2013-09-27_REVISION - M1979094HR (3)

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JD -7 Mine — Drainage Design Plan 29 4.3 Sub - Basins Receiving Stormwater Downstream of the JD -7 Mine Site Five sub - basins were delineated that receive any stormwater runoff that arrives from the upstream sub - basins (Table 11). These sub - basins are located downstream of the JD -7 Mine and contain braided ephemeral channels on the Paradox Valley floor. Significant infiltration occurs in the upper reaches of these receiving sub - basins (Section 4.7) and these sub - basins were not modeled. Table 12. Downstream Receiving Sub - Basins Sub- Area Area Basin (ft) (acres) Type Description /Type D -11 6,556,390 150.51 Disturbed & downstream North of site to East Paradox Creek D -12 16,199,550 371.89 Disturbed & downstream Valley floor, planned VW, d/s to E.Paradox Creek D -13 13,459,520 308.99 Disturbed & downstream Valley floor, soils stockpile to E.Paradox Creek D -14 18,256,190 419.10 Disturbed & downstream Valley floor, soils stockpile to E.Paradox Creek D -15 522,550 12.00 Disturbed & downstream Western edge of soils stockpile 4.4 Reach Definition Stormwater runoff from the sub - basins described above is routed through the site via natural channels, diversion ditches, borrow ditches, and perimeter ditches. The major channels and ditches are entered as reaches in the WinTR -55 run -off model. Reach characteristics are highly variable across the study area: Natural channels on the Monogram Mesa escarpment are characterized by steep gradients, very high Manning's n values, naturally steep side slopes, and a high degree of variability (non - uniformity) of channel geometry (Figure 1, Figure 12). Natural channels on the escarpment often exhibit natural armoring and are scoured into rock. Natural channels on the valley floor are characterized by lower gradients, lower Manning's n values, naturally shallow side slopes (Figure 2), and relatively uniform channel geometry except where channels become braided and diffuse (Section 4.7). Engineered channels are designed to meet the capacity and stability criteria discussed herein. The reach characteristics of engineered channels at the JD -7 Mine are described in detail in the following sections. 3 Naturally armored channels are those with d50 of the bed material greater than 5.0 mm (Hedman and Osterkamp, USGS, 1982). 4149A.130926 Whetstone Associates •