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HYDROLOGY, GEOMORPHOLOGY, AND DAM-BREAK MODELING OF THE JULY 15, 1982, LAWN LAKE DAM AND CASCADE LAKE DAM FAILURES, LARIMER COUNTY, COLORADO By Robert D. Jarrett and John E. Costa ABSTRACT At approximately 0530 Mountain Daylight Time on the morning of July 15, 1982, Lawn Lake dam, a 26-foot-high earthen dam located in Rocky Mountain National Park, Colorado, failed. The dam released 674 acre-feet of water and an estimated peak discharge of 18,000 cubic feet per second down the Roaring River valley. Three people were killed and damages totaled $31 million. The Colorado State Engineer determined that the probable cause of failure was deterioration of lead caulking used for the connection between the outlet pipe and the gate valve. The resulting leak eroded the earthfill, and progressive piping led to failure of the embankment. Floodwaters from Lawn Lake dam overtopped a second dam, Cascade Lake dam, located 6.7 miles downstream, which also failed. Cascade Lake dam, a 17-foot high concrete gravity dam, 12.1 acre-foot capacity dam, failed by toppling with 4.2 feet of water fl owi ng over its crest. The fl ood continued down the Fall River into the town of Estes Park, which received extensive damage from the overbank flow. Thi s report presents the sett i ng, a summary of the causes of the dam fail ures, the hydro 1 ogi c data, and geomorphi c effects of the fl ood. Data on dam-breach floods on high-gradient streams are limited. A dam-break computer model was used to evalute the model's capabilities on high-gradient streams, to enhance and provide supplemental hydrologic information, and to evaluate various hypothetical scenarios of dam-breach development and probable impact of the failure of Cascade Lake dam. Flood data were obtained at two gaging stations (06732500 Fall River at Estes Park and 06733000 Big Thompson River at Estes Park) and five miscellane- ous sites downstream from the Lawn Lake dam. Peak discharges were determined us i ng a vari ety of i ndi rect methods. Because of extens i ve scour and eros i on along the Roari ng Ri vel', peak di scharges were estimated from the dam-break model. Calculated peak discharges for the flood were 18,000 cubic feet per second from Lawn Lake dam, 12,000 cubic feet per second at Horseshoe Falls where Roaring River joins the Fall River, 7,210 cubic feet per second into Cascade Lake dam at the east end of Horseshoe Park, 16,000 cubic feet per second from the fai 1 ure of Cascade Lake dam, 13,100 cubi c feet per second about 1 mile downstream from Cascade lake dam, 8,520 cubic feet per second just upstream from Estes Park, 6,550 cubic feet per second for gaging station 06732500 Fall River at Estes Park, and 5,500 cubic feet per second for gaging station 06733000 Big Thompson River at Estes Park. Maximum depths ranged from 6.4 to 23.8 feet; maximum widths ranged from 97 to 1,112 feet; and mean 1