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Larry Routten and Erica Crosby -8- November 3, 1993 <br />The derived peak flows are as follows: <br />:Event Cubic Feet per Second <br />10-year, 24-hour Approximately 198 <br />25-year, 24-hour Approximately 293 <br />100-year, 24-hour Approximately 474 <br />(Tables 1-31 <br />recommend that all permanent diversions should be designed using a minimum 100-year, <br />24-hour event prediction. These estimates were derived using all available technical <br />information, and the SEDCAD software. This software is considered to be astate-of-the-art <br />package by both mine regulatory authorities and the coal mining industry. <br />Channel Desion <br />The site-specific survey data derived by the DMG were used to estimate channel gradients at <br />the West Pit location. A copy of the plot derived from the survey data is attached (Figure 6). <br />It is assumed that the existing natural channel west of the pit area will be connected with <br />newly constructed transition areas at the base of the West Pit. These transition areas are <br />located at, and above, the sediment pond located at the base of the West Pit area. This design <br />will connect the natural channel located along the western edge of the West Pit, with the <br />Magpie Creek channel just below the existing sediment pond. The sediment pond will be <br />removed. <br />The channel designs are derived using the Mannings-equation-based SEDCAD software for <br />channel design. Barfield et al. (1987) provide a description of the Mannings equation and its <br />proper use. Riprap designs were generated using SEDCAD, also with reference to the Simons <br />et al. (1982) and Urban Drainage and Flood Control District (19691 publications. <br />Figure 6 depicts the general layout of the West Pit channel work area. Channel sections are <br />designated as follows: <br />Section A - Native channel located west of the West Pit <br />Section B - Upper transition channel <br />(Approximate length = 125 feet) <br />Section C - Existing West Pit reclamation channel <br />Section D - Middle channel <br />(Approximate length = 150 feet) <br />Section E - Lower transition channel <br />(Approximate length = 125 feet) <br />