Laserfiche WebLink
CITY OF EVERETT STORMWATER MANAGEMENT MANUAL <br /> below and under each individual BMP for the application rate for each particular BMP <br /> technique.) <br /> • There is the potential that increasing the moisture content within the soil could also increase soil <br /> instability in areas with steep slopes. However,the Washington State Department of <br /> Transportation(WSDOT)has been incorporating compost-amendment in almost all of its <br /> vegetated sites since 1992 and has not experienced problems, even on the steepest sites <br /> (33 percent slope), as a result of the increased moisture holding capacity within the soils. (Note: <br /> See design criteria below for requirements of steep slope soil amendment.) <br /> Design Criteria <br /> Several options are available for creating the appropriate soil quality and depth and are listed below. <br /> There are different desired soil quality and depths depending on the function at the location that is being <br /> amended(e.g.,either a part of the stormwater drainage system, landscaped area,or lawn). Follow the <br /> option that works best with the site and then refer to Table 12.1 for estimating soil depth and height <br /> changes when applying soil amendments. <br /> OPTION 1 —Amend Existing Soils in Place: <br /> Scarify or till existing subgrade to 4 inches depth(or to depth needed to achieve a total depth of 12 inches <br /> of uncompacted soil after calculated amount of amendment is added, see specific subsections below). <br /> Entire surface should be disturbed by scarification. Do not scarify within drip line of existing trees to be <br /> retained. <br /> Within Stormwater Drainage System locations or Landscaped Areas (10 percent organic content)—Place <br /> and rototill 3 inches of composted material into 5 inches of soil(a total depth of about 9.5 inches,for a <br /> settled depth of 8 inches). As noted previously, subsoils below this layer should be scarified at least <br /> 4 inches, for a finished minimum depth of 12 inches of uncompacted soil. Rake beds to smooth and <br /> remove rocks larger than 2 inches diameter. Mulch areas with 2 inches of organic mulch. <br /> Within Lawn Areas (5 percent organic content)—Place and rototill 1.75 inches of composted material <br /> into 6.25 inches of soil (a total amended depth of about 9.5 inches,for a settled depth of 8 inches). As <br /> noted previously,subsoils below this layer should be scarified at least 4 inches,for a finished minimum <br /> depth of 12 inches of uncompacted soil. Water or roll to compact soil to 85 percent of maximum. Rake <br /> to level, and remove surface woody debris and rocks larger than 1 inch in diameter. <br /> OPTION 2—Stockpile Site Topsoils Prior to Grading for Reapplication: <br /> If placed topsoil plus compost or other organic material will amount to less than 12 inches: scarify or till <br /> subgrade to depth needed to achieve 12 inches of loosened soil after topsoil and amendment are placed. <br /> Entire surface should be disturbed by scarification. Do not scarify within drip line of existing trees to be <br /> retained. <br /> Stockpile and cover soil with weed bather material that sheds moisture yet allows air transmission, in <br /> approved location, prior to grading. <br /> Replace stockpiled topsoil prior to planting. <br /> Within Stormwater Drainage System locations or Landscaped Areas (10 percent organic content)—Place <br /> and rototill 3 inches of composted material into 5 inches of replaced soil(a total depth of about <br /> 9.5 inches, for a settled depth of 8 inches). Subsoils below this layer should be scarified at least 4 inches, <br /> for a finished minimum depth of 12 inches of uncompacted soil. Rake beds to smooth and remove rocks <br /> larger than 2 inches in diameter. Mulch areas with 2 inches of organic mulch or stockpiled duff. <br /> Volume III—Flow Control and Treatment <br /> 12-4 <br />