During risk assessments for dam safety, potential failure modes (PFM) for a dam are evaluated at specific reservoir water levels, or stages. The risk of a PFM is determined by combining the annual exceedance probability (AEP) at each reservoir stage of interest with the probability of failure at that stage, and the consequences of failure. The AEP of reservoir stages, which is the estimated probability of reaching or exceeding a specific stage on an annual basis, can be determined empirically by plotting annual maxima reservoir stages from the observed record. However, many PFMs require the estimation of pool frequencies beyond observed reservoir stages. There is usually insufficient observed data to directly estimate stage-frequency relationships for remote probabilities, such as those for water levels exceeding the top of dam. Models can be used to extrapolate reservoir stage frequency estimates beyond the observed stage data. The US Army Corps of Engineers (USACE) Risk Management Center (RMC) developed the Reservoir Frequency Analysis software (RMC-RFA), an inflow volume-based stochastic modeling platform, to facilitate stage-frequency analysis within the USACE Dam Safety Program. RMC-RFA produces a reservoir stage-frequency curve with uncertainty bounds by utilizing a deterministic flood routing model while treating the seasonal occurrence of the flood event, the antecedent reservoir stage, inflow volume, and the inflow flood hydrograph shape as uncertain variables rather than fixed values.
RMC-RFA was used to generate stage-frequency curves for eight reservoirs in the Trinity River Basin in Texas for dam safety studies in the Fort Worth District of USACE. Statistical and graphical methods were used to evaluate RMC-RFA simulation results to empirical stage-frequency curves. Based on goodness-of-fit metrics, the RMC-RFA models performed well against the observed stage data, providing confidence that the extrapolated stage-frequency curves with uncertainty bounds were reasonable.
Alessandro Parola– Hydraulic Engineer, US Army Corps of Engineers, Fort Worth District, Arlington, Texas