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CITY OF EVERETT STORMWATER MANAGEMENT MANUAL Within Lawn Areas (5 percent organic content)—Place and rototill 1.75 inches of composted material into 6.25 inches of replaced soil(a total amended depth of about 9.5 inches,for a settled depth of 8 inches). Subsoils below this layer should be scarified at least 4 inches,for a finished minimum depth of 12 inches of uncompacted soil. Water or roll to compact soil to 85 percent of maximum. Rake to level, and remove surface woody debris and rocks larger than 1 inch in diameter. OPTION 3 —Import Topsoil Meeting Organic Matter Content Standards: Scarify or till subgrade in two directions to 6 inches depth. Entire surface should be disturbed by scarification. Do not scarify within drip line of existing trees to be retained. Within Stormwater Drainage System locations or Landscaped Areas (10 percent organic content)—Use imported topsoil mix containing 10 percent organic matter(typically around 40 percent compost). Soil portion must be sand or sandy loam as defined by the USDA. Place 3 inches of imported topsoil mix on surface and till into 2 inches of soil. Place 3 inches of topsoil mix on the surface. Rake smooth and remove surface rocks over 2 inches diameter. Mulch planting beds with 2 inches of organic mulch. Within Lawn Areas (5 percent organic content)—Use imported topsoil mix containing 5 percent organic matter(typically around 25 percent compost). Soil portion must be sand or sandy loam as defined by the USDA. Place 3 inches of imported topsoil mix on surface and till into 2 inches of soil. Water or roll to compact soil to 85 percent maximum. Rake to level,and remove surface rocks larger than 1 inch in diameter. Additional Information on Soil Depth After determining the elevation to which a site must be graded for drainage and other reasons,estimation of the changes in soil depth and height need to be calculated. A final grade of the soil should range between one-half inch and 2 inches below the elevation of sidewalks,driveways, and other impervious surfaces on the site. The difference in volume of the dense verses the loose soil condition is determined by the"fluff factor"of the soil. The fluff factor of compacted subsoils in the Puget Sound Area tends to be between 1.3 and 1.4. Rototilling typically penetrates the upper 6 to 8 inches of the existing soil.Assuming only a 6-inch depth is achieved,this depth adjusted by the fluff factor will correspond to a 7.8-to 8.4-inch depth of loose soil. This loose volume would then be amended at a 2:1 ratio of loose soil to compost,corresponding to an imported amendment depth of approximately 4 inches for this example. In the loose state,both the soil and compost have a high percentage of pore spaces(volume of total soil not occupied by solids). The resulting change in elevation must account for compost settling into void spaces of the loose soil(assume 15 percent of the soils' void spaces become occupied by compost particles). After compost incorporation, the amended site will undergo some degree of compaction by the rolling procedure and the weight of the soil itself. Assume a compression factor of 1.15 for soils with a 1.3 fluff factor and 1.2 for soils with a 1.4 fluff factor(15 to 20 percent of the soils' void spaces will become occupied by compost particles.) The resulting change in elevation for a site amended to a 6-inch depth will be approximately 3 inches. Volume 111—Flow Control and Treatment 12-5