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DAM-BREAK MODELING 55 TABLE n.-Comparison of model results of a hypothetical breach width of 200 feet to the actual breach width of 65 feet Distance down. stream from Lawn Lake dlllD, in ""'" Difference from peak discharge of 2OQ..foot width, in cubic feet per ~d Peak discharge, in cubic feet per """,d Flood elevation, in "'"' Difference from muimum flood depth of 2QO.foot width, in feet Maximum flood depth, in feet 0.0 40,600 22,600 .55 38,300 21,700 (al (al 1.50 35,600 20,400 (al (al 3.83 30,400 17,800 (a) (a) b 4.73 29,100 17,300 (a) (a) 5.36 24,100 13,500 8.518.5 13.6 5.78 17,000 9,500 8,513.4 12.4 c 6.50 13,700 7,000 8,496.0 12.4 d 6.67 25,600 9,600 e 7.68 18,400 8,500 8,057.7 12.7 (a) (a) (a) (a) 2.9 2.2 1.8 2.8 7.74 18,200 8,500 8.045.4 13.4 2.4 8.78 14,700 8,000 7,866.1 14.1 3.3 f 10.28 12,700 6,500 7,701.5 12.5 2.9 gl1.45 12,100 6,000 7,582.9 9.9 2,6 h12.50 11,500 5,400 7,507.4 14.9 3.8 Average difference from maximum flood depth of 200-foot width. in feet=2.7 :~e channel erosion; computations and comparisons are not applitable. ~ynthetic ct088 section based on cross section at rive!' mile 5.36- cCascade Lake dam. dcascade Lake dam and Site 2. eSite3. 'Site 4. gSite 5. hsite6. dam embankment in a short span of time. Outflow peak discharges could have been as high as 56,000 ft'ls for a time of full breach development of 10 min (or a greater peak discharge for a shorter breach time), and a breach width of 550 ft, for these extreme conditions. EFFECTS OF CASCADE LAKE DAM FAILURE The dam-break model also was used to assess the flood wave downstream from Cascade Lake dam if the dam had not failed or was not present. This scenario was important for assessing the magnitude of damages. Observed and modeled data indicate that the failure of Cascade Lake dam probably doubled the peak discharge immediately downstream from the dam and increased the flood stage by an average of 0.6 it (table 121. The model also was run with the calibrated river hydraulics, except Cascade Lake dam was not allowed to fail. Hence, the inflow flood hydrograph to Cascade Lake dam was simply routed over the top of the dam. Similarly, since the dam was small, these results re- flected the hypothetical condition of Cascade Lake dam not being in the river system. A comparison of model results with and without the failure of Cascade Lake dam is shown in table 12. Peak discharges would have been 11,300 ft'ls less immediately downstream from the dam, to 500 ftSls less at mile 12.5, without the failure of Cascade Lake dam. Maximum flood depths would have averaged 0.6 it lower. The flood wave without failure would have reached mile 12.5, 0.3 hour later. DISCUSSION OF RESULTS Computer modeling provided a means to enhance and supplement the observed data, to evaluate the use of the model on high-gradient streams, to evaluate the relative accuracy of the results, and to evaluate various alter- native dam-breach scenarios. The model, properly calibrated, worked well in high-gradient streams com- pared to the observed data. It is important to note that there also were errors associated with the indirect peak- discharge measurements and flood-wave traveItimes (particularly in the Roaring River). Problems of oper- ating the model in such complex high-gradient channels were overcome by minor changes to the preflood base flows, and with maj or increases of n-values. The state of flow was very important in hydraulic routing. Subcritical flows from the extremely large flow