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APPENDIX 2 <br />ADEQUACY OF POND SIZING <br />The Mid -Term Permit Review of the Fish Creek Tipple/Loadout Operation <br />indicated Twentymiie Coal Company needed to evaluate the adequacy of Pond <br />E. As outlined in the 13 October 1987 letter, the Division's concern <br />involved operation discharge, spoil spring flow, and the 10 -year, 24-hour <br />event. <br />Twentymile Coal Company has reviewed the Pond E design calculations in <br />order to determine if the pond was adequately sized and if values used in <br />the calculations were appropriate. The review had indicated the tributary <br />area was erroneously recorded as 24 acres of disturbed and 11 acres of <br />undisturbed. Actually, the disturbed acreage is 22.8 and undisturbed <br />acreage is 12.7. Using these values results in a lower curve number of 81 <br />rather than 82 which, consequently, alters the amount of expected runoff <br />(1.42 acre-feet versus 1.52 acre-feet). Adding sediment storage, operation <br />discharge and spoil spring flow to the 1.42 acre-feet of runoff storage, <br />indicates the required design storage is 2.04 acre-feet. <br />Since the as -built storage is 2.19 acre-feet, the pond is adequately <br />sized to accommodate runoff volume from the 10 -year 24-hour event, <br />sedimentation storage, operation discharge, and spoil spring flow. <br />The following calculations support the adequacy of the Pond E sizing. <br />Assumptions have been used with the exception of the contributing drainage, <br />as previously discussed. <br />8-2-a Revised 10/19/88 <br />