2008-05-08_PERMIT FILE - C1981018A (3)

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• instantaneous peak discharge (Figure 7 and ES-1027, NRCS TP-149) using a type It storm distribution. Runoff flow rates for designing diversion ditches were determined in the same manner as above using a 10-year, 24-hour storm in accordance with Section 4.05.3. Runoff flow rates for the Refuse Disposal Area diversion ditches were determined using a 100-year, 24-hour storm. With the exception of Refuse Area Pond RPSA, outlets for the sedimentation ponds typically include a principal drop-inlet spillway and an earth emergency spillway. RPSA was constructed with a combination principal and emergency spillway as depicted in Map 80A. Principal spillways are constructed of corrugated metal pipe (CMP) and consist of a riser extending to the top of the combined sediment and runoff storage pool and a conduit extending through the embankment (see the pond design drawings, Maps 34, 78, 79, 80 and 80A). Detention time was determined as the difference between the inflow and outflow hydrograph centroids after appropriate routing. The DEPOSITS sedimentation pond model developed by the University of Kentucky was used to route the inflowing hydrograph (generated by the WASH subroutine) through the ponds to determine detention times. Resulting pond performance information is contained in Illustrations 4, 5, 6, and 7. All primary spillways were sized to pass the peak flow resulting from the 10-year, 24-hour rainfall event using charts contained in Chapter 6 of the NRCS Field Engineering Manual. A maximum • head of 1.0-foot above the riser crest was allowed during passage of the design peak flow. The peak flow used for initial design was the unrouted inflow determined by methods described in TP-149. This inflow rate was compared with the peak inflow rate generated by the WASH subroutine of the DEPOSITS model to determine if any adjustment in spillway sizing was necessary. In all cases, the larger of the routed flow hydrograph generate by WASH or the peak flow generated using TP-149 was used for final primary spillway sizing, resulting in a somewhat conservative size estimate for the spillway. Final primary spillway sizing took into account orifice flow into the riser perforation, orifice and wier flow into the riser, and pipe flow through the spillway conduit. All risers are fitted with a splitter-type antivortex device and trash rack at the entrance, as suggested by Chapter 6 of the NRCS Field Engineering Manual. Specific details can be found on the pond drawings shown on Maps 34, 78, 79, 80 and 80A. All emergency spillways were designed to safely pass the unrouted peak inflow rate resulting from the 25-year, 24-hour rainfall event. The emergency spillway on pond DP-1 was also sized to pass the unrouted peak inflow rate resulting from the 100-year, 24-hour storm. Spillway dimensions were based on criteria set forth in Section 4.05.3 and in NRCS Technical Release TR-35 (LID Method of Reservoir Flood Routing). All spillways are constructed in non-fill earth material and are appropriately lined with riprap to allow a maximum outflow velocity of 12 feet per second during passage of the design peak flow, thereby reducing erosional hazards. • Mid-Term Review 2002 (8/2002) V-11