In this case study, two hydrographs are assumed; namely, peak flows to 120,000 cfs and 600,000 cfs. A baseflow of 5,000 cfs and 40,000 cfs was used for hydrographs with peaks of 120,000 and 600,000 cfs respectively for all K-634 simulations. Both hydrographs are assumed to increase linearly from zero (or the base flow) to the peak flow rate at time of 1-hour, and then decrease linearly to zero (or the baseflow) at time of 6-hours (see figure 4 inset). The study channel is assumed to be a 1000 feet width rectangular section of Manning's n equal to 0.040, and various slopes in the range of 0.001__0.01. Figures 4 shows the comparison of modeling results. From the figure, various flood depths are plotted along the channel length of up to 10-miles. Two reaches of channel lengths of up to 30-miles are also plotted in figure 4 which correspond to a slope = 0.0020. In all tests, grid spacing was set at 1000-feet intervals. Time steps were 0.01 hours for K-634 and 7.2 seconds for DHM.
From figure 4 it is seen that the diffusion model provides estimates of flood depths that compare very well to the flood depths predicted from the K-634 model. For downstream distances at up to 30 miles, differences in predicted flood depths are less than 3 percent for the various channel slopes and peak flow rates considered. In figures 5 and 6, good comparisons between the diffusion hydrodynamic and the K-634 models are observed for water depths and outflow hydrographs at 5 and 10 miles down stream from the dam-break site. It should be noted that the test conditions are purposefully severe in order to bring out potential inaccuracies in the diffusion hydrodynamic model results. Less severe test conditions should lead to more favorable comparisons between the two model results. Although offsets do occur in timing, volume continuity is preserved when allowances are made for differences in baseflow volumes. |

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