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Mineral Joe Mine - Drainage Design Plan 25 <br />or spillway. In contrast, the SWRP at the Mineral Joe Mine is designed to fully retain the 10 -year 24 -hour <br />storm and may retain even larger storms (Section 5.4), and is a dry pond for part of the year. The complete <br />capture and subsequent evaporation results in the Mineral Joe Mine SWRP being effective for both <br />dissolved, suspended, and colloidal constituents as well as the settleable solids fraction. In the WinTR55 <br />model, however, the SWRP is modeled as a wet storage pond. The pond surface is set to the maximum <br />surface area of the excavated pond, at the height of the overflow outlet. Therefore, the WinTR55 model <br />conservatively predicts the maximum flow over the spillway if the pond structure were full at the beginning <br />of the storm event. Structure parameters for the existing pond condition are shown in Table 14. <br />Table 14. Structure Parameters <br />Surface <br />Height <br />Surface <br />Reach Area @ <br />Above <br />Area @ <br />Pipe Head on Weir <br />Identifier Crest <br />Crest <br />Ht Above <br />Diameter Pipe Length <br />(ac) <br />(ft) <br />(ac) <br />(in) (ft) (ft) <br />------------------------------------------------------------------------------- <br />SWRP -Inlet 0.068 <br />0.5 <br />.072 <br />10 <br />The purpose of the pond is to retain storm water from the 10 -year 24 -hour storm event and slow its velocity <br />such that any suspended sediment is deposited and contained onsite. The stormwater catchment is <br />currently unlined. It is recommended that the pond remain unlined, to facilitate routine maintenance and <br />sediment removal. Inspections and maintenance are performed in accordance with the Stormwater <br />Management Plan. <br />Figure 11. Photograph of Stormwater and Sediment Retention Pond <br />(February 2012) <br />4148B.131029 Whetstone Associates <br />