Understanding Heavy Tails of Flood Peak Distributions

Statistical distributions of flood peak discharge often show heavy tail behavior, that is, extremefloods are more likely to occur than would be predicted by commonly used distributions that have exponentialasymptotic behavior. This heavy tail behavior may surprise flood managers and citizens, as human intuitiontends to expect light tail behavior, and the heaviness of the tails is very difficult to predict, which may lead tounnecessarily high flood damage. Despite its high importance, the literature on the heavy tail behavior of flooddistributions is rather fragmented. In this review, we provide a coherent overview of the processes causingheavy flood tails and the implications for science and practice. Specifically, we propose nine hypotheses onthe mechanisms causing heavy tails in flood peak distributions related to processes in the atmosphere, thecatchment, and the river system. We then discuss to which extent the current knowledge supports or contradictsthese hypotheses. We also discuss the statistical conditions for the emergence of heavy tail behavior based onderived distribution theory and relate them to the hypotheses and flood generation mechanisms. We reviewthe degree to which the heaviness of the tails can be predicted from process knowledge and data. Finally, werecommend further research toward testing the hypotheses and improving the prediction of heavy tails.