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I BMP C209: Outlet Protection Purpose Outlet protection prevents scour at conveyance outlets and minimizes the III potential for downstream erosion by reducing the velocity of concentrated stormwater flows. I Conditions of use Outlet protection is required at the outlets of all ponds,pipes, ditches, or other conveyances, and where runoff is conveyed to a natural or manmade drainage feature such as a stream, wetland, lake,or ditch. I Design and The receiving channel at the outlet of a culvert shall be protected from Installation erosion by rock lining a minimum of 6 feet downstream and extending up Specifications the channel sides a minimum of 1—foot above the maximum tailwater elevation or 1-foot above the crown, whichever is higher. For large pipes I (more than 18 inches in diameter), the outlet protection lining of the channel is lengthened to four times the diameter of the culvert. • Standard wingwalls, and tapered outlets and paved channels should also be considered when appropriate for permanent culvert outlet protection. (See WSDOT Hydraulic Manual, available through IWSDOT Engineering Publications). • Organic or synthetic erosion blankets,with or without vegetation, are usually more effective than rock, cheaper, and easier to install. IMaterials can be chosen using manufacturer product specifications. ASTM test results are available for most products and the designer can choose the correct material for the expected flow. • With low flows, vegetation(including sod) can be effective. • The following guidelines shall be used for riprap outlet protection: 1. If the discharge velocity at the outlet is less than 5 fps (pipe slope less than 1 percent),use 2-inch to 8-inch riprap. Minimum Ithickness is 1-foot. 2. For 5 to 10 fps discharge velocity at the outlet(pipe slope less than 3 percent),use 24-inch to 4-foot riprap. Minimum thickness is 2 feet. 3. For outlets at the base of steep slope pipes(pipe slope greater than 10 percent), an engineered energy dissipater shall be used. • Filter fabric or erosion control blankets should always be used under riprap to prevent scour and channel erosion. I • New pipe outfalls can provide an opportunity for low-cost fish habitat improvements. For example, an alcove of low-velocity water can be I created by constructing the pipe outfall and associated energy dissipater back from the stream edge and digging a channel, over- widened to the upstream side, from the outfall. Overwintering juvenile Iand migrating adult salmonids may use the alcove as shelter during 4-80 Volume ll—Construction Stormwater Pollution Prevention February 2005