In recent decades, the urban environment has undergone significant changes in both appearance and function, primarily attributed to the pervasive impact of human activities ^{[1][2][3][4][5]}[1,2,3,4,5]. Urban sprawl, the dispersion of natural resources, and environmental issues such as the proliferation of impermeable surfaces, amplified stormwater runoff, soil modifications, water and air quality deterioration, and pavement hydrology alterations ^{[1][6][7][8][9]}[1,6,7,8,9] are just a few examples of the challenges faced by urban centers. Additionally, climate change and biodiversity loss have made cities more vulnerable to environmental hazards, further exacerbating these challenges ^[5][10][11][5,10,11].

Indeed, urban regeneration is gaining popularity as a viable approach to mitigating the adverse impacts of human activities in urban environments. By rethinking urban water management and adopting sustainable solutions, urban centers can minimize the detrimental effects of human activities on the environment, enhance environmental quality, and promote sustainable development ^[3][12][13][3,12,13].

Retrofitting low-impact development (LID) practice is a possible strategy to alleviate the negative impacts of urbanization on the natural environment ^[4][7][13][4,7,13]. LID represents an innovative approach to stormwater management that aims to control rainfall runoff near its source. Several effective techniques, including rain gardens, permeable pavements, rain barrels, permeable infiltration trenches, bio-swales, and tree box filters, can be employed for rainwater management. The primary principle behind LID is to maintain hydrological conditions similar to those that existed naturally before urban development [6].

LID practices aim to minimize the impact of urbanization on the environment while also improving the functionality of urban landscapes. By employing LID techniques, it is possible to reduce stormwater runoff, improve water quality, and restore the natural hydrological regime of urban areas. Recent studies have also shown that experimental simulations confirm that decentralized coupled LID-GREI systems offer the best performance in terms of trade-offs between the lowest life-cycle economic costs, hydraulic reliability, and technical and operational resilience when compared to the use of grey infrastructure in cities only ^{[14][15][16][17][18]}[14,15,16,17,18]. Further methodological research and refinement are still needed to develop the evaluation framework for sustainable urban drainage systems with a variety of LID risks, but more thorough studies have paved the way for LID practice optimization and retrofitting. Ahiablame et al. [6] conducted a comprehensive investigation into the efficacy of various LID practices, specifically focusing on rain barrels, rain gardens, permeable pavement, green roofs, and swale systems. Their analysis, derived from meticulous field and laboratory studies, evaluated the performance of these practices in terms of hydrology and water quality. Concurrently, Beecham and collaborators undertook a quantitative and qualitative assessment of the performance of extensive and intensive living walls and green roofs. Drawing from a plethora of numerical and experimental investigations, they delineated several optimization methodologies for enhancing plant performance [19]. Further, the review by Kaykhosravi et al. [20] encompassed a survey of 11 distinct models, elaborating on the features and hydrological and hydraulic modeling components that are instrumental in gauging the performance of LID and green infrastructure. Retrofitting LID practices offers a practical solution to the negative consequences of urbanization and offers the potential for the regeneration of degraded urban environments. LID practices encompass three crucial processes: collection, delivery, and cleaning to effectively manage stormwater and improve the natural hydrology of urbanized areas [8]. The collection mechanism mitigates the runoff from storms and enhances waterways in the surrounding environment [5]. The delivery component channels stormwater to other systems that capture and retain precipitation, which can then be directed to a pond and undergo filtering treatment to ensure the availability of clean and potable water ^[8][21][8,21]. The retention of rainwater not only replenishes local water supplies but also restores the aquifers of groundwater. Additionally, the cleaning process filters and purifies rainwater that contains impurities and suspended solids to promote the production of clean and wholesome water [8]. Furthermore, improving water quality and biodiversity are pivotal performance metrics for LID practice. By reducing flow velocities, novel technologies, such as vegetative swales, can effectively absorb suspended particles and metal pollutants [22]. The implementation of LID practice has proven to be effective in managing storm runoff quantity, controlling floods, improving water quality, enhancing natural habitats, reducing construction and maintenance costs, achieving economic and social benefits, and improving community aesthetics and livability ^{[6][23][24][25][26]}[6,23,24,25,26].

Among the simplest and most straightforward LID practices that can replace traditional curb and gutter drainage systems are bio-swales, which are extensively used in urban villages, green spaces, parks, industrial lands, and roads [27]. Bio-swales are garnering increased attention as a key component of LID practice in stormwater management. These shallow, grass-lined, often flat-bottomed channels, also known as vegetated swales, grassed swales, infiltration swales, bio-swales, bio-filters, and filter strips, receive flow laterally through vegetated side slopes and have a multitude of functions. They not only collect and reduce stormwater but also enhance urban amenities, improve stormwater quality, and promote urban biodiversity. Swales are employed to tackle several stormwater management challenges and rely on processes such as infiltration, sedimentation, filtration, and biological processes ^{[6][28][29][30]}[6,28,29,30]. Vegetated swales can be implemented in areas with relatively steep longitudinal slopes [31].

Zhou ^[36][51] posited that while bio-swales have long been acknowledged for their role in providing localized stormwater transport and controlling the quantity and quality of runoff, their potential contribution to the restoration of predevelopment hydrology, as well as the provision of ecological services in peri-urban areas, is significant. These wet swales play a crucial role in mitigating the impacts of climate change ^[35][50]. Catchment hydrology and the water cycle benefit from bio-swales through the restoration of natural hydrological abstractions, such as infiltration and evapotranspiration, which are essential factors in regulating climate. Furthermore, bio-swales are effective in reducing the speed of runoff and are particularly beneficial to streets with traditional curb and gutter layouts. Bio-swales serve as a viable urban facility with which to tackle future weather extremes ^[50][51][64,65], including changes in rainfall intensity and precipitation. When compared to conventional storm sewer systems, bio-swales are superior in minimizing runoff flow volumes and peaks while being better equipped to convey stormwater in open channels ^[52][53][66,67].

The impact of bio-swales on the hydroclimate in winter climates is a subject of concern. Various regression studies, both single and multivariate, have shown that surface temperature, hydraulic loading, and to a lesser extent, snow depth have a significant effect on the reduction in winter peak flow and volume. While in summer, although the underlying soil’s moisture content plays a crucial role in explaining the variation in performance, it has little effect on infiltration during winter ^[54][68]. Therefore, bio-swales in cold regions must be designed to carry out two additional tasks: roadside snow storage and meltwater control ^[55][56][57][69,70,71]. When planning bio-swales for colder climates, it is essential to consider the more demanding conditions to which they are subjected, such as shorter growing seasons, frozen ground, and exposure to road salt ^[48][62].

Modeling

In order to effectively design and manage bio-swales, modeling their performance under different conditions is imperative. Planning and design professionals can leverage modeling tools to evaluate the hydrologic and water quality performance of bio-swales and optimize their design, operation, and use to meet desired objectives. However, due to their complexity, dynamic nature, and an incomplete understanding of the physical, chemical, and biological processes occurring within them, bio-swales pose a challenge to model accurately. Numerical models represent a useful approach for modeling bio-swales by simulating the flow of water and pollutants through the swale using mathematical equations. Such models offer the ability to assess the effectiveness of various design parameters, such as soil type, vegetation type, and swale depth, and can be used to evaluate the impacts of various land use scenarios on stormwater runoff. Additionally, numerical models allow for the assessment of the advantages of bio-swales in improving water quality in terms of pollutant removal rates. On the other hand, physical models replicate the swale and the surrounding environment in a laboratory setting, providing more detailed and accurate data on the performance of bio-swales. However, constructing and operating physical models can be more expensive and time-consuming than numerical models. In order to model bio-swales accurately, an understanding of the physical, chemical, and biological processes that occur within these systems is essential. Precise knowledge of the flow rate and direction of stormwater runoff within a bio-swale can optimize its design and effectiveness in mitigating the quantity and improving the quality of stormwater runoff. In particular, modeling software must upgrade its stormwater quality components to the same level as water quantity components, as suggested by some researchers ^[13][58][13,72]. This emphasizes the importance of modeling the effectiveness of bio-swales in improving stormwater quality. As urban green infrastructure continues to expand, bio-swale modeling remains a crucial tool. The sophistication and accuracy of modeling tools are expected to increase, enabling the optimization of bio-swales to provide maximum benefits for the environment and society ^[59][36].

Water Quality

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].

Biodiversity

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].