Research Needs on Flood Characteristics

Flood risk is nature related. Flood uncertainty is investigation related. The former can be changed only by changing the characteristics of floods. The latter can be changed only by more observation and investigation. The question to be answered yet is whether or not there is a physical upper bound to flood magnitude. The concept of probable maximum precipitation (PMP) seems to imply that there is an upper bound to flood magnitude. Flood characteristics are estimated by one or more methods: frequency curves, transfer of information (joint probability), regional data, paleohydrology, Bayesian and PMF groups of methods. The contemporaneous aspects of analysis of flood characteristics are related to reliability of estimation of floods of small exceedence probabilities in the range 10-2 to 10-7, whether there is an upper bound to floods and whether it is feasible to attach a probability value to probable maximum flood (PMF). To eventually answer these basic questions, three investigations are needed: (1) study of properties of the upper tail of probability distributions of floods; (2) use of regional data for drawing either the envelopes of largest floods or the curves of average largest flood characteristics for given sample sizes in the region, with probabilities attached to these envelopes or average curves; and (3) finding of the composite probability of PMF by studying the aggregated probabilities of random variables which are “maximized” in the process of computing PMP and PMF. Flood characteristics change with time due to changes in river basins. The need exists for methods of estimation of flood characteristics over periods of time of 25–100 years, particularly for planning flood mitigation measures over an extended future.