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powder magazines at numbers 1 and 4 mines and the rock tunnel and <br /> the lower storage bin at No . 4 mine. <br /> Coal Basin sediment pond storage volumes have been <br /> designed using the Universal Soil Loss Equation and a standard <br /> 0.1 acre foot for each acre disturbed (Appendix II-B-7) . Each <br /> impoundment is designed to detain the flow for a minimum of 24 <br /> hours . Embankments are designed for a 1 . 0 foot freeboard during <br /> a 25 year/24 hour event (Appendix II-B-7) . The dam widths have <br /> been designed in accordance with the equation (H=35) /5 where H is <br /> the height in feet from the upstream toe to the top. Slopes <br /> where the combined upstream slope is not less than lv: 5h are <br /> stablized by vegetation. <br /> The spillways have been designed to safely convey <br /> the runoff event from a 25 year/24 hour storm and constructed <br /> with a 1 . 0 foot freeboard above the principle spillway. The <br /> principle spillway is designed to function only when the storm <br /> event exceeds the 10 year/24 hour storm. <br /> The embankment ponds have been constructed using <br /> the following procedures: <br /> - material placed in the fill has been kept <br /> free of detrimental amounts of rock, roots <br /> and debris; <br /> - backfill material around culverts and <br /> antiseep collars has been placed at approximately <br /> the same rate in order to prevent uneven loading; <br /> �6� <br />