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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 asemi-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 <br />• Riprap sizes were determined using the SEDCAD 4.0 subroutine for designing riprap lined channels, <br />utilizing either the OSM/Simons Method or the PARER Method. <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 D50 is selected by the program with an internally applied safety factor. <br />The PARER method compares the flow velocity to laboratory determined permissible velocities for <br />particle sizes. The PARER 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. Given the uncertainty in determining riprap sizes for steep grades using the PARER method, the <br />median riprap size was increased 3 inches as a safety measure to ensure a stable channel. <br />4.0 RESULTS <br />Based on the previously discussed input parameters and methodologies, the emergency spillways, <br />. temporary and permanent channels for the Colowyo mine site were designed and/or evaluated for <br />Exh. 7-14-3 Revision Date: 7/25/06 <br />Revision No.: TR-63 <br />