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<br />deposition of rock and debris at their watershed terminus. Rock <br />carried and deposited from the canyon of the Upper Breach had <br />numerous rocks sized 2'x 2'x 2' demonstrating the great force and <br />quantity of water at this location. <br />Both Colowyo and T&K's independent calculations show near <br />identical flow values for the Lower and Upper Breach Zones. Using <br />T&K calculated flow values, the high water flow carried by the <br />South Collection Ditch prior to breaching at the Lower and Upper <br />Breach locations was 132.0 cfs and 99.7 cfs respectively. Bear in <br />mind that the SEDCAD sizing for the South Collection Ditch, <br />obtained by joint Colowyo and DMG modeling in accordance with the <br />regulations, shows a maximum design flow of 6.99 cfs. Also, please <br />note T&K's comment that "both sections checked would get into <br />supercritical flow therefore turbulence and waves are likely. This <br />would probably cause overtopping and possible breaching at flows <br />somewhat less than computed." <br />A comment regarding the SEDCAD utility function software <br />used to calculate flow and velocity is warranted. Due to inherent <br />limiting velocity and tractive force factors in the SEDCAD utility <br />function software for erodible channel design, flow and velocity <br />calculations could not be performed using the Erodible channel <br />design. It was necessary to use the SEDCAD Nonerodible channel <br />design utility function then input the appropriate representative <br />soil condition Manning's n factor. This explains the Nonerodible <br />Channel Design title at the top of the SEDCAD calculation sheets. <br />It is hoped that this information, taken collectively <br />with additional information detailed in the prior letter of June 2, <br />1993, allows DMG to agree that the failures cited in NOV C-93-077 <br />are attributable to a rainfall event of tremendous proportions and <br />not due to non-compliance, faulty design, or lack of maintenance on <br />diversion structures by Colowyo. <br />Thank you for your time and consideration, please feel <br />free to contact me should any further questions arise. <br />Sincerely, <br />~/i{ilY~ ~ CJl~ifc~i~ <br />Juan P. Garcia <br />Hydrologist/Environmental Engineer <br />Enclosures <br />xc: R. G. Atkinson <br />J. A. Kiger <br />Central Files <br />