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Engineers Surveyors Planners
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June 8, 2018
Project No. 18054
Drainage Assessment
3111 Mountain View Ave. N.
Renton, Washington 98056
The project is located at the above address just north of Coleman Point on Lake Washington.
The Site is a 0.55 acre lot zoned R-8. The lot is inclined down to the west with average grades
of 8-12%. The western portion is mapped as a Landslide Hazard Area. The lot is served by
City of Renton sewer and water. Total lot area is 24,088 s.f., of which approximately 11,370 s.f.
is upland. Currently the lot is developed with a single family residence, driveway/ parking area,
a boat launch driveway to the lake and typical lawn/ landscaping. Existing impervious areas
include 2,498 s.f. of roof area, 3,353 s.f. of driveway and 221 s.f. of walks and patios for a total
of 6,072 s.f.
The proposed driveway, walkway, patios and walls are approximately 1,242 s.f., and the house
roof is 3,979 s.f. with a total impervious coverage of 5,221 s.f. The proposed development will
mainly be in the location of the existing residence and driveway. Shoreline restoration planting
and lawn is proposed for the landscaped area between the residence and Lake Washington.
The upland area of the lot is less than 22,000 s.f. so the project will implement Small Lot BMP
requirements and is required to mitigate an impervious area equal to 10% of the lot area
(1,137 s.f.). Per the attached Geotechnical Engineering Study, infiltration is not recommended
for the site due to poor soils. The Project proposes to provide a green roof for a portion of the
roof area (1,263 s.f.). Basic Dispersion is proposed for that portion of the roof not served by
the green roof (1,450 s.f.).
In order to prevent erosion and trap sediments within the project site, the following BMPs will
be used approximately as shown on the ESC plan:
• Clearing limits will be marked by fencing or other means on the ground.
• The existing paved driveway will be used as a stabilized construction entrance and
stockpile area.
• Silt fencing and straw wattles will be placed along the western limit of construction
between disturbed areas and Lake Washington.
• Mulch will be spread over cleared areas of the site that are not being worked. Mulch will
consist of air-dried straw and chipped site vegetation.