2009-09-21_PERMIT FILE - C1981010A (43)

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• Spillways Each sediment pond at Trapper incorporates either a combination of a principal and emergency spillway, or 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, 24-hour precipitation event. Where a combination of a principal and emergency spillway is used, the last pond in each series shall safely pass a 10-year, 24-hour precipitation event. All sediment ponds built at Trapper prior to 1986, with the exception of Coyote, have standpipes as principal spillways. The configuration 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 respect to the embankment and emergency spillways is shown in Figure 4.8-4. This type of spillway 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 constructed at one side of the sedimentation pond embankment. The locations of the spillways are shown in Figure 4.8-5, Plan View of Typical Sedimentation Pond Embankment and Spillways. 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 semi-circular corrugated metal flume anchored in a concrete base as the principal spo way. This design evolved into constructing a concrete lined V-notch or flat bottom channel in place of the corrugated hatr--pipe design. With each of these designs, the half-pipe, V-notch or flat bottom channel is excavated and constructed in the center of a typically flat bottom emergency spillway. • Details of these designs are shown in pages 4-195 thnr 4-196b, and in AppendtK Q, Section XIX. Recent designs are based on providing for a single open-channel spillway pursuant to Section 4.05.6(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-tern infrequent flows at non-erostve velocities where sustained flows are not expected. The typical configuration of this design is shown on page 4-196c, Typical Single Open-Channel Spillway Configuration. Unless otherwise approved by the'Division, the side slopes of all emergency spillways and the side slopes of single open-channel spillways will be 3H:1 V with the bottom designed to slope downstream at 1%. Emergency spillways and single open-channel spillways are constructed such that the minimum elevation of the top of the settled embankment shall be at least 1.0 That above the water surface on the pond with emergency spillway flowing at design depth. • Pe, -y 4-192 __p?A'f 13 1998