Hydrologic Issues for Dams - Association of State Dam Safety Officials

If tw o or three independent flood events within a period of three months or so are to becons idered, then their probability of occurrence should be about 10 -6 or so. Assuming theflood events in a region to be a Poisson process, the probability that three 100-year floodevents would occur in a period of three months, would be about 3 x 10 -9 ; the probabilitythat three 50-year flood events would occur in a period of three months, would be about2.1x 10 -8 ; and the probability that three 25-year flood events would occur in a period ofthree months, would be about 1.67x 10 -7 . This suggests that three 25-year floods in a 3-month period may have a probability equivalent to that of the PMF. One or three 25-yearfloods may or may not cause scour and erosion comparable to a single PMF event. Also,additional research is needed to estimate scour and erosion for different types of rock,concrete, vegetation, and soil protection due to long-term exposure to high velocities orexposure to successively occurring high velocities several times during a given period.Determination of a long-duration PMF requires estimation of the long-duration PMP.Methods to determine one-month or 5-month duration storm events equivalent to thePMP have yet to be determined. Available information includes PMP durations of onlyup to 4 days or so.Conclusion and Research NeedsDetermination of a reasonably safe DBF for a dam should be based on economic andseveral intangible factors associated with the consequences of dam failure. Economicconsequences may be estimated using analytical approaches outlined in this paper.Evaluation of the relative significance of all tangible and intangible factors is subjective.A combination of the delphi and fuzzy-set approaches may be useful in considering allrelevant factors with due weightage assigned to each based on input from the interestedor impacted parties.The methods and assumptions used for hydrologic analyses related to risk-based designsof dams are inherently subjective. As a result, different hydrologists may obtain widelydifferent results from risk analysis for one and the same dam. Research efforts mustfocus on reducing this inherent subjectivity so that results of risk analyses performed bydifferent hydrologists are as close to one another as practicable. This requiresdevelopment of guidelines and methods for each specific computational element of riskanalysis. Research needs relevant to this goal are indicated below:• Identification of methods to identify categories of dams where risk-basedhydrologic design is acceptable and preferable to that based on industry or agencystandards related to size and hazard classification. It must be recognized thatdesigns based on standards imply a certain degree of protection to designers,operators, and owners against liability for failure consequences.• Identification of consistent methods and assumptions related to PMFdetermination including magnitude, duration, and sequencing of PMP andPaper 11 - Prakash 54