1991-05-16_REVISION - C1981035 (2)

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A curve number of 72 was used for the westside as compared to 76 used on the east side. The east drainage area incorporates nearly all of the mine site. This area is devoid of vegetation and has exposed soil surfaces with above normal compaction. Both of these factors result is less precipitation abstraction from infiltration and retention, thus the west side area should exhibit a lower curve number. Flows were routed through the pond using the Modified Puls method assuming level pool conditions. As previously stated, the regulations require a design sequence where the pond, initially empty, receives the inflow of the 10yr and 25yr events, back to back. The volume of the 10yr-24hr must be stored in the pond and the 25yr-24hr flow must pass the emergency spillway. The 10yr-24hr volume for the West Pond is 1.56 AF and the peak , discharge for the 25yr-24hr storm is about 5 cfs. See computer runs in appendices 6 and 7, respectively. Using the same annual watershed sediment yield as estimated for the east drainage, 0. 1 AF, the resulting total required West Pond volume is 1.66 AF. The existing pond does not have this volume and must be enlarged. In order to drain the pond in the required 48 hours, a circular orifice with a 3 inch diameter is required. Documentation of the results can be found in the computer runs in the appendix 8. Pond Geometry In order to' achieve the required volume of 1. 66 AF, the West Pond was extended 8 ft. to the north and deepened. In addition the embankments were redesigned to meet the MLRB 5: 1 total slope regulation as discussed previously. As with the East Pond, the inslope is 2 : 1 and the outslope is 3 : 1 around the entire pond. Assuming a level top of embankment the inslope height is 12 . 3 ft and the outslope height varies around the pond. On the north side the outslope height is 10 ft, on the east side it averages 4-5 ft, and on the west side where it joins the natural hillside it is about 1 ft. A small drainage channel will be formed between the embankment and the natural hillside to drain water away from the embankment. The topwidth is 8 ft. See figure 4 . The elevation of the top of the West Pond embankment, based on an assumed data, is 210.0, the spillway crest is at 208.7 and the bottom of the pond is at 197.7 . The volume reserved for sediment is 0. 1 AF and corresponds to elevation 198.7. The volumes were calculated using the same techniques as described for the East Pond (appendix 9) . Outlet Works Outlet works for the West Pond are similar to those specified for the East Pond. They include a low level orifice and a sharp- 10