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Flood estimation using isolated event simulation models requires selection of a suitable combination of design storm and antecedent conditions. Current practice in storm drainage design is to adopt arbitrarily a storm duration, profile, and catchment wetness and to assume that the return periods of rainfall depth and flood peak are equal. This paper describes how sensitivity analysis may be used to examine the relationship between rainfall and flood return periods and, thereby, to determine systematically a suitable set of design inputs which give a peak runoff of the required return period. This process permits this particular model to be used with confidence in design as well as in simulation.

The effect of urban land cover on catchment flood response is evaluated using a lumped rainfallrunoff model to analyse flood events from selected UK catchments with mixed urban and rural land use. The present study proposes and evaluates a series of three extensions to an existing model to enable a better representation of urban effects, namely: an increase in runoff volume, reduced response time and a decrease in baseflow (resulting from decreased infiltration). Based on observed flood events from seven catchments, cross-validation methods are used to compare the predictive ability of the model variants with that of the original unmodified model. The results show that inclusion of urban effects increases the predictive ability of the model across catchments, despite large between-event variability of model performance. More detailed investigations into the relationship between model performance and individual event characteristics (antecedent soil moisture, rainfall duration, depth and intensity) did not reveal systematic inabilities of the model to reproduce certain types of events. Finally, it is demonstrated that the new extended model has the ability to simulate urban effects in accordance with the expected changes in storm runoff patterns.

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