9205 4TH AVE W 2025-12-03 - Laserfiche WebLink

July 2, 2008 Duplex Buildings L&A Job No. 8AO67 Page 15 about 3 to 3.5 feet below the existing ground surface into the layer of weathered soils underlying the site. The vegetation to be planted in rain gardens may comprise red twig dogwood, rushes, sedges, salmonberry and twinberry. Ornamentals such as royal ferns, big -leaved ligularias or various primroses are also suitable for rain gardens. The downstream rims of the rain garden ponds should be slightly lower than the upstream rims to allow for emergency overflow of water onto a well vegetated downhill area of the site. A 3 feet wide by 2-to-6 inch thick layer of rock spall pad, constructed of 2-to-4-inch rock spalls, should be constructed between the downstream ends of the rain gardens and the downhill vegetated areas for erosion control. Distribution Trenches with Vegetated Flowpaths Stormwater may also be discharged into distribution trenches then dispersed in sheet flows onto well -vegetated flowpaths. The length of a distribution trench should be 10 feet for 700 s.f. of impervious surface served, and should be increased proportionally for impervious surface over 700 s.f. The vegetated flowpath should be at least 25 feet long in the direction perpendicular to the distribution trench and should be located in areas not steeper than 15% grade. The distribution trenches should be located on the downhill side of adjacent buildings, and should be set back at least 5 feet from buildings and property lines. The trenches should be at least 24 inches wide by 30 inches deep. A typical section of the distribution trench and flow path system is shown on Plate 7. The downstream rim of a distribution trench should be a few inches lower than its upstream rim, and should be level to spread the overflowing water from the trench into a sheet flow onto a well -vegetated flowpath. The side walls of the distribution trench should be lined with a layer of non -woven filter fabric. The trench should then be filled with clean washed 3/4 to 1-1/2 inch gravel or crushed rock to within about 8 inches of the finish grade. A 6- LIU & ASSOCIATES, INC.