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2.0 Sediment Ponds <br />2.1 Design Criteria <br />Pond "H" was moved slightly upstream from the location marked on Map <br />7, Drainage and Sediment Control Plan to avoid old spoil piles on the north <br />and to decrease the reugired embankment length. Pond "L" is an existing <br />pond that takes advantage of a high natural ridge on the east and road em- <br />bankment on the north. <br />Sediment ponds "A" and "K" are designed according to the following <br />size criteria: storage volume, 10-year, 24 hour runoff .volume plus a mini- <br />mum sediment volume (Table 2.1); emergency spillway and principal spillway <br />must have a combined capacity to pass the 25-year, 24-hour peak flow (Table <br />2.i); principal spillway must provide detention time of 24 to 36 hours for <br />the 10-year, 24-hour event. <br />Peak flow estimates are from the S.O.A.P. report for Pryor Mine <br />(Table 2.2). Recommended values for design (Method 3) are: 10-year, 24-hour <br />peak flow, 376 cfs/1024 acres, or 0.37 cfs/ac; and 25-year, 24-hour peak <br />flow, 600 cfs/1024 acres, or 0.59 cfs/ac. These peak flows were obtained <br />using the SCS method and are substantiated by results of Method4, which is <br />a U.S.G.S regression based on active channel capacity. <br />Runoff yield estimates were obtained from the MULTSED computer model <br />results in the G.E.C. Minerals S.O.A.P. report (Tables 2.3 and 2.4). An aver- <br />age of the three values for SW-l, SW-2, and SW-3 modeled by MULTSED (Table 2.5) <br />was used for the Pryor minesite. For the 10-year runoff event, 0.03 ac-ft/ <br />ac will be used. <br />The MULTSED model is a state-of-the-art physical process water and <br />sediment routing model developed with EPA and Forest Service funding. The model <br />uses infiltration data from actual field tests to calculate excess rainfall. <br />This is a critical component and one of the main reasons this model is prefer- <br />able to SCS for calculating runoff yield. SCS curve numbers have never been <br />tested on western soils. Water routing is performed analytically by applying <br />the method of characteristics to the kinematic wave approximation to the mo- <br />mentum and continuity equations. The model is discussed in detail in the <br />"Preliminary Data Base for Application of MULTSED Program, G.E.C. Minerals, <br />Inc." submitted to CMLRD under S.O.A.P. <br /> <br />2 <br />2 `-t' I a <br />