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42.05 APPLICATION FOR PERMIT FOR SIIRFACE OR IINDERGROUND MINING ACTIVITIES -- <br />MINIMUM REQUIREMENTS FOR OPERATION AND RECLAMATION PLANS. <br />2.05.6 Mitigation of Surface Coal Mining Operation Impacts. (Cont'd.) <br />(3) (b) (i) (Cont'd.) <br />• <br />i <br />perhaps thirteen percent of the annual yield. <br />In the undisturbed state, no near-surface flow <br />component existed in eight of ten years. <br />The storm flow that produces erosion is spotty <br />and normally does not cover large areas. Ac- <br />cordingly, sediment yields from small water- <br />sheds vary greatly in time and space. In the <br />vicinity of Orchard Valley Mine, several years <br />might pass without significant erosion activi- <br />ty. However, one large storm can account for <br />virtually all of the long-term sediment pro- <br />duction. Fortunately, such storms have a rel- <br />atively long recurrence interval as illustrat- <br />ed by Figure 2.05-10 in the Surface Drainage <br />Appendix, Volume 8. A 10 year rainfall event <br />will produce a peak flow of approximately 30 <br />c.f.s. at Node 22 (see Map 8-1). The volume <br />of direct runoff from this storm is approxi- <br />mately 0.8 acre-foot. A five-year rainfall <br />event will produce a peak at Node 22 of per- <br />haps 17 c.f.s. and a volume of less than a <br />quarter acre-foot. As previously discussed, <br />the sedimentation ponds up stream of Node 22 <br />(see Map 8-1) would control the ten-year, 24- <br />hour storm event. <br />The fourth element of the drainage plan is the <br />waste disposal area sedimentation pond. This <br />pond was constructed according to designs pre- <br />pared by Geo-Hydro f,onsulting, Inc. with re- <br />visions to enlarge the pond made by CWI engi- <br />neers in the 1983 Waste Disposal Area designs. <br />Refer to the Waste Disposal Appendix in Volume <br />6A for further information on the pond design. <br />Run-off from the waste disposal area is di- <br />verted to the waste disposal area sedimenta- <br />tion pond by drainage ditches constructed <br />around the perimeter of the disturbed area as- <br />sociated with the waste rock pile. Also, <br />flows from Node 1, which had previously been <br />Revised 08/01/84 <br />Revised 12/26/84 <br />Revised 10/01/87 <br />1~4 <br />