Probabilistic Extreme Flood Hydrographs That Use PaleoFlood Data for Dam Safety Applications

Extreme flood hydrographs are needed to evaluate dam safety issues for situations where the reservoir inflow peak discharge is greater than the maximum spillway capacity, the reservoir has large surcharge storage, and/or the reservoir has dedicated flood control space. Flood runoff hydrographs integrate the drainage basin and channel response to precipitation, given some initial, variable state of moisture throughout the watershed. To conduct risk analyses and dam safety evaluations, probability estimates for extreme floods are required. Probabilistic extreme flood hydrographs are developed to assess the adequacy of the spillway and reservoir flood/surcharge space to temporarily store a portion of the flood volume, and to attenuate or pass the hydrograph peak without overtopping the dam. The hydrographs and probability estimates are used in risk analyses for dam safety. These hydrographs can also be used to establish reservoir operating rules and determine diversions needed for construction. This report documents statistical and rainfall-runoff techniques used to develop probabilistic extreme flood hydrographs for dam safety risk analyses. A probabilistic extreme flood hydrograph is defined in this report as one that preserves a peak discharge exceedance probability and dependence between volume and peak for a fixed duration. An extreme flood is considered to have an Annual Exceedance Probability (AEP) of 0.005 or less. There are many methods of estimating extreme flood runoff hydrographs, such as unit hydrograph approaches (e.g., Chow et al., 1988), continuous rainfall-runoff modeling (e.g., Bradley and Potter, 1992), and statistical techniques (e.g., USACE, 1975a). These and other extreme rainfall-runoff methods are discussed in-part by NRC (1988), Pilgrim and Cordery (1993), and Nathan and Weinmann (1999). Other than a method presented by Nathan and Weinmann (1999), these references do not describe methods that can be used by practitioners to estimate both extreme floods (peak, volume, duration) and associated probabilities for dam safety. The methods and examples presented in this report are an attempt to bridge part of that gap.