Runoff in the suburbs

Posted on March 23, 2015

The ubiquity of impermeable surfaces in urban environments means that relationships between rainfall and runoff in built-up areas are very different from those in rural catchments. While a lot of hydrological research has been carried in both urban and rural environments, much less is known about rainfall-runoff relationships in suburban areas, and the relevance of urban and rural flow models to suburban catchments is poorly understood. To help fill this knowledge gap, rainfall- runoff relationships for two catchments – one urban and the other suburban – in the area of Nantes, France, were compared. Surface imperviousness was 38% for the Gohards basin (urban) and 11% for the Chézine basin (suburban). Data on 140 local rainfall events in 2001-2007 were analysed. When relative runoff (i.e., runoff as a proportion of rainfall) was plotted against pre-event base flow, there was a clear difference between the two catchments. In the suburban catchment, relative runoff increased steadily with base flow. In contrast, in the urban catchment, there was a similar steady increase in relative runoff at low base flows but a much faster rate of increase at higher base flows. Because base flow is a measure of a catchment’s wetness, it appeared that the suburban basin behaved like an urban area when the catchment soil was dry, but became much more like a natural catchment as wetness increased. This interpretation was supported by comparing hydrographs for the two catchments, which showed that timings of the various discharge peaks could be related to contributions of water from different parts of a basin. Only the urban parts of the suburban catchment contributed to runoff under dry conditions. Therefore hydrological models need to reflect the fact that, in suburban areas, runoff patterns can vary widely depending on soil wetness as well as the distribution of impermeable surfaces.

Reference: Furusho, C. et al. 2014. Analysis of the hydrological behaviour of an urbanizing basin. Hydrological Processes 28, 1809–1819.