Bio-swales have gained significant attention as an effective means of stormwater management in urban areas, reducing the burden on conventional rainwater management systems. Bio-swales have the capacity to mitigate flood risk, reduce nonpoint source pollution, and enhance biodiversity. The performance of bio-swales is influenced by factors such as water quality, vegetation characteristics, substrate heterogeneity, and age, as identified by existing research. Nevertheless, critical knowledge gaps remain that need to be addressed in future research.
Influencing factors
Numerous studies have shown that swales are hydrologically effective at reducing runoff volumes, particularly during small storms [32][33][34][35][36][37][38][39][40][47,48,49,50,51,52,53,54,55]. Peak runoff rates can be reduced by 4 to 87% and runoff volumes by 15 to 82%. In a seminal study by Fassman [41][56] conducted in Auckland, New Zealand, the hydraulic performances of bio-swales were meticulously examined over the course of 42 distinct rainfall events. The research revealed a significant decrement in both peak flow and volume for storm events measuring less than 25 mm, underscoring the efficacy of bio-swales in managing stormwater runoff. Abida and Sabourin [42][57] undertook an empirical investigation in Canada, constructing five vegetated swales to ascertain their infiltration potential. Their findings elucidated a distinct temporal pattern in the infiltration rate. Initially, this rate experienced an exponential decay, but as time progressed, it plateaued, ultimately stabilizing at a constant value. Specifically, after an initial input of 130 mm/hr, a steady infiltration rate of 10 mm/hr was reached within a 20-min timeframe. This research collectively underscored the profound influence of bio-swales on urban hydrology, demonstrating their instrumental role in mitigating stormwater runoff and enhancing infiltration rates. However, the extent of the variation in swale hydrologic performance can be attributed to several factors, such as initial soil moisture conditions [38][53], soil characteristics [43][44][33,58], channel roughness, grass height and density [32][45][47,59], infiltration [33][36][38][48,51,53], compaction of the swale bed during construction [46][47][60,61], and maintenance [35][50]. When correctly sized, swales can efficiently transport stormwater runoff from various types of storms, with the most frequent type of storm having a 10-year recurrence interval [48][62]. The parameters of the rainfall event, including its duration, intensity, and preceding dry days, as well as those of the contributing drainage area, such as surface area, slope, land cover, and drainage mode, all determine the formation of runoff discharging into the bio-swale facility. The facility outflow is formed as a result of the runoff and the direct rainfall across the bio-swale footprint. Overall, swales are an effective tool for reducing runoff volumes and peak runoff rates, but their performance can be influenced by various factors. The appropriate design, sizing, construction, and maintenance are all crucial for achieving the desired hydrologic performance [49][63].
Climate
Influencing factors
The accumulation of pollutants on catchment surfaces during dry weather, which is attributable to dry atmospheric deposition and land-use practices, poses a significant challenge to roadside bio-swales. These pollutants were transported into the bio-swales by various means, such as wind, vehicle-induced turbulence, street sweeping, and snow removal activities. The potential for runoff during wet weather to displace previously accumulated pollutants, coupled with the addition of pollutants to the atmosphere through wet deposition, presents a second source of pollutants for bio-swales [59][36]. Consequently, two significant sources contribute to the influx of pollutants in bio-swales: (i) the runoff from the contributing drainage area and (ii) atmospheric deposition, both wet and dry, including rain falling directly on the bio-swales facility. During wet weather, some pollutants are carried into the stormwater facilities from nearby contributing drainage areas, while others are splashed or blown into the water [59][36]. Five primary factors affected the performance of roadside bio-swales, namely, vegetation type, percentage of vegetation cover, treatment length of bio-swales, slope, and soil type [33][37][58][60][48,52,72,73].
Research in the area of roadside bio-swales has been relatively limited due to the difficulty of altering the characteristics of soil and slope, which are largely determined by the surrounding environment [61][62][74,75]. The effectiveness of bio-swales is heavily influenced by the type of soil and the regulation of water flow into and through it. An experiment conducted in Florida found that dry soils with good drainage and high infiltration rates were associated with the significant removal of total metal, nitrogen, and phosphorus loads in two vegetated filter strips [63][76]. The slope of a grass swale is another critical factor impacted by the local environment. Steeper slopes result in faster water flow through the swale, significantly reducing the time for water infiltration into the soil. This ultimately lowers the efficiency of the bio-swale, as steeper slopes limit the time required for dislodging suspended particles from the water column. Therefore, to achieve higher infiltration rates, it is necessary to slow down the slope of the bio-swale, allowing water to flow through it for longer periods and, thus, increasing the time available for the infiltration process [37][52].
The efficiency of a bio-swale is heavily influenced by the length of its treatment, which determines the duration of water storage within the system [32][64][47,77]. Yu et al. [37][52] highlighted that treatment length is the primary factor impacting the performance of bio-swales. A longer treatment length results in increased water retention, which facilitates higher rates of pollutant removal through prolonged plant interaction. Studies have indicated that bio-swales longer than 100 m are particularly effective in removing pollutants from road runoff. Vegetation is another crucial factor that significantly affects bio-swale performance. The choice of plant species can have a profound impact on the treatment outcomes, with flood-proof species being the most effective in roadside ditches. It is critical for plants to maintain adequate biomass density and height in waterlogged environments [65][80]. A greenhouse study of 20 flood-tolerant plant species revealed that the genera Carex, Melaleuca, and Juncus produced the most significant reductions in pollutant production, while Leucophyta, Microlaena, and Acacia produced the lowest decreases [65][80]. However, plant selection alone is not sufficient, as the appropriate plant density is also necessary for optimal treatment performance. The monitoring of six roadside bio-swales over two years in central Texas demonstrated that effective solids removal decreased rapidly as vegetation density increased above 90% coverage [60][73].
Soil pollution
Soil pollution from contaminants is an important concern when it comes to water quality. Urban runoff contains a range of pollutants such as heavy metals, suspended particles, pathogens, and nutrients. In order to manage polluted runoff from various sources, such as roads, highways, parking lots, and roofs, swales have been employed to control the quantity and quality of the runoff [66][67][68][34,82,83]. Swales achieve attenuation of stormwater flow rates and peaks through the absorption of water by the grass-soil medium, thereby leading to two treatment mechanisms: increased settling and filtration through swale soils. Swales are primarily designed to carry runoff from severe storm events, with runoff from smaller events mostly or completely infiltrating into swale soils [36][43][33,51]. By promoting stormwater infiltration in swale channels, incoming pollutants are immobilized in swale channels or soils, thus reducing the conveyed pollution [30][45][30,59]. The impact of stormwater runoff pollution on soil chemistry in swales has been extensively studied, revealing the contamination of soils by traffic-derived pollutants like metals and polycyclic aromatic hydrocarbons. Areas with heavy traffic volume or stop-and-go traffic are especially susceptible to increased pollution severity [45][59]. Although bacteria and pathogens are not typically significant pollutants in highway or road runoff, other stormwater control measures are typically more effective at removing bacteria than bio-swales [48][62].Micro level
The interdependence of plants, soil, and micro-organisms in bio-swales is of paramount importance to their overall effectiveness. Soil and plants work in concert to absorb stormwater, while soil bacteria play a critical role in facilitating the water and nutrient uptake of plants. Furthermore, the involvement of micro-organisms as an extended component of plant phenotype is essential to assist plants in adapting to the frequent drying and wetting cycles inherent in bio-swale soils. This symbiotic relationship can improve plant survival and longevity in these systems [69][70][71][88,89,90]. The inflow of stormwater into bio-swales can result in the accumulation of pollutants and excessive nitrogen levels in the soil, which can have detrimental effects. Nonetheless, recent studies have indicated that bio-swale soils contain significant concentrations of microbial genes that are associated with contaminant degradation, which suggests that microbes may have the ability to ameliorate the harmful effects of these pollutants. Furthermore, the siting of bio-swales and the plant species selected for planting can have a substantial influence on the assembly and function of the artificial ecosystem’s soil microbial communities. Within each bio-swale, bacterial and fungal communities were discovered to be significantly clustered by bio-swale and plant species, indicating that soil microbial composition is subject to microenvironmental controls and that plant composition has an impact on microbial assemblages within bio-swales [72][91]. Macro level Biodiversity is an essential component of healthy ecosystems and is critical for maintaining ecological balance and functionality. The adoption of bio-swales could provide numerous benefits to biodiversity from a macro-ecological perspective. In urban areas, the use of bio-swales could reduce nonpoint pollutant sources resulting from decreased rainfall effluence, thus protecting the region’s ecosystems and maintaining water circulation. Furthermore, bio-swales could contribute to the mitigation of climate change impacts by cooling cities and providing green spaces that protect biological diversity and habitats. They could also enhance microclimates, improve the quality of land, water, and the atmosphere, and reduce carbon emissions [73][92]. Although ecological assessments of bio-swales are relatively sparse, studies found that converting traditional planting strips on urban roads into bioretention swamps enhanced invertebrate communities [74][93]. Significant parameters in this regard included vegetation structure, such as coverage and number of flowering plants and slope characteristics. This finding indicated the potential for bio-swales to provide additional benefits to biodiversity in urban areas [65][80].