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3. Hydrology and Sediment Control for River Mining <br />Sediment control for the permit areas is provided by a berm on the far northeast part of the permit <br />area which leads to a collection ditch. These two items collect runoff from all disturbed areas on the <br />site and also minor undisturbed area between the permit area and the Animas Valley Consolidated <br />Ditch. This ditch is located slightly west of the permit area. The collection ditch delivers all flows <br />to the sediment pond located at the south part of the permit area. This pond will be expanded into a <br />the lake which will be mined in the year 2002 and is the subject of the following subsection. The <br />drainage area that drains to the pond lake is shown on Map C-2. This area is 24.9 acres. It is demon- <br />strated below that the area of the pond is adequate to handle the entire runoff for the 100 year 24 <br />hour event, therefore, no discharge permit is needed. <br />Area = 24.9 acres <br />Curve number = 74 (porous gravel) Little topsoil. <br />100 year 24 hour rainfall = 3.6 inches <br />Runoff @ CN 74 = 1.31 inches <br />Total volume = 1.31 inches x 1 fr/12 in x 24.9 acres = 2.72 acre-feet or 118,406 cubic feet. <br />This volume will be entirely stored in the lake/sediment pond, which will have a minimum of 4 feet <br />of storage from the highest water storage level (6664 up to the bottom of the emergency spillway <br />(b660'). The area of the lake at these elevations is approximately 3.9 acres, therefore the volume <br />available is 3.9 acres x 4 feet height = 15.6 acre-feet. This is more than adequate to contain the 100 <br />year event. It would likely contain the 1000 year event. Additional sediment control items are dis- <br />cussed in the next subsection on the lake since the lake will be dewatered during mining. <br />Thomas Pit 2/02 10 <br />