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Twentymile Coal Company 6RT Borehole Pad & Access Road Drainage and Sediment Control System Design <br />gravel noncolloidel. This channel material has a Manning's n value of 0.025 and a limiting velocity of <br />4.0 feet per second (fps). The modeled surface water velocity was determined to be non - erosive. The <br />SEDCAD Channel Utility report is attached. <br />Summary <br />During construction of the 6RT Borehole pad, ASC's will effectively reduce suspended sediment and <br />settleable solids concentrations below NPDES requirements permit limits. Requirements include that the <br />peak total suspended solids (TSS) concentration not exceed 70 mg/L and that the peak settleable solids <br />(SS) concentration never exceed 0.5 mL /L. The access road and borehole pad should be surfaced with <br />coarse gravel and a 20 -inch diameter straw wattle (or sediment log) should be placed down - gradient of <br />the disturbance. The straw wattle (or sediment log) should be placed across the contour down - gradient <br />from the access road and borehole pad; each end of the straw wattle (or sediment log) should be turned <br />up- gradient and wrapped around the borehole pad as depicted in Exhibit 1. The straw wattle is designed <br />to treat stormwater runoff resulting from the 10 -year, 24 -hour storm; this includes 0.26 acres of <br />disturbance from the access road and borehole pad, 0.02 acres of Routt County Road 33, and 0.19 acres of <br />undisturbed/reclaimed watershed. <br />The maximum design runoff from the entire watershed is predicted to be 0.01 ac ft at a maximum <br />discharge peak rate of 0.31 cubic feet per second (cfs). This stormwater runoff volume should be <br />intercepted and treated down - gradient from the site by the straw wattle (or sediment log). The modeled <br />inflow TSS concentration into the straw wattle (or sediment log) is 2,673 mg/L, and outflow TSS <br />concentration from the straw wattle (or sediment log) is 27 mg/L. SEDCAD results show an inflow SS <br />concentration into the straw wattle (or sediment log) of 2.06 mL/L and an outflow SS concentration of <br />0.01 mL /L. Storrnwater runoff will be uniformly distributed as sheet flow over the borehole pad. <br />SEDCAD modeling resulted in a maximum water depth of 0.03 ft with a velocity of 0.17 fps that is well <br />below the limiting velocity of 4.0 fps for coarse gravel surface material. <br />Providing traditional runoff control (i.e. sediment ponds) to accept runoff from this area would require the <br />disturbance of a larger area than the access road and borehole pad themselves. The combined disturbed <br />acreage for the County road, access road and borehole pad is very small (0.28 acres), and the borehole <br />pad will be reclaimed upon completion of the 6RT Thickener Underflow Pipeline system. The straw <br />wattle (sediment log) should remain in place until vegetation is sufficiently established to control runoff, <br />erosion, and sediment loss that are similar to background conditions. <br />Water & Earth Technologies, Inc. Page 3 of 4 October 23, 2009 <br />