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C~ <br />J <br />Sediment pond and Collection Ditc <br />One sediment pond will be constructed near the low point of the <br />permit area to contain runoff from all disturbed areas. Exhibit <br />IUC shows the watershed areas draining to the pond, the collection <br />ditches, the disturbed area and the node and branch designations <br />for the SEDCAD design. <br />As can be seen from the Exhibit, the eastern mine area runoff <br />will be gathered by Collection Ditch f2 and will b• transferred <br />by trough over the Engleville Qulch to Collection Ditch it which <br />will deliver this runoff to the sediment pond. <br />Also, it is seen that no diversion ditches are included above the <br />mine area in the design. The undisturbed area above the mine <br />disturbance is relatively small and can be easily handled by the <br />sediment pond. Experience has shown that diversion ditches of <br />this nature on steep ground are both difficult to maintain and <br />difficult to reclaim after mine shutdown. <br />• The following criteria were used in the design of the sediment <br />pond. The pond will: <br />1. provide storage for the maximum anticipated inflow from a <br />10 year - 24 hour precipitation event plus a maximum 3 year <br />sediment volume. <br />2. use a trapezoidal spillway which is designed to safely <br />discharge the runoff from he 25 year - 24 hour storm. <br />3. provide for the elevation of the crest of the dam to be <br />at least 1.5 feet above the maximum water elevation with the <br />spillway flowing at design depth. <br />4, use a 4.0 inch diameter trickle tube principal spillway <br />to dewater the pond. This pipe will also allow 24 hour <br />detention time for the 10 year - 24 hour storm volume and at <br />the same time, dewater the pond in 72 hours. <br />5. minimize the amount of disturbed area. <br />6. be cleaned when the sediment volume reaches 60~ of the <br />design 3 year accumulation level. Cleanout will be provided <br />• by a piece of construction equipment operating inside the <br />pond at an area of stable, shallow slope. <br />18 <br />