► We review the impact of stormwater management on baseflow in peri-urban areas. ► Relevant techniques to assess and understand baseflow changes are not yet mainstream. ► Baseflow changes depend on ...both physiographic and urban development characteristics. ► The natural flow paradigm provides a framework to guide baseflow management. ► Source-control systems modelling will help understand their impacts on baseflow.
While infiltration source-control technologies are increasingly used to manage the volume, rate and quality of stormwater runoff, there is little guidance on their role and impact on baseflow. This review addresses the impacts of urbanisation on baseflow in peri-urban catchments, with the aim to better understand the potential role of stormwater infiltration source-control technologies in restoring pre-development baseflows. We analyse the physiographic and anthropogenic factors that affect the baseflow response to urbanisation. We also suggest that observed uncertainties in these baseflow responses may arise from inconsistencies in site assessment methodologies, including measurement techniques and selection of indicators. We use the natural flow paradigm to propose catchment-scale baseflow objectives and illustrate potential barriers in translating these catchment-scale objectives to the site scale. Finally, we examine the function of source-control stormwater infiltration techniques in light of both design and environmental parameters (e.g. climate, soil properties). Although we conclude that source-control technologies have potential to mitigate the impact of urbanisation on baseflow hydrology, the complexity of subsurface flow processes makes it difficult to model the effects of the implementation of several stormwater management techniques on catchment baseflow. We thus suggest that the adoption of a clear framework for baseflow assessment in pre- and post-development states, along with fundamental research on the translation from site-scale processes to catchment-scale effects, are essential research steps to guide future stormwater management for baseflow in peri-urban catchments.
This paper examines the scientific evidence underpinning four hypotheses about the role of plants in bioretention systems: (i) Planted systems are more effective than unplanted systems, (ii) Plant ...species differ in their effectiveness, (iii) Native species are more effective than exotic ones, (iv) Diverse systems are more efficient than monocultures.
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•Manuals are often based on hypotheses on plant role in bioretention, not evidences.•Plants have a positive effect on permeability and nitrogen removal.•Right plant species is crucial for nitrogen not TSS, phosphorus, metals removal.•Native plants may help biodiversity, but not hydrologic or treatment performances.•Diverse plantings can be more resilient, but will not necessarily perform better.
Plants are essential components of bioretention systems, with bioretention design-guides around the world providing extensive advice on the role of selection of plants to maximize system performance and sustainability. Four principal hypotheses regarding the role of plants have been identified in bioretention design manuals: (i) Planted systems are more effective than unplanted systems, (ii) Plant species differ in their effectiveness, (iii) Native species are more effective than exotic ones, (iv) Diverse systems are more efficient than monocultures. This paper examines the extent to which these hypotheses are supported by the scientific literature. Comparison of planted and unplanted systems show that increased permeability and hydraulic conductivity, as well as removal of nitrogen, are the main benefits of the presence of plants in bioretention. Knowledge on their positive effect on hydrocarbons remains fragmented, although there is evidence from phytoremediation studies in other plant-based technologies. Choosing the right species makes a difference in hydraulic performance and nitrogen removal, with root traits being identified as important predictors of performance. No scientific results can support the hypothesis that native plants or diversely-planted systems offer better performance than systems planted with fewer species or with exotic species. Questions remain regarding the plant-microbe interaction in the bioretention context, the role of biomacropores in pollutant migration or the differential impact of plant choice on performance.
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.
The management of urban stormwater has become increasingly complex over recent decades. Consequently, terminology describing the principles and practices of urban drainage has become increasingly ...diverse, increasing the potential for confusion and miscommunication. This paper documents the history, scope, application and underlying principles of terms used in urban drainage and provides recommendations for clear communication of these principles. Terminology evolves locally and thus has an important role in establishing awareness and credibility of new approaches and contains nuanced understandings of the principles that are applied locally to address specific problems. Despite the understandable desire to have a 'uniform set of terminology', such a concept is flawed, ignoring the fact that terms reflect locally shared understanding. The local development of terminology thus has an important role in advancing the profession, but authors should facilitate communication between disciplines and between regions of the world, by being explicit and accurate in their application.
Vegetated biofiltration systems (biofilters) are now a well-established technology for treatment of urban stormwater, typically showing high nutrient uptake. However, the impact of high temporal ...variability of rainfall events (further exacerbated by climate change) on nitrogen and phosphorus removal processes, within different biofiltration designs, is still unknown. Hence, a laboratory-based study was conducted to uncover mechanisms behind nutrient removal in biofilters across different drying and wetting regimes. Two sets of experimental columns were based on (1) the standard biofiltration design (unsaturated zone only), and (2) combination of unsaturated and saturated (submerged) zone (SZ) with additional carbon source. Columns were watered with synthetic stormwater according to three drying and wetting schemes, exploring 1, 2, 3, 4 and 7-week drying. Hydraulic performance, soil moisture and pollutant removal were monitored. The results show that hydraulic conductivity of SZ design experiences less change over time compared to standard design, due to slower media drying, crack formation and lower plant die-off. Varied drying lengths challenged both designs differently, with 2-week drying resulting in significant drop of performance across most pollutants in standard design (except ammonia), while SZ design was able to retain high performance for up to four weeks of drying, sustaining microbial and plant uptake. Increased oxygenation of SZ columns during short-term drying was beneficial for ammonia and phosphorus removal. While SZ design showed better performance and quicker recovery for nitrogen removal, in regions with inter-rain event shorter than two weeks, the standard design (no saturated zone, no carbon source) can achieve similar if not better results.
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•1- to 7-week drying and wetting in standard and submerged zone (SZ) biofilters.•SZ design showed more stable hydraulic performance due to slower drying.•SZ design showed better performance and quicker recovery for nitrogen removal.•Standard design achieved better TP and NH3 removal under 14-day (and shorter) ADWP.•Biofilters with 5–10% moisture content in top zone optimal for nutrient removal.
A large-scale laboratory study was conducted to test the influence of design and operating conditions on the lifespan of stormwater biofilters. The evolution of hydraulic conductivity over time was ...studied in relation to a number of key design parameters (media type, filter depth, vegetation type, system sizing, etc). The biofilters were observed to clog over time, with average hydraulic conductivity decreasing by a factor of 3.6 over the 72 weeks of testing. The choice of plant species appears to have a significant effect on the rate of decrease in permeability, with plants with thick roots (e.g. Melaleuca) demonstrating an ability to maintain permeability over time. Other species studied, with finer roots, had no such beneficial effects. As expected, small systems relative to their catchment (and thus which are subjected to high loading rates) are more prone to clogging, as increases in hydraulic and sediment loading can lead to extremely low hydraulic conductivities. Sizing and the appropriate choice of vegetation are thus key elements in design because they can limit clogging, and therefore, indirectly increase annual load treated by limiting the volume of water bypassing the system.
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► We test the influence of design parameters on clogging in stormwater biofilters. ► We find consistent decreases in hydraulic conductivity over time. ► Thick-rooted vegetation can reduce or even prevent clogging. ► Sizing to reduce loading rates and vegetation selection are critical to avoiding clogging.
► Conventional urban stormwater drainage fundamentally alters stream hydrology. ► Methods for reducing stormwater pollution typically only attenuate peak flows. ► Stream protection requires a more ...complete approach to flow-regime restoration. ► Near-natural flow regimes are achievable using existing strategies at small scales. ► Both infiltration and harvesting of stormwater are required to achieve this.
Conventional approaches to stormwater management for environmental protection fail because they do not address all of the changes to the flow regime caused by conventional stormwater drainage. In this paper, we contrasted the hydrologic effects of two conventional approaches to urban stormwater management – (a) drainage-efficiency focused and (b) pollutant-load-reduction focused – identifying their shortcomings and contrasting their hydrologic outcomes with those of a proposed alternative approach focused on restoring important elements of the natural flow regime. Under conventional approaches, both high-flow and low-flow hydrology remain perturbed. We suggest that urban stormwater management should emphasize the restoration or protection of natural hydrologic processes at small scales, with the aim of restoring natural flow regimes at larger scales downstream. We therefore suggest that, despite recent advances in managing stormwater to reduce pollutant loads and peak flow rates, a more complete approach is needed, one which includes as a goal the restoration or protection of ecologically important elements of the pre-development hydrograph. We propose an approach, flow-regime management, which aims as much as possible to restore and protect ecological structure and function of urban streams by retaining the pre-urban frequency of untreated storm flows, reducing the total stormwater runoff volume through evapotranspiration or harvesting, and delivering filtered flow rates to match pre-urban baseflow rates. We note, however, that the cumulative effects of urban stormwater management at smaller scales on catchment-scale hydrology are not yet fully understood.
Green roofs are a promising engineered ecosystem designed to reduce stormwater runoff and restore vegetation cover in cities. Plants can contribute to rainfall retention by rapidly depleting water in ...the substrate, however, this increases the risk of plant drought stress. This study determined whether lower plant density or preferentially redirecting rainfall to plants on green roofs could reduce drought stress without reducing rainfall retention. Plant density was manipulated, and metal structures were installed above the substrate surfaces to redirect the flow of rainwater towards plants (runoff zones). Green roof modules were used to test three plant density treatments: unplanted, half-planted (10 plants/m2) and fully-planted (18 plants/m2), and two runoff zone treatments which were installed in unplanted and half-planted modules. It was expected that 1) green roofs with greater plant density would experience more drought stress (i.e., lower leaf water status), and 2) green roofs with runoff zones would show higher ET and hence retention compared with those without runoff zones, as water will be directed to plants (run-on zones), facilitating growth. Contrary to the hypothesis, evapotranspiration (ET) and rainfall retention were similar for half-planted and fully-planted modules, such that ∼82 % of applied rainfall was retained. While both vegetation treatments dried out the substrates before rainfall was applied, the fully-planted modules dried out quicker and showed significantly lower leaf water status than half-planted modules. This indicates that planting at lower density may reduce plant drought stress, without reducing rainfall retention. Installing runoff zones marginally reduced ET and rainfall retention, likely due to shading by the runoff zone structures reducing evaporation from the substrate. However, runoff also occurred earlier where runoff zones were installed as they likely created preferential flow paths that reduced soil moisture and therefore ET and retention. Despite reduced rainfall retention, plants in modules with runoff zones showed significantly higher leaf water status. Reducing plant density therefore represents a simple means of reducing plant stress on green roofs without reducing rainfall retention. Installing runoff zones on green roofs is a novel approach that could reduce plant drought stress, particularly in hot and dry climates, albeit at a small cost of reduced rainfall retention.
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•The effects of plant cover on green roof evapotranspiration and retention were studied.•Runoff zones were also installed to redirect rainfall onto plants.•For half-planted and fully-planted treatments, ET and retention were similar.•Redirecting rainfall onto plants reduced stress and minimally reduced retention.
Urban stormwater is a crucial resource at a time when climate change and population growth threaten freshwater supplies; but there are health risks from contaminants, such as toxic metals. It is ...vitally important to understand how to use this resource safely and responsibly. Our study investigated the extent of metal contamination in vegetable crops irrigated with stormwater under short- and long-term conditions. We created artificially aged gardens by adding metal-contaminated sediment to soil, simulating accumulation of metals in the soil from irrigation with raw stormwater over zero, five and ten years. Our crops--French bean (Phaseolus vulgaris), kale (Brassica oleracea var. acephala), and beetroot (Beta vulgaris)--were irrigated twice a week for 11 weeks, with either synthetic stormwater or potable water. They were then tested for concentrations of Cd, Cr, Pb, Cu and Zn. An accumulation of Pb was the most marked sign of contamination, with six of nine French bean and seven of nine beetroot leaf samples breaching Australia's existing guidelines. Metal concentration in a crop tended to increase with the effective age of the garden; but importantly, its rate of increase did not match the rate of increase in the soil. Our study also highlighted differences in sensitivity between different crop types. French bean demonstrated the highest levels of uptake, while kale displayed restrictive behaviour. Our study makes it clear: irrigation with stormwater is indeed feasible, as long as appropriate crops are selected and media are frequently turned over. We have also shown that an understanding of such risks yields meaningful information on appropriate safeguards. A holistic approach is needed--to account for all routes to toxic metal exposure, including especially Pb. A major outcome of our study is critical information for minimising health risks from stormwater irrigation of crops.
Biofiltration systems are a recommended and increasingly popular technology for stormwater management; however there is a general lack of performance data for these systems, particularly at the field ...scale. The objective of this study was to investigate the hydrologic and pollutant removal performance of three field-scale biofiltration systems in two different climates. Biofilters were shown to effectively attenuate peak runoff flow rates by at least 80%. Performance assessment of a lined biofilter demonstrated that retention of inflow volumes by the filter media, for subsequent loss via evapotranspiration, reduced runoff volumes by 33% on average. Retention of water was found to be most influenced by inflow volumes, although only small to medium storms could be assessed. Vegetation was shown to be important for maintaining hydraulic capacity, because root growth and senescence countered compaction and clogging. Suspended solids and heavy metals were effectively removed, irrespective of the design configuration, with load reductions generally in excess of 90%. In contrast, nutrient retention was variable, and ranged from consistent leaching to effective and reliable removal, depending on the design. To ensure effective removal of phosphorus, a filter medium with a low phosphorus content should be selected. Nitrogen is more difficult to remove because it is highly soluble and strongly influenced by the variable wetting and drying regime that is inherent in biofilter operation. The results of this research suggest that reconfiguration of biofilter design to manage the deleterious effects of drying on biological activity is necessary to ensure long term nitrogen removal.
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