Oral Presentation 9th Australian Stream Management Conference 2018

Urban streams and sediments: informing better management through recent findings (#87)

Kathryn L Russell 1 , Geoff J Vietz 1 , Tim D Fletcher 1
  1. School of Ecosystem and Forest Science, The University of Melbourne, Burnley, VIC, Australia

Sediment supply from urban catchments to streams is a key input to support the geomorphic integrity and ecological health of streams, but also produces a maintenance burden in ponds and estuaries downstream. Urban stormwater runoff is known to increase sediment transport capacity, enlarge channels and deplete substrates. Urban sediment supply regimes may enhance or mitigate this effect, but are poorly understood at present. In this paper we provide a summary of recent research on urban sediment regimes, with a focus on coarse-grained sediments, and their role in stream channel change and management. Mature urban catchments are often thought to be have low sediment production, as land surfaces are sealed and erosion is stabilised. Our research demonstrates this is not the case. Urban catchments have highly dynamic geomorphic processes, driven by high levels of sediment connectivity between sources (e.g. infill construction areas, gravel surfaces, bare soil patches) and the drainage network. The increase in supply from the catchment to the stream appears to be outweighed by the increase in sediment transport capacity (i.e. catchment runoff and flow energy). Sediments are transported efficiently into underground pipes and through the channel, producing much higher bedload and suspended transport rates than background conditions. The key to allowing bed sediments to persist in urban streams, and reducing sediment-maintenance requirements, is to address the flow regime. However, interventions to reduce stormwater runoff from the catchment (e.g. source control measures such as rainwater tanks and rain gardens) can also reduce sediment supply, constraining recovery potential. We recommend that designers and managers of source control measures explicitly consider coarse-grained sediment bypass, or replenishment of sediments to streams. This will improve the ability of streams to provide ecological and social values, including habitat and human accessibility, and reduce economic burdens such as sediment-maintenance and stabilisation.

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