Laserfiche WebLink
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 worts 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 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 Westem Washington <br />Volume 11 - Chapter 4 - Page 357 <br />