Green infrastructure drainage of a commercial plaza without directly connected impervious areas: a case study.

A paired-catchment study of two adjacent commercial areas in northern Sweden, one with Green Infrastructure (GI) storm drainage and the other with a conventional storm sewer system, served to evaluate the hydrological performance of both drainage systems and demonstrate advantages of GI. The GI catchment avoided directly connected impervious areas by diverting runoff from a parking lot to a cascade of three infiltration features, a fractured rock strip draining onto a sloping infiltration area, followed by a collector swale. Both catchments were monitored over 4 years by measuring rainfall, runoff and, in the vicinity of the swale, soil water content and groundwater levels. For frequent storms, the median GI efficiencies in reducing runoff volumes and peak flows, and extending peak flow lags, were 96, 99 and 60%, respectively, compared to conventional drainage. The storm rainfall depth, initial soil water content, increases in intra-event soil water storage and groundwater levels had statistically significant effects on either runoff volume or peak flow reductions. No effects were found for storm rainfall intensity and duration, antecedent dry days, and initial groundwater levels. The study demonstrated that GI drainage can be successfully applied even in the challenging environment of a subarctic climate.

Metal enrichment of soils in three urban drainage grass swales used for seasonal snow storage.

Enrichment of soils in three urban drainage swales by metals associated with traffic sources was investigated in a cool temperate climate with seasonal snow. Such swales differed from those not exposed to snow by receiving additional pollutant loads from winter road maintenance involving applications of salt and grit, use of studded tires, and storage and melting of polluted snow cleared from trafficked areas into swales. Among the swales studied, swale L2 in the downtown was the oldest (built around 1960), drained runoff from a road with the highest traffic intensity, and exhibited the highest mean concentrations of most of the metals studied (Pb, Cu, Zn, Cr, Cd, Ni, Co, V, Ti, and W). In the case of Pb, this exceedance was about an order of magnitude: 71 mg/kg DW in L2, compared to about ~8 mg/kg DW in L1 and L3, both built in 1979. Among the metals originating from local geology, barium (Ba) was found in the swales and the grit material at high concentrations of ~650 mg/kg DW and 700-1000 mg/kg DW, respectively. Such concentrations exceeded the Swedish EPA guideline limits of 300 mg/kg DW for less sensitive soil use. The sequential extraction analysis of samples from swale L2 indicated that Ba was mostly in the immobile residual fraction (90%). The absence of clear decline in metal concentrations with distance from the trafficked surfaces suggested that stored snow was another source of metals partly balancing spatial distribution of metals in swale soils.

Next generation swale design for stormwater runoff treatment: A comprehensive approach.

Swales are the oldest and most common stormwater control measure for conveying and treating roadway runoff worldwide. Swales are also gaining popularity as part of stormwater treatment trains and as crucial elements in green infrastructure to build more resilient cities. To achieve higher pollutant reductions, swale alternatives with engineered media (bioswales) and wetland conditions (wet swales) are being tested. However, the available swale design guidance is primarily focused on hydraulic conveyance, overlooking their function as an important water quality treatment tool. The objective of this article is to provide science-based swale design guidance for treating targeted pollutants in stormwater runoff. This guidance is underpinned by a literature review. The results of this review suggest that well-maintained grass swales with check dams or infiltration swales are the best options for runoff volume reduction and removal of sediment and heavy metals. For nitrogen removal, wet swales are the most effective swale alternative. Bioswales are best for phosphorus and bacteria removal; both wet swales and bioswales can also treat heavy metals. Selection of a swale type depends on the site constraints, local climate, and available funding for design, construction, and operation. Appropriate siting, pre-design site investigations, and consideration of future maintenance during design are critical to successful long-term swale performance. Swale design recommendations based on a synthesis of the available research are provided, but actual design standards should be developed using local empirical data. Future research is necessary to identify optimal design parameters for all swale types, especially for wet swales.

Processes improving urban stormwater quality in grass swales and filter strips: A review of research findings.

Increasing interest in urban drainage green infrastructure brings attention to grass swales and filter strips (GS&GFS) and their role in stormwater management. While the understanding of the hydrology and hydraulics of these stormwater control measures is adequate for current needs, there are knowledge gaps in understanding the water quality processes in GS&GFS and such a finding motivated preparation of the review paper that follows. The review revealed that most of the empirical studies of GS&GFS flow quality focused on the removal of pollutants associated with road runoff, and particularly solids, with relatively few studies addressing nutrients, traffic associated hydrocarbons, oxygen demanding substances, chloride, and faecal indicator bacteria. The reported results suffer from limitations caused by experimental conditions often representing a steady flow used to irrigate GS&GFS and generate runoff, non-submerged flows, no lateral inflows along swale side slopes, constant dosing of solids, emphasis on larger-than-typical solids, incomplete descriptions of experimental conditions, and limited attention to experimental uncertainties. Besides settling, other treatment processes, like adsorption/desorption, plant uptake, chemical precipitation and microbial degradation are often acknowledged, but without attempting to quantify their effects on flow quality. The modelling of GS&GFS flow quality would be beneficial for an improved understanding of green urban drainage infrastructure, but currently it is infeasible without a better knowledge of stormwater quality processes in GS&GFS facilities.

Info-Gap robustness pathway method for transitioning of urban drainage systems under deep uncertainties.

In the urban water cycle, there are different ways of handling stormwater runoff. Traditional systems mainly rely on underground piped, sometimes named 'gray' infrastructure. New and so-called 'green/blue' ambitions aim for treating and conveying the runoff at the surface. Such concepts are mainly based on ground infiltration and temporal storage. In this work a methodology to create and compare different planning alternatives for stormwater handling on their pathways to a desired system state is presented. Investigations are made to assess the system performance and robustness when facing the deeply uncertain spatial and temporal developments in the future urban fabric, including impacts caused by climate change, urbanization and other disruptive events, like shifts in the network layout and interactions of 'gray' and 'green/blue' structures. With the Info-Gap robustness pathway method, three planning alternatives are evaluated to identify critical performance levels at different stages over time. This novel methodology is applied to a real case study problem where a city relocation process takes place during the upcoming decades. In this case study it is shown that hybrid systems including green infrastructures are more robust with respect to future uncertainties, compared to traditional network design.

Source-Based Modeling Of Urban Stormwater Quality Response to the Selected Scenarios Combining Future Changes in Climate and Socio-Economic Factors.

The assessment of future trends in urban stormwater quality should be most helpful for ensuring the effectiveness of the existing stormwater quality infrastructure in the future and mitigating the associated impacts on receiving waters. Combined effects of expected changes in climate and socio-economic factors on stormwater quality were examined in two urban test catchments by applying a source-based computer model (WinSLAMM) for TSS and three heavy metals (copper, lead, and zinc) for various future scenarios. Generally, both catchments showed similar responses to the future scenarios and pollutant loads were generally more sensitive to changes in socio-economic factors (i.e., increasing traffic intensities, growth and intensification of the individual land-uses) than in the climate. Specifically, for the selected Intermediate socio-economic scenario and two climate change scenarios (RSP = 2.6 and 8.5), the TSS loads from both catchments increased by about 10 % on average, but when applying the Intermediate climate change scenario (RCP = 4.5) for two SSPs, the Sustainability and Security scenarios (SSP1 and SSP3), the TSS loads increased on average by 70 %. Furthermore, it was observed that well-designed and maintained stormwater treatment facilities targeting local pollution hotspots exhibited the potential to significantly improve stormwater quality, however, at potentially high costs. In fact, it was possible to reduce pollutant loads from both catchments under the future Sustainability scenario (on average, e.g., TSS were reduced by 20 %), compared to the current conditions. The methodology developed in this study was found useful for planning climate change adaptation strategies in the context of local conditions.