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Gossard Pond <br />This sediment control plan addresses the watershed tributary to the Gossard Pond. The Gossard <br />Pond receives water from the Gossard Loadout area, including several subwatersheds including <br />the coal stockpiles and coal preparation areas, and some direct inflow from water used during <br />wash down of the Gossard crushing facility. The Gossard Pond is a non -discharging structure <br />that is designed to contain the 100 -year, 24-hour storm event as demonstrated herein. <br />Please see Volume 2D, Exhibit 7 for the methodologies and assumptions utilized in the Gossard <br />Pond SEDCAD model and the basis for utilization of the curve numbers in the models. A curve <br />number of 74 was selected for the majority of the contributing subwatersheds. This is believe to <br />be a more representative curve number than a disturbed curve number of 85 since the Gossard <br />Loadout area subwatershed are broken up including ever changing loose unconsolidated coal <br />stockpiles due to the shipping of coal. The subwatershed and corresponding acreages used in <br />this SEDCAD model are presented on Figure 1, and Figure 2 provides the as -built configuration <br />of the Gossard Pond. <br />Colowyo washes down the Gossard primary crusher on an as -needed basis. The water that is <br />used during wash down is routed down to two concrete structures that capture the coal fines, and <br />once full of water, discharge the water from the concrete structures directly into the pond. <br />SEDCAD does not have the ability to model this additional inflow directly; therefore, the <br />methodology suggested by SEDCAD's primary author, Pam Schwab, is to model the impact of a <br />constant inflow into the Gossard Pond through a dummy structure. <br />In the current version of SEDCAD, a fixed flow can only be inputted by inserting a "dummy" <br />upstream pond with a watershed large enough to produce a "tank" flow, and then setting the <br />output of the dummy reservoir as a constant "User Defined" outflow curve, independent of pool <br />elevation in the dummy pond. A flow of 0.223 cfs (100 gpm) was conservatively utilized, which <br />is a volume of water well in excess of any wash down inflows that may be encountered in the <br />Gossard Pond. The synthesized 100 gpm inflow was then dropped directly into Gossard Pond in <br />the SEDCAD model. Further, Colowyo does not wash down the Gossard primary crusher daily, <br />so the 100 gpm is very conservative estimate compared to actual activities occurring. <br />SEDCAD also allows the user to override the customary starting pool. In this case for the 100 <br />year storm event, the starting pool is assumed to be at 6,388, to account for the 100 gpm already <br />being in the pond when the storm event would commence. <br />The volume of the pond was inputted into the SEDCAD model, along with its spillway details. <br />The model watersheds were inputted as a series of sub -watersheds, each with its own acreage, its <br />own flow response parameters (slope, distance, time of concentration), and the specific runoff <br />curve numbers noted above. The 100 year, 24 hour storm was then applied to the composite <br />watershed, and routed down to the sediment pond. <br />The results of the runoff calculations and synthesized constant inflow are presented in the <br />attached SEDCAD model outputs. As noted, the peak elevation for this modeled storm event is <br />Page 1 Revision Date: 4/29/20 <br />Revision No.: TR -138 <br />