2023-11-27_PERMIT FILE - C1981019 (11)

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Exh. 7-23B-1 Revision Date: 9/18/23 Revision No.: TR-160 Exhibit 7, Item 23, PART B Section 25 Pond Sediment Control During Mining Introduction Exhibit 7, Item 14 in Volume 2D describes the hydrologic methodology used in sediment pond, permanent post-mine channels, and temporary conveyance channel demonstrations. Runoff curve numbers assigned to the undisturbed and/or reclaimed lands in various stages of mining and reclamation have been selected in accordance with Table 1 in the Introductory Text for Exhibit 7 in Volume 2D or as further described below. For channels protected by a riprap liner, selection of minimum riprap size is done using the Simons/OSM method in SEDCADTM. For channels to be protected by a vegetative liner, the permissible velocities are also determined using SEDCADTM routines. SEDCADTM demonstrations with two distinct time frames have been evaluated in detail for the Section 25 Pond. Exhibit 7, Item 23, Parts I addresses sediment control for the Section 25 Pond during the post mining timeframe including post mine drainage channels. This exhibit (Exhibit 7, Item 23, Part B) address sediment control during the mining timeframe for the Section 25 Pond and provides a demonstration for a temporary channel configurations. The assumed condition during mining is the temporary spoil pile is constructed to its maximum configuration and the entire area from the temporary spoil pile and native areas below are reporting to the Section 25 Pond. Based on recommendations provided in a February 2015 telephone conversation with Dr. Richard Warner of the University of Kentucky Biosystems and Agricultural Engineering Department, and a coauthor of the SEDCADTM Design Manual and User’s Guide, the "Cover Factor" appropriate for loose dumped spoil is between 0.40 and 0.45. The higher value was conservatively selected for use in this analysis. When advised that we are using curve number of 74 for runoff from loose dumped spoils, as per the approved Introductory Text of Volume 2D, Dr. Warner pointed out that a value of 74 is probably an overly conservative number, and a value on the order of 65 would be more appropriate based on his experience and measurements made over the years at numerous coal mines. A curve number of 74 was nevertheless used for spoil piles in the SEDCADTM modeling included in this submittal. Selection of SEDCADTM input parameters L and S which are needed for sediment generation in the sedimentation evaluation analysis are selected on a case basis for each subwatershed. The MUSLE (Modified Soil Loss Equation) is used to estimate sediment generation at the source. Selection of the parameters R, K, LS C and P is described in the literature. LS is derived via an equation that includes a "length" parameter "L", and a "slope" parameter "S". However, LS is not the simple product of those two terms. Both "L" and "S" are based on the topography of the subwatershed in question with certain limitations provided in the literature as well as SEDCADTM users' training courses. First a representative slope is determined from the topography, and this sets the parameter "S". The "length" parameter "L" is more complicated as it represents the longest slope that water will have to flow before encountering a concentration point, such as a rivulet of a distinct slope change. SEDCADTM instructors advise that regardless of the topography, "L" can ever practicably exceed about 200 feet.