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EXHIBIT 7, ITEM 23, PART M <br />Sediment Control for Initial Disturbance <br />Coal Stockpile Pond Using Total Containment <br />Exhibit 7, Item 23 Part J discusses the operation and performance of Coal Stockpile Pond during <br />the initial disturbance phase. Modeling associated with this Part M assumes that both the Middle <br />Pond and the Section 36 Pond are not constructed downstream of the Coal Stockpile Pond (Part <br />J). Due to certain regulatory issues, it may be necessary to delay the construction of both the <br />Middle Pond and the Section 36 Pond. This Part M describes how as an alternative, upstream <br />disturbances can occur if the Coal Stockpile Pond is constructed and operated as a total <br />containment structure. <br />It should be noted that this Part M modeling is only necessary prior to the time that the Section <br />36 Pond and the Middle Pond are completed, and is assumed to be a temporary very short <br />duration condition. All structures associated with this Part M are also temporary except the Coal <br />Stockpile Pond SEDCAD modeling associated with this Part M is similar to but slightly <br />different than the modeling provided in Part J, mainly due to the method used to network the <br />model. <br />As noted previously it is assumed that this Part M is temporary short term condition. If non <br />Division regulatory constraints are alleviated, construction of the Middle Pond and Section 36 <br />Pond can commence. If that occurs, Colowyo will continue to develop this Part M fully as <br />described below, and will also construct the Section 36 Pond and Middle Pond also. If this <br />situation occurs this would make a conservative model, as Part J only requires a small sump in <br />the Coal Stockpond Area to be constructed to contain all runoff from the facilities area. After <br />July 15, Colowyo will begin to construct the Section 25 Pond, and at that time Parts J and M of <br />this exhibit will no longer be necessary. Once that condition occurs Colowyo will submit a <br />minor revision to the permit to remove Parts J and M since they are both temporary conditions. <br />Developing Site Access <br />As an initial step, it will be necessary to construct a temporary topsoil haulage route to access the <br />Coal Stockpile Pond and the associated topsoil stockpile area. Exhibit 7-23, Figure MI shows <br />the location of this haulage route. Typically, this temporary haulage route will be approximately <br />40 feet in width to allow two way traffic. A portion of this route is not within the drainage area <br />of the Coal Stockpile Pond. In this case, a temporary subgrade sump and associated small v - <br />ditch will be built. The haulage route inside the drainage area of the Coal Stockpile Pond will <br />have erosion control silt fence or s -fence installed down gradient during stripping of topsoil and <br />construction of the Coal Stockpile Pond and associated ditches. This silt fence or s -fence will be <br />removed once the collector ditches and Coal Stockpile Pond are fully constructed. <br />Temporary Sump <br />The first SEDCAD model provides the 10 year 24 hour design for the temporary sump. As <br />shown, this design event provides about 0.21 ac -feet of runoff from the approximately 21.9 acres <br />of watershed. Given the results of the SEDCAD model, the sump needs to have 3H:1V <br />sideslopes, with a top length of 85 feet by top width of 60 feet, a depth of approximately 3 feet to <br />store design storm. The temporary incised sump will completely contain all runoff from this <br />Exh. 7-23M-1 Revision Date: 5/17/17 <br />Revision No.: TR -111 <br />