School of Agriculture, Food and Ecosystem Sciences - Research Publications
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ItemTree water-use strategies to improve stormwater retention performance of biofiltration systems(PERGAMON-ELSEVIER SCIENCE LTD, 2018-11-01)Biofiltration systems are highly valued in urban landscapes as they remove pollutants from stormwater runoff whilst contributing to a reduction in runoff volumes. Integrating trees in biofilters may improve their runoff retention performance, as trees have greater transpiration than commonly used sedge or herb species. High transpiration rates will rapidly deplete retained water, creating storage capacity prior to the next runoff event. However, a tree with high transpiration rates in a biofilter system will likely be frequently exposed to drought stress. Selecting appropriate tree species therefore requires an understanding of how different trees use water and how they respond to substrate drying. We selected 20 tree species and quantified evapotranspiration (ET) and drought stress (leaf water potential; Ψ) in relation to substrate water content. To compare species, we developed metrics which describe: (i) maximum rates of ET under well-watered conditions, (ii) the sensitivity of ET and (iii) the response of Ψ to declining substrate water content. Using these three metrics, we classified species into three groups: risky, balanced or conservative. Risky and balanced species showed high maximum ET, whereas conservative species always had low ET. As substrates dried, the balanced species down-regulated ET to delay the onset of drought stress; whereas risky species did not. Therefore, balanced species with high ET are more likely to improve the retention performance of biofiltration systems without introducing significant drought risk. This classification of tree water use strategies can be easily integrated into water balance models and improve tree species selection for biofiltration systems.
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ItemUrban sediment supply to streams from hillslope sources(ELSEVIER, 2019-02-25)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.
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ItemGlobal sediment yields from urban and urbanizing watersheds(ELSEVIER SCIENCE BV, 2017-05-01)Streams with urban watersheds are almost universally subject to degradation, largely driven by changes to flow and sediment inputs from the watershed. However, the impact of urbanization on sediment yields of urban watersheds is poorly understood. We undertook a comprehensive review of global responses of fine-grained and coarse-grained sediment yields to different phases of urbanization and compared them to a long-standing conceptual model. The summarized yields showed a great deal of variability, but were consistent with the widely-used conceptual model for watersheds with active construction. Importantly, however, the yields for established urban areas tended to be higher than previously assumed, and tended to remain higher than background levels. This is most likely because the urban drainage network has a very high sediment transport efficiency and because the increased runoff in urban watersheds is very effective at eroding the available sediment sources (mainly infill development, urban decay and renewal, and gravel surfaces in parks and gardens). The updated model provided here will assist in informing the extent to which sediment supply to stormwater drainage systems and urban streams needs to be addressed to assist the protection and restoration of streams in urban watersheds.
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ItemRestoring in-stream habitat in urban catchments: Modify flow or the channel?(WILEY, 2019-01)Abstract Urban streams have almost universally altered physical habitat conditions due to excess stormwater run‐off. This includes changes to in‐channel hydraulics and channel morphology. Restoration of in‐channel habitat has two main levers: address the hydrology or channel morphology. Both variables impact in‐stream habitat, but understanding the relative role of hydrologic and morphologic change remains a challenge. This study uses two‐dimensional hydraulic modelling to examine the relative roles of flow and channel morphology in setting hydraulic conditions. We investigated four test scenarios involving the combinations of urban versus natural hydrology and urban versus natural channel morphology. The analysis investigated three ecologically relevant hydraulics characteristics: bed mobilization, retentive habitat, and floodplain inundation, using Shields stress, shallow slow‐water habitat (SSWH) area, and floodplain inundation area hydraulic metrics, respectively. The results indicate substantial differences in hydraulic conditions between the two reaches. The urban reach showed increased bed mobility potential and SSWH availability plummeted as flow increased, whereas the natural channel showed a relatively stable bed with substantially more SSWH at most flows. Floodplain inundation frequency was low in the urban channel with decreased duration. Scenarios examined suggest that hydraulic conditions are highly sensitive to channel morphology relative to flow regime. This suggests that once channel form has been degraded, mitigating urbanization impacts on flow regime cannot maintain “natural” channel hydraulics. Management approaches therefore must protect channel morphology from change. Where the channel has already been fundamentally altered, opportunities for channel morphology rehabilitation need to be considered.
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ItemEffect of urbanization on stream hydraulics(WILEY, 2018-09)Abstract Urbanization results in major changes to stream morphology and hydrology with the latter often cited as a primary stressor of urban stream ecosystems. These modifications unequivocally alter stream hydraulics, but little is known about such impacts. Hydraulic changes due to urbanization were demonstrated using two‐dimensional hydrodynamic model simulations, comparing urban and non‐urban stream reaches. We investigated three ecologically relevant hydraulic characteristics: bed mobilization, retentive habitat, and floodplain inundation, using hydraulic metrics bed shear stress, shallow slow‐water habitat (SSWH) area, and floodplain inundation area. We hypothesized that urbanization would substantially increase bed mobilization, decrease retentive habitat, and due to increased channel size would decrease floodplain inundation. Relative percent area of bed disturbance was 4 times higher, compared with that of the non‐urban stream at bankfull discharge. SSWH availability rapidly diminished in the urban stream as discharge increased, with SSWH area and patch size 2 times smaller than the non‐urban stream for a frequently occurring flow 0.7 times bankfull discharge. Floodplain inundation decreased in frequency and duration. These results demonstrate changes in hydraulics due to urbanization that may impact on physical habitat in streams. New “water sensitive” approaches to stormwater management could be enhanced by specification of hydraulic regimes capable of supporting healthy stream habitats. We propose that a complete management approach should include the goals of restoration and protection of natural hydraulic processes, particularly those that support ecological and geomorphic functioning of streams.
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ItemUrban Stormwater Runoff: A New Class of Environmental Flow Problem(PUBLIC LIBRARY SCIENCE, 2012-09-19)Environmental flow assessment frameworks have begun to consider changes to flow regimes resulting from land-use change. Urban stormwater runoff, which degrades streams through altered volume, pattern and quality of flow, presents a problem that challenges dominant approaches to stormwater and water resource management, and to environmental flow assessment. We used evidence of ecological response to different stormwater drainage systems to develop methods for input to environmental flow assessment. We identified the nature of hydrologic change resulting from conventional urban stormwater runoff, and the mechanisms by which such hydrologic change is prevented in streams where ecological condition has been protected. We also quantified the increase in total volume resulting from urban stormwater runoff, by comparing annual streamflow volumes from undeveloped catchments with the volumes that would run off impervious surfaces under the same rainfall regimes. In catchments with as little as 5-10% total imperviousness, conventional stormwater drainage, associated with poor in-stream ecological condition, reduces contributions to baseflows and increases the frequency and magnitude of storm flows, but in similarly impervious catchments in which streams retain good ecological condition, informal drainage to forested hillslopes, without a direct piped discharge to the stream, results in little such hydrologic change. In urbanized catchments, dispersed urban stormwater retention measures can potentially protect urban stream ecosystems by mimicking the hydrologic effects of informal drainage, if sufficient water is harvested and kept out of the stream, and if discharged water is treated to a suitable quality. Urban stormwater is a new class of environmental flow problem: one that requires reduction of a large excess volume of water to maintain riverine ecological integrity. It is the best type of problem, because solving it provides an opportunity to solve other problems such as the provision of water for human use.
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ItemA suburban sediment budget: Coarse-grained sediment flux through hillslopes, stormwater systems and streams(Wiley, 2019-10-01)Sediment in urban stormwater systems creates a significant maintenance burden, while a lack of coarse‐grained bed sediment in streams limits their ecological value and geomorphic resilience. Gravel substrates, for example, provide benthic habitat yet are often scoured from the channel bed only to end up in a detention basin or treatment wetland. This dual problem of both ‘too much’ and ‘too little’ coarse‐grained sediment reflects a watershed sediment budget that is profoundly altered. We developed a conceptual urban coarse‐grained (>0.5 mm) sediment budget across three domains: hillslopes (urban land surfaces), the built stormwater network and stream channels. We then quantified key sources, sinks and storages for a suburban case study, using a combination of hillslope and in‐channel monitoring, and interrogation of local government records. Around 36% of the sediment supplied to the stormwater network reached the catchment outlet, a level of sediment delivery much higher than observed in similar‐sized natural catchments. The remainder was deposited in the sediment cascade and either stored, or extracted and removed from the catchment (e.g. material deposited in sediment ponds and gross pollutant traps). Conventional urban drainage networks are characterized by high hillslope sediment supply and low storage, resulting in efficient sediment delivery. Channel erosion, deposition in (and extraction from) pipes and channels, and floodplain deposition are small compared to sediment transport through the cascade. An understanding of the sediment budget of urban headwater catchments can provide stormwater and waterway managers with the information they need to address specific sediment problems such as sedimentation in stormwater assets and geomorphic recovery of urban streams.
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ItemUrban catchment runoff increases bedload sediment yield and particle size in stream channels(Elsevier, 2018-09-01)Physical degradation of urban rivers negatively impacts the environmental and social values they provide, and imposes significant financial costs on waterway management agencies. While the impact of urban stormwater runoff on streams is well recognised, the influence of altered bed sediment regimes on urban stream geomorphology is poorly understood. This study reports bedload sediment yields and bedload particle size distributions measured with sediment traps in nine small streams in eastern Melbourne, Australia, across a gradient of urbanization. We assessed relationships between the yield and size of bedload sediment and measures of catchment urbanization (including total imperviousness, effective imperviousness, road density and pipe density) and hydrology (measured through flow gauging at each site). Bedload yields were greater and bedload sediments were coarser-grained in more urbanized catchments. Bedload yields were strongly related to drainage connection of the urban land surface to the stream (captured by measures such as effective imperviousness and pipe density). The increase in bedload yield and calibre in urban catchments was driven mainly by the increase in sediment-transporting runoff from connected impervious surfaces. This study found no evidence of urban land cover severely limiting coarse-grained sediment supply. Most of the bedload material appeared to originate from imported sediment sources (e.g. construction and surfacing materials) in upland urban hillslope areas, which were connected to the channel by efficient transport reaches such as pipes and rock-lined channels, rather than from channel-derived erosion. Findings of this study suggest a rethink of the coarse-grained sediment-supplying potential of urban catchments, which has long been assumed to be low due to sealing of the land surface with hard materials.
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ItemNo Preview AvailableMANAGEMENT OPTIONS TO ADDRESS DIFFUSE CAUSES OF HYDROLOGIC ALTERATION(ACADEMIC PRESS LTD-ELSEVIER SCIENCE LTD, 2017)
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