DAM BREACH ANALYSIS SIMULATION ON THE LOWER SUSQUEHANNA RIVER

摘要

Recent advances in unsteady flow modeling enhance the ability of engineers to predictthe effects of dam failures and other unanticipated releases on low-lying areas, therebyreducing the risk to lives and property. Using a HEC-RAS one-dimensionalhydrodynamic model of the lower Susquehanna River, the authors re-created the flood ofrecord (Hurricane Agnes in June 1972), simulated the Probable Maximum Flood (PMF),and estimated downstream impacts associated with the failure of Safe Harbor Dam insouthern Pennsylvania under both sunny day and PMF conditions.Using cross section data obtained from FEMA, the authors created a HEC-RAS model ofthe river, beginning 7.6 miles below Safe Harbor at Holtwood Dam, and extending 10.7miles above Safe Harbor to Wrightsville. Using as-built drawings, the model was revisedto include Safe Harbor Dam and its 31 spillway gates. A rating curve for the concretespillway, flashboards and rubber dams originally in use at Holtwood Dam was used toestablish downstream boundary conditions. The model was then calibrated based onwater surface elevations observed during Hurricane Agnes, and at lesser discharges.Using a revised rating curve based on current conditions at the downstream facility(flashboards in place on entire spillway), this calibrated model was then used to estimatedownstream stages and discharges associated with various failure scenarios at SafeHarbor Dam. For the sunny day breach, a 0.20 hour time to failure was assumed and thedam’s spillway gates are closed, with all flow directed through the Safe Harborpowerhouse prior to the breach. The PMF breach, like the sunny day, is also based on a0.20 hour time to failure. The results of the analysis were used to update the EmergencyAction Plan, including the downstream inundation maps.