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$2.05 APPLICATION FOR PERMIT FOR SURFACE OR IINDERGROUND MINING ACTZVITIES -- MINIMUM REQUIREMENTS FOR OPERATION AND RECLAMATION PLANS. 2.05.6 Mitigation of Surface Coal Mining Operation Impacts. (Cont'd.) (3) (b) (i) (Cont'd.) in a 100-year, 24-hour storm, defined as 2.8 in. The areas have been graded and localized drainage ditches and culverts constructed to safely and efficiently dispose of water. For calculations of drainage ditch and culvert ca- pacities, see the Surface Drainage Appendix in volume 8, The storm design criteria in the lower basin has been somewhat relaxed due to diversion channel design for Probable Maximum Thunder- storm. The appropriate lower basin design storm is more properly related to the 100- year storm recurrence interval. By definition, the 100-year storm is that storm which occurs at an average interval of 100 years. Note that [he definition refers to an average interval, hence [here is no guaran- • tee that a storm Of this magnitude cannot oc- cur, say, twice in ten years. There is about a one percent probability ("OPP") of a 100- year storm being exceeded in any given year. The "OPP" should be the smallest design storm foc installations where high hazard potential exists, defined as installations located where failure would result in loss of life, serious damage to buildings, utilities and transporta- tion facilities. The drainage system would limit damage to pri- mary structures, limit erosion and prevent slope instability due to ponding and subse- quent saturation of benches. Significant erosion might be expected from the large storms. This is to be tolerated only during extra high flows. Because all ditches are existing and follow the sides of roads, however, normal run-off flows will not be at erosion-causing velocities. The sedimentation ponds were constructed to adeouately store the sediment generated from this drainage plan. Revised 08/01/84 Corrected 10/12/86 Revised 10/01/87 101