HomeMy WebLinkAboutL2STA-TIR-Final20150415Lake to Sound Trail - Segment A
Final Technical Information Report
Drainage and Floodplain
Prepared for
King County
Division of Capital Planning and Development
Facilities Management Division, DES
King County Administration Building
500 Fourth Avenue, Room 320
Seattle, Washington 98104
Prepared by
Parametrix
719 2nd Avenue, Suite 200
Seattle, WA 98104
T. 206.394.3700 F. 1.855.542.6353
www.parametrix.com
CITATION
Parametrix. 2015. Lake to Sound Trail - Segment A
Final Technical Information Report
Drainage and Floodplain. Prepared by Parametrix, Seattle, Washington. April 2015.
CERTIFICATION
The technical material and data contained in this document were prepared under the supervision and direction of the undersigned, whose seal, as a professional engineer licensed to practice
as such, is affixed below.
Prepared by Craig Buitrago, P.E.
Checked by Julie Brandt, P.E.
Approved by Jenny Bailey, Project Manager
TABLE OF CONTENTS
1. OVERVIEW 1-1
1.1 PROPOSED PROJECT 1-1
1.2 EXISTING SITE CONDITIONS 1-1
1.3 DEVELOPED SITE CONDITIONS 1-13
2. CONDITIONS AND REQUIREMENTS 2-1
3. OFFSITE ANALYSIS 3-1
3.1 STUDY AREA DEFINITION AND MAPS 3-1
3.2 RESOURCE REVIEW 3-1
3.3 FIELD INSPECTION 3-1
3.3.1 TDA 1 (A-Line Stations 1+00 to 14+82) 3-2
3.3.2 TDA 2 (C-Line Stations 201+11to206+50) 3-2
3.3.3 TDA 3 (B-Line Stations 101+77 to 110+00) 3-2
3.3.4 TDA 4 (B-Line Stations 110+00 to 132+50) 3-3
3.3.5 TDA 5 (B-Line Stations 132+50 to 143+17) 3-3
3.4 DRAINAGE SYSTEM AND PROBLEM DESCRIPTIONS 3-3
3.5 MITIGATION OF EXISTING OR POTENTIAL PROBLEMS 3-3
4. FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN 4-1
4.1 EXISTING SITE HYDROLOGY (PART A) 4-1
4.2 DEVELOPED SITE HYDROLOGY (PART B) 4-1
4.3 PERFORMANCE STANDARDS (PART C) 4-2
4.3.1 Flow Control 4-2
4.3.2 Conveyance System 4-3
4.3.3 Water Quality Treatment 4-3
4.4 FLOW CONTROL SYSTEM (PART D) 4-3
4.5 WATER QUALITY (PART E) 4-4
5. CONVEYANCE SYSTEM ANALYSIS AND DESIGN 5-1
5.1 MONSTER ROAD STORM DRAIN SYSTEMS 5-1
5.2 TDA 3 CATCH BASIN 5-1
5.3 TDA 4 BOX CULVERT 5-1
6. SPECIAL REPORTS AND STUDIES 6-1
6.1 FLOODPLAIN ANALYSIS 6-1
6.2 STREAM DISCIPLINE REPORT 6-1
6.3 CRITICAL AREA STUDY (DRAFT) 6-1
6.4 VEGETATION AND WILDLIFE DISCIPLINE REPORT (DRAFT) 6-1
7. OTHER PERMITS 7-1
8. CSWPPP ANALYSIS AND DESIGN 8-1
9. BOND QUANTITIES, FACILITY SUMMARIES, AND DECLARATION OF COVENANT 9-1
10. OPERATIONS AND MAINTENANCE MANUAL 10-1
11. REFERENCES 11-1
LIST OF FIGURES
1-1 TIR Worksheet 1-2
1-2 Site Location 1-10
1-3 Drainage Basins, Subbasins, and Site Characteristics 1-11
1-4 Soils 1-12
LIST OF TABLES
1-1 Effective Impervious Area Comparison 1-14
2-1 Summary of Core and Special Requirements 2-1
4-1 Comparison of Peak Runoff Rates 4-4
APPENDICES
A Project Design Drawings
B Land Cover Area Calculations
C Offsite Analysis - Resource Review and Site Visit Photos
D KCRTS Runoff Modeling Documentation
E Box Culvert Design Calculations
F Grading Within the Floodplain - Cut and Fill Calculations
G King County Surface Water Design Manual Operation and Maintenance Excerpts
ACRONYMS AND ABBREVIATIONS
ADS Advanced Drainage Systems
BMP best management practice
BNSF Burlington Northern Santa Fe
cfs cubic feet per second
CSWPPP construction stormwater pollution prevention plan
EIA effective impervious area
FEMA Federal Emergency Management Agency
FHWA Federal Highway Administration
GIS geographic information system
KCRTS King County Runoff Time Series
KCSWDM King County Surface Water Design Manual
M&O maintenance and operations
NEPA National Environmental Policy Act
NPDES National Pollutant Discharge Elimination System
NRCS Natural Resources Conservation Service
Renton Addendum City of Renton 2010 Amendments to the 2009 King County Surface Water Design Manual
Segment A Lake to Sound Trail Segment A
TIR technical information report
TDA threshold discharge area
Tukwila Addendum City of Tukwila Infrastructure Design and Construction Standards
WSDOT Washington State Department of Transportation
OVERVIEW
Proposed Project
King County, together with the Cities of Renton and Tukwila, the Washington State Department of Transportation (WSDOT), and the Federal Highway Administration (FHWA), is proposing to
develop a 1.2-mile-long trail segment of what will ultimately be the 16-mile-long Lake to Sound Trail. This trail, known as the Lake to Sound Trail Segment A (Segment A) or the Two Rivers
Trail, will provide non-motorized access to recreation and employment centers for a variety of users such as bicyclists, pedestrians, runners, wheelchair users, and skaters.
This technical information report (TIR) provides stormwater management documentation for Segment A and has been prepared in accordance with the 2009 King County Surface Water Design
Manual (KCSWDM, King County 2009a). The TIR Worksheet (Figure 1-1) provides a general overview of the components of this report. A site location map is presented in Figure 1-2. Design
drawings for the proposed project are provided in Appendix A.
Existing Site Conditions
The Segment A project corridor lies within the Black River drainage basin, as shown on Figure 1-2. The corridor extends from the Green River Trail along an informal dirt footpath south
of the Black River, crosses beneath the Burlington Northern Santa Fe (BNSF) and Union Pacific railroad trestles, crosses Monster Road southeast of the Monster Road bridge, crosses over
the Black River east of the Monster Road bridge, and follows an existing gravel maintenance road along the north bank of the Black River and the Black River Riparian Forest until reaching
Naches Avenue SW. Five threshold discharge areas (TDAs) have been identified for the site, as shown on Figure 1-3.
Topography in the site area is relatively flat; therefore, portions of the proposed trail in TDA 1 pass through the Black River floodplain. Surface runoff from most of the site travels
as sheet flow discharging directly to the Black River (TDAs 1 and 3) or to wetlands in the Black River Riparian Forest (TDAs 4 and 5). Runoff from the portion of the proposed trail
that approaches and crosses the existing Monster Road (TDA 2) is collected into the storm drain systems on the north and south sides of the roadway. The north Monster Road storm drain
system drains to an existing ditch that discharges to the Black River on the southeast side of the bridge. The south Monster Road storm drain system is conveyed southwest of Monster
Road and continues onto private property adjacent the Black River. Northeast of the project area near the Black River Riparian Forest, a steep hillside drains towards the proposed site.
There are four existing culverts in this area (TDAs 3, 4, and 5) that convey surface water flows beneath the existing gravel maintenance road. Soils in the project area (Figure 1-4)
were classified west (TDAs 1, 2, and 3) and east (TDAs 4 and 5) of Monster Road. The west area, including Monster Road, is identified as Newberg silt loam—hydrologic soil group A/B—while
the east area is Woodinville silt loam—hydrologic soil group D (NRCS 2013).
Figure 1-1. TIR Worksheet
Page 1
Page 2
Page 3
Page 4
Page 5
Page 6
Page 7
Page 8
Figure 1-2. Site Location
Figure 1-3. Drainage Basins, Subbasins, and Site Characteristics
Figure 1-4. Soils
Developed Site Conditions
The proposed Segment A trail is designed to integrate into the existing landscape with as little alteration as possible. As previously stated, the paved trail will follow an existing
dirt path and maintenance road. The trail will generally follow existing contours and maintain the current surface water flow patterns and discharge locations. Specific elements that
will be included in the Segment A project are:
Constructing a 12-foot-wide asphalt pavement trail with 2-foot-wide shoulders and 1-foot-wide clear zones from the edge of the pavement (approximately 2.9 acres of impervious surface)
Performing minor grading to construct the trail (approximately 1,410 cubic yards of cut and 2,980 cubic yards of fill, disturbing an area of approximately 0.72 acres outside the proposed
trail footprint).
Performing ground improvements, which will disturb an area of approximately 0.17 acres, to mechanically improve the physical properties of weak soils in the vicinity of the proposed
trail bridge footings.
Constructing an undercrossing feature (a fenced canopy supported by posts) beneath two railroad bridges to protect trail users from potential falling debris
Installing a new trail bridge over the Black River to the east of the existing Monster Road Bridge, which cannot be improved to safely accommodate the envisioned trail use
Installing a pedestrian-actuated signal crossing of Monster Road south of the bridge
Installing approximately 250 feet of sidewalk improvements at the southwest approach to Monster Road
Building a retaining wall near the south approach to Monster Road
Building three retaining walls north of the proposed trail bridge over the Black River to minimize fill impacts to adjacent areas
Constructing up to two 10-foot by 20-foot pull-out rest areas (one at the northern perimeter of the Black River Riparian Forest and one just west of the Black River Pump Station)
Installing one box culvert and five new catch basins
Building two retaining walls, one on each side of the box culvert to avoid impacts to adjacent wetlands
Installing split-rail fencing and plantings to minimize the potential for disturbance to sensitive wildlife
Restoring the remaining disturbed areas with native plantings and soil amendments
The Segment A alignment and profile will be adjusted and walls may be designed to minimize cut and fill impacts. Table 1-1 provides a comparison of the existing and proposed effective
impervious area (EIA) for each TDA within the project site. Dispersion best management practices (BMPs) will be used along the trail to maintain the existing sheet flow stormwater patterns
wherever possible. The effective impervious fractions applied to the actual total impervious areas within each TDA are 0 percent of total impervious for portions where full dispersion
is applicable, 50 percent of total impervious where basic
dispersion will be used, and 100 percent of total impervious where no dispersion is feasible. A detailed summary of impervious area calculations for each TDA is provided in Appendix
B.
Table 1-1. Effective Impervious Area Comparison
TDA1
Station
Length
(mile)
Total Area (acre)
EIA2 (acre)
Discharge Location
Start
End
Existing
Proposed
Tukwila
Green River Trail to Monster Road (A-Line Segment)
1a
1+00
7+65
0.13
0.36
0.01
0.13
Sheet flow to Black River
Renton
Green River Trail to Monster Road (A-Line Segment) - continued
1b
7+65
14+20
0.12
0.42
0.03
0.18
Sheet flow to Black River
Monster Road Bridge (C-Line Segment)
2
201+11
206+50
0.10
1.05
0.85
0.87
Ditch outfall to Black River
Monster Road to Naches Avenue (B-Line Segment)
3
101+77
110+00
0.16
0.52
0.12
0.07
Sheet flow to Black River
4
110+00
132+50
0.43
1.36
0.14
0.07
Sheet flow to wetlands
5
132+50
143+17
0.20
0.64
0.05
0.15
Sheet flow to wetlands
Total Project
1.20
1.47
1 TDA = Threshold Discharge Area
2 EIA = effective impervious area
CONDITIONS AND REQUIREMENTS
Both the City of Tukwila and the City of Renton have adopted the KCSWDM with jurisdictional addenda. Therefore, the Segment A trail stormwater design follows the guidelines provided
in the KCSWDM, the City of Renton 2010 Amendments to the 2009 King County Surface Water Design Manual (Renton Addendum, Renton 2010), and the City of Tukwila Infrastructure Design and
Construction Standards (Tukwila Addendum, Tukwila 2010).
All three manuals identified above apply requirements based on the size of a project. The Segment A project will result in greater than 2,000 square feet (approximately 0.05 acre) of
new and replaced impervious surface (Table 1-1); therefore, the project requires a full drainage review. According to Table 1.1.2.A in the KCSWDM, projects subject to a full drainage
review must meet eight core requirements and five special requirements. In addition, the 2010 Renton Addendum lists a sixth special requirement that must be reviewed. The core and special
requirements were evaluated for each TDA and summarized in Table 2-1. The details of the analysis, including the basis of hydrologic and hydraulic design, are discussed in subsequent
sections of this TIR.
Table 2-1. Summary of Core and Special Requirements
Core and Special Requirements
Proposed Stormwater Management Approach
TIR Section
CR1: Discharge Location
Preserve existing discharge locations to Black River and riparian wetlands.
4
CR2: Offsite Analysis
Offsite analysis complete.
3
CR3: Flow Control
TDA 1a is subject to Tukwila Level 2 Conservation Flow Control Standard (match flow durations from one-half of 2-year through 50-year frequencies).
TDAs 1b to 5 are subject to the Renton Peak Rate Flow Control Standard (Level 1/Basic Flow Control) matching existing conditions, (match the 2-, 10-, and 100-year peak rate flows).
The Segment A project is subject to a flow control exemption because it will result in no more than 0.1 cubic feet per second (cfs) increase in the 100-year peak flow rate in each TDA.
4
CR4: Conveyance System
Typically, runoff will sheet flow from the paved trail to the adjacent gravel shoulder and then to existing drainage pathways. At Monster Road, the project will install connections
to the existing roadway conveyance systems to maintain existing drainage patterns.
A new pedestrian bridge will be constructed to cross over the Black River east of the Monster Road Bridge. The bridge is designed to have 3 feet of freeboard from 100 year floodplain
to bottom of superstructure elevation.
A new box culvert at B-Line Station 126+00 will be designed to convey discharge from offsite areas at the 25-year peak flow with 6 inches of freeboard. Near B-Line 105+00, one new catch
basin will be installed to extend an existing pipe.
5
CR5: Erosion and Sediment Control
The construction stormwater pollution prevention plan (CSWPPP) will be prepared and submitted to the Cities of Tukwila and Renton prior to construction as a separate document.
8
CR6: Maintenance and Operations (M&O)
King County will provide maintenance and operations for the trail.
10
CR7: Financial Guarantees and Liability
King County will provide financial information at the time of permit application.
9
CR8: Water Quality
Non-motorized trails are considered non-pollution generating impervious surfaces and water quality treatment is not required. Therefore, a water quality analysis is not required for
the Segment A trail.
4
SR1: Other Adopted Requirements
No area-specific requirements apply to this project.
n/a2
SR2: Flood Hazard Area Delineation
The trail is located within the 100-year floodplain in TDA 1.
6
SR3: Flood Protection Facilities
The trail design provides onsite compensatory storage for trail grading within the 100-year floodplain in TDA 1.
6
SR4: Source Control
This special requirement is not applicable to this project because it does not meet the commercial development permit threshold.
n/a
SR5: Oil Control
This special requirement is not applicable to this project because the trail is not a pollution-generating surface and does not meet the high-use site threshold.
n/a
SR6: Aquifer Protection Area1
This special requirement is not applicable to this project because the trail is not in an Aquifer Protection Area.
n/a
1 Special Requirement 6 is specific to the 2010 Renton Addendum.
2 n/a = not applicable
OFFSITE ANALYSIS
This section presents the Level 1 downstream analysis performed for the site in compliance with KCSWDM Core Requirement Number 2. The intent of the downstream analysis is to identify
potential offsite flooding, erosion, water quality, and/or other hydrology-related issues that may be created or aggravated by the proposed project; evaluate issues identified; and
take appropriate measures to prevent creating or aggravating potential problems. The elements of the downstream analysis are described in the sections below.
Study Area Definition and Maps
The study area includes the proposed Segment A project site, Wetlands 1 through 5, and the overland flow path to the Black River. As shown on Figure 1-3, the study area lies within the
Black River Basin in the Springbrook 17 and 18 Subbasins. The study area extends through the cities of Tukwila and Renton, within Section 13, Township 23N, Range 04E.
Resource Review
The King County geographic information system (GIS) was accessed through the web-based iMAP tool (King County 2013) to identify past studies, known drainage problems, sensitive areas,
wetlands, migrating river studies, King County-designated water quality problems, and floodplain/floodway studies within the study area. Map sets were reviewed for information on land
use, zoning, surface water features, stormwater services, groundwater, critical areas, and shorelines (Appendix C). Based on the review of the King County GIS information, the following
water resource-related elements exist within the study area:
Black River and Green River
Black River Riparian Forest
Floodway and 100-year floodplain
High susceptibility to groundwater contamination within TDA 1
Shoreline designation along the Green River in TDA 1
National Pollutant Discharge Elimination System (NPDES) discharge point in TDA 1
In addition, the Green River is classified as having a Shoreline of Statewide Significance, and the Black River and Springbrook Creek are city-designated Shorelines [Renton Shoreline
Master Program 4-3-090-B].
Field Inspection
Parametrix performed a Level 1 field inspection of the trail corridor on February 12, 2013. The weather was overcast and cool. The purpose of the field inspection was to identify any
drainage or erosion problems downstream of the project corridor. The field inspection was also used to verify TDA boundaries and their connections 0.25 mile downstream of their trail
discharge location. Photographs from the field inspection are provided in Appendix C.
TDA 1 (A-Line Stations 1+00 to 14+20)
TDA 1 begins within the city of Tukwila from Station 1+00 to Station 7+65 between the Green River and the soccer fields at Fort Dent. Land cover consists of well-established grass and
planted trees with an access road composed of a compacted tire track. Soils are classified as Newberg silt loam, hydrologic soil group A/B (NRCS 2013). The ground surface is relatively
flat with small undulations. Runoff in this area infiltrates into the soil on the south side of the proposed trail. Any runoff on the north side of the proposed trail that does not
infiltrate will flow into the Black River. Two railroad trestles and one overhead water line are located between Station 6+95 and Station 8+00. The trestles form a cover over this area
and intercept the rainfall.
The portion of TDA 1 from Station 7+65 to Station 14+82 lies within the city of Renton. Land cover in this area is established grass, shrubs, and trees surrounding a compacted dirt path/access
road. Runoff between Stations 7+65 and 10+50 tends to sheet flow to the north towards the Black River. From Stations 10+50 to 14+20, runoff sheet flows along the path/access road and
then discharges at approximately Station 10+50 to the north and the Black River.
TDA 2 (C-Line Stations 201+11to206+50)
TDA 2 is located along Monster Road and includes areas south and north of the road. The south approach consists of the KP Corporation driveway and roadway, sidewalk, and wide shoulder
associated with Monster Road. The north approach consists of the vegetated area between Monster Road and the south embankment of the Black River, where the proposed pedestrian bridge
approach and abutment will be installed and the pedestrian bridge will cross over the Black River.
Monster Road consists of a 60-foot to 90 foot-wide paved area (roadway, sidewalk, and existing bridge) that will not change from the existing impervious land cover. Runoff from the portion
of the proposed trail that approaches and crosses the existing Monster Road (TDA 2) is collected into the storm drain systems on the north and south sides of the roadway. The north
Monster Road storm drain system drains to an existing ditch that discharges to the Black River on the southeast side of the bridge. The south Monster Road storm drain system is conveyed
southwest of Monster Road and continues onto private property adjacent the Black River. Runoff from TDA 2 sheet flows southeast on top of the bridge and is collected in a piped conveyance
system that discharges to a roadside ditch north of Monster Road, adjacent to C-Line Station 203+70. The ditch flows northwest adjacent to the bridge and discharges to the Black River.
Land cover and related runoff quantities will not change for TDA 2; therefore, the piped conveyance system and roadside ditch are expected to have adequate capacity for the proposed
project.
TDA 3 (B-Line Stations 101+77 to 110+00)
TDA 3 follows along the existing gravel access roadway from Station 101+77 to Station 110+00. Soils in TDA 3 are classified as Woodinville silt loam, hydrologic soil group D (NRCS 2013).
Drainage along the roadway in TDA 3 tends to sheet flow to the southwest along the gravel road and then discharges to the south into the Black River. An ecology block barrier exists
between the existing 12-foot-wide gravel road and the Black River to the south from Station 101+00 to the point at which the access road splits near Station 105+00. Beyond the split
at Station 105+00, the proposed project follows the alignment to the north, where the existing access road narrows to approximately 10 feet. Wetland 5 is located on the northwest side
of the gravel roadway between Stations 102+25 and 106+00. Drainage from this area enters a catch basin, is conveyed beneath the access road through a 12-inch culvert near Station
105+18, and discharges into the Black River. Wetland 6 is located on the southeast side of the trail between Stations 105+90 and 107+10. Drainage in this area sheet flows diagonally
across the gravel roadway from the northwest to the southwest towards Wetland 6. A slight ridge exists at Station 110+00 where TDA 2 ends and TDA 3 begins.
TDA 4 (B-Line Stations 110+00 to 132+50)
Topography in TDA 4 gently slopes from the northwest to the southeast and runoff sheet flows to nearby wetlands. Soils are classified as Woodinville silt loam, hydrologic soil group
D (NRCS 2013). Beginning at Station 110+00, the gravel roadway narrows to approximately 6 feet in width and then narrows to 4 feet around Station 124+00. A low point is located near
Station 126+00 where a small drainage ditch (approximately 12 inches wide) crosses from the north to the south and discharges into Wetlands 3 and 4. The main contribution of flow in
this ditch is from a 12-inch culvert located on the north side of the road that directs drainage under the railway. The gravel road gently slopes uphill towards Station 132+50 where
there is a slight high point. Water from the nearby wetlands slowly flows through the Black River Riparian Forest to the Black River, which is approximately 300 to 1,000 feet south
of the gravel roadway. The water then flows towards the Black River Pump Station dam to the west, then to the convergence point with the Green River.
TDA 5 (B-Line Stations 132+50 to 143+17)
The ground surface in TDA 5 is relatively flat, with a slight downward slope to the south towards the Black River. Soils are classified as Woodinville silt loam, hydrologic soil group
D (NRCS 2013). Runoff from the existing access road/path discharges to the west/southwest to the Wetland 1/2 Complex, which is a series of interconnected wetlands located on either
side of the existing road. The width of the access road/path is approximately 4 feet wide throughout TDA 5. The elevation along the project alignment is very flat between Stations 132+50
and 138+00, holding at approximately 28 feet. Runoff in this area slowly sheet flows to the southwest. A catch basin with a solid lid is located near Station 133+00 on the north side
of the gravel road. This catch basin is connected on the north to an 18-inch ADS (Advanced Drainage Systems high-density polyethylene) pipe and on the south to a 24-inch ADS pipe. This
system appears to convey drainage from the north side of the railway to the Wetland 1/2 Complex located approximately 90 feet to the south of the gravel road. Near Station 139+50, a
12-inch culvert connects the Wetland 1/2 Complex across the access road. Drainage from the wetlands reaches the Black River approximately 600 feet to the southwest. Water flows to the
west in the Black River where it joins with drainage from TDAs 2 and 3 more than 0.25 mile downstream.
Drainage System and Problem Descriptions
The existing drainage systems consists of the cross culverts, and Monster Road conveyance systems described in Section 3.3. There appears to be adequate capacity and no apparent drainage
problems.
Mitigation of Existing or Potential Problems
No existing problems were identified and no potential problems are anticipated. Therefore, no mitigation is proposed.
FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN
The flow control analysis and water quality evaluation for the project is presented in the following sections.
Existing Site Hydrology (Part A)
Existing soil types, land cover, and runoff patterns are described for each TDA in Section 3.2. This section describes runoff modeling assumptions and site parameters used in analyzing
the existing site hydrology.
Drainage Area Delineations
TDA boundaries for the project vicinity were delineated based on existing site topography. Within each identified TDA, the site boundaries were delineated based on the proposed cut-and-fill
line. Some clearing may take place beyond the cut-and-fill line; however, these areas will be restored to vegetated cover. The vegetated areas beyond the cut-and-fill line were not
included in the runoff modeling analysis.
Land Cover
Existing land cover within the site boundary for each TDA was identified based on field investigation and review of aerial photography. The existing cover in TDA 1 was generally modeled
as forest, with the exception of the existing railroad crossings, which were modeled as impervious surface. The existing compacted dirt path/road in TDA 1 was not differentiated from
the forested area in the model. TDA 2 consists of the impervious surface of Monster Road and the vegetated Black River embankment where the bridge approach and abutment will be installed.
The existing land cover was modeled as impervious surface area and till forest. In TDAs 3, 4, and 5, the existing gravel maintenance road was considered impervious surface with an effective
impervious fraction of 0.50 based on Table 3.2.2.E of the KCSWDM. The ineffective impervious portion of the maintenance road was modeled as grass and the remaining area in TDAs 3, 4,
and 5 was modeled as forest.
Soils
Soil types were sorted into hydrologic soil groups based on Natural Resources Conservation Service classifications (NRCS 2013) and model soil types based on Table 3.2.2.B of the KCSWDM.
Newberg silt loam (TDAs 1, 2, and 3) is considered hydrologic soil group A/B and was modeled as till. Woodinville silt loam (TDAs 4 and 5) is considered hydrologic soil group D and
was modeled as till.
Developed Site Hydrology (Part B)
The proposed condition is shown on the design drawings provided in Appendix A. Modeling assumptions and site parameters used in analyzing the proposed site hydrology are discussed in
this section.
Drainage Area Delineations
The proposed trail will maintain the existing drainage patterns along the trail corridor. Therefore, proposed drainage areas will match existing drainage areas.
Land Cover
Within each TDA, proposed land cover was determined by separating the total site area into (1) the portion that will be the proposed trail, and (2) everything else, which will be shrub/forest.
The area of the proposed trail was calculated as 16-foot width (the 12-foot-wide paved trail and two 2-foot-wide gravel shoulders) multiplied by the length of the TDA segment. The trail
and gravel shoulders are considered new impervious area based on the KCSWDM definition. The total impervious area proposed for each TDA was assigned an effective impervious percentage
based on the flow dispersion BMP planned for that portion of the trail. In TDA 2, remaining areas outside of the proposed trail and the impervious surface of Monster Road were modeled
as grass. For all other TDAs, remaining areas within the proposed site boundary will be shrub/forest and were modeled as forest.
BMP Adjustments
Following the guidelines in Table 1.2.3.C of the KCSWDM, impervious portions of the trail that will be fully dispersed were modeled as forest, while portions of the trail that will receive
basic dispersion were modeled as 50 percent impervious and 50 percent grass.
Performance Standards (Part C)
This section discusses applicable design standards from the KCSWDM and the Renton and Tukwila Addenda.
Flow Control
Standards
The Cities of Renton and Tukwila have designated area-specific flow control standards for each of their jurisdictions. The portion of the project within the city of Tukwila (TDA 1a)
is subject to a Level 2 Conservation Flow Control Standard matching existing conditions, which requires proposed target surfaces to match existing runoff flow durations for one-half
of the 2-year frequency through the 50-year frequency (Tukwila 2010). The remainder of the project area is located within the City of Renton and is subject to a Peak Rate Flow Control
Standard (Level 1/Basic Flow Control) matching existing conditions, which requires runoff from proposed target surfaces to match the 2-, 10-, and 100-year peak rate runoff flows for
existing conditions (Renton 2010).
Target Surfaces
Flow control requirements apply to areas identified as target surfaces. Target surfaces for Basic (Peak) Flow Control Areas include new impervious and new pervious surfaces that are
not fully dispersed. Target surfaces for Conservation Flow Control Areas include areas not fully dispersed that are new impervious surface; new pervious surface; existing impervious
surface added since January 8, 2001; and certain types of replaced impervious surface.
Flow Control Exemption
In both jurisdictions, projects are exempt from constructing flow control facilities for a given TDA if the 100-year peak runoff flow from the proposed target surfaces will be within
0.1 cfs of the existing 100-year peak runoff flow.
Conveyance System
The Segment A project will maintain the existing drainage patterns of dispersed sheet flows from the trail surface within TDAs 1, 3, 4, and 5. For off-site runoff that passes through
the project site, existing culverts and storm pipes will either remain undisturbed or be extended to account for the trail alignment and width. TDA 2 is located on Monster Road, and
the proposed trail and sidewalk improvements will require additions to the existing conveyance systems to maintain existing drainage patterns. Also, one new box culvert will be constructed
in TDA 4 in Renton at the existing ditch location near B-Line Station 126+00 to convey the off-site drainage to the opposite side of the trail and discharge in the same location as
the existing ditch.
The proposed pedestrian bridge and abutments are designed to be above the 100-year floodplain elevation. Construction activities will occur outside of the Black River ordinary high water
mark (OHWM) and project improvements will not change the shape of the Black River channel. Therefore, no conveyance or hydraulic analyses were performed for the pedestrian bridge crossing
over the Black River. Further discussion of the pedestrian bridge is provided in Section 5.2.
The existing onsite conveyance systems will not experience a change in contributing flow characteristics as a result of the proposed project, therefore, the proposed modifications to
the Monster Road conveyance systems and trail culverts do not require conveyance capacity analysis. Similarly, the proposed box culvert will replace an existing ditch that flows across
the current access road near station. The contributing runoff will not change as a result of the trail project, therefore, culvert will be sized to convey at least as much capacity
as the existing ditch.
Water Quality Treatment
Water quality treatment is required by the Cities of Renton and Tukwila for runoff from pollution-generating surfaces. The Segment A project will not include any pollution-generating
surfaces; therefore, no water quality treatment is proposed.
Flow Control System (Part D)
Flow Control Facility Exemption
The proposed Segment A project will maintain 100-year peak flows for each TDA within the 0.1 cfs threshold and will therefore be exempt from requirements to construct flow control facilities.
The King County Runoff Time Series (KCRTS, King County 2009b) software was used to simulate runoff flows from existing and proposed conditions (including dispersion BMPs) for the Segment
A project. Table 4-1 summarizes the modeling results. Detailed results of flow modeling for each TDA are provided in Appendix D.
Table 4-1. Comparison of Peak Runoff Rates
TDA
Total Area (acre)
Existing Cover (acre)
Proposed Cover (acre)
100-Year Peak Runoff (cfs)
EIA1
Grass
Shrub/ Forest
EIA1
Grass
Shrub/ Forest
Existing
Proposed
Change
1
0.78
0.04
-
0.73
0.31
0.18
0.29
0.1
0.2
0.1
2
1.05
0.85
-
0.21
0.87
0.18
-
0.4
0.4
0
3
0.52
0.12
-
0.40
0.14
0.07
0.31
0.1
0.1
0
4
1.36
0.14
-
1.22
0.07
0.02
1.27
0.1
0.1
0
5
0.64
0.05
-
0.60
0.15
0.12
0.37
0.1
0.1
0
1 EIA = effective impervious area
Flow Dispersion BMPs
The Segment A project will employ basic and full dispersion BMPs wherever possible. The following design requirements apply:
Basic Dispersion:
The dispersion device is sheet flow provided by the 12-foot-wide paved trail and 2-foot-wide gravel shoulders.
A 10-foot-wide vegetated flow path is provided to meet the requirements of the sheet flow dispersion method.
The dispersion flow path is less than 15 percent in grade, covered in native vegetation, and located between the trail and the downstream drainage feature (wetlands and/or Black River).
There are no landslide hazard areas or septic systems downstream and the dispersion will not create flooding or erosion impacts downstream.
Full Dispersion:
The dispersion device is sheet flow provided by the 12-foot-wide paved trail and 2-foot-wide gravel shoulders.
A 100-foot-wide vegetated flow path is provided to meet the requirements of the sheet flow dispersion method.
The dispersion flow path is less than 15 percent in grade, covered in native vegetation, and located between the trail and a downstream drainage feature (wetlands and/or Black River).
There are no landslide hazard areas or septic systems downstream and the dispersion will not create flooding or erosion impacts downstream.
The locations of dispersion techniques within each TDA are listed in Appendix B.
The Segment A trail, along with the proposed sheet flow dispersion components, will be located within wetland buffers in TDAs 3, 4, and 5. As discussed in the critical area report (Parametrix
2015b), the proposed trail alignment will result in some impacts to adjacent wetland buffers; therefore, a wetland mitigation site will be developed. The critical area report details
the impacts study, the selection of the mitigation site, and the size of the mitigation site.
Water Quality (Part E)
The Segment A project will not include any pollution-generating surfaces; therefore, no water quality treatment is proposed.
CONVEYANCE SYSTEM ANALYSIS AND DESIGN
The Segment A project will maintain the existing dispersed sheet flow drainage patterns within each TDA and leave all existing cross-culverts in place. The project will include the new
conveyance system components discussed below.
Monster Road Storm Drain Systems
Two existing conveyance systems are located on Monster Road: one on the north side of the road, and another along the gutter line of the south sidewalk, in front of the KP Corporation
property (TDA 2).
As shown on the design drawings presented in Appendix A, the project will install a new ramp and pedestrian-actuated signal crossing at the southeast end of the Monster Road bridge (C-Line
Station 202+50). To accommodate the new flow line that will be created by the pedestrian safety improvements, a lateral connection with the 12-inch storm drain pipe will be established
by installing one new catch basin in the gutter and a second catch basin in the existing conveyance system on the north side of Monster Road.
The south trail approach to Monster Road will follow the existing Monster Road sidewalk and a new sidewalk will be installed further north. The new sidewalk will create a new gutter
flow line and a depressed area southwest of the trail ramp. To accommodate for the modified runoff patterns, one new catch basin will be installed in the depression between the sidewalk
and the trail southwest of the trail ramp (C-Line Station 202+37), and a second catch basin will be installed in the gutter near the driveway entrance to the KP Corporation property
(C-Line Station 203+34). Collected runoff will be conveyed via 12-inch storm drain pipe from the catch basin near the trail ramp (C-Line Station 202+37) to the catch basin in the new
gutterline (C-Line Station 203+34), and re-connect to the existing storm conveyance system in the existing catch basin located south side of the trail near C-Line Station 203+33.
Drainage problems have not been documented in this area and the contributing area will be less than or equal to the existing area; therefore, 12-inch storm drain pipes similar to the
existing pipe systems will have adequate capacity to convey runoff. As a result, no calculations were performed.
Pedestrian Bridge
As previously discussed, the proposed pedestrian bridge and abutments will be constructed above the Black River 100-year floodplain elevation.
The Black River 100-year floodplain elevation at the proposed pedestrian bridge is calculated at 22.57 feet (NAVD 88) using the 1995 Flood Insurance Rate Map (FIRM). The trail surface
across the bridge is 6-feet above the floodplain elevation, which allows up to 3-feet in bridge depth from top of trail to the bottom of any bridge element, plus 3-feet of clearance
from the bottom of any bridge element to the 100-year floodplain elevation. Additionally, the north and south pedestrian bridge abutments will be installed outside of the 100 year floodplain
elevation. The 3- foot clearance to base of bridge meets the bridge clearance design requirements for rivers where the 100-year peak flow exceeds 100 cfs (Section 6.03.F of the King
County Road Standards, Section 6.02.F). The project does not propose changes to the Black River channel and all work will be done outside of the ordinary high water mark (OHWM). As
a result, the project does not anticipate the trail bridge improvements to have conveyance impacts to the Black River, and no calculations were performed.
Floodplain elevations are further discussed in Section 6.1, and the bridge design drawings are found in Appendix A.
TDA 3 Catch Basin
The project will install a new catch basin in TDA 3 near B-Line Station 105+05 with a 12-inch storm drain pipe to tie into the existing catch basin. The existing catch basin collects
offsite runoff and overflow from Wetland 5. The proposed trail cannot avoid covering this catch basin; therefore, it will be raised to grade and a solid locking lid will be installed.
To maintain existing flow patterns, the new catch basin will be installed west of the trail perpendicular to the existing catch basin. This area does not have documentation of drainage
problems and the contributing area will be less than or equal to the existing area. Therefore, a 12-inch storm drain pipe extending the existing pipe will have adequate capacity to
convey runoff. As a result, no calculations were performed.
TDA 4 Box Culvert
The project will construct one new box culvert in Renton at the existing ditch location near B-Line Station 126+00 to convey the upstream drainage to the opposite side of the trail and
discharge in the same location as the existing ditch. The existing ditch is 1 foot wide and 1 foot deep with 2:1 side slopes and a longitudinal slope of 5.26 percent. The maximum capacity
of this ditch is 13.29 cfs. No signs of erosion or drainage problems were evident during the downstream analysis. Because there will be no change in the area contributing runoff to
the culvert, the project did not perform a separate drainage basin analysis for contributing flow, and will instead design a culvert that will convey at least 13.29 cfs when half full.
Based on stakeholder negotiations, a concrete box culvert with a 48-inch rise and 40-inch width will be installed. The culvert will be set flat and countersunk with streambed gravel
with a depth of approximately 2 feet at the upstream end and approximately 1 foot at the outfall. The culvert will convey 29.6 cfs at 1 foot deep (1 foot of freeboard) and 59.2 cfs
at full flow capacity. Culvert analysis and design documentation is provided in Appendix E.
SPECIAL REPORTS AND STUDIES
Floodplain Analysis
Floodplain impacts include any site activity that will place material at or below the 100-year floodplain elevation within a floodplain boundary. The Green and Black River floodplain
boundaries and elevations within the vicinity of the Segment A project were identified based on 1995 Federal Emergency Management Agency (FEMA) Federal Insurance Rate Map 53033C, Panels
0975F and 0976F (FEMA 1995). The project team depicted the floodplain boundary in the design drawings and floodplain figures using City of Renton GIS Data (Renton 2015), which is a
digital interpretation of the 1995 FEMA FIRM. The floodplain boundary is shown on Figure 1-3 and in detail in Appendix F.
Approximately 1,050 feet of the Segment A trail alignment is located within the Green River and Black River floodplains in TDA 1, from the connection to the Green River Trail at A-Line
Station 1+00 to Station 11+50. In TDA 3, approximately 350 feet of the proposed trail alignment is located in the Black River floodplain from B-Line Station 101+77 to approximately
Station 105+50. The remainder of the trail in TDA 3 and the entire length in TDA 4 is located adjacent to the floodplain. The trail in TDA 5 is located in the floodplain from B-Line
Station 132+50 to the end of the project at Naches Avenue (B-Line Station 143+17). The FEMA floodplain elevation west of the Black River Pump Station (A-Line Station 1+00 through 17+76,
and B-Line Station 101+78 through 105+50 is 19 feet NGVD 29, which translates to 22.57 feet NAVD 88 in the project datum. East of the Black River Pump Station, the FEMA floodplain elevation
is 15 feet NGVD 29, which translates to 18.57 feet NAVD 88. Based on floodplain elevations, the proposed trail is beneath the 100-year floodplain elevations in TDA 1 from A-Line Station
1+00 to Station 11+50. The remainder of the trail will be constructed above the floodplain elevations.
The proposed vertical alignment of the trail will approximate existing grade as close as possible while providing smooth transitions for ADA compliance and positive drainage towards
the river. Between A-Line Stations 1+00 and 12+25, approximately 217 cubic yards of fill will be placed and approximately 242 cubic yards of excavation will be conducted. The net result
of the project will remove approximately 25 cubic yards of material below the floodplain elevation. Detailed calculations are provided in Appendix F.
Stream Discipline Report
In support of the WSDOT Environmental Classification Summary form for the NEPA Documented Categorical Exclusion process, this report describes streams and aquatic resources in the project
vicinity, including two fish-bearing streams. This report also evaluates potential impacts to streams and stream buffers from the proposed project, presents avoidance and minimization
measures included in the project design, and discusses conceptual mitigation for unavoidable impacts (Parametrix 2015a).
Critical Area Study
A critical area report (Parametrix 2015) has been prepared to evaluate the proposed trail impacts to the wetlands and associated buffers along the trail corridor in Renton. Because of
the amount of wetland buffer impacts created by the new trail alignment, there will be a wetland mitigation site. The critical area report details the impacts study, the selection of
the mitigation site, and the size of the mitigation site.
Vegetation and Wildlife Discipline Report
This report provides information in support of the WSDOT Environmental Classification Summary form for the National Environmental Policy Act (NEPA) Documented Categorical Exclusion process
by describing vegetation and wildlife resources in the project vicinity, evaluating potential impacts to critical areas from the proposed project, and presenting mitigation for potential
impacts (Parametrix 2015c).
OTHER PERMITS
Other permits required for this project include:
City of Tukwila Shoreline Master Program
City of Renton Shoreline Conditional Use Permit
NPDES Construction Stormwater General Permit
CSWPPP ANALYSIS AND DESIGN
The construction stormwater pollution prevention plan (CSWPPP) will be prepared and submitted to the Cities of Tukwila and Renton prior to construction as a separate document.
BOND QUANTITIES, FACILITY SUMMARIES, AND DECLARATION OF COVENANT
The Segment A project is a public improvement project led by King County within the public county right-of-way. Quantities will be prepared for the work elements pertaining to the project,
and an engineer’s estimate will be provided to establish an opinion of cost for the project. However, bonding worksheets usually prepared for private developer projects have not been
developed for this public county project.
OPERATIONS AND MAINTENANCE MANUAL
Operation and maintenance of the Segment A trail will be provided by King County staff after completion of the project. Applicable operation and maintenance guidelines from the KCSWDM
are provided in Appendix G.
REFERENCES
FEMA (Federal Emergency Management Agency). 1995. Flood Insurance Rate Map for King County, Washington, and Incorporated Areas. Map 53033C, Panels 0957F and 0976F. Effective September
29, 1989; Revised May 16, 1995.
King County. 2009a. King County Surface Water Design Manual. King County Department of Natural Resources and Parks. January 2009.
King County. 2009b. King County Runoff Time Series (KCRTS) Hydrologic Simulation Model for Implementing the Runoff-Files Methodology, Version 6.0. King County Department of Natural Resources
and Parks Water and Land Resources Division. March 2009.
King County. 2012. King County Geographic Information System Floodway and Floodplain data layers. Available at: http://www5.kingcounty.gov/gisdataportal/. Data generated 2012.
King County. 2013. King County iMap Interactive Mapping Tool. Accessed at: http://www.kingcounty.gov/operations/gis/Maps/iMAP.aspx. May 2013.
NRCS (Natural Resources Conservation Service). 2013. Web Soil Survey Application. Accessed at: http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm. April 2013.
Parametrix. 2015a. Lake to Sound Trail – Segment A: Stream Discipline Report. Prepared for King County, Seattle, Washington. April 2015.
Parametrix. 2015b. Lake to Sound Trail – Segment A, Final Critical Area Study. Prepared for King County Parks Division. Seattle, Washington. April 2015.
Parametrix. 2015c. Lake to Sound Trail – Segment A: Vegetation and Wildlife Discipline Report. Prepared for King County, Seattle, Washington. April 2015.
Renton, City of. 2010. Amendments to the King County Surface Water Design Manual. City of Renton Public Works Department, Surface Water Utility. February 2010.
Renton, City of. 2015. GIS Data. FEMA Floodplain Boundary. http://rentonwa.gov/government/default.aspx?id=29887. Data Downloaded May 30, 2015.
Tukwila, City of. 2010. Infrastructure Design and Construction Standards. City of Tukwila Public Works Department. Fourth Edition, revised April 2010.
NRCS (Natural Resources Conservation Service). 2013. Web Soil Survey. Accessed at: http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm. April 2013.
Project Design Drawings
Land Cover Area Calculations
Offsite Analysis - Resource Review and Site Visit Photos
KCRTS Runoff Modeling Documentation
Box Culvert Design Calculations
Grading Within the Floodplain - Cut and Fill Calculations
King County Surface Water Design Manual Operation and Maintenance Excerpts