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D = PL0.2x~? <br />(P+0.8xS) <br />where: <br />Q =accumulated direct runoff <br />P =accumulated rainfall (from design storm) <br />S =potential maximum retention <br />= 1000 - 10 <br />RCN <br />RCN =runoff curve number <br />Runoff calculations for ponds designed before mine construction are included in Illustrations 4, 5, 6, <br />and 7. <br />The available pond capacities were determined by the average end area method from planimetered <br />areas of contours shown on the drawings. All ponds were sized according to the requirements of <br />Section 4.05.6. <br />The peak runoff rates to the ponds for the 25-year, 24hour and 100-year, 24hour storms were <br />initially determined for preliminary design purposes from the graphical procedures for determining <br />instantaneous peak discharge (Figure 7 and ES-1027, NRCS TP-149) using a type II storm <br />distribution. Runoff flow rates for designing diversion ditches were determined in the same manner <br />as above using a 10-year, 24-hour storm in accordance with Section 4.05.3. Runoff flow rates for <br />the Refuse Disposal Area diversion ditches were determined using a 100-year, 24-hour storm. <br />With the exception of Refuse Area Pond RPSA, outlets for the sedimentation ponds typically include <br />a principal drop-inlet spillway and an earth emergency spillway. RPSA was constructed with a <br />combination principal and emergency spillway as depicted in Map 80A. Principal spillways are <br />constructed of corrugated metal pipe (CMP) and consist of a riser extending to the top of the <br />combined sediment and runoff storage pool and a conduit extending through the embankment (see <br />the pond design drawings, Maps 34, 76, 79, 80 and 80A). <br />Detention time was determined as the difference between the inflow and outflow hydrograph <br />centroids after appropriate routing. The DEPOSITS sedimentation pond model developed by the <br />University of Kentucky was used to route the inflowing hydrograph (generated by the WASH <br />subroutine) through the ponds to determine detention times. Resulting pond performance <br />information is contained in Illustrations 4, 5, 6, and 7. <br />• All primary spillways were sized to pass the peak flow resulting from the 10-year, 24-hour rainfall <br />event using charts contained in Chapter 6 of the NRCS Field Engineering Manual. A maximum <br />(Permit Renewal #3, 8199) V-12 <br />