The rerun periods of detrimental effects ate often used as design criteria in urban storm water management. Considerable uncertainty is associated with the models used. This is either ignored or pooled with the inherent event to event variation such as rainfall depth It is here argued that uncertainty and inherent event to event variation should be treated separately, in providing engineers and managers with the distributions of return periods. It is then possible to base management decisions on knowledge of both the expected return periods and their corresponding confidence limits. It is further argued that the traditional pooling of inherent variation and uncertainty leads to meaningless rearm period curves with little engineering value.