Risk Assessment of Post-Fire Floods on Dams and Their Floodplains
The rapidly changing global climate and the increased dependence on infrastructure networks make our society vulnerable to natural disasters. Decision makers need to understand the intensity of potential natural disasters to take necessary actions to minimize their impacts. In Southwest United States, wildfires are increasing in frequency and magnitude. The literature review shows limited studies in evaluating the impacts of post-wildfire floods on civil infrastructures and residential zones. Earth dams are vulnerable to post-wildfire floods. The increased post-wildfire runoff volumes due to changes in soil characteristics and reduced vegetation could result in overtopping failure of an earth dam (dam-break scenario), and the accumulation of sediment and debris flow could reduce the capacity of the reservoir (no-dam break scenario). In this study, a framework to evaluate the impacts of post-fire floods on earth dams is proposed. First, pre and post-wildfire runoff volumes are estimated considering a distribution of runoff coefficients found in the literature, different watershed burnt areas and historic rainfall data. Second, based on these runoff volumes, potential dam overtopping failure is modeled using WMS: SMPDBK developed by National Weather Services. The model predicts downstream flooding due to dam failure. The dam-break results are interpolated with HAZUS (developed by Federal Emergency Management Agency) inventory data to assess the downstream economic, environmental and social impacts. Finally, the impacts of dam failure and no-dam failure scenarios are evaluated with inputs from Hazus results and from an interview to a dam safety manager about disaster response alternatives and procedures. The framework is demonstrated using three earth dams in the Southwest United States. The results showed that with increased fire intensity and post-fire rainfall, increase in impacts on earth dams due to increased runoff and sediment yields resulting in a potential dam failure and thereby increased impacts on its floodplain. These impacts are integrated into a decision matrix and a decision tree that could be used to prioritize dams and high hazard zones in the watershed.
Risk Assessment, Post-fire Floods, Dams, Floodplains
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