VS 1 Site PlanGig Harbor, WA 983356622 Wollochet Dr. NW.OFFICE (253) 858-3636BARB M (253) 549-3344105.01'35.01'130.00'60.00'
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TO: City of Yelm
DATE: March 16, 2022
FROM: Quinten Foster, Project Engineer
Matt Weber, PE
Tacoma - (253) 383-2422
PROJECT NO.: 2210152.10
PROJECT NAME: Mill Pond
(Includes Vista and Meadows)
SUBJECT: Mill Pond On-lot Infiltration Trenches
This memorandum discusses the drainage requirements for the Mill Pond site development projects and
discusses how on-lot infiltration is feasible and follows the City requirements. The project is located at the
northeast corner of 104th Place SE and Mill Road SE, in Yelm, Washington . The project proposes that each lot
will have an individual infiltration trench to handle roof runoff from each future building. The table below correlates
minimum required trench sizes to a range of potential impervious areas. WWHM was used to size the on -lot
trenches. Refer to Exhibit 1 for an example calculation from WWHM on how minimum trench size was
determined. Exhibit 2 is a reference to the Pierce county standard detail which will be followed for this project as
well.
TRENCH SIZE
Impervious Proposed (SF) Bottom Area Required (SF)
2,400 48
2,600 51
2,800 54
3,000 57
3,200 63
3,400 66
3,600 69
3,800 75
4,000 78
AHBL, Inc.
Quinten Foster,
Project Engineer
Q:\2021\2210152\10_CIV\NON_CAD\REPORTS\On-Lot Infiltration Memo\20220316 Storm Memo 2210152.10.docx 03/16/2022
SLOPE= 0.1% MIN-
a PERFORATED
PIPE
c�LEVEL
� I LEVEL
VARIES
COMPACT BACKFILL
OR TOPSOIL
2' MEN.
PROFILE
ROOF DRAIN —
OVERFLOW —
SPLASH BLOCK
S' MIN.
SEE RESIDENTIAL SEDIMENT
CONTROL STRUCTURE
DETAIL THIS PAGE
10' MIN.
6" PERFORATED PIPE (4"
ACCEPTABLE FOR SINGLE FAMILY
ROOF DOWNSPOUT SYSTEMS)
WRAP TRENCH ENTIRELY
WITH FILTER FABRIC
4j," -Y,' NON FRACTURED DRAIN ROCK
EQUIV. TO WSDOT SPEC. 1103,12(5)
SOLID LOCKING
BAND FINE MESH I
SCREEN TO PIPE
WI S.S. BANDING /--T4'DIA.
BAND PIPE SUPPORTS
TO SIDEWALL W1
STAINLESS STEEL
(SS_) BANDING (2
PLACES, TYP.)
BAND FINE MESH
SCREEN TO PIPE _
Wl SS. BANDING
RESIDENTIAL SEDIMENT CONTROL
STRUCTURE
NOTE:
SEE TABLE 3.5 IN VOLUME III FOR TRENCH LENGTH.
0 RESIDENTIAL ROOF DOWNSPOUT SYSTEM
ORDINANCE: 2021-xxx
Pierce County
Planning and Public Works SECTION A - 11.1 EFFECTIVE DATE: 77112021
2401 S 351h St.
Tacoma, Washington 95409