Probabilistic Dam Breach Analysis for Pennsylvania's Largest Dam Breach Inundation Limits

ABSTRACT ONLY - George B. Stevenson Dam, situated within Sinnemahoning State Park (Pennsylvania) is a High Hazard flood control structure owned by Pennsylvania Department of Conservation and Natural Resources
(DCNR) and has the largest inundation limits of any dam located in Pennsylvania. A probabilistic dam break analysis was conducted for the George B. Stevenson Dam at the request of DCNR to determine the appropriate dam breach parameters to be used when generating the flood inundation limits in the receiving streams and river reaches. The probabilistic analysis was performed by first selecting a minimum and maximum range for the breach width, breach side slope, and time to breach parameters based on published equations from MacDonald and Langridge (1984), USBR (1988), Von Thun & Gillette (1990), Froehlich (1995), and Froehlich (2008) methodologies. The three breach parameters were randomly selected from the range and the breach was routed using an unsteady HEC-RAS model to determine the maximum water surface elevation and discharge at specific location. The scenarios were continued until the calculated mean peak discharge and water surface for scenarios varied by less than 0.01% with the addition of the results from the most recent scenario. A total of 100 breach scenarios were analyzed for the project. The peak discharges in the scenarios ranged from 1,000,000 to 1,660,000 cfs (mean discharge was 1,300,000 cfs) and peak water surface elevations ranged from 816.9 to 831.4 feet (mean was 823.7). The results from this analysis showed that peak discharge and water surface elevations are sensitive to the breach parameters, which can vary based on breach type, dam type, dam composition, dam construction, etc. This probabilistic approach provided an in-depth sensitivity analysis on parameter selection and would be extremely beneficial for large and small dams when there is uncertainty on the parameters, or unknowns regarding the dam.