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<br />Eighty-six channel/valley cross sections were used to model the <br />107 miles downstream of the dam included in the study. For the <br />most part, the data for these cross sections was obtained from <br />U. S. Geological Survey (USGS) flood profile and flood insurance <br />studies performed in the County in the early to mid 1970's. <br />Orthophoto contour maps at a scale of I inch equals 400 feet <br />with a five foot contour interval were developed in conjunction <br />with the USGS studies, and were used in this dam breach study. <br /> <br />Cow Creek Canyon, a reach approximately 31 miles in length, was <br />not included in the USGS studies and there was no orthophoto <br />coverage for this 'reach. Nine of the 16 cross sections which <br />lie in the Cow Creek Canyon reach (below West Fork) were field <br />surveyed in 1981 as part of preliminary studies for the <br />Galesville project. The other 7 cross sections (below Glendale <br />to West Fork) were taken from USGS 15 minute series <br />quadrangles. <br /> <br />Manning's Un" channel/valley roughness <br />using the USGS study data as a guide. <br />field observation was used as guide in <br />values. <br /> <br />values were selected <br />For Cow Creek Canyon, <br />selecting roughness <br /> <br />,) <br /> <br />Computer printouts for three different assumptions of initial <br />flow conditions at dam failure are included as Appendix C. <br />These initial conditions are: 1) Low water ~y fair weather; 2) <br />probable maximum flood (PMF); 3) 1964 flood - . <br /> <br />Other than initial flow and pool elevation, all input parameters <br />for the three failure cases are the same. This input is <br />included with the printout for the fair weather failure. The <br />time of breach formation was taken as 10 minutes in an effort to <br />simulate a sudden2;ailure of the RCC dam. The breach shape <br />(refer to Fig. 1 - ) was selected as rectangular having a 300 <br />foot width, roughly the width of the base of the dam, and a <br />bottom elevation equal to the channel bottom at the tailwater of <br />the dam. This selection of breach time/shape in no way is <br />intended to reflect upon the desi9n or integrity of the dam. It <br />is merely an attempt to simulate to some degree a catastrophic <br />failure of the dam which could perhaps result from sabotage or <br />an act of war. <br /> <br />Relatively high values of manning's Un" (0.1) were used in the <br />first 54 miles below the dam. This was necessary to obtain a <br />solution with the program. Problems arose with convergence and <br />negative area computations. While these roughness values may <br />appear somewhat high, it is not considered to seriously effect <br />the resulting estimate of the dam failure flood. The probable <br />large debris load and energy losses below the dam would tend to <br />justify higher than normal roughness values. <br /> <br />1/ <br /> <br />1964 flood <br />event). <br />Figures on <br /> <br />(Regional accepted as approximating a IOO-year <br /> <br />~/ <br /> <br />pages 7 to 11 <br /> <br />B 3 of 11 <br />