2024-03-15_PERMIT FILE - C1981019 (6)

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Page 1 Revision Date: 3/11/24 Revision No.: TR-162 Appendix Exh. 7-14G Gossard Pond This sediment control plan addresses the watershed tributary to the Gossard Pond. The Gossard Pond receives water from the Gossard Loadout area, including several subwatersheds including the coal stockpiles and coal preparation areas, and some direct inflow from water used during wash down of the Gossard crushing facility, and from a ditch routing water from a small portion of Haul Road A. The Gossard Pond is a non-discharging structure that is designed to contain the 100-year, 24-hour storm event as demonstrated herein. Please see Volume 2D, Exhibit 7 for the methodologies and assumptions utilized in the Gossard Pond SEDCADTM model and the basis for utilization of the curve numbers in the models. A curve number of 74 was selected for the majority of the contributing subwatersheds. This is believe to be a more representative curve number than a disturbed curve number of 85 since the Gossard Loadout area subwatershed are broken up including ever changing loose unconsolidated coal stockpiles due to the shipping of coal. The subwatershed and corresponding acreages used in this SEDCADTM model are presented on Figure 1, and Figure 2 provides the as-built configuration of the Gossard Pond. Colowyo washes down the Gossard primary crusher on an as-needed basis. The water that is used during wash down is routed down to two concrete structures that capture the coal fines, and once full of water, discharge the water from the concrete structures directly into the pond. SEDCADTM does not have the ability to model this additional inflow directly; therefore, the methodology suggested by SEDCADTM primary author, Pam Schwab, is to model the impact of a constant inflow into the Gossard Pond through a dummy structure. In the current version of SEDCADTM, a fixed flow can only be inputted by inserting a "dummy" upstream pond with a watershed large enough to produce a "tank" flow, and then setting the output of the dummy reservoir as a constant "User Defined" outflow curve, independent of pool elevation in the dummy pond. A flow of 0.223 cfs (100 gpm) was conservatively utilized, which is a volume of water well in excess of any wash down inflows that may be encountered in the Gossard Pond. The synthesized 100 gpm inflow was then routed directly into Gossard Pond in the SEDCADTM model. Further, Colowyo does not wash down the Gossard primary crusher daily, so the 100 gpm is very conservative estimate compared to actual activities occurring. SEDCADTM also allows the user to override the customary starting pool. In this case for the 100-year storm event, the starting pool is assumed to be at 6,388, which was determined by Colowyo’s staff experience managing the Gossard Pond, to account for the 100 gpm already being in the pond when the storm event would commence. The volume of the pond was inputted into the SEDCADTM model, along with its spillway details. The model watersheds were inputted as a series of sub-watersheds, each with its own acreage, its own flow response parameters (slope, distance, time of concentration), and the specific runoff curve numbers noted above. The 100-year, 24 hour storm was then applied to the composite watershed, and routed down to the sediment pond.