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56 LAWN LAKE DAM AND CASCADE LAKE DAM FAILURES, COLORADO TABLE 12.-Comparison of model results with and without the failure of Cascade Lake dam DistlUlC8 downstream. ~ Lawn Lllke dam, in miles Difference in peak discharge from model with failUl'e, in cubic feet per """'. Peak discharge, without. failure, in cubic feet per """'. Floode1evation without Cascade Lake dam failure. "'foot Maximum flood depth, infeot Difference in mu- imum depth from model with failure, in feet a 6.67 6.700 11.300 b 7.68 6.300 3.600 8,054.1 9.4 -0,8 7.74 6,200 3,500 8,042.3 10.3 ,.7 8.78 5,800 900 7,863.0 11.0 ,.8 ClO,28 5,700 500 7,699.1 10.1 '.5 d11.45 5.600 500 7,580.7 7.7 ,.4 "12.50 5.600 500 7,504.3 11.8 -,7 Average difference in maximum flood depth from model with failure, in feet=O.6 Rcascade Lake dam. bSite 3. cSite4. dsite5. aSite 6. resistance appeared reasonable and provided the best comparison with the observed data. As table 8 indicates, field-selected n-values had to be increased by an average of 78 percent for high-flow resistance and energy losses. Calibration with n-values meant a tradeoff in minimizing the differences of peak discharge, traveltime, and water-surface elevations. As n-values were reduced, peak discharges were increased, but traveltime was faster and water-surface elevations were lower. Conversely, as n-values were increased, peak discharges were decreased, but traveltime was slower and water-surface elevations were increased. Flow conditions were unknown in the Roaring River, and the effects of debris and channel changes were unknown; the model assumed clear water. However, in the Roaring River and on the Fall River immediately downstream from Cascade Lake dam, total water and debris discharge probably was much greater than the model indicated. Geomorphic and sedimentologic evi- dence at Horseshoe Falls and downstream from Cascade Lake dam indicates that at these two locations the water flood bulked up with enough sediment and debris to temporarily create a noncohesive, coarse-grained, tur- bulent, sediment gravity flow. For these boulder berms to have formed, sediment loads must have been at least 50 to 60 percent of the flood flow, by volume (Costa, 1984). Model results indicated moderate flood-wave at- tenuations in the Roaring River; however, probably lit- tle attenuation occurred in this steep reach. The model appeared to have difficulty reproducing results im- mediately downstream from Cascade Lake dam, a very small-capacity dam. Although the model reasonably reproduced the Cascade Lake dam peak outflow, the hydraulic routing component attenuated peak flows too much (table 9). All modeling results indicated that the effects of different breach scenarios decreased with distance downstream. THE FLOOD AFTERMATH Since 1890, 130 known dam failures have occurred in Colorado (Colorado Water Conservation Board, 1983). Floods from these failures have resulted in small loss of life, but large property losses. Because of the relatively small volume of water released from Lawn Lake and Cascade Lake dams, and because Lake Estes impounded the flood, flooding lasted only a few hours. However, impacts were severe. Surprising- ly few fatalities occurred as a result of the flood, be- cause of several positive factors related to the flood warning. The number of people at risk upstream from Cascade Lake dam at the time of failure was limited to about 25 to 30 camped along the Roaring River, and probably fewer than 20 people in Horseshoe Park_ Therefore, in the first 6.75 mi downstream from Lawn Lake, probably fewer than 50 people were at risk. Because it is National Park property, few structures existed in the flood plain. However, the National Park Service indicated about 275 people were camped inAspenglen Campground downstream from Cascade Lake dam. The Estes Park Chief of Police estimated that 4,000 to 5,000 residents and tourists were in the flood plain; they could have been potential flood victims in the reach from Cascade Lake dam to Lake Estes (fig. 1). This section of the report summarizes the human element and the damages resulting from the flood. ~"""".