Semi-explicit modelling of watersheds with urban drainage systems

Abstract

In rainfall-runoff modelling of urbanized and semi-urbanized watersheds, the urban drainage systems considerably influence runoff propagation time. In small scale watersheds, the drainage network may be modelled explicitly. In contrast, for larger watersheds, most hydrological models are based on a rough representation of the effects of drainage systems, thus failing to represent the rapidly-varying real flow dynamics. Therefore, a trade-off methodology has been developed to account for impervious surfaces and drainage effects accurately, without the need for modelling the entire drainage network in detail. Undrained impervious areas have been distinguished from drained ones. Rain falling on the former has been discharged as overland flow, whereas flow on the later has been routed separately using "virtual pipes", which enable a simplified process-oriented modelling of the drainage network. The methodology has been applied to a 130 km2 Belgian catchment, resulting in the simulation of fast flow peaks, which do not appear when the effect of the drainage network is neglected.