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2.1.8 Element 8: Stabilize Channels and Outlets <br /> Where site runoff is to be conveyed in channels, or discharged to a stream or some other natural <br /> drainage point, efforts will be taken to prevent downstream erosion. <br /> The project site is located west of the Cascade Mountain Crest. As such, all temporary on-site <br /> conveyance channels shall be designed, constructed, and stabilized to prevent erosion from the <br /> expected peak 10 minute velocity of flow from a Type 1A, 10-year, 24-hour recurrence interval <br /> storm for the developed condition. Alternatively, the 10-year, 1-hour peak flow rate indicated by <br /> an approved continuous runoff simulation model, increased by a factor of 1.6, shall be used. <br /> Stabilization, including armoring material, adequate to prevent erosion of outlets, adjacent stream <br /> banks, slopes, and downstream reaches shall be provided at the outlets of all conveyance systems. <br /> Provide stabilization, including armoring material, adequate to prevent erosion of outlets, <br /> adjacent stream banks, slopes, and downstream reaches,will be installed at the outlets of all <br /> conveyance systems. <br /> List and describe BMPs: <br /> •Outlet Protection(BMP C209) <br /> Purpose <br /> Outlet protection prevents scour at conveyance outlets and minimizes the potential for <br /> downstream erosion by reducing the velocity of concentrated stormwater flows. <br /> Conditions of Use <br /> Outlet protection is required at the outlets of all ponds,pipes, ditches, or other conveyances, <br /> and where runoff is conveyed to a natural or manmade drainage feature such <br /> as a stream,wetland, lake, or ditch. <br /> Design and Installation Specifications <br /> The receiving channel at the outlet of a culvert shall be protected from erosion by rock lining <br /> a minimum of 6 feet downstream and extending up the channel sides a minimum of <br /> 1—foot above the maximum tailwater elevation or 1-foot above the crown, whichever is <br /> higher. For large pipes (more than 18 inches in diameter), the outlet protection lining of <br /> the channel is lengthened to four times the diameter of the culvert. <br /> • Standard wingwalls, and tapered outlets and paved channels should also be considered <br /> when appropriate for permanent culvert outlet protection. (See WSDOT <br /> Hydraulic Manual, available through WSDOT Engineering Publications). <br /> • Organic or synthetic erosion blankets,with or without vegetation, are usually more <br /> effective than rock, cheaper, and easier to install. Materials can be chosen using <br /> manufacturer product specifications. ASTM test results are available for most <br /> products and the designer can choose the correct material for the expected flow. <br /> • With low flows,vegetation(including sod) can be effective. <br /> Page I16 <br />