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Spillways <br />Sediment Pond 009 has two seper ate spillw eye; a principal and an emergency. Please see <br />Ezhi bit 13-7A for location end design of spillways. The principal spillway consists of en <br />18-inch diameter corrugated metal pipe (CMP) with a trash rack at the inlet. This pipe <br />will keep the pond level at the 7086.3 elevation. Maximum elevation (peek stage) above <br />the principal spillway with the passage at the 10-year, 24-hour event will be 7086.8, this <br />leaves 2.8 feet to the emergency spillway. <br />The embankment portion of the emergency spillway was underleyed with a geotextile as shown <br />on Exhibit 13-7A. Riprap will be used to protect the channel in the event of a discharge <br />through the spillway. <br />The emergency spillway outflow channel consists of a vegetated, trapezoidal channel. In <br />order to protect the channel, in the unlikely event of a discharge prior to vegetation <br />establishment, the channel will be mechenicelLy stabi Lized. This was conducted in the <br />following manner. The channel will be shaped to the design geometry (see Table 13-15-7). <br />The channel wee seeded with a combination of intermediate wheatgrass, tall fescue and <br />basin wildrye. This seed mix will provide the long term erosion control (see Addendum C). <br />In order to provide erosional stability prior to vegetation estebLishment, a product <br />called Lendstrand was applied to the seeded area. Lendstrand roving fibers are continuous <br />fibrilated polypropylene strands applied with the aid of an air compressor and applicator <br />gun et a rate of 0.25 to 0.35 lbs/sy. These fibers ere then tacked together and to the <br />surf ate by spraying pine tar (ROadOyl) or asphalt emulsion on the fibers. Tests conducted <br />an Lendstrand/ROadOyl end Lendstrand/Asphalt Emulsion indicate that the application, prior <br />to vegetation establishment, een withstand velocities up to 9 FPS. The application rate <br />will be about 0.25 gallons per square yard for RoadOyl and about 0.35 gallons per square <br />yard for asphalt emulsion. <br />The channel was designed with the aid of the SEDCAD+ computer program. The closest <br />vegetation type listed in SEDCAD+ resembling the species listed above is tall fescue. <br />Stability end capacity for this grass type is listed es B. As stated earlier, prior to~a <br />"good stand of Brass," the Lendstrand will protect the channel during passage of the <br />25-year, 24-hour precipitation event. Peek stage elevation with the passage of the <br />23-year storm is 7090.4, this leaves 2.8 feet of freeboard to the top of embankment. <br />Please refer to Addendums A end B for capacities of both spi llweys during passage of the <br />10 end 25-year storm events. Table 13-SA-7 and Addendum C presents the design details for <br />the principal end emergency spi llwey. <br />13-SA-9 Revised 08/17/92 <br />