HomeMy WebLinkAboutReviewed RS_Southport Office Roundabout Drainage Memo_180403_v1.pdfApril 3, 2018
City of Renton-Utility Systems Division
1055 South Grady Way
Renton, WA 98057
RE Southport Office Permit Revision to Stormwater Plan
Dear Ian Fitz-James:
I am writing to inform you of the changes to the stormwater plan resulting from the permit revision to the
Southport Office project to add a small roundabout to the onsite private roadway Southport Bristol Avenue.
The permit revision proposes to add a roundabout to decrease traffic queuing concerns related to hotel and
office events that occur during daily commuting times. The proposed revision increases the overall impervious
area of the project by 317 square feet (see figures attached that compare proposed revision conditions to
permitted conditions).
The increase in impervious area will have a negligible effect on the functionality of the stormwater system. The
water quality vault has volume sufficient to treat stormwater runoff from 4.33 acres of impervious surface. The
project proposes to route only 3.51 acres of site area to the water quality vault.
The project is exempt from flow control requirements through the direct discharge to Lake Washington so the
effects on flow control is not a concern.
The water quality system for the area being revised consists of a water quality vault that treats the runoff from
road, parking, sidewalks, and landscape areas. The vault is designed under the assumption of 100%
impervious area for the area it treats. The overall area routed to the water quality vault is not changing under
this revision, so the small increase in percentage of impervious area does not affect the design of the water
quality system. The water quality plan, water quality areas, and supporting calculations from the TIR are
attached for your review. The complete TIR has been submitted for your review as well.
I hope you will consider the small magnitude of this design revision and its negligible impact on the stormwater
design and permit the revision without further analysis and review.
Sincerely,
COUGHLIN PORTER LUNDEEN, INC.
Bart Balko, PE
Project Manager
TRUE NORTH
8-1-1
re you
-424-5555
GROUND
CE (USA)
BRISTOL PROPERTY
PUGET SOUND ENERGY PROPERTY SHEET x03LEGEND
Description
Sidewalk
Impervious Area
Roadway
Impervious Area
Pervious Area
Extents of
ASI 023 Revisions
Quantity
4,005 SF
22,005 SF
2,744 SF
28,754 SF
PERMITTED PROPOSED CONDITIONS
TRUE NORTH
8-1-1
re you
-424-5555
GROUND
CE (USA)
BRISTOL PROPERTY
PUGET SOUND ENERGY PROPERTY SHEET x03ROUNDABOUT REVISION PROPOSED CONDITIONS
LEGEND
Description
Sidewalk
Impervious Area
Roadway
Impervious Area
Pervious Area
Extents of
ASI 023 Revisions
Quantity
3,398 SF
22,929 SF
2,427 SF
28,754 SF
ROUNDABOUT ADDITION
(TOTAL INCREASE OF 317
SF OF IMPERVIOUS AREA)
Scale: 1"=100'
FIGURE 5A
PROPOSED WATER QUALITY
LEGEND
Description
Area Routed to
Ex Wetvault
Area Routed to
Ex Filterra
Quantity
3.51 Acres
2.55 Acres
SHEET x01BOEING PROPERTY
BRISTOL PROPERTY
BRISTOL PROPERTY
PUGET SOUND ENERGY PROPERTYSHEET x02THE HOTEL AT SOUTHPORT
SHEET x03SHEET x04EXISTING WATER
QUALITY VAULT TO
TREAT 3.51 ACRES
EXISTING STORMFILTER
TO TREAT 2.55 ACRES
2001 SOUTHPORT MASTER PLAN2001 MASTER PLAN WATER QUALITY
EXISTING WETVAULT CALCULATIONSFROM 2001 SOUTHPORT MASTER PLAN
EXISTING WETVAULT CALCULATIONS
FROM 2001 SOUTHPORT MASTER PLAN
Southport Wetvault Sizing Calculations
Following the procedure outlined in 6.4.1.1 of the King County Stormwater Design Manual:
Step 1: Identify required wetpool volume factor
𝑒=3 (𝑟𝑖𝑛𝑎𝑒 𝑎𝑎𝑟𝑖𝑎)
Step 2: Determine rainfall (R) for the mean annual storm
𝑅𝑟𝑖𝑛𝑒 𝐹𝑖𝑒𝑢𝑟𝑒 6.4.1.𝐴,
𝑅=0.47"= 0.039′
Step 3: Calculate runoff from the mean annual storm
𝑅𝑜=𝑅(0.9𝐴𝑖+0.25𝐴𝑡𝑑+0.10𝐴𝑡𝑑+0.01𝐴𝑜)
𝐴𝑖=190,000 𝑅𝐹
𝐴𝑡𝑑=0.00 𝑅𝐹
𝐴𝑡𝑑=0.00 𝑅𝐹
𝐴𝑜=0.00 𝑅𝐹
𝑅𝑜=( 0.039 𝐹𝑅)(0.9)(190,000 𝑅𝐹)
𝑅𝑜=6,669 𝐶𝐹
Step 4: Calculate required wetpool volume
𝑅𝑏=𝑒𝑅𝑜
𝑅𝑏=(3)(6,669 𝐶𝐹)
𝑅𝑏=20,007 𝐶𝐹
With inside dimensions of 178.5’ x 25’, and a live storage depth of 4.8’, the provided volume of the existing
vault is approximately 21,420 CF, so,
21,420 𝐶𝐹>20,007 𝐶𝐹
𝑅𝑜𝑜𝑜𝑡𝑖𝑑𝑑𝑑>𝑅𝑜𝑑𝑜𝑡𝑖𝑜𝑑𝑑
Therefore, the existing vault is adequate for reuse as a wetvault to provide basic water quality
treatment.
Additionally, the existing wetvault must meet the requirements of City of Renton Amendment to KCSWDM
Section 6.4.2.
PROJECT Southport Hotel DESIGNED BY BGG DATE 04/01/2015
PROJECT NUMBER C140507-01 CLIENT MG2 CHECKED BY BSB SHEET OF
Wetvault sized
for 4.33 Acres
UPDATED USING EXISTING VAULT SIZE
FOR THE 2015 STORMWATER PLAN