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ATTACHMENT 7 STORMWATER
MANAGEMENT CONCEPT
SUMMARY OF STORMWATER MANAGEMENT CONCEPTS
I-405 TUKWILA TO RENTON IMPROVEMENT PROJECT - STAGE 3
This document summarizes the conceptual stormwater planning and design for the purposes of
supporting City of Renton permits for the I-405 Tukwila to Renton Improvement Project – Stage 3 (Stage
3). This work will be included with I-405 Renton to Bellevue Project – Stage 1 Project (Stage 1) as one
construction project by WSDOT through design-build contracting.
The scope of Stage 3 falls entirely within the City of Renton.
Project Description
The project will complete one more stage of the project studied in the 2008 I-405, Tukwila to Renton
Improvement Project (I-5 to SR 169 Phase 2) Environmental Assessment. The I-405, Tukwila to Renton
Improvement Project improves approximately 4 miles of I-405 between I-5 in Tukwila (M.P. 0.0) and SR
169 (M.P. 4.0) in Renton.
Other I-405, Tukwila to Renton Improvement Project stages are as follows:
• Stage 1, which added the ½-diamond interchange at SR 515 (Talbot Road) and was previously
completed.
• Stage 2, which adds HOV direct-connector ramps from southbound I-405 to southbound SR 167
and from northbound SR 167 to northbound I-405, and is under construction with an anticipated
completion in 2018.
• Stage 3 is currently funded to be constructed during the same timeframe as the I-405 Renton to
Bellevue Improvement Project – Stage 1.
• Stage 4 and other future stages are unfunded and will be completed at a time yet to be
determined.
Just east of I-5, Stage 3 improvements include widening along southbound I-405 to the inside for
approximately 500 feet to extend the southbound HOV lane west. With this widening, the HOV lane will
be extended approximately 500 feet past the split to the southbound I-405 to northbound I-5 off-
connection. Currently, the southbound HOV lane ends at the off-connection to northbound I-5.
Between SR 167 and SR 169, Stage 3 improvements include combining the two existing interior lanes
(the existing HOV and adjacent general-purpose lanes) to create a new dual lane express toll system in
both directions. Southbound I-405 will be widened to the outside including widening the southbound I-
405 bridge over the Cedar River to add one new southbound lane, and the existing northbound outside
shoulder will be converted into a northbound peak-use shoulder lane. Structures over I-405 at Cedar
and Renton Avenues will be reconstructed to accommodate the widened southbound I-405 roadway.
Note that the final design will continue to develop these stormwater management concepts. The design-
build contract will specify performance standards based on the Washington State Department of
Transportation (WSDOT) Highway Runoff Manual requirements. This design is one solution that will meet
that standard, but this information is subject to change during final design, using equivalent mitigation
measures.
STORMWATER MANAGEMENT CONCEPT
I-405 TUKWILA TO RENTON IMPROVEMENT PROJECT - STAGE 3
Project Watershed
The project is located in the lower elevations of Water Resource Inventory Area 8 – Lake Washington
Watershed. The contributing basins have undergone extensive land use changes in the form of
commercial, urban, and residential development.
Project Drainage Basins
The work falls within the Cedar River drainage basin and extends from Mile Post (MP) 3.04 to MP 4.00.
The project evaluation includes Threshold Discharge Areas (TDAs) which represent individual highway
drainage areas that outlet to a single natural discharge location or multiple natural discharge locations
that combine within ¼ mile downstream (as determined by the shortest flow path). I-405 runoff is
collected within independent WSDOT storm drainage systems. Those systems discharge on each side of
the Cedar River. They are considered one TDA because the outfalls combine in the Cedar River at the
point along the River just downstream of the I-405 crossing.
See Attachment 1 for a vicinity map, and Attachment 2 for an illustration of the TDA boundaries.
Proposal Overview
The Stage 3 stormwater concepts have changes from those that were developed to support the 2008 I
405, Tukwila to Renton Improvement Project (I 5 to SR 169 Phase 2) Environmental Assessment. The
future stages may eventually implement those stormwater management facilities, but the smaller scope
of Stage 3 does not support the advancing the larger concepts associated with full build-out. Within the
Cedar River TDA, the large constructed stormwater treatment wetland and the new discharge to the Cedar
River will not be constructed.
Stage 3 will increase the pollutant generating impervious surface area (PGIS) by 0.35 acre within the City
of Renton.
Table 2. Summary of Stormwater TDAs Associated with Discharges within Renton
Receiving
Waterbody TDA
Existing
PGIS in
TDA
(acres)
Existing
Treatment
(acres)
Proposed
PGIS in
TDA
(acres)
Proposed
Treated
PGIS
Post
Project
(acres)
Cedar River C1 (south of river) 10.15 0.00 10.50 10.50
C1 (north of river) 9.57 2.29 9.57 2.29
See Attachment 2 for an illustration of the TDA boundaries
See Attachment 3 for an illustration of the conceptual spill containment and runoff treatment facilities.
STORMWATER MANAGEMENT CONCEPT
I-405 TUKWILA TO RENTON IMPROVEMENT PROJECT - STAGE 3
Flow Control
The Cedar River is flow control exempt. The conceptual design assumes that flow control will not be
required pending final design review of WSDOT conveyance system capacities. Flow control will be
considered if the conveyance system capacity is not sufficient for post-project condition.
Runoff Treatment
The WSDOT Highway Runoff Manual provides a variety of enhanced runoff treatment BMPs that focus on
removal of sediment and metals. The conceptual design proposes to install compost-amended
biofiltration swales (CABS) runoff treatment to treat 10.50 acres of highway runoff that currently flows
through the spill containment pond located between Cedar and Renton Avenue at Mile Post (MP) 3.60.
These new facilities fall with the City of Renton Cedar Valley Aquifer Groundwater Protection Zones 1 and
will be designed in accordance with Renton Municipal Code 4-3-050 Critical Areas Regulation. A concrete
liner is anticipated, and WSDOT is working through the details of gaining approval for them to be open
top to facilitate maintenance.
The existing media filter drain type 4 runoff treatment facility will remain on west side of I-405 at MP 3.90.
The spill containment pond and runoff treatment on the north side of the Cedar River at MP 3.95 will also
remain.
Conveyance
The projects will replace many structures within the project limits in areas of pavement widening. Closed
conveyance systems will be used as the preferred conveyance method along the I-405 corridor due to the
site constraints.
Roadway widening will require new collection structures along the new edge of pavement. These new
structures will connect to the existing drainage systems. Existing conveyance will be modified as required
to satisfy water quality treatment and flow control measures. The conceptual design has only considered
conveyance as it relates to the feasibility of establishing collection areas for runoff treatment and flow
control. Detailed conveyance plans will be produced during the final design.
STORMWATER MANAGEMENT CONCEPT
I-405 TUKWILA TO RENTON IMPROVEMENT PROJECT - STAGE 3
Fish Barriers
The Stage 3 Project includes known fish barriers within the project limits. There are three culverts
currently on the Washington State Department of Fish and Wildlife (WDFW) fish barrier list:
• Culvert 42: Rolling Hills Creek at MP 3.06 is listed due to velocity
• Culvert 49: Upper Rolling Hills Creek at MP 3.06 is listed due to steep grade
• Culvert 52: Thunder Hills Creek at MP 3.06 is listed due to steep grade
Figure 1: Vicinity Map Showing Culverts in the City of Renton
The culvert crossings will be upgraded to fish passable structures by future I-405 improvement projects.
Upper Rolling Hills Creek
Rolling Hills Creek
Thunder Hills Creek
STORMWATER MANAGEMENT CONCEPT
I-405 TUKWILA TO RENTON IMPROVEMENT PROJECT - STAGE 3
ATTACHMENT 1
Stage 3 Project Vicinity
STORMWATER MANAGEMENT CONCEPT
I-405 TUKWILA TO RENTON IMPROVEMENT PROJECT - STAGE 3
ATTACHMENT 2
Threshold Discharge Area Layout
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Washington State
Department of Transportation
PLAN REF NO
FILE NAME c:\users\blackal\pw_wsdot\d0140551\XL4653_PS_DR_800_TDA_TRIP01.dgn
SUNSET BLVD
HOUSER W AY BYPASS
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Cedar River
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TALBOT ROAD (SR 515)
BENSON ROAD I-405 MP 4.0
END PROJECT
PROPOSED POST-PROJECT TDA BOUNDARIES
TUKWILA TO RENTON
I-405
IMPROVEMENT PROJECT - STAGE 3
I-405 RENTON TO BELLEVUE IMPROVEMENT PROJECT - STAGE 1
ADDITIONAL TDA BOUNDARIES CONSIDERED IN THE
RAINIER AVENUE
SR 167
STORMWATER MANAGEMENT CONCEPT
I-405 TUKWILA TO RENTON IMPROVEMENT PROJECT - STAGE 3
ATTACHMENT 3
Flow Control and Runoff Treatment Drawings
SB I-405
NB I-405
T.23N. R.5E. W.M.
1
1
I-405
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MUSEUM
CITY OF RENTON
NB I-405
SB I-405
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LIBERTY PARK
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CARCO CENTERCENTER RENTON COMMUNITY
40' X 60' VAULT
AREA = 1,715,673 SF
(SOUTH RENTON SEGMENT)
TDA - C1- CEDAR RIVER
2 - 170' X 16' CABS
IMPROVEMENT PROJECT STAGE 3
TUKWILA TO RENTON
STAGE 3 - SHEET 1
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VAULT
30'X80'
2
-
170'X
14' CABS
MP 3.30
MP 3.35
MP 3.40
MP 3.45
MP 3.50 MP 3.55
M
P
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MP
3.75 MP 3.80 MP 3.85 MP 3.90
C58
IE.: 59.20' (24 S
C
H
@ S
W)
IE.: 58.35' (24 S
C
H
@
NE)
ELEV 68.95'
ELEV 66.08'
SHEET
OF
SHEETS
.
Washington State
Department of Transportation
-
LEGEND
POST IMPERVIOUS AREA
IMPERVIOUS AREA TO BE REMOVED
NEW IMPERVIOUS AREA
DATUM
(NAVD) 88
100 200
SCALE IN FEET
0