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

July 2, 2008 Duplex Buildings L&A Job No. 8A067 Page 14 The above BMPs may be used to dispose stormwater onsite for single-family residential projects with 2,000 or more but not more than 10,000 square feet of impervious surfaces. Rain gardens or distribution trenches with flowpaths may be located on each individual lot if possible. Otherwise, it may be designed as a joint -use facility located in a designated drainage tract within the site. Stormwater colleted over impervious surfaces should be routed into a sediment control/oil-water separator structure before being released into rain gardens or distribution trenches. Rain Gardens A rain garden is constructed of a pond lined by a layer of compost -amended soil mix, with wetland vegetation planted in the pond. The slopes of the pond should be no stepper than 3H:1 V. Amended soils consist of minimum 4-inch compose tilled into the top 12 inches of soils or 12 inches of imported sand/compost blend. Water stored in the rain gardens would evaporate into the ambience, be absorbed by vegetation in the rain gardens and infiltrate into the ground through bottom of the rain gardens. The bottom of the ponds of rain gardens should be cut into the layer of weathered soils of medium -dense, silty, fine sand with a trace to some gravel underlying the topsoil. The permeability of this layer of weathered soils is generally from 2.0 to 6.0 (inches per hour). We recommend that an infiltration rate not to exceed 1.0 iph be used for the design of rain gardens in determining their required sizes. Rain gardens should be located on the downhill side of adjacent buildings and be set back at least 5 feet from buildings and property lines. Properly designed and constructed rain gardens can help filtering out oil, grease, herbicides, pesticides and fertilizers. Our. recommendations for the construction of rain gardens are presented on Plate 6. The rain gardens should be excavated LIU & ASSOCIATES, INC.