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• The following guidelines shall be used for riprap outlet protection: <br /> 2014 Stormwater Management Manual for Western Washington <br /> Volume II- Chapter 4-Page 356 <br /> 1. If the discharge velocity at the outlet is less than 5 fps(pipe slope less than 1 <br /> percent), use 2-inch to 8-inch riprap. Minimum thickness is 1-foot. <br /> 2. For 5 to 10 fps discharge velocity at the outlet(pipe slope less than 3 percent), <br /> use 24-inch to 48-inch riprap. Minimum thickness is 2 feet. <br /> 3. For outlets at the base of steep slope pipes(pipe slope greater than 10 percent), <br /> an engineered energy dissipater shall be used. <br /> • Filter fabric or erosion control blankets should always be used under riprap to prevent <br /> scour and channel erosion. <br /> • New pipe outfalls can provide an opportunity for low-cost fish habitat improvements. <br /> For example, an alcove of low-velocity water can be created by constructing <br /> the pipe outfall and associated energy dissipater back from the stream <br /> edge and digging a channel, over-widened to the upstream side, from the outfall. <br /> Overwintering juvenile and migrating adult salmonids may use the alcove as shelter <br /> during high flows. Bank stabilization,bioengineering, and habitat features may <br /> be required for disturbed areas. This work may require a HPA. See Volume V <br /> (p.765) for more information on outfall system design. <br /> Maintenance Standards <br /> • Inspect and repair as needed. <br /> • Add rock as needed to maintain the intended function. <br /> • Clean energy dissipater if sediment builds up. <br /> Page I17 <br />