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1 <br />In the case of grass -lined ditches and swales, check dams and accumulated sed- <br />iment shall be removed when the grass has matured sufficiently to protect the ditch <br />or swale unless the slope of the swale is greater than 4 percent. The area beneath <br />the check dams shall be seeded and mulched immediately after dam removal. <br />Maintenance Standards <br />. Triangular silt dams shall be inspected for performance and sediment accu- <br />mulation during and after each runoff producing rainfall. Sediment shall be <br />removed when it reaches one half the height of the dam. <br />. Anticipate submergence and deposition above the triangular silt dam and erosion <br />from high flows around the edges of the dam. Immediately repair any damage or <br />any undercutting of the dam. <br />BMP C209: Outlet Protection <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 con- <br />veyances, 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 lin- <br />ing 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 con- <br />sidered 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 />. The following guidelines shall be used for riprap outlet protection: <br />2014 Stormwater Management Manual for Western Washington <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />Volume 11 - Chapter 4 - Page 356 <br />J <br />