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• Spillways <br />Each sediment pond at Trapper incorporates either a combination of a principal and emergency <br />spillway, or a single open -channel spillway design. Regardless of the design chosen, each <br />emergency spillway or single open -channel spillway will safely pass a 25 -year, 24-hour <br />precipitation event. Where a combination of a principal and emergency spillway is used, the last <br />pond in each series shall safely pass a 10 -year, 24-hour precipitation event. <br />All sediment ponds built at Trapper prior to 1986, with the exception of Coyote, have standpipes <br />as principal spillways. The configuration of these spillways is shown in Figure 4.8-3, Typical <br />Standpipe Design. Spillway discharge rates for each of these ponds are shown in Appendix Q, <br />Section XIV. The location of these spillways with respect to the embankment and emergency <br />spillways is shown in Figure 4.8-4. This type of spillway has not been incorporated into any new <br />pond designs at Trapper since 1985. <br />The spillways built after 1985 have been designed as open channel structures constructed at one <br />side of the sedimentation pond embankment. The locations of the spillways are shown in Figure <br />4.8-5, Plan View of Typical Sedimentation Pond Embankment and Spillways. <br />Various open -channel spillway designs have been incorporated into the pond designs since <br />1985. The first generation consisted of a combination principle and emergency spillway design <br />using a semi -circular corrugated metal flume anchored in a concrete base as the principal <br />spillway. This design evolved into constructing a concrete lined V -notch or flat bottom channel in <br />place of the corrugated half -pipe design. With each of these designs, the half -pipe, V -notch or <br />flat bottom channel is excavated and constructed in the center of a typically flat bottom <br />• emergency spillway. Details of these designs are shown in pages 4-195 thru 4-196b (Figures <br />4.8-6, 4.8-6a, 4.8-6b and 4.8-6c), and in Appendix Q, Section XIX. Recent designs are based on <br />providing for a single open -channel spillway pursuant to Section 4.05.6(3)(e) of the regulations. <br />The design of single open -channel spillways makes use of either a non -erodible channel <br />designed to carry sustained flows, and/or an earth -lined or grass -lined channel designed to carry <br />short-term infrequent flows at non-erosive velocities where sustained flows are not expected. <br />The typical configuration of this design is shown on page 4-196c, Typical Single Open -Channel <br />Spillway Configuration. <br />Unless otherwise approved by the Division, the side slopes of all emergency spillways and the <br />side slopes of single open -channel spillways will be 3HA V with the bottom designed to slope <br />downstream at 1%. Emergency spillways and single open -channel spillways are constructed <br />such that the minimum elevation of the top of the settled embankment shall be at least 1.0 foot <br />above the water surface on the pond with emergency spillway flowing at design depth. <br />(vlr2-"Z 15 <br />4-192 <br />