Preliminary Estimate of Dam Break Peak Discharge and Development of Dam Failure Impact Classes

摘要

Dam break analysis and floodplain inundation mapping is usually performed to assess the flood hazards downstream of a dam. However, formal floodplain studies (such as the ones conducted for FEMA) currently do not include a dam break analysis. The floodplain management community is trying to find practical ways to consider the impacts of a dam failure in floodplain studies downstream of existing dams. The problem is that dam break modeling usually requires a relatively complicated and costly engineering study, and it is not practical to provide such a study for every floodplain. This study presents a short-cut methodology that enables preliminary estimates of peak flow discharges from a dam under probable maximum flood (PMF) with and without dam failure due to overtopping. The dam PMF inflow and outflow is estimated by an equation using the watershed area and curve number and the 6-hour Probable maximum Precipitation (PMP) depth. Also, based on previous dam break studies, a relationship is established between two dimensionless ratios: 1) ratio of the volume of PMF inflow to the maximum reservoir storage, and 2) ratio of dam break to no dam break PMF peak outflows. A new equation is developed to estimate the ratio of dam break to no dam break peak outflows. Three dam failure impact classes are proposed to categorize the overall impact of dam break in increasing the peak flows from a dam under PMF conditions. The limits of the impact classes, as well as the constant in the new equation, are evaluated for dams in Virginia but may need to be adjusted for other locations.