Abstract
Abstract. While the correspondence of rainfall return period TP and flood return period TQ is at the heart of the design storm procedure, their relationship is still poorly understood. The purpose of this paper is to shed light on the controls on this relationship examining in particular the effect of the variability of event runoff coefficients. A simplified world with block rainfall and linear catchment response is assumed and a derived flood frequency approach, both in analytical and Monte-Carlo modes, is used. The results indicate that TQ can be much higher than TP of the associated storm. The ratio TQ /TP depends on the average wetness of the system. In a dry system, TQ can be of the order of hundreds of times of TP. In contrast, in a wet system, the maximum flood return period is never more than a few times that of the corresponding storm. This is because a wet system cannot be much worse than it normally is. The presence of a threshold effect in runoff generation related to storm volume reduces the maximum ratio of TQ /TP since it decreases the randomness of the runoff coefficients and increases the probability to be in a wet situation. We also examine the relation between the return periods of the input and the output of the design storm procedure when using a pre-selected runoff coefficient and the question which runoff coefficients produce a flood return period equal to the rainfall return period. For the systems analysed here, this runoff coefficient is always larger than the median of the runoff coefficients that cause the maximum annual floods. It depends on the average wetness of the system and on the return period considered, and its variability is particularly high when a threshold effect in runoff generation is present.
Highlights
In catchments with limited streamflow data or subject to major land use changes, the estimation of the design flood, i.e., the largest flood that should be considered in the evaluation of a given project, is typically performed using the design storm procedure
In this paper we examine the effect of event runoff coefficients on the relationship between rainfall and flood return periods to shed light on design practice
In the design storm procedure the ratio of flood and rainfall return periods TQ/TP is maximised by varying storm duration
Summary
In catchments with limited streamflow data or subject to major land use changes, the estimation of the design flood, i.e., the largest flood that should be considered in the evaluation of a given project, is typically performed using the design storm procedure. Packman and Kidd, 1980; Bradley and Potter, 1992) This is a pragmatic assumption but clearly not always correct because it does not account for the role of different processes in determining the relationship between the frequencies of the design rainfall and the derived flood peak Even in this very simple situation, the mapping of return periods is not trivial: except for very particular cases, the return period of the flood peak is always smaller than the return period of the generating rainfall This is in contrast with the observations in the real world where, often, very extreme floods are produced by storms whose magnitude is not so extreme In this paper we focus on the role of the antecedent conditions of the basin expressed by the variability of the runoff coefficients
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