Hydrologic Issues for Dams - Association of State Dam Safety Officials

The PMF represents an estimated upper bound on the maximum runoff potential for aparticular watershed. In some sense, the inherent assumption is that a dam with aspillway designed to pass this flood has zero risk of overtopping.The PMF design standard has been reexamined in the past and continues to be debated asa design standard up to the present time. One of the most significant initial reevaluations1of this standard is described in NRC (1985) . In this reevaluation, base safety analysis(an incremental deterministic evaluation) and risk analysis were explored as potentialalternatives to the PMF criteria.Since this reevaluation, a considerable amount of research, which examines the use of thePMF and explores the benefits of a risk approach, has been completed. Despite thisresearch, significant hydrologic problems in dam safety analysis still need to be solved.These problems are apparent when considering: 1) mechanisms in addition toovertopping that might cause catastrophic failures of the dam; 2) the recent research intopaleoflood estimates of the largest floods occurring in the past 10,000 years; and, 3) theneed to estimate the exceedance probability of extreme floods when performing a riskanalysis.Mechanisms, other than overtopping, which have potential to cause failure of a dam, areerosion of the spillway or undermining of the spillway foundation. Certainly, theseproblems have been noticed in recent floods in the Kansas, Gila and Colorado RiverBasins. The potential for failure increases for these mechanisms as the duration of theflooding increases. Extended duration events could result in severe erosion of a spillway,with an insufficient intervening period to perform maintenance; and, consequently, causedam failure. As an extreme example, the great Upper Mississippi Basin flood of 1993included 5 major flood inflow events to Saylorville Reservoir on the Des Moines Riverover a period of 5 months. Examples like this illustrate that extended flow periods shouldbe considered in the design process, as opposed to the current application of the singlepeak hydrograph of the PMF.A possible approach to addressing this design problem is to consider estimating theprobable maximum precipitation over multi-month periods. Inflow hydrographs could bedeveloped by simulating patterns of historic precipitation proportioned to have an equalvolume to the probable maximum amount over the duration desired.Besides the volume issue, recent paleoflood evidence in the western United Statesindicates that the largest floods occurring in the past 10,000 years are significantlysmaller than PMF estimates. This difference has caused some concern with regard to themagnitude of the PMF estimates. Possible reasons for this difference between theestimates might come from problems with models used to estimate peak flows frompaleo-stage indicators or the area reduction factors used to convert point estimates of theprobable maximum precipitation (PMP) to a total storm depth.1 National Research Council, 1985. Safety of Dams, Flood and Earthquake Criteria, Committee of SafetyCriteria for Dams, National Academy Press, Washington, D.C., 321p.ix