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Dam Breach Study and Inundation Mapping of Eleven Dams Owned by Oklahoma City, Oklahoma

Shivers, Molly
The loss of life and property are a hazard downstream of a reservoir in the event of a dam failure. Inundation mapping of dam failures is required in safety documentation when the dam is considered high hazard. In the past, these maps were created as the result of a catastrophic flood; however, the technology is now available for predictive flood modeling. Eleven dams, operated by Oklahoma City, were selected for inundation mapping and modeled using Hydrologic Engineering Center River Analysis System (HEC-RAS) software to simulate two dam breach scenarios: a 75% Probable Maximum Flood (PMF) and a fair weather (sunny day) flood. The model was calibrated using the Lake Overholser model and daily mean discharge data from the USGS stream-gaging stations. A Manning�s roughness coefficient of 0.034 was used for the river channel in the calibrated model. The peak stages of six bridges along Lightning Creek were compared to an indirect step-backwater analysis of the May 8, 1993 flood (Tortorelli, 1996). HEC-RAS modeled maximum surface water difference above the streambed for Lightning Creek were within 55% of the maximum surface water difference above the streambed determined by Tortorelli (1996). The predicted flow was only 36 percent of the flow resulting from the estimated May 1993 flood. HEC-RAS flood models were combined with contour maps to determine the inundated areas downstream of each dam. The resulting maps can be used to create emergency evacuation plans.