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method, the Pader Method, for selecting riprap sizes. For background, both methods are summarized <br />below. <br />The OSM/Simons Method determines the flow induced shear stress on the channel lining and compares <br />this value to the permissible shear stress of the lining material. The permissible shear stress is defined as <br />the lowest shear force which will cause incipient motion of the D50 particle. To ensure a stable channel, <br />the minimum allowable D50 is selected by the program with an internally applied safety factor. <br />The PADER method compares the flow velocity to laboratory determined permissible velocities for <br />particle sizes. The PADER method assumes a specific gravity of 2.65 for riprap (Bureau of Land and <br />Water Conservation, 1996), which is considered appropriate for materials used by Colowyo at the mine <br />site. If the Pader Method were to be used, given the uncertainty in determining riprap sizes for steep <br />grades using this method, the median riprap size would be increased somewhat as a safety measure to <br />ensure a stable channel. <br />In some cases, as a practical consideration, a group of channels has been designed using a single <br />minimum riprap size. That size will in all cases be as large or larger than the size determined in the <br />SEDCAD analysis for each individual ditch, and will thus result in a more conservative design for some <br />of the ditches in the group. <br />6.0 REFERENCES <br />Agricultural Handbook No. 703, "Predicting Soil Erosion by Water: A Guide to Conservation Planning <br />with the Revised Universal Soil Loss Equation (RUSLE)", U.S. Department of Agriculture, Agricultural <br />Research Service, July 1996, 404 pp. <br />Barfield, Warner, & Haan (1981). "Applied Hydrology and Sedimentology for Disturbed Areas". <br />Oklahoma Technical Press, Stillwater, OK. <br />Haan, C.T., Barfield, B.J., and Hayes, J.C., 1994, 1981. "Design Hydrology and Sedimentology for Small <br />Catchments". Academic Press — Harcourt Brace & Company, San Diego. <br />Schwab and Warner (1998). " SEDCAD4 Users Manual'. Civil Software Design. Lexington, KY. <br />Natural Resources Conservation Service (1986). "Urban Hydrology for Small Watersheds (TR -.55) ". Soil <br />Conservation Service. Washington, DC. <br />Exh. 7-23-7 Revision Date: 9/23/11 <br />Revision No.: PR -03 <br />