Drainage Review.doc
Drainage Review: BLD 20090345 Jorgenson SFR 1122 Vista Pl
For this site, I reviewed the following documents:
Letter from Joseph M. Smeby, P.E., Omega Engineering, Everett, WA to Jerry
Shuster, City of Edmonds dated April 27. 2009, RE: Jorgensen SFR: 1120 Vista
Place - Drainage letter, and accompanying attachments.
Site Plan, 1122 Vista Place, pre pared by Hanson Designs, received by City of
Edmonds June 8, 2009
Cover, Jorgensen SFR, prepared by Omega Engineering, Everett, received by
City of Edmonds June 8, 2009.
The redevelopment of this parcel involves the demolition of an existing home and
driveway and construction of a new home and driveway and an infiltration system to
handle the runoff from the roof and eaves. There is information missing from the
submittal, therefore, a complete review cannot be completed by the City:
1. The letter states that the total roof area with eaves is 3,840 square feet and the
infiltration system is designed to mitigate for that area. The letter further states that
runoff from the driveway and steps will be allowed to sheet flow over the future
landscaping west of the improvements and infiltrate naturally into the existing soil. The
applicant needs to provide the following information:
Type of deck, impervious or pervious (uncovered, have boards no wider than 6”,
maintain ½” spacing, and have open loose soil beneath it). If impervious, it must
be added to the roof and eaves area.
Area of the driveway and steps. To use sheet flow dispersion for this area, no
overflow onto the neighboring property to the west will be permitted and the
following criteria apply (King County 2009, Small Site Drainage Requirements):
i) A 2-foot-wide, 4 to 6-inch deep strip of crushed rock must be provided at the edge of
the driveway to facilitate dispersal of runoff
ii) A "vegetated flow path segment" of at least 10 feet in length must be available along
the flow path that the runoff would follow upon the discharge from the strip of crushed
rock.
iii) No more than a 20-foot wide strip of impervious surface may sheet flow in this
manner unless the vegetated flow path segment is increased 10 feet for each additional
20 feet of impervious surface width or fraction thereof.
2. ECDC 18.30.30 A.1 encourages the use of infiltration for flow control for sites that
add more than 2,000 square feet of impervious surface. Exhibit B to the the code
delineates the areas where infiltration may be feasible. This site is not located within
the specified infiltration areas, therefore, as stated in Attachment B, infiltration is not
permitted. That said, if the criteria listed below are demonstrated, an exception will be
considered by the City Engineer per the procedures of ECDC 18.30.065.
More detail is needed prior to the City considering an infiltration system for this
development. City code specifies the 1992 Department of Ecology Stormwater
Management Manual for design criteria. This manual provides detailed site suitability
and evaluation criteria, mostly geared for large sites. The minimum requirements
for consideration of an infiltration system design are as follows:
The design infiltration rate can be assessed one of three ways:
USDA Soil Textural Classification: A minimum of one soils log for each proposed
infiltration location, minimum 2 feet below the bottom of the proposed infiltration
system, that describes the SCS series of the soil and the textural class of each
horizon through the depth of the log, and notes of any evidence of high
groundwater (such as mottling).The textural class for each horizon at an below
the proposed bottom of the infiltration facility should include the %sand, % silt,
and % clay for each. The design infiltration rate for the proposed facility is
smallest of the "Estimated Long Term" infiltration rates based on Table 3.7 of the
2005 Ecology Stormwater Management Manual for Western Washington for the
layers at or below the proposed bottom of the infiltration facility. All soil logs and
a drawing showing the location of the logs must be included.
ASTM Gradation Testing: If the soils for infiltration have a size greater than
D
10
or equal to 0.05 mm, Table 3.8 in the 2005 Ecology Stormwater Management
Manual for Western Washington can be used to estimate the design infiltration
rate.
The Pilot Infiltration Test (PIT) with appropriate correction factors in the 2005
Ecology Stormwater Management Manual for Western Washington
In addition:
The soils log must be prepared by or under the direction of a licensed civil
engineer, engineering geologist, or geotechnical engineer.
Trench bottom must be demonstrated to be a minimum of 1 foot above the
seasonal high groundwater level
Infiltration systems must not be placed in critical areas buffers or on slopes
greater than 25%. Infiltration systems proposed on slopes greater than 15% or
must be approved by a engineering geologist, or geotechnical engineer
Demonstrate an escape route for the runoff if it exceeds the design capacity such
that it does not cause flooding of adjacent properties or public roads.
3. The inflow to the infiltration system must be preceded by a catch basin (or yard
drain with minimum 1 foot of "catch") to settle out any solids in the runoff that may lead
to plugging of the system. The outlet pipe from the yard drain shall be a tee with 1/2"
mesh hardware cloth on both ends of the it.
Sincerely,
Jerry Shuster, P.E.
Stormwater Engineering Program Manager
City of Edmonds
121 5th Ave N.
Edmonds, WA 98020
Desk Phone: 425-771-0220 x1323
Fax: 425-672-5750
shuster@ci.edmonds.wa.us