Catchment response modifications

The quantifying of urbanisation effects on the rainfall-runoff relationship has been studied widely. Much of the work has concentrated on the modifications made to the volume and time distribution of surface water runoff hydrographs from single rainfall events. The various hydrograph parameters such as peak discharge, Qp, time to peak, tp and lag time (various definitions) are usually related to catchment characteristics, including area of impervious surfaces or proportion of area urbanised, in order to obtain quantitative rainfall-runoff relationships. A thorough description of many of the studies was given by Packman (1980). The various formulae that have resulted from individual studies are only applicable to the areas where they have been derived, and it is not advisable to use them for areas with different climates and topography. Instead, rainfall-runoff modelling is used, although most widely applied rainfall-runoff models still rely on empirical formulae for some of their parameters when they are applied to sites where there is insufficient measured data for calibration.

In the Flood Estimation Handbook (FEH; Institute of Hydrology, 1999) described in Chapter 13, the net effect of urbanisation is represented in the statistical estimation of design flows at an ungauged site by treating a catchment as if it were essentially rural and then applying adjustment factors. For moderately urbanised catchment (FEH catchment descriptor URBEXT1990 < 0.125 or URBEXT2000 < 0.150) an urban adjustment factor (UAF) is applied to estimation of the median annual flood (QMED) and the pooled growth curve. The adjustment was derived using flood data from 115 urban catchments (with URBEXT > 0.05), of which 34 were heavily urbanised (0.125 < URBEXT1990 < 0.500 or 0.150 < URBEXT2000 < 0.600). For QMED, the recommended adjustment is currently given as

QMEDurban = UAF * QMEDrural (18.1)


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