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JD -7 Mine — Drainage Design Plan 37
<br />of 0.024 and routes stormwater to Catchment 001B -A North (Figure 11). The south collection ditch is 178
<br />feet long with an average slope of 0.1461 and routes stormwater to Catchment 001B -A South. Each is
<br />constructed with a relatively uniform channel geometry in fine grained material (sandy silt) with an
<br />estimated D85 grain size of 1.0 mm. The geometry and hydraulic parameters modeled for the two UGWRP
<br />collection ditches are shown in Table 13.
<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 -7 Mine, stormwater is collected in the Pit Pool
<br />retention pond, as described in Section 4.5.1. Two stormwater and sediment retention ponds collect any
<br />sediment and runoff from the underground - waste -rock pile, as described in Section 4.5.2 below. Upstream
<br />storage ponds also work to collect sediment and reduce peak flows, but are not explicitly modeled in
<br />WinTR -55, as discussed in Section 4.5.3 below.
<br />4.5.1 Pit Pool Retention Pond
<br />Stormwater that runs off from the existing open pit (Subbasin D1) collects in an ephemeral pool behind an
<br />existing berm (Figure 10, Figure 11). The outlet from the existing Pit Pool feeds into the OPWRD
<br />diversion channel which has a very low grade. The existing Pit Pool will be expanded to the west and a
<br />spillway will be constructed to pass the 100 -yr 24 -hr storm.
<br />The expanded Pit Pool retention pond is shown in Plate 2. The pond will be excavated to an elevation of
<br />5,744 ft and the existing berm will be lengthened and built up to an elevation of 5,754 ft. An 8 -ft wide
<br />spillway will be constructed at the west end of the pit pool, with an invert elvation of 5,753 ft at the
<br />spillway crest.
<br />Table 14. Stage- Storage Data for Expanded Pit Pool Stormwater Retention Pond
<br />Volume Volume
<br />Elev Area Perimeter (ft3) (acre -ft)
<br />5,744.0 30,058.9 1,047.3 0 0.0
<br />5,745.0
<br />33,256.8
<br />1,084.5
<br />31,645
<br />31,645
<br />5,746.0
<br />36,565.6
<br />1,120.8
<br />34,898
<br />66,543
<br />5,747.0
<br />39,979.3
<br />1,155.0
<br />38,260
<br />104,803
<br />5,748.0
<br />43,495.7
<br />1,189.3
<br />41,725
<br />146,528
<br />5,749.0
<br />47,112.4
<br />1,221.3
<br />45,292
<br />191,820
<br />5,750.0
<br />50,821.0
<br />1,249.7
<br />48,955
<br />240,775
<br />5,751.0
<br />54,601.5
<br />1,275.0
<br />52,700
<br />293,474
<br />5,752.0
<br />60,279.4
<br />1,313.0
<br />57,417
<br />350,891
<br />5,753.0
<br />68,858.9
<br />1,354.3
<br />64,521
<br />415,412
<br />5,753.0
<br />Spillway invert
<br />5,755.0 114,130.6 1,718.1 181,094 596,506
<br />4.5.2 Retention Ponds 001 B -A North and 001 B -A South
<br />4.5.2.1 Current Design
<br />Two stormwater retention ponds currently collect stormwater and sediment that runs off from the
<br />underground - waste -rock pile in the mine facilities area (Figure 12, Figure 18). The purpose of the ponds is
<br />4149A.140207 Whetstone Associates
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