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A <br /> f <br /> OLRAY SILVER MINES <br /> STORMWATER DISCHARGE <br /> Mine Water Ponds#2 and #3 are assigned Outfalls 001 and 002 respectively. These ponds are <br /> designed to contain the full stormwater runoff from a 10 year 24 hour event plus sediment <br /> storage volume. <br /> Ditches that collect runoff from the entire disturbed area have been designed to direct the <br /> stormwater into these two ponds. The ponds will each has a trapezoidal emergency spillway <br /> built into the top of the embankment which will allow the ponds to safely pass the 25 year peak <br /> flows through the ponds. These ponds will only be present for the life of the mine, which is <br /> expected to be 25 years. After such time, the drainage areas leading to the ponds will be fully <br /> reclaimed. It is important to note that these areas are now almost devoid of vegetation so that <br /> Ouray Silver Mines Inc. will be significantly improving the long term reclamation of the site and <br /> lessening the sediment load to Sneffels Creek. <br /> During the life of these ponds,they will likely never discharge during winter months and since <br /> they are built to fully contain the runoff from a 10 year storm event, they are not expected to <br /> discharge the remaining of the year. <br /> The stormwater volume was determined using the calculations below as described in the <br /> "Procedures for Determining Peak Flows in Colorado", which includes and supplements <br /> Technical Release No. 55 "Urban Hydrology for Small Watersheds." <br /> Hydrologic Soil Group <br /> Group A Soils: High infiltration (low runoff). Sand, loamy sand, or sandy loam. <br /> Infiltration rate > 0.3 inch/hr when wet. <br /> Group B Soils: Moderate infiltration (moderate runoff). Silt loam or loam. Infiltration <br /> rate 0.15 to 0.3 inch/hr when wet. <br /> Group C Soils: Low infiltration (moderate to high runoff). Sandy clay loam. Infiltration <br /> rate 0.05 to 0.15 inch/hr when wet. <br /> Group D Soils: Very low infiltration (high runoff). Clay loam, silty clay loam, sandy clay, <br /> silty clay, or clay. Infiltration rate 0 to 0.05 inch/hr when wet. <br /> Runoff Curve Numbers CN for Watershed Area Draining to Mine Pond 2 14.431 acres <br /> Description Hydrologic Soil Group Area (acres) CN <br /> Disturbed Land Group C Soils 14.431 87 <br /> An area of 14.431 acres will drain to the discharge point of Pond 2 for the worst case <br /> disturbance of this area. <br /> Stormwater Management Plan 13 1 P a g e <br />