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EMERGENCY SPILLWAY, TEMPORARY AND PERMANENT <br />CHANNEL DESIGN <br />METHODOLOGIES AND ASSUMPTIONS <br />The permanent channel design dimensions were determined using Mannings equation for open channel <br />flow or the Bathurst resistance equation for large -scale roughness flow through the use of SEDCAD 4.0 <br />utilities. Riprap sizes were determined using the OSM /Simons Method option in the SEDCAD program. <br />Emergency spillway riprap sizing was determined using the 25 -year, 24 hour storm event. Temporary <br />channels were sized using the 10 -year, 24 -hour storm event. Permanent channels were designed and/or <br />evaluated using the 10 -year, 24 -hour or the 100 -year, 24 -hour storm event depending on the size of <br />watershed contributing to the channel. Watersheds in excess of one square mile were designed and/or <br />evaluated using the 100 -year, 24 -hour storm event. <br />1.0 MANNINGS EQUATION CALCULATIONS <br />The majority of the permanent channels were designed using Mannings equation assuming steady state, <br />uniform flow. The mannings roughness coefficient is calculated by the SEDCAD 4.0 program by either <br />the D50 riprap size with depth of flow and velocity, by vegetation class, or by soil type. <br />For the Colowyo mine site, a vegetation class of B was assigned and is appropriate for western state <br />grasses (Temple et at, 1987). The SEDCAD 4.0 User Manual (Warner et at, 1998) suggests that classes A <br />and B be used to determine channel capacity and classes C and D be used to determine vegetal stability. <br />By doing so the program designer suggests that larger flows tend to lay over vegetation in the channel <br />resulting in less flow resistance and a higher velocity flow. The higher velocity flow is assumed to be <br />more erosive thus suggesting a lower stability. To this end, the Colowyo channels designed with <br />vegetative linings show a capacity class B and a stability class C. <br />Hard pan and shale materials were used to simulate area soil types for bare channels according to field <br />reconnaissance by Colowyo personnel. The soil type results in a mannings roughness coefficient of 0.20 <br />with a maximum permissible flow velocity of 6.0 fps. Some of the unlined temporary channel sections <br />indicate velocities above the permissible flow velocity. Note that the modeling runs are for a very limited <br />Exh. 7 -14 -2 Revision Date: 11/3/10 <br />Revision No.: TR -87 <br />