Modelling the impact of urbanisation on flood frequency relationships in the UK

This paper investigates the effect of urbanisation on the three key statistics used to establish flood frequency curves when combining the index flood method with the method of L-moments for estimating distribution parameters, i.e. the median annual maximum peak flow (the index flood), L-CV and L-SKEW. Using an existing procedure for estimating the three statistics at ungauged sites in the UK using catchment descriptors, as-rural estimates of the three statistics were obtained in 200 urban catchments and compared with the corresponding values obtained from observed data. The (log) differences of these estimates were related to catchment descriptors relevant to the urbanisation process using linear regression. The results show that urbanisation lead to a reduction in L-CV but an increase in L-SKEW. A jackknife leave-one-out experiment showed that the adjustment factors developed here were generally better at predicting the effect of urbanisation on the flood frequency curve than the existing adjustment factor currently used in the UK.