HomeMy WebLinkAboutPermit Revision Drainage Memo Combined (2019-02-22)
February 22, 2019
City of Renton-Utility Systems Division
1055 South Grady Way
Renton, WA 98057
RE Southport Office Permit Revision to Stormwater Plan
Dear Rohini Nair:
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 amount of bus parking along the onsite private roadway Southport
Bristol Avenue. The revision is an owner-driven change to promote alternative transportation options within the
development.
The permit revision proposes to add a 100-foot long section of bus parking just north of office tower 3. The
revision also proposes to extend the current planned parking section east of office tower 3 from 60 feet long to
160 feet long. The change increases impervious area by 1,874 square feet. The effect on stormwater design is
explained in the next paragraph.
Flow Control: As previously stated, there is less than 2,000 square feet of change in overall impervious
surface. Further, the project is exempt from flow control requirements through direct discharge to Lake
Washington so the effect on flow control is not a concern.
Water Quality: 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% pollution-generating 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 impervious pollution-generating area does
not affect the design of the water quality system. The water quality vault has the capacity to treat the
stormwater runoff from 4.33 acres of pollution-generating impervious surface and the project still proposes to
route only 3.51 acres of site area to the water quality vault. The water quality plan, water quality areas, and
supporting calculations from the TIR are attached for your review.
We can provide a copy of the complete TIR for your review if desired, however the city should have this
document on file.
I hope you will consider the small magnitude of this design change and find this memorandum to be adequate
explanation and evidence that this revision will have no negative effects on the current drainage design.
Sincerely,
COUGHLIN PORTER LUNDEEN, INC.
Bart Balko, PE
Project Manager
LEGEND
g.
Call befo
D
or 1-80
UNDER
SER
Description
Impervious Area
Pervious Area
Quantity
4,515 SF
5,688 SF
CURRENT PERMITTED PROPOSED CONDITIONS
PI
V
A
B
C
D
E
F
G
HGARAGE ENTRY
FF=32.00 FF=33.50
17 1817.8
J
K
L
M
N
P
Q
R
S
T
FF=33.50
40'
REVISED PROPOSED CONDITIONS
LEGEND
g.
Call befo
D
or 1-80
UNDER
SER
Description
Impervious Area
Pervious Area
Quantity
6,389 SF
3,680 SF
40'
PI
V
A
B
C
D
E
F
G
HGARAGE ENTRY
FF=32.00 FF=33.50
J
K
L
M
N
P
Q
R
S
T
FF=33.50
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
g. 8-1- D
or 1-800-424-5
UNDERGROU
SERVICE (U
BOEING PROPERTY
BRISTOL PROPERTY
BRISTOL PROPERTY
PUGET SOUND ENERGY PROPERTY
THE HOTEL AT SOUTHPORT
EXISTING 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