In recent years, the focus of flood hazard assessment for dams has shifted to a risk informed decision making (RIDM) approach. An extensive array of tools has been developed to facilitate probabilistic flood hazard analysis (PFHA) that provide a more robust understanding of hydrologic hazards for a dam than a deterministic probable maximum flood (PMF) study. This is done by characterizing the expected behavior of key variables such as headwater and spillway flow over the full range of return intervals from common storms to extreme events comparable to the PMF.
PFHA methodologies can be divided into two general categories: 1) flow-based approaches that leverage historical inflow data and apply statistical techniques to extrapolate risk to extreme return intervals, and 2) rainfall-runoff approaches that utilize precipitation-frequency information and hydrologic modeling within the watershed to characterize flood risk. Commonly used toolsets for the flow-based approach include RMC-BestFit, RMC-RFA, and HEC-SSP, while watershed modeling tools include RMC-RRFT, HEC-HMS, HEC-WAT, and SEFM among many others.
While the suite of available tools provides great value, the abundance of options can present a challenge to dam managers seeking to identify the best approach for their unique situation. Factors such as available data, prior analysis, existing models, potential failure modes, and watershed characteristics such as upstream regulation and characterization of the storm types that drive flood hazards in the study area all come into consideration. An appropriate analysis will provide information on flood risks and associated uncertainties to support decision-making.
Through RTI’s experience executing a diverse set of flood risk studies using many different approaches, meaningful lessons have been learned in evaluating the relevant factors and selecting the best method to evaluate flood risk adequately and efficiently. The lessons learned will focus on how to evaluate the unique aspects of a study site in selecting the appropriate tool to use for efficient risk evaluation that allows dam managers to obtain appropriate confidence in study findings.
