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Trapper used SEDCAD 4 to model 20-acre study areas possessing the following sedimentologic <br /> characteristics representative of each bond release parcel: <br /> o Average slope <br /> o Slope lengths <br /> o Soil erodibility factor <br /> o Land use <br /> o Land cover <br /> This methodology allows for applying each block's actual vegetative cover data to the SEDCAD <br /> run for that block. Run for pre- and post-mining conditions, SEDCAD 4 calculated the peak <br /> settleable solids concentration of runoff flowing to a single discharge point at the down-stream <br /> end of each 20-acre study area. The Division finds use of 20-acre study areas a valid approach <br /> for modeling sedimentation in reclaimed drainages in the bond release area. <br /> The total sediment load calculated for the pre-mining block A,F,K,Z and Oak drainage 20-acre study <br /> area was 23.8 tons of sediment and for the same post-mining block, 9.6 tons of sediment. TMI's <br /> SEDCAD 4 modeling concluded that the calculated post-mining peak settleable solids generated from <br /> each block is much less than the calculated pre-mining value. The Division's review of each <br /> parameter is presented in the table below. <br /> Table 8. SEDCAD Variables and Basis for Selection. <br /> Pre-mining <br /> Paramete Trapper's Trapper's basis for selection Division's Comment <br /> r Selection pp <br /> Produces the minimum 0.5 inch A greater than 100-year event is necessary in <br /> Design inches . of runoff necessaryto modelingas SCS TR55 questions the accuracy <br /> Event 24 hours maintaining program accuracy of the Curve Number method when runoff <br /> (SL19 application Sec 1.4.2). depth is less than 0.5 inch. <br /> 48 AMC II Trapper's selected cover, soil Reasonable as er selections of cover soil <br /> CN group, hydrologic conditions and p ' <br /> 62 AMC III group, hydrologic conditions and C factor. <br /> CP factor(see below). <br /> Oak/Aspen -Sage/ Permit application,Appendix Q Reasonable as per pre-miningconditions. <br /> Grass Section XII <br /> "Fair" hydrologic Permit application,Appendix Q Reasonable as per description of"Fair" in SCS <br /> condition Section XII TR55 <br /> group <br /> Hydrologic soil Permit application,Appendix Reasonable, as per soil types on bond release <br /> 8 p pp pp Q <br /> clasification "6' Section XII blocks delineated on map M29 &consistent <br /> with soul group B. <br /> CP Factor: 0.050 <br /> Tables 2.3 &4.4-2 of permit Selection accounts for total cover and canopy <br /> application C1981010 cover of premining vegetation. <br /> Slope data on Summary pages in Slope lengths and land use description inputs <br /> Time of Concentration SEDCAD runs accurately matched field data. <br /> Permit Number C 1981010 <br /> SL19 <br />