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Peak Flow <br />The 10-year, 24-hour peak flow value is calculated with the hydrology component of the <br />SEDCAD+ model. Appendix A presents the hydrology demonstraitions. <br />Channel Slope <br />Channel slope is the most critical parameter. The steeper the slope; the lower the allowable <br />velocity. The channel slopes were determined by direct measure ie,nt from the mine permit <br />maps. ~ <br />Channel Geometry <br />Existing channels (constructed prior to 2007) were evaluated assuming a triangular, or V-shaped, <br />configuration, with a 3H:1V side slope. In certain situations, SCC designed atrapezoidal- <br />shaped channel with a bottom width of either three (3) or 10 feet and 4H:1V side slopes. <br />The design parameters for peak flow, channel slope and channel geometry are summarized on <br />the Channel Design Schedule table at the front of Appendix B. <br /> <br />Retardance Class <br />The reclamation seed mixture SCC uses to revegetate reclaimed areas results in a mixed grass <br />community. Consequently, the vegetation type selected as the model input is mixed grass. <br />The flow resistance varies by grasses type and length. Retardance classes have been divided <br />into five classes designated by A, B, C, D, and E. SCS (1979) developed guidance for selection <br />of vegetation retardance classes and they are outlined in Table 7-1i2-1, Guide to Retardance <br />Class Selection. <br />TABLE 7-12-1 <br />Guide to Retardance Class Selection <br />Stand Length of Vegetation <br />in. Retard•,mce Class <br />Good <br /> >30 A <br /> 11 - 24 B <br /> 6-LO C <br /> 2-6 D <br /> <2 'E <br />Fair <br /> >30 B <br /> 11 - 24 C <br /> 6-10 D <br /> 2-6 D <br /> <2 E <br />t` J <br />TR-41 7-12-6 ~ ~. Revised 04/07 <br />