Potential and Application of Flood Frequency Determinations

Limitations of uncertainty in estimating flood frequencies and means of minimizing the uncertainty are discussed. The value of reducing uncertainty by improved use of data is discussed in relation to risk of future flooding that cannot be eliminated or reduced except by structural measures. The value and uses of reliability criteria are much misunderstood. An arbitrary safety factor is infeasible, whereas the concept of expected probability is a logical result of considering uncertainties. This concept has been used to assess impacts of uncertainties in mean and variance of flood values. When it can be extended to include uncertainties in higher order moments, such as the skew coefficient, light might be shed on the reason why extreme events occur that are far beyond any reasonable expectation. Applications of flood frequency estimates in the flood insurance program and in general flood mitigation projects are discussed. Most important, stress is placed on the most fundamental use of flood frequency information — assessment of expected damages and benefits of all kinds for various alternative programs of flood management under all possible flood conditions.