Flood Probability Estimation

A procedure is developed for estimating design floods in situations where data are insufficient for a conventional frequency analysis. It involves the use of a compound distribution which is a weighted combination of individual two parameter distributions. Initial values are assigned to the parameters of the component distributions and their weights on the basis of subjective estimates of the mean and standard deviation of the flood peaks. The weights of the component distributions are updated with Bayes' theorem in the light of any additional information such as recorded floods, the largest flood in a number of years or a flow which has not been exceeded in a given number of years. Numerical methods are used. A computer program is developed to carry out the necessary computations and is described to illustrate how the method could be used by practicing hydrologists. Examples are presented to illustrate the procedure.