2015.
4. Estimate does not include costs to prepare the credit release documents or
any local, state or federal permit documentation.
1
Hugh Mortensen
From:Ronald Straka <rstraka@Rentonwa.gov>
Sent:Friday, December 11, 2015 11:58 AM
To:Hugh Mortensen
Cc:Allen Quynn; David Christensen
Subject:RE: Springbrook Mitigation Bank
Hugh,
The cost of credits range that we are considering $825,000 to $1,500,000. This would be what is charged for one credit
in the Wetland Bank, which is equal to one acre of a category II wetland (Ecology’s Wetland Classification System). If
you have category III wetlands then 0.85 credits are needed for every acre of impact to Category III wetlands and 0.70
credits needed for every acre of impact to Category IV wetlands. You can use this in your report regarding the City’s
Wastewater Utility’s Thunder Hills Sewer Project.
As we discussed the benefits of using the Wetland bank is that there is no replacement ratio for impacted wetlands, the
permitting process is streamlined, since there is no need to develop a mitigation plan and have it reviewed by the Corps
of Engineers and City, there is no post construction monitoring for 10‐years that would be normally required by the
Corps as part of their permit, there is no risk of the mitigation failing and having to do additional work if monitoring
shows that performance measures haven’t been met by the mitigation project and you don’t have to purchase land or
find a site to do the mitigation on. All of these thing reduce the cost and risk associated with doing concurrent
mitigation.
Thank you,
Ron Straka, P.E.
Surface Water Utility Engineering Manager
City of Renton
Renton City Hall – 5th Floor
1055 South Grady Way
Renton WA 98057‐3232
Cell: 206‐919‐4281
Office: 425‐430‐7248
Email: Rstraka@Rentonwa.gov
From: Hugh Mortensen [mailto:HMortensen@watershedco.com]
Sent: Friday, December 11, 2015 10:12 AM
To: Ronald Straka
Subject: Springbrook Mitigation Bank
Ron,
Thanks for your time on the phone just now. Any help on anticipated per‐credit costs would be much appreciated. The
project in mind contains an unavoidable small impact to a Category III wetland.
Regards,
2
HUGH B. MORTENSEN, PWS
Principal
(425) 822‐5242, Extension 107
750 Sixth Street South
Kirkland, WA 98033
http://www.watershedco.com/
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THIS PAGE WAS INITIONALLY LEFT BLANK.
v:\2002\active\2002003607\analysis\cost\mem_thunderhills_cost_final_20160304.docx
Appendix C – Easement Attainment Costs
By Stantec Consulting Services Inc.
v:\2002\active\2002003607\analysis\cost\mem_thunderhills_cost_final_20160304.docx
THIS PAGE WAS INITIONALLY LEFT BLANK.
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THIS PAGE WAS INITIONALLY LEFT BLANK.
v:\2002\active\2002003607\analysis\cost\mem_thunderhills_cost_final_20160304.docx
Appendix D – Construction Costs
By Stantec Consulting Services Inc.
v:\2002\active\2002003607\analysis\cost\mem_thunderhills_cost_final_20160304.docx
THIS PAGE WAS INITIONALLY LEFT BLANK.
PLANNING LEVEL COST ESTIMATE
Stantec Consulting Services Inc.Prepared by: SMA
11130 NE 33rd Pl, Suite 200 Date:
Bellevue, WA 98004 Revised by: ELB
Date:
OWNER:City of Renton
PROJECT: Thunder Hills Alternative Analysis Report - Alternative No. 1
Approx.
Item Description Quantity Unit Unit Cost Total Cost
Construction Estimate
1 Mobilization and Demobilization 1 LS 132,200.00$ 132,200.00$
2 Trench Excavation Safety Systems 1 LS 20,000.00$ 20,000.00$
3 Construction Survey, Staking, and As-builts 1 LS 20,000.00$ 20,000.00$
4 Traffic Control 1 LS 10,000.00$ 10,000.00$
5 Temporary Erosion/Sedimentation Controls 1 LS 50,000.00$ 50,000.00$
6 Site Clearing and Grubbing 1 LS 30,000.00$ 30,000.00$
7 12-Inch Diam. HDPE Gravity Sewer Pipe Installed under Access
Road 1,410 LF 300.00$ 423,000.00$
8 Cured-In-Place Pipe (CIPP) Rehab.1,470 LF 50.00$ 73,500.00$
9 Television Inspection of Sanitary Sewers 2,880 LF 2.00$ 5,760.00$
10 Abandon & Fill Sanitary Sewer Main 1,410 LF 15.00$ 21,150.00$
11 48-Inch Diam. Sanitary Sewer Manhole 7 EA 6,000.00$ 42,000.00$
12 Connect to Existing Sewer System 6 EA 6,000.00$ 36,000.00$
13 Pavement Restoration (Existing Entrance)400 SY 150.00$ 60,000.00$
14 New Asphalt Entrance 1,010 SY 250.00$ 252,500.00$
15 Gravel Access Road & Path 0 SF 10.00$ -$
16 Gravity Walls and Rockeries 1 LS 500,000.00$ 500,000.00$
17 Removal and Replacement of Unsuitable Foundation Material 30 TON 25.00$ 750.00$
18 Storm Culverts 50 LF 150.00$ 7,500.00$
19 Misc. Site Restoration 1 LS 100,000.00$ 100,000.00$
Subtotal 1,784,360$
9.5% 169,500$
Subtotal 1,953,860$
25%488,500$
2,442,360$
Unit costs for linear foot of new sewer pipe includes estimated cost of trench backfill
3/1/2016
Sales Tax @
Contingency @
Total Probable Construction Cost
3/11/2016
V:\2002\active\2002003607\analysis\cost\cost_alternative1.xlsx
PLANNING LEVEL COST ESTIMATE
Stantec Consulting Services Inc.Prepared by: SMA
11130 NE 33rd Pl, Suite 200 Date:
Bellevue, WA 98004 Revised by: ELB
Date:
OWNER:City of Renton
PROJECT: Thunder Hills Alternative Analysis Report - Alternative No. 2
Approx.
Item Description Quantity Unit Unit Cost Total Cost
Schedule A - Thunder Hills Improvements
1 Mobilization and Demobilization 1 LS 148,900.00$ 148,900.00$
2 Trench Excavation Safety Systems 1 LS 15,000.00$ 15,000.00$
3 Construction Survey, Staking, and As-builts 1 LS 20,000.00$ 20,000.00$
4 Traffic Control 1 LS 10,000.00$ 10,000.00$
5 Temporary Erosion/Sedimentation Controls 1 LS 50,000.00$ 50,000.00$
6 Site Clearing and Grubbing 1 LS 30,000.00$ 30,000.00$
7 12-Inch Diam. HDPE Gravity Sewer Pipe Installed under Access
Road 1,040 LF 300.00$ 312,000.00$
8 Cured-In-Place Pipe (CIPP) Rehab.1,010 LF 50.00$ 50,500.00$
9 Television Inspection of Sanitary Sewers 2,050 LF 2.00$ 4,100.00$
10 Abandon & Fill Sanitary Sewer Main 1,040 LF 15.00$ 15,600.00$
11 48-Inch Diam. Sanitary Sewer Manhole 5 EA 6,000.00$ 30,000.00$
12 Connect to Existing Sewer System 4 EA 6,000.00$ 24,000.00$
13 Pavement Restoration (Existing Entrance)400 SY 150.00$ 60,000.00$
14 New Asphalt Entrance 1,010 SY 250.00$ 252,500.00$
15 Gravel Access Road & Path 35,000 SF 10.00$ 350,000.00$
16 Gravity Walls and Rockeries 1 LS 500,000.00$ 500,000.00$
17 Removal and Replacement of Unsuitable Foundation Material 25 TON 25.00$ 625.00$
18 Storm Culvert 50 LF 150.00$ 7,500.00$
19 Misc. Site Restoration 1 LS 130,000.00$ 130,000.00$
Subtotal 2,010,725$
9.5% 191,000$
Subtotal 2,201,725$
25%550,400$
2,752,125$
Approx.
Item Description Quantity Unit Unit Cost Total Cost
Schedule B - Talbot Hill Sewer Basin Improvements (regardless of whether or not Thunder is diverted)
1 Mobilization and Demobilization 1 LS 12,900.00$ 12,900.00$
2 Trench Excavation Safety Systems 1 LS 2,500.00$ 2,500.00$
3 Construction Survey, Staking, and As-builts 1 LS 3,000.00$ 3,000.00$
4 Traffic Control 1 LS 7,000.00$ 7,000.00$
5 Temporary Erosion/Sedimentation Controls 1 LS 1,000.00$ 1,000.00$
6 12-Inch PVC Gravity Sewer Pipe Installed in ROW 110 LF 375.00$ 41,250.00$
7 Pipe Bursting under Talbot (8-Inch Pipe to 12-Inch Pipe)170 LF 450.00$ 76,500.00$
8 Television Inspection of Sanitary Sewers 280 LF 2.00$ 560.00$
9 Connect to Existing Sewer System 4 EA 6,000.00$ 24,000.00$
10 Removal and Replacement of Unsuitable Foundation Material 10 TON 25.00$ 250.00$
11 Asphalt Restoration 50 SY 115.00$ 5,750.00$
Subtotal 174,710$
9.5% 16,600$
Subtotal 191,310$
25%47,800$
239,110$
3/1/2016
Sales Tax @
Contingency @
Schedule A Total Cost
3/11/2016
Sales Tax @
Contingency @
Schedule B Total Cost
V:\2002\active\2002003607\analysis\cost\cost_alternative2.xlsx
Approx.
Item Description Quantity Unit Unit Cost Total Cost
Schedule C - Talbot Hill Sewer Basin Improvements for Thunder Hills Basin Diversion
1 Mobilization and Demobilization 1 LS 106,500.00$ 106,500.00$
2 Trench Excavation Safety Systems 1 LS 17,500.00$ 17,500.00$
3 Construction Survey, Staking, and As-builts 1 LS 22,000.00$ 22,000.00$
4 Traffic Control 1 LS 35,000.00$ 35,000.00$
5 Temporary Erosion/Sedimentation Controls 1 LS 10,000.00$ 10,000.00$
6 8-Inch PVC Gravity Sewer Pipe Installed in ROW 980 LF 325.00$ 318,500.00$
7 10-Inch PVC Gravity Sewer Pipe Installed in ROW 200 LF 350.00$ 70,000.00$
8 12-Inch PVC Gravity Sewer Pipe Installed in ROW 910 LF 375.00$ 341,250.00$
9 Television Inspection of Sanitary Sewers 2,090 LF 2.00$ 4,180.00$
10 48-Inch Diam. Sanitary Sewer Manhole 1 EA 6,000.00$ 6,000.00$
11 Rechanneling Existing Manholes 2 EA 2,000.00$ 4,000.00$
12 Connect to Existing Sewer System 7 EA 6,000.00$ 42,000.00$
13 Removal and Replacement of Unsuitable Foundation Material 40 TON 25.00$ 1,000.00$
14 Asphalt Restoration 4,000 SY 115.00$ 460,000.00$
Subtotal 1,437,930$
9.5% 136,600$
Subtotal 1,574,530$
25%393,600$
1,968,130$
4,959,365$
Unit costs for linear foot of new sewer pipe includes estimated cost of trench backfill
Sales Tax @
Contingency @
Schedule C Total Cost
Total Probable Construction Cost (Schedule A + Schedule B + Schedule C)
V:\2002\active\2002003607\analysis\cost\cost_alternative2.xlsx
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v:\2002\active\2002003607\analysis\cost\mem_thunderhills_cost_final_20160304.docx
Appendix E – Operation and Maintenance Costs
By City of Renton & Stantec Consulting Services Inc.
v:\2002\active\2002003607\analysis\cost\mem_thunderhills_cost_final_20160304.docx
THIS PAGE WAS INITIONALLY LEFT BLANK.
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