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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 Maruungs equation for open <br />channel flow or the Bathurst resistance equation for large-scale roughness flow through the use of <br />SEDCAD 4.0 utilities. Riprap sizes were determined using either the OSM/Simons Method or the <br />PADER method. <br />Emergency spillway riprap sizing was deterrrtirred using the 25-year, 24 hour storm event. Temporary <br />channels were sized ruing the 10-year, 24-hour storm event. Permanent channels were designed <br />and/or evaluated using the 10-year, 24-hour or the 100-year, 24-hour storm event depending on the <br />size of watershed contributing to the channel. Watersheds in excess of one square mile were <br />designed and/or evaluated ruing the 100-year, 24-hour store event. <br />1.0 MANNINGS EQUATION CALCULATIONS <br />• The majority of the permanent channels were designed using Mannings equation assuming steady <br />state, uniform flow. The mannings roughness coefficient is calctilated by the SEDCAD 4.0 program <br />by either 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 er al, 1987). The SEDCAD 4.0 User Manual (blamer er al, 1998) suggests that <br />clazses A and B be used to determine channel capacity and classes C and D be used to deternine <br />vegetal stability. By doing so the program designer suggests that larger flows tend to lay o~ <br />vegetation in the channel resulting in less flow resistance and a higher veloary flow. The higher <br />velocity flow is assumed to be more erosive thus suggesting a lower stability. To this end, the <br />Colowyo channels designed with 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 <br />field reconnaissance by Colowyo personnel. The soil type results in a mannings roughness <br />coefficient of 0.20 with a maximum permissible flow velocity of 6.0 fps. Some of the unlined <br />temporary channel sections indicate velocities above the permissible flow velocity. Note that the <br />u <br />Page I <br />