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<br />ae <br />a maximum elevation of 8700 feet and an average elevation of 6500 feet and <br />the application of this method to the study area elevation is questionable. <br />Assuming that the sediment ponds are designed for the 10-year, 24-hour. <br />rainfall-runoff volume plus the three-year sediment volume, no discharge from <br />' the ponds will occur until this initial storage is used. Once this volume is <br />used, discharge from the sediment ponds will occur at a maximum rate equal to <br />the snowmelt runoff rate at any particular time. The actual discharge rate <br />out of the sediment ponds will be less than the flow rate coming into the <br />ponds due to storage within the reservoir. However, if ve estimate the de- <br />tention time within the sediment ponds assuming the discharge into the ponds <br />equals the discharge out for the peak snowmelt runoff rate, the detention times <br />~. are on the order of hundreds of hours. Since snowmelt generally has a small <br />sediment yield, the detention times provided will be more than adequate to <br />~' allow settling of the suspended sediment. The discharge from the sediment <br />' ponds during the snowmelt runoff would be over the emergency spillway which is <br />• designed for the 25-year, 24-hour peak discharge and is more than adequate to <br />handle the peak discharge due to snowmelt. <br />The snowmelt runoff volumes estimated are conservative. Due to the char- <br />acteristics of snowmelt runoff and the detention times provided, design of the <br />sediment pond structures for the 10-year, 24-hour rainfall event plus the three- <br />year sediment yield will provide adequate control of the snowmelt runoff and <br />maintain the hydrologic balance. <br />6.3.2 Rainfall Volume for 10-Year, 24-Hour Storm Event <br />The design volume and spillway capacities for the existing and proposed <br />sediment pond structures given in the mine permit for the proposed permit bound- <br />ary (February 17, 1981) and in the permit amendment (October 1979) are compared <br />to the HULTSED results. The design volumes are compared to the 10-year, 24-hcur <br />runoff volume plus the estimated three-year sediment yield. The spillway capa- <br />cities are compared to the 25-year, 26-hour peak discharge of the watershed unit <br />draining into a given sediment pond. The location of the existing and proposed <br />sedinent ponds are shown on Hydrology ?lap 2, Appendix A. Special considerations <br />are discussed for Sediment Pond "D" and proposed Sediment Pond "E". <br />. As previously stated, the storage capacity or area above the embankment of <br />Sediment Pond "D' has been removed by the progress of mining. Previous to this <br />state of mining, Sediment Pond "D" intercepted the runoff which flows to drainage <br />