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HYDROLOGY, GEOMORPHOLOGY, AND DAM-BREAK MODELING OF THE JULY 15, 1982, LAWN LAKE OAM AND CASCADE LAKE DAM FAILURES, LARIMER COUNTY, COLORADO By Robert 0, 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 Ri ver valley. Three peop 1 e were ki 11 ed and damages totaled $31 mill i on. The Co 1 orado State Engi neer determi ned that the probab 1 e 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 toppl ing with 4.2 feet of water flowing over its cresL The flood continued down the Fall River into the town of Estes Park, which received extensive damage from the overbank flow. Thi s report presents the setting, a summary of the causes of the dam failures, the hydrologic data, and geomorphic effects of the flood, 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 vari ous hypothet i ca 1 scenari os of dam-breach deve 1 opment and probab 1 e 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 using a variety of indirect methods. Because of extensive scour and erosion along the Roari ng Ri ver, 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 fail ure of Cascade Lake dam, 13,100 cubi c feet per second about 1 mil e downstream from Cascade Lake dam, 8,520 cubi c 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; maxi mum wi dths ranged from 97 to 1,112 feet; and mean 1