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• Scilhvavs Each sediment pond at Trapper incorporates either a combination of a principal and emergency spillway, ar a single open-channel spillway design. Regardless of the design chosen, each emergency spillway or single open-channel spillway will safely pass a 25-year, 24hour precipitation event. Where a combination of a principal and emergency spilhvay is used, the last pond in each series shall safely pass a 10-year, 24hour precipitation event. All sediment ponds built at Trapper prior to 1988, with the exception of Coyote, have standpipes as prtnGpal spillways. The confguretion of these spillways is shown in Figure 4.8-3, Typical Standpipe Design. Spillway discharge rates for each of these ponds are shown in Appendix Q, Section XIV. The location of these spillways with respell to the embankment and emergency spillways is shown in Figure 4.8-4. This type of spilMray has not been incorporated into any new pond designs at Trapper since 1985. The spillways built after 1985 have been designed as open channel structures censWded at one side of the sedimentation pond embankment. The locations of the spillways are shown in Figure 4.&5, Plan View of Typical Sedimentation Pond Embankment and Spiihvays. Various open-channel spillway designs have been incorporated into the pond designs since 1985. The first generation consisted of a combination principle and emergency spillway design using a semFdreular corrugated metal flume anchored in a connate base es the principal sprlway. This design evolved into constructing a concrete lined V-notch or flat bottom channel in place of the corrugated half pipe design. Wdh each of these designs, the half-pipe, V-notch or flat bottom channel is excavated and consWded in the center of a typicaly flat bottom emergency spillway. • Details of these designs are shown in pages 4195 thru 4196b, and in Append'a Q, Sedion XIX. Recent designs are based on providing for a single open-channel spillway pursuant to Sedion 4.05.8(3)(e) of the regulations. The design of single open-channel spillways makes use of either a non-erodible channel designed to carry sustained flows, and/or an earth-lined or grass-lined channel designed to carry short-tens infrequent flows at non-erosive velocities where sustained flows are not effected. The typical configuration of this design is shown on page 4198c, Typical Single Open-Channel SpilNvay Configuration. Unless otherwise approved by the~Division, the side slopes of all emergency spilways and the side slopes of single open-channel spilways will be 3H:1 V with the bottom designed to slope downstream at 1%. Emergency spilways and single open-channel spillways are constructed such that the minimum elevation of the top of the setged embankment shall be at least 1.0 foot above the water surface on the pond with emergency spillway flowing at design depth. • P~-y 4192 __p~A'f 13 1998 _