2009-06-08_REVISION - M1997032 (25)

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Section 2 Drainage Control Design • S _ 1000 _ 10 CN According to the U.S. Department of Agriculture (USDA) TR 55 report (USDA 1986), the major factors that determine the CN include the hydrologic soil group (HSG), cover type, treatment, hydrologic condition, and antecedent runoff condition (ARC). HSG of the Van 4 Mine area was determined based on USDA, Natural Resources Conservation Service (MRCS) soil map (refer Appendix C). The soil type of the mine area is classified as "Monogram Loam", which has a moderately high rate of water transmission (0.20-0.60 in/hr). Therefore, based on the soil type and soil drainage class, the mine area was classified as HSG "B". The CN for newly graded areas, average ARC, and HSG of "B" is 86. For P equal to 3.0 inches and CN equal to 86, S equals 1.63 and Q equals 1.66 inches. Therefore, CDM used 1.66 inches as the design runoff for the berm design calculation. The total runoff volume from Basin 2 was calculated by multiplying the basin area by the design runoff (1.66 inches). Then the calculated total runoff volume was divided by available retention area within the surface facilities area to calculate the minimum required berm elevation of Basin 2. The calculated minimum berm elevation is 1.5 feet, which includes 1-foot of freeboard. An existing berm encloses Basin 2. However, CDM recommends increasing the height of the existing berm, where it is less than 1.5- feet high. CDM also recommends extending the berm along the crest of the DRA as illustrated in Exhibits A and B, to capture runoff in Basin 2 completely. Details for these calculations are shown in Table 2-4. The total runoff volume from Basin 3 was also calculated by multiplying the basin area by the design runoff (1.66 inches). Then the calculated total runoff volume was divided by available flat retention area along the toe of the DRA to calculate the minimum required berm elevation of Basin 3. The calculated minimum berm elevation is 1.5 feet, which includes 1-foot of freeboard. An existing berm (or water bar) encloses Basin 3 along the toe of the DRA. However, CDM recommends increasing the height of the existing berm, where it is less than 1.5-feet high. CDM also recommends extending the berm along the toe of the DRA as illustrated in Exhibits A and B, to capture runoff in Basin 3 completely. Details for these calculations are shown in Table 2-4. Table 2-4 Calculation Summary for Berms Basin Basin Area 100-yr, 24-hr Storm Runoff Runoff Volume Retention Area Berm Height Calculated Freeboard Design s -ft ac in in ft' s -ft ft ft ft 2 153,788 3.53 3 1.66 21,274 63,924 0.33 1.00 1.33 3 30,467 0.70 3 1.66 4,215 13,084 0.32 1.00 1.32 2-4 7164986-Denison Mines%Task Order 1 - Sunday Mines Drainage StudoVan 4 Mine Drainage Study%Report%Van4 Mine Drainage Report.doc