PERMFILE58447 - Laserfiche WebLink

Home Browse Search

Pa1~ J'AE-fs HYDROLOGIC AND WATER QUALITY DOCUIIIENTATION POWER BOREHOLE PAD SMALL AREA EXEMPTION TWENTYMILE COAL MINE INTRODUCTION Twentymile Coal Company (TCC) plans to construct a 200' x 200' bore hole pad at the coal mine as shown on Figure I. Borehole Pad Location. The following hydrologic and water quality modeling documentation is provided to support a small area exemption (SAE) for the pad. A borehole will be drilled from the surface to the underground workings. An electrical power disiribwion line will be dropped through the borehole to the underground mine. The pad will be constructed near the ridge top. The watershed area above the pad is' very small and undisturbed, and therefore, will produce insignificant runoff. The 10-year, 24-hour storm was used in the design that is appropriate For this application. The drill pad will be relatively flat and graded to drain towards the North at a slope of about 0.5`70. Topsoil from the pad area will he pushed into berms surrounding the pad (see Figure I ). The topsoil beans will he seeded with the previously approved temporary seed miz. A 10' wide grass fiber has been estimated to be adequate to treat runoff from the drill pad. The area down, gradient of the pad is undisturbed and is a mixture of grass and sagebrush. Since the distance from the pad to the nearest drainage is more than 200 feet, the grass/sagebrush should he more than adequme to treat any runoff from • the pad. The drill pad surface will be covered with gravel. Hydrology and grass filter efficiency was modeled using the SEDCAD9 computer model consistent with methodologies presented in previous submittals. Sediment calculations were based on disturbed area particle size distributions presented in previous submittals and shown on the accompanying SEDCAD4 computer printouts. 1.0 Bore Hole Pad Area The drill pad will be approximately 200' x 200' which equals 0.92 acres'. Hvdroloev Inputs Storm Event: 10-year, 2d-hour - I.8 in Soil Type: Gravel Ground Cover: Disturbed Cover Type: N/A CN: 89 All other hydrology inputs are shown on the attached SEDCAD3 computer printouts. Sedimentoloev Inputs K factor: 0.10 C factor: I.0 P factor: 0.9 Ground: Disturbed (gravel)