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the down-stream end of each 20-acre study area. SEDCAD+ was run for pre- and post-mining <br />conditions. <br />10-year, 24-hour model storm. A 10-year. 24-hour rainstorm was modeled in the SEDCAD+ runs. <br />Such storms contribute only a small fraction of the total annual tlows in the drainages where the <br />bond release parcels are located, but it is during these short-lived storms that most of the erosion and <br />sediment transport occurs. The erosion is caused mainly by concentrated overland flow on hillslopes <br />and channelized flow in the reconstructed drainageways. <br />Division's review of Trapper's SEDCAD+ parameters. Trapper used baseline data for selecting <br />parameters for the SEDCAD+ runs ofpre-mining conditions. Trapper used recent data in the bond <br />release submittal and annual reclamation reports for the runs of post-mining conditions. The <br />Division reviewed all of the parameters and found that one set of Trapper's parameters reasonably <br />represents the conditions in the field for the pre-mining conditions and another set that Trapper used <br />reasonably represents post-mining conditions. The Division found that other sets of parameters that <br />Trapper selected were not reasonable. The Division's review is summarized in a memorandum to <br />the bond release file dated November 23, 1998. <br />Trapper's conclusion from SEDCAD+. Trapper's SEDCAD+ modeling concluded that the <br />computed postmining peak settleable solids generated from each block is much less than the <br />computed premining value. Trapper considers that the lower postmining sediment yields aze a result <br />of a large amount of vegetative cover on the reclaimed lands. <br />Need for independent verification of sediment yield rates predicted by Trapper's SEDCAD+ <br />runs. The Division identified three limitations to the curve number method that SEDCAD+ uses <br />for predicting runoff and sediment yield. These limitations are applicable in Trapper's SEDCAD+ <br />modeling; consequently, the Division employed the following three alternative methods for checking <br />the reasonableness of Trapper's SEDCAD+ results. <br />1. The Division's calculations of the Universal Soil Loss Equation for pre- and post-mining <br />conditions. Using parameters based on pre-mining and post-mining data, the Division <br />calculated average annual soil loss per acre from the bond release parcels to be 1.06 tons for pre- <br />mining conditions and 0.45 tons for post-mining conditions. These figures represent only gross <br />soil loss, and do not account for differing sediment delivery rates from disturbed lands compazed <br />to undisturbed lands. The Division predicted 10% less delivery from undisturbed lands as <br />explained in the Memorandum to the bond release file dated November 23, 1998. Reducing the <br />pre-mining gross soil loss rate by ]0% gives apre-mining net sediment yield rate of 0.9~ <br />ton/acre/year, which is still more than the 0.45 post-mining sediment yield rate. <br />2. The Division's calculations of post-mining sediment yield based on surveys of sediment <br />accumulated in Trapper's ponds. The Division used pond sediment survey data for calculating <br />the amount of sediment that the bond release parcels have yielded over the last several years. <br />These post-mining sediment yield rates, based on pond sediment surveys, are less than the pre- <br />mining 0.95 ton/acre/year rate calculated using the USLE, adjusted for greater post-mining <br />Trapper Mine Page 9 12/14/98 <br />Phase II Bond fielease <br />