Areal rainfall correction coefficients for small urban catchments

The spatial rainfall variability can have a large effect on the prediction of discharges in urban drainage systems. Areal rainfall correction coefficients are often used to cope with this problem. This paper makes a clear distinction between two types of areal correction coefficients: one for correction of historical data at given time moments and one for rainfall at given return period levels. The first type can be used within applications of calibration of models of urban drainage systems, while the second type is needed for design purposes. The second type is most often found in the literature and referred to as the “areal reduction factors”. This paper shows that for small catchment areas and limited rainfall intensities, they also may reflect an increase and not always a reduction. Therefore, they are called more generally “areal correction coefficients” in this paper. Both types of coefficients were derived for the Flanders region in Belgium. They were based on simulations with a spatial rainfall generator. The areal correction coefficients were considered dependent on the size of the catchment, the rainfall aggregation level, and the rainfall intensity (linked to the return period through IDF-relationships). The dependencies in time and space make a correct application of correction coefficients to the combined hydrologic-hydrodynamic modelling of urban drainage systems and the design of these systems not obvious, as pointed out by the paper.