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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 <br />when subjected to 10 -year and 25 -year, 24 -hour design storm events. Due to the lack of ability for <br />SedCAD to directly incorporate base flow conditions, i.e. B -seam discharge, SWS #4 was added to the <br />structure representing Dewatering Pond #1. The acreage of SWS #4 was adjusted to result in an <br />increase in peak flow out of this structure of 100 gpm (0.22 cfs). Greater flow rates are permitted in the <br />NPDES permit but would not occur until site stabilization with vegetation. <br />Several items should to be pointed out concerning the design and model. <br />1. Only two of the dewatering ponds were used in the SedCAD model. Three are planned for the <br />initial construction. Surface areas for these ponds were incorporated into SWSs for Dewatering <br />Pond #1. <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 <br />those presented due to the longer flow distances and associated delays in flow caused by the <br />upstream 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 <br />to the site stabilization and treatment of the inflows. <br />6. The emergency spillway of the Last Chance Pond consists of the termination of the north end of <br />the berm for the embankment at an elevation one foot above the '0.00' elevation for the flume. <br />While use of this spillway is not projected by either scenario modeled it provides a failsafe <br />routing for flow under extreme storm events or the clogging of the primary spillway by animals <br />such as muskrats. This spillway empties back onto the flat with about a 3% slope. <br />Technical Revision #68 (05/2011) <br />