Flows that cause geomorphic change in urban streams and what stormwater control measures can do

Abstract

Reduced infiltration and evapotranspiration from impervious areas of urban watersheds once covered by vegetation, and stormwater runoff directed through drainage networks, can lead to ten times more water “doing work” on stream channels. Recent research demonstrates the potential for stormwater control measures (SCMs) to address the causes of urban stream degradation, by retaining and reducing (through consumption or evapotranspiration) some of this excess urban runoff. But can SCMs also address flow patterns to the extent that flows above geomorphic thresholds, such as those responsible for bed and bank erosion, are mitigated? We review the literature on hydrologic metrics related to geomorphic change in streams, with a focus on urban systems. We also use hydraulic models and hydrologic data sets for sites throughout Melbourne, Australia, to associate geomorphic thresholds with standard hydrologic indices. We identify hydrologic metrics that may be used to inform the management of SCMs in a way that reduces geomorphic change in receiving waters. A major uncertainty is the conjunctive changes to sediment supply under urbanization, and the role this plays in stream morphology.

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