On the role of the runoff coefficient in the mapping of rainfall to flood return periods

摘要

While the correspondence of rainfall return period T_P and flood returnperiod T_Q is at the heart of the design storm procedure, theirrelationship is still poorly understood. The purpose of this paper is to shedlight on the controls on this relationship examining in particular the effectof the variability of event runoff coefficients. A simplified world withblock rainfall and linear catchment response is assumed and a derived floodfrequency approach, both in analytical and Monte-Carlo modes, is used. Theresults indicate that T_Q can be much higher than T_P of the associatedstorm. The ratio T_Q /T_P depends on the average wetness of the system. In adry system, T_Q can be of the order of hundreds of times of T_P. Incontrast, in a wet system, the maximum flood return period is never more thana few times that of the corresponding storm. This is because a wet systemcannot be much worse than it normally is. The presence of a threshold effectin runoff generation related to storm volume reduces the maximum ratio ofT_Q /T_P since it decreases the randomness of the runoff coefficients andincreases the probability to be in a wet situation. We also examine therelation between the return periods of the input and the output of the designstorm procedure when using a pre-selected runoff coefficient and the questionwhich runoff coefficients produce a flood return period equal to the rainfallreturn period. For the systems analysed here, this runoff coefficient isalways larger than the median of the runoff coefficients that cause themaximum annual floods. It depends on the average wetness of the system and onthe return period considered, and its variability is particularly high when athreshold effect in runoff generation is present.