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Sediment volume at the pond was determined using the Universal Soil Loss <br />• Equation and the projected life of the pile. These calculations are included in <br />the final design section of Appendix 4-B. <br />A theoretical detention time of 24 hours is provided for runoff resulting from a <br />10-year, 24-hour precipitation event. An emergency spillway is provided to <br />safely pass the discharge from a 25-year, 24-hour precipitation event. Spillway <br />details are shown on Drawing D-4-12. <br />The emergency spillway is a trapezoidal earth structure with a minimum freeboard <br />of 2.3 feet. The top width of the dam is 10 feet and height from the upstream <br />toe to the crest of the dam is 10 feet. The total pond capacity is 3.2 acre <br />feet. <br />Both upstream and downstream slopes are Iv:2.5h. Stability analysis of the dam <br />is discussed in the geotechnical investigation by Chen and Associates, <br />Appendix 4-C. <br />• Construction of a sediment pond dam will require that an existing dam structure <br />be removed, the entire base area cleared of all trees and other organic matter, <br />and the foundation surface scarified. No surface will be sloped steeper than <br />lv:2h. Fill with consist of locally available material as specified in the <br />geotechnical report in Appendix 4-C. Fill will be spread in horizontal layers <br />no more than 8 inches deep and will be compacted to 95 percent dry density. <br />The pond and dam will be constructed under the supervision of a registered <br />engineer. All areas, including embankments, disturbed by construction will be <br />stabilized by a combination of vegetation and riprap. Typical cross-sections <br />showing riprap placement are on Drawing D-4-10. <br />• <br />4-108 <br />