How alternative urban stream channel designs influence ecohydraulic conditions.

Streams draining urban catchments ubiquitously undergo negative physical and ecosystem changes, recognized to be primarily driven by frequent stormwater runoff input. The common management intervention is rehabilitation of channel morphology. Despite engineering design intentions, ecohydraulic benefits of urban channel rehabilitation are largely unknown and likely limited. This investigation uses an ecohydraulic modeling approach to investigate the performance of alternative channel design configurations intended to restore key ecosystem functioning in urban streams. Channel reconfiguration design scenarios, specified to emulate the range of channel topographic complexity often used in rehabilitation are compared against a reference 'natural' scenario using ecologically relevant hydraulic metrics. The results showed that the ecohydraulic conditions were incremental improved with the addition of natural oscillations to an increasing number of individual topographic variables in a degraded channel. Results showed that reconfiguration reduced excessive frequency of bed mobility, loss of habitat and hydraulic diversity particularly as more topographic variables were added. However, the results also showed that none of the design scenarios returned the ecohydraulics to their reference conditions. This indicate that channel-based restoration can offer some potential changes to hydraulic habitat conditions but are unlikely to completely mitigate the effects of hydrologic change. We suggest that while reach-scale channel modification may be beneficial to restore urban stream, addressing altered hydrology is critical to fully recover natural ecosystem processes.

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.

Can catchment-scale urban stormwater management measures benefit the stream hydraulic environment?

The potential for catchment-scale stormwater control measures (SCMs) to mitigate the impact of stormwater runoff issues and excess stormwater volume is increasingly recognised. There is, however, limited understanding about their potential in reducing in-channel disturbance and improving hydraulic conditions for stream ecosystem benefits. This study investigates the benefits that SCM application in a catchment have on in-stream hydraulics. To do this, a two-dimensional hydraulic model was employed to simulate the stream hydraulic response to scenarios of SCM application applied in an urban catchment to return towards pre-development hydrologic pulses. The hydraulic response analysis considered three hydraulic metrics associated with key components of stream ecosystem functions: benthic mobilization, hydraulic diversity and retentive habitat availability. The results showed that when applied intensively, the developed SCM scenarios could effectively restore the in-stream hydraulics to close to natural levels. Compared to an unmanaged urban case (no SCMs), SCM scenarios yielded channels with reduced bed mobility potential, close to natural hydraulic diversity and improvement of retentive habitat availability. This indicates that mitigating the effect of stormwater driven hydrological change could result in significant improvements in the physical environment to better support ecosystem functioning. We therefore suggest that intensive implementation of SCMs is an important action in an urbanizing catchment to maintain the flow regime and hydraulic conditions that sustain the 'natural' stream habitat functioning. We propose that stormwater management and protection of stream ecosystem processes should incorporate hydraulic metrics to measure the effectiveness of management strategies.

River Bank Erosion and the Influence of Environmental Flow Management.

Environmental flows aim to influence river hydrology to provide appropriate physical conditions for ecological functioning within the restrictions of flow regulation. The hydrologic characteristics of flow events, however, may also lead to unintended morphologic effects in rivers, such as increases in riverbank erosion beyond natural rates. This may negatively impact habitat for biota, riparian infrastructure, and land use. Strategic environmental flow delivery linked to monitoring and adaptive management can help mitigate risks. We monitor riverbank condition (erosion and deposition) relative to environmental flows on the Goulburn River, Victoria, Australia. We describe the process of adaptive management aimed at reducing potential impacts of flow management on bank condition. Field measurements (erosion pins) quantify the hydrogeomorphic response of banks to the delivery of planned and natural flow events. Managed flows provide opportunities for monitoring riverbank response to flows, which in turn informs planning. The results demonstrate that environmental flows have little influence on bank erosion and visual perceptions in the absence of monitoring are an unreliable guide. This monitoring project represents a mutually beneficial, science-practice partnership demonstrating that a traditional 'know then do' approach can be foreshortened by close collaboration between researchers and managers. To do so requires transparent, often informal lines of communication. The benefits for researchers-a more strategic and targeted approach to monitoring activities; and benefits for the practitioners-reduced time between actions and understanding response; mean that a learn by doing approach is likely to have better outcomes for researchers, stakeholders, the public, and the environment.