ALBERT

Description: The urban stream syndrome is an almost universal physical and ecological response of streams to catchment urbanization. Altered channel geomorphology is a primary symptom that includes channel deepening, widening and instability. While the common approach is to treat the symptoms (e.g. modifying and stabilizing the channel), many stream restoration objectives will not be achieved unless the more vexing problem, treating the cause, is addressed in some way. Research demonstrates that the dominant cause of geomorphic change in streams in urban catchments is an altered flow regime and increase in the volume of stormwater runoff. Thus, managers can choose to treat the symptoms by modifying and controlling the channel to accommodate the altered flow regime, or treat the cause by modifying the flow regime to reduce the impact on channel morphology. In both cases treatments must, at the least, explicitly consider hydrogeomorphology—the science of the linkages between various hydrologic and geomorphic processes—to have a chance of success. This paper provides a review of recent literature (2010 to early 2015) to discuss fluvial hydrogeomorphology in the management of streams subject to urbanization. We suggest that while the dominant approach is focused on combating the symptoms of catchment urbanization (that we refer to as channel reconfiguration), there is increasing interest in approaches that attempt to address the causes by using stormwater control measures at a range of scales in the catchment (e.g. flow-regime management). In many settings in the oft-constrained urban catchment, effective management of stream morphology may require multiple approaches. To conclude, we identify five research areas that could inform urban hydrogeomorphology, one of the most challenging of which is the extent to which the volume of excess urban stormwater runoff can be reduced to mitigate the impact on stream geomorphology.

Description: There is now widespread recognition of the degrading influence of urban stormwater runoff on stream ecosystems and of the need to mitigate these impacts using stormwater control measures. Unfortunately, however, understanding of the flow regime requirements to protect urban stream ecosystems remains poor, with a focus typically on only limited aspects of the flow regime. We review recent literature discussing ecohydrological approaches to managing urban stormwater and, building on the natural flow paradigm , identify ecologically relevant flow metrics that can be used to design stormwater control measures to restore more natural magnitude, duration, timing, frequency and variability of both high and low flows. Such an approach requires a consideration of the appropriate flow and water quality required by the receiving water, and the application of techniques at or near source to meet appropriate flow regime and water quality targets. The ecohydrological approach provides multiple benefits beyond the health of urban streams, including flood mitigation, water supply augmentation, human thermal comfort, and social amenity. There are, however, uncertainties that need to be addressed. Foremost is the need to define ecologically and geomorphically appropriate flow regimes for channels which have already been modified by existing land use. Given the excess of water generated by impervious surfaces, there is also an urgent need to test the feasibility of the natural flow paradigm in urban streams, for example using catchment-scale trials.

Description: The geomorphic and ecological degradation of streams resulting from urbanization is well recognized. Recent reviews have hypothesized variability in ecological response to urbanization among stream types and among regions with varying climate and catchment soil and geology. Testing such hypotheses will be aided by the use of consistent metrics of the primary processes that drive urban-induced degradation of streams. We thus developed an urban typology distinguishing the common classes of urban land and water management that are hypothesized to have different effects on stream ecosystems. In developing the typology, we identify and emphasize the importance of step changes (thresholds) in the hydrologic consequences of urbanization, the often-unrecognized massive increase in drainage density of urban catchments, the difficulty in setting reference condition for many cities and the resulting difficulties in comparing responses to urbanization among cities. We critically assess the evidence for variability in responses to urbanization and its causes, and conclude that there are few studies demonstrating intra- or inter-regional differences in response to urbanization that have adequately accounted for the influence of potential differences in urban land and water management. We use the urban typology to propose metrics to allow such accounting, which we argue is necessary to advance management for stream protection and restoration.