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2-41 <br />2.4 Mining Hydrology <br />The objective of related studies was to provide an evaluation of the hydrologic impact <br />that the project will have on the mine area, and to serve as a basis for final design of diversion <br />and sedimentation control facilities. <br />2.4.1 Surface Water Hydrologic Controls <br />The surface runoff area has been subdivided into "disturbed" and "undisturbed" areas. <br />Runoff from most disturbed areas will be conveyed into a sedimentation pond capable of <br />containing the run-off from a 10-year event with a spillway system designed to handle the peak <br />flow generated by the 25-year storm event. Dewatering will be through a primary spillway pipe <br />with a normally closed valve and will be dewatered into East Salt Creek. Runoff from one area <br />addressed in Section 2.4.4.2 will be treated with alternate control measures. Near the portal, <br />runoff from the undisturbed areas will be diverted around the disturbed area, and conveyed to <br />the receiving ephemeral channel. Near the Coal mine waste facilities area, runoff from the <br />undisturbed areas will be routed to the sediment pond. Figure 2.2-3 depicts the flow of water <br />associated with disturbed and undisturbed areas. Please see Appendix N for a detailed analysis <br />- probable hydrologic consequences due to the Coal Mine Waste Pile. <br />The undisturbed area near the portal was divided into sub-basins designated C-1 <br />through C-4 and C-6 through C-15. The disturbed area is divided into sixteen sub-basins, <br />designated D-1 through D-16. These sub-basins during mining are shown in Figures 2.2-3; their <br />areas and hydrologic parameters are given on Table 4.2-10. The sub-basins vary in size from <br />0.5 acres of disturbed area up to 322 acres for the main McClane Creek drainage, see Figure <br />4.2-1a. Rainfall values fora 10-year and 25-year, 24-hour storm were taken from the NOAH <br />atlas; runoff was estimated according the Soil Conservation Service procedures, as given in <br />Procedures for Determining Peak Flows in Colorado and SedCad. <br />Runoff Calculations for the reclaimed configuration of the mine site are shown on Table <br />4.2-11. Table 3.1-1 contains the calculations for ditch and rip rap sizing. To provide the <br />necessary detail to evaluate the drainage basin boundaries after mining, the areas are <br />delineated on Figures 4.2-1b and 3.1-1. <br /> <br />M°Clane Volume 1 Rev 04/08 <br />