2016-08-08_PERMIT FILE - C1981019A (8)

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EROSION AND SEDIMENT CONTROL PLAN INTRODUCTION This plan consists of the various subparts described in the Table of Contents. It addresses the active mining phase through the time period when the temporary spoil pile has been hauled back to the pit. It provides modeling and design information for diversions, ditches, sediment ponds, other sediment control measures. The post -mining hydrology is described elsewhere in Volume 18, Exhibit 7, Item 25. The hydraulic analysis and modeling has been based on the SEDCAD ® model software developed and marketed by Civil Software Design. Other hydrologic and hydraulic calculations and fundamental design assumptions used to model the proposed Colowyo Mine diversion and sedimentation structures are fully described the Introductory Text of Volume 2D in the document entitled "Methodologies And Assumptions For Sedimentation Pond Design Evaluations". SEDCAD4 is a comprehensive, computer-based model used to evaluate storm water, erosion, and sediment control management techniques. The SEDCAD4 model calculates run-off and peak flow via a numerical modeling technique based on user inputs of a design storm event (i.e., rainfall amount and duration, selection of rainfall distribution, and convolution increment). Hydrographs are developed on a sub -watershed basis with the input of area, time of concentration, SCS Curve Number, and the selection of 1 of 3 dimensionless double triangle unit hydrograph shapes. The results of the SEDCAD4 analyses are presented as attachments in each of the individual facility design sections. It should be noted that two distinct time frames have been evaluated in detail for the sediment pond performance analysis. These are addressed in Exh. 7-23, Part C, which deals with the time frame shortly after mining has commenced, and again in Exh. 7-23, Part I, which deals with a time frame near the end of the operation. An overview of the two scenarios and an explanation as to why two time frames were viewed as necessary is presented below. Because it was not clear that the early term condition evaluated in Exh. 7-23, Part C would be the critical condition as regards to sediment retention performance of the Section 25 and 26 ponds, Colowyo developed modeling of the late term case, Exh. 7-23, Part I. It should be noted that the physical dimensions and properties of the ponds themselves are identical in both cases. A detailed review of Exh. 7-23, Part C and Exh. 7-23, Part I will show that all that changes is the amount and manner of sediment delivered to them. Note also that the ponds satisfy the 0.5 ml/1 settleable solids performance standards in both cases, although again, the sediment inflow and outflow results are different, as would be expected. In the Exh. 7-23, Part C scenario, there will be a large pile of spoil located immediately upstream of the pond in each drainage. However, a mitigating factor in this time frame is the fact that that the majority of the watershed runoff does not report directly to the ponds because much of the runoff will be intercepted by the pit. So the sediment concentration would be expected to be higher, but the total volume of inflow would be lower. Both factors affect the trap efficiency of the ponds. In the Exh. 7-23, Part I scenario, the lower portions of the watershed are largely revegetated except for the bottommost lift of the former spoil pile's footprint. This location happens to be immediately above the ponds. The remainder of the removed spoil pile footprint will be in various states of revegetation growth by the time of this scenario. Much of the regraded pit area will also be reclaimed in this time frame. In this scenario, the pit will no longer intercept the runoff, and it all reports to the pond. For that reason, the series of upstream stock ponds was included, to accomplish the flow attenuation that was previously accomplished by the pit in the Exh. 7-23, Part C scenario." Exh. 7-23-1 Revision Date: 2/15/16 Revision No.: PR -04