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The peak design flow for each terrace was determined using the <br />SEDIMOT II computer model. In accordance with Section 4.05.3 of the CMLRB <br />rules and regulations, all of the terraces have been designed to safely <br />pass the peak flow of the 10-yr, 24-hr storm event. The structures are <br />permanent, convey runoff as an ephemeral channel and have watersheds less <br />than one square mile. The calculations to determine flow parameters were <br />taken Erom Section IV, Surface Mining Water Diversion Design Manual, <br />Simons, Li & Associates, Inc.(1982). <br />Figure 4.6-M2 shows typical terrace channel cross sections. Table <br />4.6-M3 provides the design specifications for each of the terraces. <br />Appendix 4.6-M(b) contains a description of the various SEDIMOT II input <br />parameters, the SEDIMOT II worksheets for hydrograph routing between <br />structures and time of concentration within a watershed, and the computer <br />simulation output summary sheets. <br />CONSTRUCTION 9CHBDQLB <br /> The schedule for the hydrologic reconstruction plan consists of four <br /> major phases. The first phase pertains to the construction of the lower <br /> portion of Terrace 6. Approximately 1,000 feet of Terrace 6 will be <br /> constructed upslope from the Center-'Rest Ridge drainage channel as remedial <br /> action to abate Not ice of Violation C-90-041. The second phase pertains to <br /> the construction of all of the check dams along the watercourse between <br />Center Ridge and West Ridge. The third phase will include the construction <br />of terraces along the final graded slopes of the West Ridge mining area. <br />The fourth and final phase will consist of completing the construction of <br />the terraces along the upper slopes of the West Ridge area after the final <br />pit area has been backfilled and final graded to the approved postmining <br />topography. <br />Other than building approximately 1,000 feet of Terrace 6 in Phase I, <br />construction of the terraces will not be started until all the check dams <br />have been built. Proceeding in this manner will ensure the stability of <br />the watercourse prior to introducing concentrated flows from the West Ridge <br />mining area. Implementation of the plan will commence not later than ten <br />(10) working days following approval of the plan, unless inclement weather <br />or unfavorable ground conditions preclude construction activities. <br />• <br />6 <br />March 31,1994 <br />