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Seattle Public Utilities -
Broodstock Collection
Facility Retrofit
Drainage Report
Revision No. # 1
60 % Draft
January 31, 2020
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SPU Broodstock Collection Facility Retrofit Drainage Report
McMillen Jacobs Associates i Rev. No. 1/Jan 2020
Table of Contents
1.0 INTRODUCTION ................................................................................................................................. 3
1.1 Existing Site Conditions ........................................................................................................... 3
2.0 PROJECTS REQUIRING DRAINAGE REVIEW ................................................................................ 3
3.0 SITE IMPROVEMENTS ...................................................................................................................... 4
3.1 Impervious Surface .................................................................................................................. 4
3.1.1 Site Access Improvements and Access Ramp Reconfiguration ......................................... 4
3.1.2 Concrete Sill ........................................................................................................................ 5
3.1.3 Total Impervious Surface .................................................................................................... 6
3.2 Land Disturbing Activity ........................................................................................................... 6
3.3 Drainage Pipes or Ditches ....................................................................................................... 8
3.4 Hazard Areas ........................................................................................................................... 8
3.5 Redevelopment Project ............................................................................................................ 8
4.0 Drainage Review ................................................................................................................................ 9
4.1 Drainage Review Core Requirements ................................................................................... 10
4.1.1 Core Requirement #1: Discharge at the Natural Location ................................................ 10
4.1.2 Core Requirement #2: Offsite Analysis ............................................................................. 10
4.1.3 Core Requirement #3: Flow Control ................................................................................. 10
4.1.4 Core Requirement #4: Conveyance System .................................................................... 11
4.1.5 Core Requirement #5: Construction Stormwater Pollution Prevention ............................. 11
4.1.6 Core Requirement #6: Maintenance and Operations ....................................................... 11
4.1.7 Core Requirement #7: Financial Guarantees and Liability ............................................... 11
4.1.8 Core Requirement #8: Water Quality Facilities ................................................................. 11
4.1.9 Core Requirement #9: On-site BMPs ............................................................................... 12
4.2 Drainage Review Special Requirements ............................................................................... 12
4.2.1 Special Requirement #1: Other Adopted Area-Specific Requirements ............................ 12
4.2.2 Special Requirement #2: Flood Hazard Area Delineation ................................................ 12
4.2.3 Special Requirement #3: Flood Protection Facilities ........................................................ 12
4.2.4 Special Requirement #4: Source Control.......................................................................... 12
4.2.5 Special Requirement #5: Oil Control ................................................................................. 12
4.2.6 Special Requirement #6: Aquifer Protection Area ............................................................ 12
5.0 Conclusions ..................................................................................................................................... 13
6.0 REFERENCES .................................................................................................................................. 13
SPU Broodstock Collection Facility Retrofit Drainage Report
McMillen Jacobs Associates ii Rev. No. 1/Jan 2020
Distribution
To: Michael Norton, PE Carol Volk
Seattle Public Utilities Seattle Public Utilities
Mary Weber, PE
Seattle Public Utilities
From: Derek Nelson, P.E.
McMillen Jacobs Associates
Prepared By: Noah Hornsby, P.E.
McMillen Jacobs Associates
Reviewed By: Derek Nelson, P.E.
McMillen Jacobs Associates
Revision Log
Revision No. Date Revision Description
0 05/17/2019 30% Draft Submittal
1 01/31/2020 60% Draft Submittal
SPU Broodstock Collection Facility Retrofit Drainage Report
McMillen Jacobs Associates Rev. No. 1/Jan 2020
1.0 INTRODUCTION
The purpose of this Drainage Report is to provide any necessary drainage computations for the Seattle
Public Utilities Broodstock Collection Facility (SPU BCF) replacement project, with sufficient information
that an unbiased third party can review the report and determine that all applicable standards have been
met. All computations within the drainage report will be presented in a rational format with sufficient
information included to allow reviewers to reproduce the results provided. This draft report is based on
the 60% design drawings.
1.1 Existing Site Conditions
The SPU BCF is located on River Mile (RM) 1.7 of the Cedar River (Renton, WA) immediately upstream
of the Interstate 405 Bridge. The only permanent component of the existing facility is a substrate rail,
perpendicular to the flow. The rail consists of buried angle iron with winged struts extending 3 feet both
upstream and downstream, and anchored to the river bed using rebar stakes 2-3 feet in length. Access to
the site is currently made via a concrete access ramp on the south bank of the river.
2.0 PROJECTS REQUIRING DRAINAGE REVIEW
The City of Renton Surface Water Design Manual (SWDM) ( City of Renton, 2017), Section 1.1.1, states
that a drainage review is required for all proposed projects that meet the following conditions:
The project adds or will result in 2,000 square feet or more of new impervious surface, replaced
impervious surface, or new plus replaced impervious surface, OR
The project proposes 7,000 square feet or more of land disturbing activity, OR
The project proposes to construct or modify a drainage pipe/ditch that is 12 inches or more in
size/depth, or receives storm water runoff or surface water from a drainage pipe/ditch that is 12
inches or more in size/depth, OR
The project contains or is adjacent to a flood, erosion, or steep slope hazard area as defined in RMC
4-3-050, or projects located in a land slide area, OR
The project is a redevelopment project proposing $100,000 or more of improvements to an existing
high-use site.
The drainage review requirements list will be compared to the proposed project features and a determination
of whether a drainage review will be required for SPU BCF replacement project will be made.
SPU Broodstock Collection Facility Retrofit Drainage Report
McMillen Jacobs Associates Rev. No. 1/Jan 2020
3.0 SITE IMPROVEMENTS
3.1 Impervious Surface
The City of Renton defines an impervious surface as modified or compacted surfaces like roofs, pavement,
gravel, dirt or the improvement from one surface to a more compacted surface. Additionally, any pervious
surface that modifies the runoff characteristics would also be considered a new impervious surface. The
proposed project includes improvements to site access on the south bank of the river, reconfiguration of the
existing access ramp, and construction of an in-stream concrete sill to attach and operate the new temporary
picket barrier and fish collection facility.
3.1.1 Site Access Improvements and Access Ramp Reconfiguration
It was assumed that SPU will need to utilize a larger crane than is currently in use at the BCF to
accommodate the size and weight of the new temporary picket system. To accommodate the larger crane,
the access road leading to the access ramp from the Cedar River Trail Trailhead parking lot had to be
widened. Using the Autoturn software, and assuming a 100-foot crane with a wheel base of 23’-10”, it was
determined that the access road needed to be widened by 3 feet. The additional square footage will be added
to the north side and run the full length of the access road as shown in Figure 1. See Table 1 for total square
footage of impervious surface added and material used.
The access ramp reconfiguration is shown in Figure 1, and consists of demolishing a portion of the existing
access ramp and reconstructing it approximately 19 feet upstream of its current location to match the
alignment of the new in-stream concrete sill. A 1-foot thick concrete wing wall will be included on the
upstream side of the access ramp to connect the ramp to the sill, and provide an area for the walkway
between the bank and the new trap box to be anchored. The “hammerhead” portion of the access ramp will
also be expanded and extended approximately 12 feet in the upstream direction to accommodate the turning
radius of the larger crane. See table 1 for total square footage of new and/or replaced impervious surface
and materials used.
Table 1. New and Replaced Impervious Surface
Design Component Work Type
Final
Surface
Type
Area
(Sq. Ft)
Existing Access Ramp Concrete Removal Pervious 653
New Access Ramp Concrete Placement Impervious 1,148
New access Ramp Grasscrete Placement Impervious 355
New Access Road Concrete Placement Impervious 419
Total New Impervious Surface 1,922
Net Increase to Impervious Surface 1,269
SPU Broodstock Collection Facility Retrofit Drainage Report
McMillen Jacobs Associates Rev. No. 1/Jan 2020
Figure 1. Access Ramp Reconfiguration
3.1.2 Concrete Sill
The concrete sill will be constructed in-stream between the new Access ramp on the south bank and the
existing rock wall on the north bank as shown in Figure 2. The dimensions of the sill will be 84 feet wide
by 21 feet long resulting in a total area of 1,764 square feet as illustrated in the design drawings. Scour
protection consisting of riprap with a D50 of approximately 8 inches will be included on the upstream and
downstream sides of the sill and will extend for 8 feet in both directions resulting in a total square footage
of 1,344 square feet. The concrete sill and scour protection will be submerged under the existing stream
flows following construction. Since the stream would already be considered an impervious surface, the
concrete sill and scour protection will not be included in the total increase to impervious surfaces
calculation.
SPU Broodstock Collection Facility Retrofit Drainage Report
McMillen Jacobs Associates Rev. No. 1/Jan 2020
Figure 2. Concrete Sill and Scour Protection
3.1.3 Total Impervious Surface
Based on the information presented in this Section 3, the total area of new or replaced impervious surface
for this project is 1,922 square feet with a total net increase of 1,269 square feet. This does not meet the
2,000 square-foot design threshold defined by the City of Renton SWDM Section 1.1.1 for projects
requiring a drainage review.
3.2 Land Disturbing Activity
Land disturbing activity includes demolition, construction, clearing, grading, filling excavation, and
compactions. It is any activity that results in a changes to the existing soil cover or existing soil topography
(City of Renton, 2017). For the SPU BCF project as proposed, this would include all of the areas described
in the impervious surface section plus the area dewatered for construction and the construction staging
areas. Construction will take place over two consecutive seasons so the river will be dewatered in two
phases as shown in Figure 3 and Figure 4. The total area dewatered will be approximately 9,500 square feet
depending on the contractors means and methods. The contractor staging areas will occupy approximately
5,400 square feet. The total combined area of land disturbing activity for the proposed construction will be
Concrete Sill
Scour Protection
SPU Broodstock Collection Facility Retrofit Drainage Report
McMillen Jacobs Associates Rev. No. 1/Jan 2020
approximately 17,500 square feet. This area exceeds the 7,000 square-foot design threshold defined by the
City of Renton SWDM Section 1.1.1 as requiring a drainage review.
Figure 3. Cofferdam Dewatering Plan - Phase 1
SPU Broodstock Collection Facility Retrofit Drainage Report
McMillen Jacobs Associates Rev. No. 1/Jan 2020
Figure 4. Cofferdam Dewatering Plan - Phase 2
3.3 Drainage Pipes or Ditches
There are no drainage pipes or ditches of any type or size that will be constructed, installed, or modified for
the proposed project. This criterion does not apply.
3.4 Hazard Areas
This project is located within a flood hazard area as defined by the Renton Municipal Code, Title 4, and
Chapter 3. The sill and he majority of the Access ramp and access road lie in an area designated by FEMA
flood hazard zone AE which is defined as having 1% chance of being inundated in any given year. This
meets the 4th criterion for requiring a drainage review in the City of Renton SWDM Section 1.1.1.
3.5 Redevelopment Project
This project does propose $100,000 or more in improvements to the existing facility. However, this project
is not being constructed in an existing high-use site. This criterion does not apply.
SPU Broodstock Collection Facility Retrofit Drainage Report
McMillen Jacobs Associates Rev. No. 1/Jan 2020
4.0 Drainage Review
The City of Renton provides a flow chart for determining the type of drainage review required for a specific
project. The flow chart is presented here in Figure 5. Based on the information presented in Section 3.0 and
the path shown through the flow chart, a full drainage review will be required for the SPU BCF project.
The full drainage review requires that the proposed project be in compliance with the full range of core and
special requirements outlined in the City of Renton SWDM Sections 1.2 and 1.3 and summarized in the
following sections.
Figure 5. City of Renton SWDM Drainage Review Flow Chart
SPU Broodstock Collection Facility Retrofit Drainage Report
McMillen Jacobs Associates Rev. No. 1/Jan 2020
4.1 Drainage Review Core Requirements
4.1.1 Core Requirement #1: Discharge at the Natural Location
Core requirement #1 requires that all storm water runoff and surface water from a project must be
discharged at the natural location and not diverted onto or away from downstream properties. The SPU
BCF project as proposed would not significantly alter the location or mode of discharge from the existing
project site. The City of Renton SWDM lists three drainage requirements that a proposed project must
comply with as applicable. Requirements 2 and 3 apply to projects located in a landslide hazard drainage
area or containing a landslide hazard, steep slope hazard area, or an erosion hazard area. The SPU BCF is
not located in an area identified as containing any of these hazards and is exempt from drainage
requirements 2 and 3. Discharge requirement 1 dictates how to deal with concentrated runoff when no
conveyance system exists at the downstream property line. The SPU BCF does not include any proposed
areas to generate a concentrated runoff condition. Core requirement #1 would not apply to the proposed
SPU BCF design.
4.1.2 Core Requirement #2: Offsite Analysis
The SPU BCF project is exempt from this requirement. Per the City of Renton SWDM section 1.2.2, a
proposed project is exempt from core requirement #2 if any of the following are true:
The City of Renton determines there is sufficient information for them to conclude that the project
will not have a significant adverse impact on the downstream and/or upstream drainage system,
OR
The project adds less than 2,000 square feet of new impervious surface, AND less than ¾ acre of
new pervious surface, AND does not construct or modify a drainage pipe/ditch that is 12 inches or
more in size/depth or that receives runoff from a drainage pipe/ditch that is 12 inches or more in
size/depth, AND does not contain or lie adjacent to a landslide hazard, steep slope hazard area, or
erosion hazard area as defined in RMC 4-3-050, OR
The project does not change the rate, volume, duration, or location of discharges to and from the
project site (e.g., where existing impervious surface is replaced with other impervious surface
having similar runoff-generating characteristics, or where pipe/ditch modifications do not change
existing discharge characteristics).
The SPU BCF project only adds 1,269 square feet of new or replaced impervious surface as described in
Section 3.1, and 653 square feet of new pervious surface. Additionally, no drainage pipes or ditches of any
size are being constructed, removed, or altered in any way.
4.1.3 Core Requirement #3: Flow Control
The SPU BCF project is exempt from this requirement. Per the City of Renton SWDM section 1.2.3, a
proposed project is exempt from core requirement #3 if it meets the following criteria:
Less than 5,000 square feet of new plus replaced impervious surface will be created, AND
Less than ¾ acres of new pervious surface will be added.
SPU Broodstock Collection Facility Retrofit Drainage Report
McMillen Jacobs Associates Rev. No. 1/Jan 2020
Based on the areas of new plus replaced impervious surface described in Section 3.1 and the addition of no
new impervious surface to the project, the SPU BCF meets this criteria and is exempt from core requirement
#3.
4.1.4 Core Requirement #4: Conveyance System
The SPU BCF is exempt from this requirement. There are no new pipe systems, culverts, ditches/channels,
tightline systems, or bridges being installed or constructed for this project. Additionally, there are no
existing onsite or offsite conveyance systems within the project limits identified in the design drawings.
4.1.5 Core Requirement #5: Construction Stormwater Pollution Prevention
Core Requirement #5 states that “ all proposed projects that will clear, grade, or otherwise disturb the site
must provide erosion and sediment controls to prevent, to the maximum extent practical, the transport of
sediment from the project site to downstream drainage facilities, water resources, and adjacent properties”.
Additionally, spill controls are required to prevent, reduce, or eliminate the discharge of pollutants to on-
site or adjacent stormwater systems or watercourses. The SPU BCF design drawings show the minimum
erosion and sediment control measures required for construction of this project including;
Silt fences at the bottom of all riverbank slopes and around construction staging areas,
Concrete washout areas in the south staging area,
The City of Renton standard erosion and sediment control notes and recommended construction
sequence, and
Requirements that the contractor develop a comprehensive Construction Stormwater Pollution
Prevention Plan that meets this City of Renton requirements outline in the SWDM.
4.1.6 Core Requirement #6: Maintenance and Operations
To be completed at 90%
4.1.7 Core Requirement #7: Financial Guarantees and Liability
The SPU BCF is exempt from this requirement. There are no permanent flow control/water quality
facilities, conveyance systems, erosion control, or road drainage included in the design of this project. No
drainage facilities as defined by the City of Renton SWDM will be constructed as part of this project.
4.1.8 Core Requirement #8: Water Quality Facilities
The SPU BCF is exempt from this requirement. The City of Renton SWDM provides four possible
exemptions from core requirement #8. The first exemption, and the one that applies to this project, states
that a proposed project or any threshold discharge area within the site of a project is exempt if it meets all
of the following criteria:
SPU Broodstock Collection Facility Retrofit Drainage Report
McMillen Jacobs Associates Rev. No. 1/Jan 2020
Less than 5,000 square feet of new plus replaced pollution generating impervious surface (PGIS)
that is not fully dispersed will be created, AND
Less than ¾ acres of new pollution generating pervious surface (PGPS) that is not full dispersed
will be added.
There are no PGIS or PGPS planned for this project. Also, based on the areas of new plus replaced
impervious surface described in Section 3.1 and the addition of no new impervious surface to the project,
the SPU BCF meets this criteria and is exempt from core requirement #8.
4.1.9 Core Requirement #9: On-site BMPs
To be completed at 90%
4.2 Drainage Review Special Requirements
4.2.1 Special Requirement #1: Other Adopted Area-Specific Requirements
The project site location for the SPU BCF is covered in the Lake Washington/Cedar/Sammamish Watershed
(WRIA 8) Chinook Salmon Conservation Plan. Implementation of habitat protection and restoration
projects described in the plan is voluntary as described in the Site-Specific Projects section of the plan. For
the SPU BCF project, the City of Renton SWDM will be the guidance document for this report.
4.2.2 Special Requirement #2: Flood Hazard Area Delineation
To be completed at 90%
4.2.3 Special Requirement #3: Flood Protection Facilities
The SPU BCF is exempt from this requirement. This project will not rely on any existing flood protection
facility for protection against hazards posed by erosion or inundation. This project will also not modify or
construct a new flood protection facility.
4.2.4 Special Requirement #4: Source Control
The SPU BCF is exempt from this requirement. There is no commercial building or commercial site develop
permit required for this project.
4.2.5 Special Requirement #5: Oil Control
The SPU BCF is exempt from this requirement. The development of this project site will not include high-
use site characteristics, does not involve $100,000 or more to a high use site, and all impervious and
pervious surface improvements fall below the threshold for this requirement.
4.2.6 Special Requirement #6: Aquifer Protection Area
The SPU BCF project is located in Zone 1 of the City of Renton’s Aquifer Protection Area (APA) as
identified in RMC 4-3-050 and Section 15B of the SWDM. The project as designed does not include any
open facilities, on-site BMPs that rely on infiltration, or open conveyance systems. The requirements of
special requirement #6 have been met.
SPU Broodstock Collection Facility Retrofit Drainage Report
McMillen Jacobs Associates Rev. No. 1/Jan 2020
5.0 Conclusions
To be completed at 90%
6.0 REFERENCES
City of Renton, 2017. Surface Water Design Manual. City of Renton Public Works Department, Surface
Water Utility, December 12, 2016.