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B -Seam Dewatering Treatment System <br />General Notes on Design, Operation and <br />SedCAD Analysis <br />The B -Seam Dewatering System area (see Map 157) was analyzed using SedCAD to evaluate the area when <br />subjected to 10 -year and 25 -year, 24 -hour design storm events. Due to the lack of ability for SedCAD to <br />directly incorporate base flow conditions, i.e. B -seam discharge, SWS #4 was added to the structure <br />representing Dewatering Pond #1. The acreage of SWS #4 was adjusted to result in an increase in peak <br />flow out of this structure of 200 gpm (0.44 cfs). Greater flow rates are permitted in the NPDES permit but <br />would not occur until site stabilization with vegetation. <br />Several items should to be pointed out concerning the design and model. <br />1. Only three of the dewatering ponds were used in the SedCAD model. Eight ponds are proposed. <br />Surface areas for these ponds were incorporated into SWSs for the system. <br />2. Modeling represents conditions worst case conditions immediately following construction. <br />3. Routing was ignored in the SedCAD model. Actual peak flows are expected to be lower than those <br />presented due to the longer flow distances and associated delays in flow caused by the upstream <br />ponds and flow through the artificial wetlands. <br />4. Gradients within the artificial wetlands are very low and would result in sediment reductions <br />which are only partially accounted for by this 'screening' SedCAD model by use of the 'P' factor <br />for one of the subwatersheds. Actual reductions in sediment loads through the wetlands were <br />not accounted for. <br />5. Increased efficiency of the system is expected as the vegetation matures and contributes more to <br />the site stabilization and treatment of the inflows. <br />6. The spillway system for the Last Chance Pond is a two tier combined system. It consists an 'H' <br />flume for the primary with emergency spillway formed from the level immediately above the <br />flume. <br />7. The 'H' flume was selected for the primary spillway for its ability to accurately measure flows <br />under low flow and high flow conditions. A 1.0' H flume has a reported range from 0.31 to 861.7 <br />gpm (0.0007 to 1.92 cfs). <br />Minor Revision #162 (09/2014) <br />