2012-03-27_GENERAL DOCUMENTS - C1981010 (2)

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the SEDCAD 4 calculates the expected sediment load in runoff from a 100 -year precipitation event. In the USLE comparison, only the "C" factor is varied between the pre - mining and the post - mining calculations; all other variables are constant. The pre - mining calculation uses baseline vegetation cover values from the permit application (Table 2.3 -13 and Table 4.4 -2). The postmining calculation uses vegetation cover data collected in 2010 for the disturbed area. In the review of the original SL -12 application, Division staff questioned the USLE value for cover (C value) used by TMI for post- mining. TMI staff agreed with the Division and subsequently updated the USLE calculations accordingly. For the bond release parcels in SL -12, TMI calculated a pre - mining sediment yield rate of 3.12 tons per acre per year and a post - mining rate of 2.21 tons per acre per year, suggesting that the post- mining rate is less than the pre - mining rate. The Division finds that the variables used in the calculations are reasonable for making the comparison; therefore, the comparison appears technically valid. Furthermore, the variables are similar to those the Division has found to be reasonable in TMI's previous bond releases, and the lands in SL -12 are similar to some of the lands in previous bond releases. In the SEDCAD 4 comparison, TMI utilized baseline data for selecting parameters for the SEDCAD 4 run with pre - mining conditions and 2010 data in the bond release submittal for the run with post - mining conditions. The factors that differ between the pre - mining condition and the post - mining condition include the curve numbers and the control factors. The Division's review of each of the parameters shows that the parameters reasonably represent the conditions in the field for the pre- mining conditions and the post - mining conditions. TMI ran SEDCAD 4 for a 20 -acre study area having characteristics representative of the bond release parcels both before and after mining. SEDCAD 4 calculated the sediment load in runoff flowing to a single discharge point at the downstream end of the study area. The Division believes use of a 20 -acre study area is a valid approach for modeling sedimentation in the reclaimed drainages in the bond release area because it averages actual vegetative cover data. The total sediment load calculated for the pre - mining 20 -acre study area was 30.3 tons of sediment. The total sediment load calculated for the post - mining 20 -acre study area was 29 tons of sediment. The primary reason for the result is the difference in the Control Practice factors. For the post- mining conditions, the ground cover is greater than pre - mining conditions, resulting in a lower C factor. Furthermore, surface conditions after mining and reclamation will be roughened using contour seeding and contour ditches. Observations made during the bond release inspection, as well as observations made during previous inspections, indicated that the vegetation on the reclaimed slopes had stabilized the soil. There was no major erosion or soil instability observed. The Division's field Trapper Mine Page 8 March 27, 2012 Phase 11 Bond Release (SL -12)