Relative roles of flow and sediment supply in driving urban stream degradation and limiting recovery

The physical condition of streams in urban catchments is primarily driven by flow and sediment supply, both of which are controlled by altered land cover. Despite documented increases in coarse-grained sediment supply from urban catchments compared to natural conditions, urban streams tend to be depleted of bed sediment. Much of this apparent contradiction can be attributed to an increase in sediment transport capacity, but the relative roles of altered flow and sediment supply remain poorly understood. To better understand the role of sediment supply, we constructed a catchment-scale budget of coarse-grained (> 0.5 mm) sediment for a typical suburban headwater stream, based on data from Melbourne, Australia. Hillslope sediment supply was computed from monitoring of sediment inputs to stormwater drains, while yield from the channel was estimated from bedload monitoring. We also explored transport capacity (the amount of sediment that can be moved) and competence (the ability to move particles of a certain size) in streams with varying catchment urbanization levels, using hydraulic models to compute stream power and shear stress as surrogates. Urban hillslope coarse-grained sediment supply to the stormwater network was plentiful, furnished mostly by landscaped (lawn/garden/gravel) areas (54%) and lot-scale construction areas (35%). Coarse-grained sediment yield at the catchment outlet was 40% of hillslope supply, with the remainder stored in, or exported (cleaned out) from, the stormwater network or channels, or deposited on the floodplain. Channel erosion was minor based on our observations of stable, rock-lined channels. Despite increased sediment supply, transport capacity and competence are greatly increased in urban streams, leading to persistent net sediment supply limitations. Connectivity of the land surface to the stream channel by stormwater pipes drives both elevated flows and sediment supply. Measures to improve water quality and flow regimes which reduce connectivity (e.g. constructed wetlands, rain gardens), must also consider sediment supply, allowing adequate coarse sediment to pass. Otherwise direct replenishing of sediment in the channel will be required if our well-intentioned measures for managing urban stormwater are going to support habitat and prevent stream erosion.