Urban sediment supply to streams from hillslope sources

Coarse-grained sediments supplied to a stream, in concert with the flow regime, play an important role in channel form and functioning, but are poorly understood in urban catchments. Improved knowledge of coarse-grained (>0.5 mm) sediment sources and supply rates will underpin strategies to mitigate impacts of urbanization on streams. We quantified key hillslope (i.e. non-channel) sources of sediment in urban areas by monitoring coarse-grained sediment yields from nine street-scale stormwater catchments over one year. From our observations, we developed a suburban hillslope sediment budget and a conceptual model of the response of hillslope coarse-grained sediment supply to different levels of urbanization. Coarse-grained sediment supply from the urban land surface was substantial. The highest unit-area yields came from infill construction sites (2800 kg/ha/yr), followed by gravel surfaces (740 kg/ha/yr), grass/mulch surfaces (84 kg/ha/yr), then impervious surfaces (21 kg/ha/yr), with the latter still producing yields far above background conditions. In typical suburban catchments grass and mulch surfaces and construction areas were key sources, with gravel and impervious surfaces making smaller contributions. Small source areas were important, for example construction produced 32% of sediment from 0.5% of the area. Connectivity of sediment sources to impervious surfaces, and hence to drainage systems, was important in driving sediment yields. Our conceptual model indicates that hillslope coarse-grained sediment supply increases with urbanization from natural to suburban conditions as connectivity increases, then declines with higher levels of urbanization as sources become scarcer. Impervious surfaces provide sources and supply pathways of coarse sediment, but also increase sediment transport capacity, causing severely supply-limited conditions and reducing the persistence of bed sediments in streams. When reducing hydrological connectivity to address the urban flow regime, consideration should be given to maintaining coarse-grained sediment supply through bypass or replenishment arrangements, to help reduce stream degradation and maintain form and functioning.