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Rail Loop Pond <br />This sediment control plan addresses the watershed tributary to the Rail Loop Pond. The Rail <br />Loop Pond receives water from the Gossard Loadout area, including several subwatersheds <br />including the train loading tunnel, roads, the Gossard Pond outslope, and some direct inflow <br />from water used during wash down of the train loading tunnel. The Rail Loop Pond is a non - <br />discharging structure that is designed to contain the 100 -year, 24-hour storm event as <br />demonstrated herein. <br />Please see Volume 2D, Exhibit 7 for the methodologies and assumptions utilized in the Rail <br />Loop Pond SEDCAD model and the basis for utilization of the curve numbers in the models. <br />The subwatershed and corresponding acreages used in this SEDCAD model are presented on <br />Figure 1, and Figure 2 provides the as -built configuration of the Rail Loop Pond. <br />Colowyo washes down the train loading tunnel on an as -needed basis. The water that is used <br />during wash down flows on the ground out of the tunnel and topographically it is routed to the <br />Rail Loop Pond. SEDCAD does not have the ability to model this additional inflow directly; <br />therefore, the methodology suggested by SEDCAD's primary author, Pam Schwab, is to model <br />the impact of a constant inflow into the Rail Loop 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 0.02 cfs (10 gpm) was conservatively utilized, which is <br />a volume of water well in excess of any wash down inflows that may be encountered in the Rail <br />Loop Pond. The synthesized 10 gpm inflow was then dropped directly into Rail Loop Pond in <br />the SEDCAD model. Further, Colowyo does not wash down the train loading tunnel daily, so <br />the 10 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 6,347, to account for the 10 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 />6,351', two feet below the emergency spillway elevation of 6,353'. Thus, the Rail Loop Pond <br />can fully contain the 100 year, 24 hour storm event without discharging. <br />Page 1 Revision Date: 4/29/20 <br />Revision No.: TR -138 <br />