Laserfiche WebLink
9 <br /> <br /> <br /> <br />The total acreage of the pre-mining Flume Gulch watershed is approximately 5,800 acres. The pre-mine <br />Flume Gulch total watershed acreage within the Trapper Mine permit area is 1,878 acres and increases <br />by approximately 236.8 acres post-mining to an acreage of 2,114.3, an increase of approximately 12.6%. <br />In order to understand the changes in the drainage flows and sediment generation as a result of the <br />increased acreages, Mattern also reviewed the pre- and post-mining drainage densities and sediment <br />generation. Table 4 shows the drainage densities pre- and post- mining and Table 5 shows the <br />predicted USLE and SEDCAD outputs. <br />Table 4 Flume Gulch Pre-Mine and Post-Mine Comparative Hydrology Characteristics <br /> Pre-Mining Post-Mining Pre-Mining Post-Mining <br /> <br />Drainage <br />Density <br />Drainage <br />Density <br />Average <br />Gradient <br />Average <br />Gradient <br />Sub-Drainage (mi/m2) (mi/m2) (%) (%) <br />Grouse 4.8 4.0 8.3 8.2 <br />Sage 3.4 6.5 9.1 10.0 <br />Oak 3.8 3.1 11.4 11.9 <br />West Flume 4.5 4.1 13.5 14.5 <br />Middle Flume 2.8 3.4 10.4 9.8 <br />East Middle Flume 4.4 3.1 10.8 8.8 <br />East Flume 5.6 4.5 9.4 8.3 <br />Overall Average 3.8 3.6 10.6 10.3 <br /> <br />Table 5 Flume Gulch Pre-Mine and Post Mine Sediment Yield Projections for the L-Pit <br />AOC Variance <br /> <br />USLE Results <br />(annual tons/acre) <br />SEDCAD Total <br />Area (acres) <br />Overall Sediment <br />Yield (tons) <br />Sediment Yield Per <br />Acre (tons <br />Pre-Mining 2.44 771 872.8 1.13 <br />Post-Mining 0.70 1,038 765.9 0.74 <br /> <br />Based on the Mattern evaluation, the sub-drainage watersheds would be slightly larger overall. The <br />watershed characteristics would be very similar with the same overall drainage densities and similar <br />drainage gradients. The post-mining AOC variance topography shows an overall decrease in sediment <br />yield per acre based on the SEDCAD evaluation. <br />Drainage patterns would be designed to blend with surrounding drainage patterns and would not <br />represent a change from the surrounding drainage patterns. Figure 3 shows the proposed drainage <br />patterns for the AOC variance post-mining topography for the L-Pit area, the drainage patterns within <br />other reclaimed areas of the Flume Gulch drainage and the surrounding areas. As shown on the figure <br />the proposed post-mining drainage pattern blends with the surrounding drainage patterns. <br />There would not be any significant changes in the post-mining watershed areas that would impact <br />seasonal or flood flows from the post-mining reclaimed areas or adversely affect surrounding ecology or <br />any existing or planned use of surface water or groundwater.