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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 per- <br /> cent), 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 per- <br /> cent), an engineered energy dissipater shall be used. <br /> . Filter fabric or erosion control blankets should always be used under riprap to pre- <br /> vent scour and channel erosion. <br /> . New pipe outfalls can provide an opportunity for low-cost fish habitat improve- <br /> ments. For example, an alcove of low-velocity water can be created by con- <br /> structing 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 shel- <br /> ter 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 /> BMP C220: Storm Drain Inlet Protection <br /> Purpose <br /> Storm drain inlet protection prevents coarse sediment from entering drainage systems <br /> prior to permanent stabilization of the disturbed area. <br /> Conditions of Use <br /> Use storm drain inlet protection at inlets that are operational before permanent sta- <br /> bilization of the disturbed drainage area. Provide protection for all storm drain inlets <br /> downslope and within 500 feet of a disturbed or construction area, unless conveying g Y 9 run- <br /> off entering catch basins to a sediment pond or trap. <br /> Also consider inlet protection for lawn and yard drains on new home construction. These <br /> small and numerous drains coupled with lack of gutters in new home construction can <br /> add significant amounts of sediment into the roof drain system. If possible delay <br /> installing lawn and yard drains until just before landscaping or cap these drains to pre- <br /> 2014 Stormwater Management Manual for Western Washington <br /> Volume ll- Chapter 4 -Page 357 <br />