2020-07-01_PERMIT FILE - C1981019 (2)

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Rail Loop Pond This sediment control plan addresses the watershed tributary to the Rail Loop Pond. The Rail Loop Pond receives water from the Gossard Loadout area, including several subwatersheds including the train loading tunnel, roads, the Gossard Pond outslope, and some direct inflow from water used during wash down of the train loading tunnel. The Rail Loop 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 Rail Loop Pond SEDCAD model and the basis for utilization of the curve numbers in the models. The subwatershed and corresponding acreages used in this SEDCAD model are presented on Figure 1, and Figure 2 provides the as -built configuration of the Rail Loop Pond. Colowyo washes down the train loading tunnel on an as -needed basis. The water that is used during wash down flows on the ground out of the tunnel and topographically it is routed to the Rail Loop Pond. SEDCAD does not have the ability to model this additional inflow directly; therefore, the methodology suggested by SEDCAD's primary author, Pam Schwab, is to model the impact of a constant inflow into the Rail Loop Pond through a dummy structure. In the current version of SEDCAD, 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 0.02 cfs (10 gpm) was conservatively utilized, which is a volume of water well in excess of any wash down inflows that may be encountered in the Rail Loop Pond. The synthesized 10 gpm inflow was then dropped directly into Rail Loop Pond in the SEDCAD model. Further, Colowyo does not wash down the train loading tunnel daily, so the 10 gpm is very conservative estimate compared to actual activities occurring. SEDCAD 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 6,347, to account for the 10 gpm already being in the pond when the storm event would commence. The volume of the pond was inputted into the SEDCAD 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. The results of the runoff calculations and synthesized constant inflow are presented in the attached SEDCAD model outputs. As noted, the peak elevation for this modeled storm event is 6,351', two feet below the emergency spillway elevation of 6,353'. Thus, the Rail Loop Pond can fully contain the 100 year, 24 hour storm event without discharging. Page 1 Revision Date: 4/29/20 Revision No.: TR -138