PERMFILE69489 - Laserfiche WebLink

Home Browse Search

• Getty Mining Company March 26, 1985 Page 3 appears reasonable. We did an independent check on the sedi- ment yield calculations using the Universal Soil Loss Equation and arrived at a similar sediment yield. However, it should be pointed out that the 3-year sediment yield used in design (0.02 ac-ft/ac) is for a watershed whose characteristics (especially slope and length) are different from that draining to Sedimentation Pond F. We could not duplicate the hydraulic calculations used to route the 10-year, 24-hour inflow hydro- graph through Pond F, so these calculations were redone (See Appendix 3). The as-built Sedimentation Pond F could store up to 2.24 acre- feet of water and sediment between elevations 95.0 and 99.5 feet, which is the crest of the 18-inch riser. This is the storage volume available to store 3 year of sediment and runoff from the 10-year, 24-hour design storm. Because we did not have adequate mapping to estimate the elevation-capacity curve above elevation 99.5, we extrapolated this curve to obtain a total storage at the emergency spillway crest (eleva- tion 100.0) of about 2.86 acre-feet, and at the dam crest (elevation 101.3) of about 4.4 acre-feet. The storage between the emergency spillway crest and dam crest (1.3 Eeet) would be available to attenuate the peak discharge from the 25-year, 24-hour precipitation of 2.2 inches used to design the emer- gency spillway. IMPACT OF PROPOSED FACILITIES As shown in Appendix 1, the contributing drainage area to Sedimentation Pond F watershed including the new facilities, would be about 60 acres. The 60 acres is bordered on the north by the railroad, on the south by the access/haul road, on the east by the ridge between Pond F and Pond G, and on the west by the proposed new coal stockpile, screening and crushing building, conveyors, and loadout. New collection channels will carry runoff and sediment from the proposed facilities to Sedimentation Pond F. For purposes of this analyses, all sixty acres contributing to Sedimentation Pond F were assumed to be disturbed. Using the same design criteria as for the original Sedimenta- tion Pond F design, the runoff from the 10-year, 24-hour rainfall of 1.8 inches over the 60-acre contributing drainage area would be 0.95 acre feet. In addition, an estimated 1.2 acre-feet of sediment also would be discharged from the 60 • acres over a 3-year period resulting in a combined water and