Flood routing analysis for a failure of Mackay Dam

The objective is to provide basic information necessary to establish the appropriate level of protection for Idaho National Engineering Laboratory (INEL) facilities from possible flooding of the Big Lost River due to a failure of Mackay Dam. The results of this study will provide the basis for assessing and developing a flood protection system for the INEL. The National Weather Service computer code DAMBRK was used to model the hydrologic consequences of a failure of Mackay Dam. Seismic, structural, and hydrologic overtopping failures of the dam were examined. The flood wave from each dam failure scenario was calculated and routed through the Big Lost River Valley to determine the maximum flooding potential downstream. Results are tabulated in the form of peak flood elevation, peak flow, wave arrival time, and maximum water velocity for several cross sections along the valley. For the conditions assumed, the study indicates flooding on parts of the INEL due to the failure of Mackay Dam. This flooding is of shallow depths and involves low water velocities, and thus poses little threat of structural damage to INEL facilities. Results of the study are not to be construed to reflect on the structural integrity of Mackay Dam inmore » any way. In conjunction with the dam failure analysis, the potential for snowmelt runoff near the INEL facilities was evaluated in a separate study, presented in Appendix A. Local runoff resulting from the simultaneous occurrence of melting snow and heavy rains does not pose a threat to the facilities if accepted practices of local runoff control are followed.« less