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duration, final closure scenario. Upon arriving closer to these scenarios, Colowyo will re- evaluate the <br />adequacy of these channels. <br />2.0 BATHURST EQUATION CALCULATIONS <br />Most of the flow resistance in channels with large -scale roughness is derived from the form drag of the <br />roughness elements and the distortion of the flow as it passes around roughness elements. Consequently, <br />a flow resistance equation for these conditions has to account for skin friction and form drag. Because of <br />the shallow depths of flow and the large size of the roughness elements, the flow resistance will vary with <br />the relative roughness area, roughness geometry, Froude number, and Reynolds number. Bathurst's <br />experimental work quantified these relationships in a semi - empirical fashion (Chen a at, 1988). The <br />results estimate flow depth and flow induced shear stresses, which determine the required dimensions of <br />the designed channel. Velocity is not determined for channels calculated with the Bathurst equation, as <br />velocity through the channel varies dramatically depending on proximity to riprap particles and is not <br />directly used in the determination of adequate riprap size. <br />3.0 RIPRAP SIZING AND VEGETATIVE LINER DESIGN <br />Unless specifically stated otherwise, riprap sizes were determined using the SEDCAD 4.0 subroutine for <br />designing riprap lined channels utilizing the OSM /Simons Method. SEDCAD also provides another <br />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 />Exh. 7 -14 -3 Revision Date 12/10/12 <br />Revision No.: TR -97 <br />