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<br />Brian Munson -3- November 17, I98I <br />Sediment yield to each of the ponds was estimated by estimating the concentration <br />of suspended solids in runoff water and multiplying it by the annual runoff <br />volume. No information is given to substantiate the estimates of annual run- <br />off or suspended solids concentrations. On a per-acre basis, the sediment <br />storage volumes of each of the ponds is approximately 0.0033 acre-feet per <br />acre of disturbance. From estimates generated by other mining companies in <br />the same general locale as the Grassy Creek mine, this figure appears to be <br />extremely low. <br />The Rockcastle Company states that the SCS advised them against using the <br />USLE to estimate sediment yield. This method is widely accepted as a means of <br />determining sediment yield from surface mines. While this Division realizes <br />that the USLE has deficiencies in surface mine sediment yield estimation, we <br />feel it is a good tool for design purposes. <br />Recently, this Division has accepted a value based solely upon a ratio of <br />the mean annual precipitation. N. Kri6hnamurthi of Utah International presented <br />a case for this approach, citing work done by Curtis in Kentucky and other <br />works. The Rockcastle Company may wish to read the appropriate sections of <br />Utah International's Trapper mine application for detail on the justification <br />of the method. This method is rather simplistic and ignores several factors <br />which may increase or decrease the actual sediment yield. Estimate the mean <br />annual precipitation and multiply that value by 0.00166 to obtain the three- <br />year sediment yield in acre-feet per acre of disturbance. <br />The estimate by The Rockcastle Company uses a value of 10,000 mg/liter as an <br />estimate of sediment concentrations in runoff water. While they claim that <br />this is conservative or an overestimate, values of sediment concentrations from <br />surface mined lands in Colorado run as high as 90,000 mg/liter. The estimate <br />of annual runoff is not substantiated and the Division has no information <br />which shows the figure to be appropriate. <br />This Division requires The Rockcastle Company to either use another method for <br />sediment yield calculations which can be substantiated, or to produce <br />evidence to substantiate the estimates given. Those ponds which are pre- <br />existing, approved by this Division in writing and certified by a professional <br />engineer, need not be redesigned for sediment storage inadequacies. <br />None of the ponds has any means for dewatering the 10-year, 24-hour storm <br />volume. This leads to a series of problems associated with Colorado State <br />Water Law and pond embankment stability. This Division has the policy of <br />requiring dewatering systems at the elevation of the 1000 sediment storage <br />volume. This device should be designed to provide between 24 and 36 hours <br />of detention time. <br />Not having dewatering devices leads to other problems associated with State <br />Water Law. Any structure which may store over 2 AF of water for 36 hours or <br />more must have water storage rights obtained for its operation. In order to <br />obtain these rights, TRC will have to show that some means (i.e., pump, <br />dewatering device) exists on site capable of releasing the stored water within <br />36 hours. The Rockcastle Company may avoid the problems associated with water <br />