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DocuSign Envelope ID: F52521 E7-10C1-43F9-B62F-C1 E599A5D572 <br /> The P-Factor—when examining erosion by itself,is commonly taken as 1.0, <br /> si nce thi s assumes that no sped al "practi ces" (i.e.terraci ng, contouri ng, etc.) <br /> wi I I be used. Yet, the use of si I t fences or other storm water management/ <br /> sediment control practices may be integrated into the RUSLE using a P-factor <br /> that is less than 1.0, which reflects the effectiveness of the sediment control <br /> practice in removing sediment from runoff. <br /> Sample Problem A steep slope is to be protected from erosion using RECP. The 3H:1V <br /> 6.17a slope is 100 feet long and comprised of silty loam. The RUSLE will <br /> be used to evaluate the effectiveness of RECP in limiting annual soil <br /> loss. Following are the inputs to the RUSLE equation from the U.S. <br /> Department of Agriculture: <br /> R = 250 <br /> K= 0.33 <br /> LS = 6.2 <br /> P = 1.0 (assuming no sediment control) <br /> From Table 6.17a: <br /> Cunprotected = 1.00 <br /> Cprotected, year 1 = 0.03 <br /> Cprotected, year 2+ = 0.005 <br /> Aunprotected = 250 x 0.33 x 6.2 x 1.0 x 1.0 = 511 tons/acre/year <br /> Aprotected, year 1 = 250 x 0.33 x 6.2 x 0.03 x 1.0 = 15 tons/acre/year <br /> Aprotected, year 2+ = 250 x 0.33 x 6.2 x 0.005 x 1.0 = 3 tons/acre/year <br /> This example shows that vegetation, protected by an RECP, is 97 <br /> percent effective in reducing erosion in the first year and 99.5 percent <br /> effective in the longer-term. <br /> Table 6.17b aids in selecting an appropriate type of RECP for the <br /> project-specific slope. <br /> Drainage Channels Concepts—Permissible shear design is commonly used <br /> to determine if a channel I i ner is stable. This method requires the input of an <br /> appropriate expected flow rate(discharge)as well as the determination of flow <br /> depth. A broader presentation of channel design is located in Appendix 8.05, <br /> Design of Stable Channels and Diversions. <br /> The design flow rate will be based on local storm frequency design standards <br /> and flow depth is calculated - commonly using Manning's equation. With <br /> these inputs the designer can then perform a permissible shear design, which <br /> compares the permissible shear of the prospective I i ner materials to the <br /> expected flow-induced shear as calculated using the equation below. <br /> Tc =YDS <br /> where: <br /> Y=unit weight of water(62.4lb/ft3) <br /> D=depth of flow(ft) <br /> S =channel slope(ft/ft) <br /> 6.17. Rev.6/06 <br />