2022-07-12_PERMIT FILE - C1981019A (18)

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Exh. 7-23J-3 Revision Date: 4/28/22 Revision No.: TR-154 Section 26 Pond Permanent Post Mine Channels The Section 26 Pond watershed is comprised of two post-mining channels which include Bear Draw and Lower Bear Draw. The locations of these post mine channels are presented on Map 41B and Figure 7-23J-4, and the channel profiles are presented on Map 33C. The area contributory to Bear Draw and Little Bear Draw are less than one square mile; therefore, the modeled storm event required by rule is the 10-year 24-hour event. However, for built in conservatism, both channels will are modeled against the 25-year 24-hour storm event. The upper reaches of Bear Draw will be a vegetative lined while the lowest most portion will be riprap lined. Lower Bear Draw will be protected with riprap over the entire length of the channel. Two stock ponds, BD-1 and BD-2 will be constructed inline in the Bear Draw to reduce peak flows from larger storm events and provide a water source to support the post mine land use. Lower Bear Draw will be constructed inside the reclaimed footprint of the Section 26 Pond. Therefore, in the 25-Year 24-Hour model, it is assumed the entire watersheds reporting to both The Sidehill and Section 26 Ponds has been approved for Phase II bond release and both ponds are no longer necessary for sediment control. A SEDCADTM model (25 Year 24 Hour Channel Demonstration model) has been included which evaluates the peak flow for each of the channel. The minimum channel configurations are summarized below Name Peak Flow (CFS) Average Slope (%) Channel Type Side Slopes Minimum Depth (Feet) Erosion Protection Lower Bear Draw 27.04 9.5 Trapezoidal 10’ bottom 3H:1V 4.0 Riprap, D50 = 6” Bear Draw 0+00 to 15+00 30.03 10.8 Trapezoidal 10’ bottom 3H:1V 4.0 Riprap, D50 = 6” Bear Draw 15+00 to 34+00 60.49 2.8 Trapezoidal 10’ bottom 3H:1V 4.0 Vegetation Bear Draw 34+00 to 81+10 35.80 7.1 Trapezoidal 10’ bottom 3H:1V 4.0 Vegetation