Northstream Lane_831_White_Stomwater.doc
CITY OF EDMONDS
PLAN REVIEW COMMENTS
ENGINEERING DIVISION
(425) 771-0220
City Website: www.ci.edmonds.wa.us
DATE: October 14, 2008
TO: File
FROM: Jerry Shuster, P.E., Stormwater Engineering Program Manager
RE: Application #: BLD20071046
Project: White Residence
Project Address: 831 Northstream Lane
On September 16, 2008 a correction notice was created for this site requiring the
applicant to provide stormwater controls for the site since, with the unauthorized
replacement of the driveway, the new impervious surface area exceeded the 2,000 square
feet threshold in ECDC Chapter 18.30.
The owner’s contractor, Mr. Lee Elliot (Carpentry by Elliott) had several meeting with
Engineering Staff expressing his displeasure with this requirement and, initially, we were
not able to work out a mutually acceptable solution. On October 1, the owner was sent a
letter detailing the specific steps that needed to be taken to meet the requirements.
Through their Contractor, the applicant chose to use infiltration to satisfy the stormwater
control requirements. This being the case, the letter required an engineered test to
determine the infiltration rate of the on-site soils to size the proposed trench to handle the
runoff from the 760 square foot driveway. This soils test would also be used to assess the
adequacy of the on-site drywell infiltration system installed by Mr. Elliott to handle an
addition 1,680 square feet of new impervious area.
On October 9, 2008, I revived an e-mail from Mr. Elliott with an attachment from a
certified on-site wastewater system designer, Mr. Craig DeClercq. This attachment
documented a “perk test” conducted by Mr. Craig DeClercq at the White residence. The
document described the subsurface soils to a depth of 40 inches below ground surface
(bgs). From 6 to 40 feet bgs the report described the material as “loamy medium sand.”
The perk test was conducted per the EPA Manual (1980) and the resulting infiltration rate
was 2 inches per minute.
The King County Surface Water Design Manual (1998) has Small Site Drainage
Requirements in Appendix C. This appendix has standard designs for infiltration
trenches and drywells. For an infiltration trench, the Manual recommends 30 feet of
trench per 1,000 square feet of impervious area for medium sands. Using this as a guide,
for the 760 square foot driveway, this scales to approximately 23 feet.
Mr. Elliott proposed a 14 foot trench with no pipe or yard drain. I told Mr. Elliott the
driveway runoff should first flow to a yard drain to remove any sediment then flow
through a 4 inch perforated pipe in a trench 20 feet long. The ¾ inch washed rock in the
trench should be wrapped in a non-woven geotextile. He agreed and I red-lined his
submittal.
The King County manual recommends a drywell filled with 2 cubic yards of washed rock
for every 1,000 square feet of impervious area for soils that are medium sands. Scaling
this for the 1,680 square feet of new impervious area, this equates to approximately 3.4
cubic yards of washed rock. On September 23, 2008, Mr. Elliott submittal a
correspondence attesting that he place 5 cubic yards of washed rock into the drywell that
serves this 1,680 square feet of new impervious area. This is adequate rock in the
drywell for this area.