Revision Storm Rev 1.pdfCity of Edmonds
121 5" AVENUE NORTH • EDMONDS, WA 98020 • (425) 771-0220 FAX(425) 771-0221
Website: www.ci.edinonds.wa,us
DEVELOPMENT SERVICES DEPARTMENT
Planning • Building • Engineering
Permit #: BLD20080299
Plan Review Corrections
Date: April 21, 2008
Project Name/Address: Hovde Drainage 123830 - 101 PL W
Contact Person/Address/Fax:
Mary Ann Hovde 1 Fax 206-533-1014
Reviewer: Don Fiene Division: Engineering
During review of the subject submittal it was found that the following information, corrections, or
clarifications will need to be addressed:
1. Need to provide explanation on how percolation tests were performed. A *minimum* of three
tests are required If performed per the first 4 bullets on the attached testing procedure guidelines,
note as such and attach the copy.
2. Need a detail of the system. Attached is an acceptable standard.
3. Show the formula used or use the formula from the attached city handout (show your work).
4. Provide a narrative describing what the system is designed for, with special emphasis on
impervious surface calculation.
Please resubmit three copies of the revised plans/documents to a Development Services Coordinator. You
may contact me at 425-771-0220 if you have specific questions regarding these plan corrections.
DA'Z'E mailed/faxed/emailed 4121108 PAGE 1 OFA 4
SECTION 5A INFILTRATION FACILITIES
the infiltration facility, and at least one test hole should reach the water table. If the water table is very
deep, the test hole need not extend more than one-fourth the maximum width of the pond below the
bottom of a pond, or more than 5 feet below the bottom of a tank. If there is any question about the actual
wet -season water table elevation, measurements shall be made during the period when the water level is
expected to be at a maximum.
Any requirements related to steep slope, landslide hazard, or other sensitive area impacts should also be
addressed in the soil study.
The geotechnical engineer shall provide a report stating whether the site is suitable for the proposed
infiltration facility, and shall recommend a design infiltration rate (see "Design Infiltration Rate"
below).
M, D3
4J4914 etkr, tieiit, (AST 385).
test
S;rrru
of two ,,solls,logs,shal,],beoaiiied for each tank and for each 10,000 square feet (plan
g
V16*4 6f`Prd(p6§ed'�and inhitn'ftionj` surface area, Soils shall be logged for a minimum 'of 5 feet
below the bottom of each proposed infiltration facility. The logs shall describe the SCS series of the
Q, soil, indicate the textural class of the soil horizons throughout the depth of the log, note any evidence
of high groundwater level (such as mottling), and estimate the maximum groundwater elevation, if
within the limits of the log.
In the past, many infiltration facilities have been built which have not performed as the designer intended.
This has resulted in flooding and substantial public expenditures to correct problems. Monitoring of
actual facility performance has shown that the full-scale infiltration rate is far lower than the rate
determined by small-scale testing. Actual measured facility rates of 10% of the small-scale test rate have
been seen. It is clear that great conservatism in the selection of design rates, is needed, particularly where
conditions are less than ideal. Ideally, the design infiltration rate will be determined using an analytical
groundwater model to investigate the effects of the local hydrologic conditions on facility performance,
but this may be excessively costly for small projects,
A simplified method may be used for determining the design infiltration rate by applying correction factors
to the measured infiltration rate. The correction factors account for uncertainties in testing, depth to the
water table or impervious strata, infiltration receptor geometry, and long-term reductions in permeability
9/1/98 5-54 1998 Surface Water Design Manual
5. 1.1 DOWNSPOUT INFILTRATION SYSTEMS
FIGURE 5.1.1.A TYPICAL DOWNSPOUT INFILTRATION SYSTEM
roof
PLAN VIEW drain
NTS 4" rigid or 6" fiexible
perforated pipe
----------------------------------------
infiltration trench sump w/solid lid
PLAN VIEW / roof drain
NTS
overflow
4" rigid or 6" flexible splash block
r perforated pipe
6" awl:/ %iY/: '!iZ/i✓/�/a%/✓/,Y'i:li?%i?%a
-Q --n. ------
-------------- -----„-------- .__--_
Q a v
9° a washed rock 4° Qo t 3' m�in 5.0' min
12"'
1 1/2"-3/4"
•• drn in
------------------ - -- 9 p - 4-p ° 4 -�`.
fine mesh screen CB sump w/solid lid
10' min
varies
A
filter fabric
6"
as °(a
p°pa °po
24"
12"
C ° (3
\O °
�--- 24'�--�I
SECTION A
NTS
compacted backfill
/. rigid er 6" flexible
perforated pipe
washed rock
1 112"-314"
1998 Surface Water Design Manual 911198
5-7
Infiltration Facility Sizing
In select cases, the City of Edmonds will allow infiltration of drainage for proposed
developments or redevelopments. In all cases, properties proposing to install infiltration
facilities must be located close to Type A soils as indicated on the SCS Soil Survey of
Snohomish County Area Washington, and percolation tests performed on the property must
indicate a minimum infiltration rate of 0.0333 inches/minute. In all cases, the final decision to
allow infiltration will rest with the City Engineer.
In cases where infiltration has been allowed, the City of Edmonds allows infiltration facility
sizing based on the following formula:
_ (0.054P,00Ar)
ARI
10("o)
where:
ARI =Area required for infiltration (ft);
P100 =100 -year, 24-hour design storm total precipitation (3.0 inches in City of Edmonds);
A.,. =Tributary area served by infiltration facility(ft);
Zn =Design infiltration rate determined by percolation tests (in/min — cannot exceed 0.333
in/min).
Infiltration facilities must be designed to provide the area required for infiltration, as determined
by the above formula.
Page 1 of 1 Infiltration siAng.DOC