HEC-RAS Dam Break Modeling of Gwinnett County’s NRCS Facilities
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The potential flood risk caused by dam failure is often more severe and can behave very different to that of natural flooding events. The tragedy of dam failure is all too familiar to Georgia with the failure of the Kelly Barnes dam near Toccoa Georgia which resulted in 39 deaths in the early hours of November 6th 1977. Floodplain maps issued by FEMA only show the 1% and 0.2% annual chance floods and assume all dams and culverts function perfectly and therefore the risk of dam break is often forgotten. Recent developments in the United States Army Corps of Engineers (USACE) Hydrologic Engineering Center River Analysis System (HEC-RAS) has included the addition of unsteady 1-dimenstional hydraulic modeling and dam breach modeling. Gwinnett County’s recent flood study program and spatial database developments has provided steady state HEC-RAS models and supporting spatial data that have provided the foundation for the creation of unsteady HECRAS models for dam break analysis. The conversion from a steady state flood study model to a dynamic dam break model required some careful modifications to account for differences in flood routing methodologies between the two model states. These unsteady models have been used to simulate both sunny day failures and storm in progress failures at all 14 of Gwinnett County’s NRCS facilities. These models have enabled flood inundation maps to be created which include peak elevation, floodwave arrival time and floodwave time to the peak for both sunny day and storm in progress failures. This has identified homes and businesses at risk enabling emergency evacuation plans to be created.