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/~/, ~ ~l~ <br />5' E • ~X~,,,P,,,v(- $ r~l ~~~ I ll l ll l ll uu ~u <br />999 <br />HYDROLOGIC DESIGN DOCUMENTATION <br />REFUSE DISPOSAL PILE BENCH CHANNELS <br />TWENTYMILE COAL MINE <br />INTRODUCTION <br />The following hydrologic and design documentation is provided for the bench channels for the proposed <br />refuse disposal pile. Twentymile Coal mine (Twentymile) proposes to construct drainage channels at 50' <br />(vertical) intervals along the side slope of the refuse disposal pile. The channels will be 20' wide and have <br />an average slope of 0.5%. Side slopes of the channels will be about 2.8H:1 V on the pile side. and 20H:1 V <br />on the out-slope side. (refer to refuse pile typical details) <br />Hvdroloev Inputs <br />The largest bench channel watershed area was used for design purposes. This area equaled 5.9 acres. A <br />curve number (CN) of 86 was used to represent a disturbed condition of fairly coarse material. <br />Disturbed <br />• <br />~- <br />Soil Type: <br />Hyd. Soil Group: <br />Ground Cover: <br />Cover Type: <br />CN: <br />Design Storm: <br />Results <br />coal refuse, coarse <br />disturbed <br />N/A <br />86 <br />IOOyr/24hr, 2.6 in. <br />SEDCAD4 modeling show a peak Flow of 4.7cfs from the I OOyr/24hr return interval storm. The channel <br />design (attached), show that the planned bench channels will contain and safely pass the peaA flow. <br />Hydrology inputs are shown on the attached SEDCAD4 computer printouts. <br />t~lApPgOVED JA`s 0 5 1999 <br />