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. Depending on the mining and reclamation sequence, there is a year of highest <br />runoff for the pond drainage area. The year of highest runoff was found by weight <br />averaging corresponding curve numbers and acres for the different conditions in the <br />drainage area to find the year of highest volume of runoff. Table 4.6-23 summarizes <br />the total runoff for each year. Since Year 2 has the highest runoff volume, it was <br />used as the design year. Table 4.6-24 shows a typical calculation to determine the <br />runoff fora drainage area and Table 4.6-25 shows the runoff from the different areas <br />for The yearly sequences. This was then summarized into Table 4.6-23. Table 4.6- <br />26 designates the storage needs of the area due to pumping from the pits. Exhibit <br />4.6-17 designates the drainage area conditions that would correspond 1o the year of <br />highest runoff. <br />4.6.4.3 Sediment Pond Design for West Ridge Drainage Area <br />The sediment yield and peak runoff values were used to formulate a sediment <br />pond design that would contain the highest year of runoff from a 10 year - 24 hour <br />storm event, pump storage, and would provide ten years of sediment storage below The <br />• primary spillway. The total storage volume needed to build the pond as a permanent <br />pool pond with a pipe located above the combined sediment storage level, highest <br />runoff level from Year 2, and pump storage is 32.44 ac/ft and can be seen in Table <br />4.6-27. The peak flow calculation due to a 10 year - 24 hour storm event for the area <br />can be seen in Table 4.6-28 for Year Two and included in this table are check designs <br />of peak flows for The area in years four and seven. This is due to contemporaneous <br />reclamation decreasing the ponding effect of the drainage area. Exhibit 4.6-18 is an <br />aerial photo mop of the area designating the ponding effect of area in Year Two. The <br />primary discharge pipe was sized for a peak flow of 45.0 cfs from the "Handbook of <br />Steel Drainage and Highway Construction Products," 1977. A 36" corrugated steel <br />mitered culvert with inlet control and a HW/D ratio of 1.3 proved to be adequate for <br />the peak flow without water passing through the emergency spillway. The peak flow <br />calculation for The emergency spillway due to a 100 year - 24 hour storm event con <br />be seen in Table 4.6-29. <br />• <br />Revised 7-81 4.6-74 <br />