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see Exhibit 13-7 for location and design of spillways. The principal spillway will <br />consist of an 18•inch diameter corrugated metal pipe (CMP) with a trash rack a[ the inlet. <br />This pipe will keep [he pond level at the 7086.5 elevation. Maximum elevation (peak <br />stage) above the principal spillway with [he passage a[ the 10-year, 24-hour event will be <br />7087.0, this leaves 3.0 feet to the emergency spillway. <br />The embankment portion of the emergency spillway will be underlain with a geotex[ile as <br />shown on Exhibit 13-7. Riprap will be used to protect [he channel in the event of a <br />discharge through the spillway. Construction standards for [he remaining portion of the <br />emergency spillway are listed below. <br />The emergency spillway outflow channel will consist of a vegetated, trapezoidal channel. <br />In order to protect the channel, in the unlikely event of a discharge prior to vegetation <br />establishment, [he channel will be mechanically stabilized. This will be conducted in the <br />following manner, The channel will be shaped to the design geometry (see Table 13-15-7). <br />The channel will Chen 6e seeded with a combing ciao of intermediate wheatgrass, call fescue <br />and basin wildrye. This seed mix will provide the long term erosion control (see Atldendum <br />• C). In order to provide erosional stability prior to vegetation establishment, a product <br />called Landstrand will be applied to the seeded area. landstrand roving fibers are <br />continuous fibrilated polypropylene strands applied with the aid of an air compressor and <br />applicator gun at a rate of 0.25 to 0.35 lbslsy. These fibers are then tacked together <br />and [o the surface by spraying pine tar (ROadOyl) or asphalt emulsion on the fibers. <br />Tests conducted on Landstrand/ROadOyl and landstrand/Asphalt Emulsion indicate that the <br />application, prior to vegetation establishment, can withstand velocities up to 9 FPS- The <br />application rate will be about 0.25 gallons per square yartl for RoadOyl and about 0.35 <br />gallons per square yard for asphalt emulsion. <br />The channel was designed with the aid of the SEDCAD+ computer program. The closest <br />vegetation type listed in SEOCAD+ resembling the species listed above is tall fescue. <br />Stability and capacity for this grass type is listed as B. As stated earlier, prior to a <br />"good stand of grass," the Landstrand will protect [he channel during passage of the <br />25-yea r, 24-hour precipitation even[. Peak stage elevation with [he passage of the <br />25-year storm is 7091.0, this leaves 3.0 feet of treeboa rd to the top of embankment. <br />Please refer to Addendums A and B far Capacities of both spillways during passage of the <br />10 and 25-year storm events. Table 13-5-7 and Addendum C presents the tlesign details far <br />• the principal and emergency spillway. <br />13-5-9 Revised 04/20/92 <br />