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JD -6 Mine — Drainage Design Plan 24 <br />e , <br />Figure 10. Photograph of Lower Diversion Structure North (LDS -N) <br />4.5 Storage Structures <br />Storage structures are stormwater management facilities typically designed to impound water, control <br />sediment, and delay downstream flooding. At the JD -6 Mine, a stormwater and sediment retention pond <br />(referred to in the TR55 and HydroCAD® models as "SWRP ") collects sediment and runoff from roads <br />and the mine facilities area, as described in Section 4.5.1 below. Upstream storage ponds also work to <br />collect sediment and reduce peak flows, but are not explicitly modeled in TR55 as discussed in Section <br />4.5.3 below. <br />4.5.1 Stormwater and Sediment Retention Pond (SWRP) <br />The SWRP collects stormwater and sediment that runs off from the mine facilities area (or Duggan Adit <br />staging area) and access road. The existing SWRP is shown in Figure 11 and its location relative to mine <br />facilities is shown in Figure 4. The planned SWRP expansion is discussed in Section 4.5.2. <br />The SWRP was surveyed in May, 2013. The pond is routinely maintained, and the fine sediment that <br />collects in the pond is regularly removed to preserve storage capacity. As of May 28, 2013, the pond was <br />dry and had a maximum possible surface area of 0.068 acres. The pond surface dimensions are <br />approximately 45 ft by 85 ft, with a total usable pond volume of 12,512 W (0.287 acre -ft). <br />The SWRP is implemented as an in -line storage structure in the WinTR55 model. The TR55 model <br />assumes that the SWRP is a wet detention pond, which is considered an effective BMP for the removal of <br />particulates (the settleable solids fraction of pollutant load) and only minimally effective in the removal of <br />dissolved or colloidal constituents. "Classic' stormwater management practices assume that dry ponds <br />have little documented direct water quality benefits due to scouring of bottom sediments. Wet ponds are <br />designed to facilitate sediment removal and to allow decanted water to pass through the structure via a pipe <br />4148A.131028 Whetstone Associates <br />