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<br />design sediment volume. This is a conservative estimate since the volume <br />below the lowest orifice is partly filled with sediments in most years. <br />1 <br />Evaporation from ponds which do not go dry was estimated by applying an <br />evaporation rate to the area of the pond at the elevation of the emergency <br />spillway. Determination of pond surface area was made from 1 to 200 scale <br />topographic maps flown after the ponds were built. <br />Evaporation at the minesite is approximately 32 inches a year, as taken <br />from U.S. Weather Bureau Technical Paper 37 (Kohler, Nordenson, and Baker, <br />1959). Evaporation from ponds was first estimated as 32 inches times the pond <br />surface area. For each pond, if evaporation volumes calculated in this manner <br />were greater than the design sediment volume, it was assumed that the pond <br />went dry and the design sediment volume was used as the estimated evaporation. <br />Evaporation from ponds which are known to have year-round inflow from beaver <br />ponds was calculated by the area times evaporation rate method. Specific <br />estimates of evaporative losses are set forth in Table 1. Ponds L, N, Q, R, <br />and S are referenced in Permit 79-171 but were never built. They have not <br />been considered in these calculations. <br />Table 1. Evaporative Losses From Sedimentation Ponds <br />1. Ponds which dry up seasonally: <br />design sediment volume = evaporation = 26.7 acre-feet <br />2. Ponds flowing year-round: <br />area X evaporation <br />31.5 acres X 32 inches g4.0 acre-feet <br />12 inches per foot <br />Total Evaporation = 110.7 acre-feet <br />-2- <br />