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Review

Unlocking the Positive Impact of Bio-Swales on Hydrology, Water Quality, and Biodiversity: A Bibliometric Review

by
Tong Chen
1,
Mo Wang
1,2,*,
Jin Su
2,3,* and
Jianjun Li
1
1
College of Architecture and Urban Planning, Guangzhou University, Guangzhou 510006, China
2
Guangdong Provincial Ecological Restoration Engineering Technology Research Center, Guangzhou 510006, China
3
Faculty of Civil Engineering and Built Environment, University Tun Hussein Onn Malaysia, Parit Raja 86400, Pahat, Johor, Malaysia
*
Authors to whom correspondence should be addressed.
Sustainability 2023, 15(10), 8141; https://doi.org/10.3390/su15108141
Submission received: 25 March 2023 / Revised: 12 May 2023 / Accepted: 12 May 2023 / Published: 17 May 2023
(This article belongs to the Section Sustainable Urban and Rural Development)
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Abstract

:
Bio-swales have gained significant attention as an effective means of stormwater management in urban areas, reducing the burden on conventional rainwater management systems. Despite this increasing interest, a comprehensive assessment of the status of bio-swale research is lacking. In response, this article employs two powerful information-visualizing software tools, the “Bibliometrix” R package and “CiteSpace”, to conduct a quantitative investigation of 323 English language sources published in the Web of Science prior to 2022. The objective is to provide a comprehensive examination of bio-swale research from multiple perspectives, shedding light on current advancements and future research trends. The findings reveal (1) a persistent annual increase in bio-swale-related publications and (2) the predominant focus on regulating services, such as hydrology, water quality, and biodiversity, with hot topics within these areas, including the influencing factors, climate, modeling, soil contaminants, and biodiversity at both macro and micro levels. Furthermore, our study concludes that future research necessitates interdisciplinary and interterritorial collaboration, a broader focus that encompasses the social, economic, ecological, and engineering aspects of bio-swales, and the adoption of diverse research methodologies. Given the currently limited research on biodiversity in bio-swales, this area holds the potential to become a future research hotspot. By harnessing the insights and findings of our study, researchers can gain a more profound understanding of the current state of bio-swale research and devise effective strategies to further propel this critical area of study.

1. Introduction

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]. 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] 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].
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].
Retrofitting low-impact development (LID) practice is a possible strategy to alleviate the negative impacts of urbanization on the natural environment [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]. 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]. 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].
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]. Vegetated swales can be implemented in areas with relatively steep longitudinal slopes [31].
Revitt et al. [32] identified three distinct types of grass drainage swales: standard swales, dry swales, and swales with check berms. Standard swales are shallow, open ditches that are vegetated and allow runoff to flow through and into permeable soils. Conversely, dry swales have engineered soil bottoms and include an underdrain pipe to enhance infiltration and drainage. Additionally, the inclusion of check-berms at regular intervals can further enhance the infiltration of swale channels [33]. Swales are capable of receiving lateral inflows over their side slopes as well as longitudinal inflows from upstream sources. Furthermore, grass filter strips can pretreat lateral inflows by accepting overland drainage flows [34,35,36].
Despite previous comprehensive studies on bio-swales, there are gaps in the literature regarding the lack of dynamic evolution procedures, the clustering of research topics, and research status frameworks. In order to address these gaps and the need for disciplinary development, this article employs bibliometric methods to analyze research papers on bio-swales. The main objective is to provide a comprehensive knowledge base on the fundamental traits, literature co-citation analysis, research hotspots, and frontier developments in bio-swales using graphic knowledge maps. This review synthesizes the benefits and advantages of bio-swales in controlling hydrology, water quality, and other factors while also making recommendations for future research directions on bio-swales. This study aims to advance the field of bio-swales and promote their widespread application in urban settings to address environmental challenges (Figure 1).

2. Date and Methods

2.1. Literature Search

A literature review was conducted using the Web of Science database using the keywords “grass swale”, “bio-swale”, and “infiltration swale” to identify relevant journal articles, proceedings, and reports. The abstracts of the selected papers were evaluated for their relevance to the topic of swales, and complete texts were downloaded if deemed pertinent. Furthermore, the reference lists of downloaded papers were examined to uncover additional relevant articles, and this process was repeated until no further relevant papers were identified. In total, 323 papers were scrutinized in detail. The search was restricted to articles published until June 2022 to ensure a current representation of the available literature.

2.2. Data Processing

To conduct the quantitative analysis, the “Bibliometrix” R package and “CiteSpace” were employed. The bibliometric tool “Bibliometrix” is an open-source tool for scientometrics, which is powerful and suitable for use with other analytic techniques. This software program established data matrices for co-citation, coupling, collaborative analysis, and co-word analysis and had a quick analysis speed [37]. “CiteSpace”, another bibliometric tool used in this study, is based on the idea of “co-occurrence clustering” for the visualization and scientific analysis of literature. It extracts keywords, subject terms, disciplines, domain categories, etc., at a thematic level, then reconstructs them according to the type and strength of the links between information units, forming networks of different meanings, and then discovers the implicit patterns and laws of knowledge structures in specific disciplines and domains. Another important factor in choosing “CiteSpace” was the ability to output a more aesthetically pleasing and clean image [38,39]. Therefore, “Bibliometrix” was used to extract data at the macro level of the literature, such as annual publication rates, country of authorship, most cited authors, and cited literature, etc., while “CiteSpace” was used to identify key points of development in a field.
Hotspot mining was conducted to form the theme of this article. The “Bibliometrix” analysis was powerful and relatively full-featured, making it suitable for use with other analytic techniques. Meanwhile, “CiteSpace” was used to identify implicit patterns and laws of knowledge structures in specific disciplines and domains. By using these quantitative analysis tools, this study offered a coherent knowledge base on the fundamental traits, literature co-citation analysis, research hotspots, and frontier developments in bio-swales together with graphic knowledge maps.
To identify pivotal moments in the evolution of bio-swale research over time, this study employed the “Bibliometrix” software package to visualize trends in the publication dates of the collected literature. Additionally, a co-citation network was created using CiteSpace 5.7. R5 software to analyze the literature, with node size reflecting the frequency of citations [40,41]. The keywords of the co-citation network are then clustered since phrases composed of groups of words are more representative than words [42,43]. Figure 1 outlines the framework of the present study. Prior research has demonstrated the efficacy of co-citation analysis in revealing the intellectual structure and research frontiers within a given discipline [38,44]. For example, Small et al. [44] employed co-citation analysis to map the intellectual structure of information science, thereby identifying new research topics and the waning relevance of old ones over time. Likewise, Chen [38] employed co-citation analysis to identify research frontiers in library and information science, finding them to be closely linked to technological advancements and their practical applications. The use of co-citation analysis as a tool for tracking research subjects and their changes over time has been well-established in previous studies. However, it is important to recognize that this logical framework is subject to error due to the temporal lag between academic disciplines and the publication of research. As a result, key works and emerging research frontiers may not be immediately discovered due to publication delays.
After using “Bibliometrix” in the first stage to derive some rough status, a visual comparison of the different directions of bio-swale research was then made using CiteSpace 5.7. R5. The following were the primary procedures for using CiteSpace (5.7. R5): (1) The author imported all 323 of the chosen documents in text format into CiteSpace. (2) The author entered the following information into CiteSpace: the period range was 1995 to 2021, and the time slice was 1 year. The themes examined in the documents included titles, abstracts, and keywords. (3) After using CiteSpace we get a hot topic about the whole bio-swales field. The previous process was then repeated to filter out the relevant studies on its different services individually for visual image generation. (4) A comparison of the knowledge network maps that were created could immediately inform us of the state of the field’s research, the overall evolution of the field in recent years, and its potential future directions.

3. Results

3.1. Status, Development Trends and Hot Topics

As of June 2022, the analysis of 323 studies on bio-swale yielded a total of 6752 citations, averaging 20.9 citations per paper. The oldest bio-swale study dates back to 1995, with only a small number of publications between 1990 and 2000. However, the number of articles grew rapidly after 2000. From 2000 to 2010, the number of published documents remained relatively stable, laying the groundwork for comprehending the LID concept and investigating research methodologies. There was a significant increase in the number of documents from 2010 to 2021, with a peak of 36 counts in 2019. The slight decline in publications in the past three years may be attributed to the impact of the global epidemic. Nonetheless, based on prior research, bio-swale has caught the attention of researchers, and more detailed and in-depth studies are anticipated. The number of citations in articles also increased every year, with the exception of a slight decline in 2020 and 2022. Both the number of publications and the number of articles cited are expected to rise gradually in the coming years, reflecting the growing interest in this topic (Figure 2).
The identification of the top fifteen countries or regions associated with the 339 papers was accomplished by utilizing the inherent functionality of the database’s “Analyse search results” feature. It is crucial to acknowledge that the inclusion of multiple authors from various countries within a single paper can lead to duplicated counts, resulting in a cumulative figure that exceeds the initial dataset of 323 articles. The top five countries with the most publications on bio-swale are the United States, China, Australia, Germany, and the United Kingdom, with 124, 49, 30, 21, and 16 sources, respectively (Figure 3). This data indicates that research attention is related to national policy. LID is a specific technology in urban drainage developed in the United States in the 1990s to address flood disasters and urban non-point source pollution. The United States has the closest co-operation with other countries in terms of national cooperation. After nearly 20 years of development, bio-swale has become a widely adopted urban green rainwater infrastructure technology in the United States and many developed countries. In 2014, China began implementing the engineering construction practice of low-impact rainwater system development in sponge cities with the financial support of the state.
Based on the data presented in Figure 4, six of the most productive authors in the field of bio-swale research were identified. These authors, including Davis AP, Jamil E., Kim H., Stagge JH., Deletic A., and Hunt WF, were found to have made significant contributions to the development of the field. In addition, a number of other scholars in the domain were also noted to have played a critical role in advancing bio-swale research. In Figure 4, the most cited documents were listed based on the number of citations. Of these sources, Deletic A’s literature was the most co-cited, with a total of 51 citations. Notably, Deletic A’s work, published in 2006, was found to be particularly influential in shaping the discourse surrounding bio-swale research. The median citation count stands at 32.5, markedly lower than the citation numbers for the three most cited articles. This conspicuous discrepancy underscores the academic community’s acknowledgment of the seminal contributions by Delete A., Stagge JH., and Davis AP (Figure 4). Their work has evidently resonated profoundly within the scholarly milieu, engendering significant academic discourse and thereby accruing a high volume of citations.

3.2. Research on Regulating Services

3.2.1. Hydrology

After a rigorous screening process, 135 hydrological research papers were identified. The majority of authors hailed from the United States (142), followed by China (33), Australia (23), Brazil (20), Germany (16), Sweden (13), and the UK (13), accounting for 44, 10, 7, 6, 5, 4, and 4% of the total literature across all countries, respectively. Most countries began focusing on hydrological research after 2000. The co-occurrence analysis provided a comprehensive understanding of the field’s development trends by identifying emerging research hotspots and analyzing ongoing studies [45].
The knowledge network of co-occurring keywords comprised 373 nodes and 1613 connections, as shown in Figure 4. Excluding the words related to conceptual definitions, popular keywords in the field included “removal”, “pipe system”, “infiltration device”, “climate change”, “cold climate” and “best management practice runoff”. The connecting lines in Figure 5 underscored the intricate inter-relationships among the keywords, indicating the complexity of their effects. Moreover, the citation burst analysis revealed shifts in hotspots and new trends in specific research areas [46].
Figure 5 depicts the co-occurrence clusters of keywords in bio-swale research, which encompass diverse subjects classified by definitions, influencing factors, research methodologies (such as model calculation and factorial design), and effects. This wide-ranging array of keyword clusters can serve as a valuable resource for exploring the trends in this field’s evolution. Keywords, in conjunction with their respective clusters, represent the forefront of the research domain. Analyzing the temporal dynamics and frequency of keywords can assist scholars in identifying crucial research hotspots. The literature on hydrological research, spanning from 2000 to 2021, can be classified into three principal hotspots, namely (1) factors affecting hydrologic performance, (2) climate, and (3) modeling. In the following sections, a concise summary of these three hotspots is provided.
(1)
Influencing factors
Numerous studies have shown that swales are hydrologically effective at reducing runoff volumes, particularly during small storms [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 [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 [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 [53], soil characteristics [33,58], channel roughness, grass height and density [47,59], infiltration [48,51,53], compaction of the swale bed during construction [60,61], and maintenance [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 [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 [63].
(2)
Climate
Zhou [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 [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 [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 [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 [68]. Therefore, bio-swales in cold regions must be designed to carry out two additional tasks: roadside snow storage and meltwater control [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 [62].
(3)
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 [13,72]. 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 [36].

3.2.2. Water Quality

The investigation of bio-swales in relation to water quality has garnered considerable attention from scholars and policymakers worldwide. The United States, China, France, Australia, and the UK stand out as the countries with the most publications on this subject, comprising roughly 54.6, 13.1, 5.8, 4.2, and 4.2% of all sources examined, respectively. The majority of articles focused on bio-swales are found within the country of the author’s affiliations. Although water quality research began later than hydrology, the two fields share several co-occurring keywords, as the subsequent literature considers water quality alongside hydrology. Since 2005, bio-swales have captured the interest of researchers and policymakers alike as a sustainable solution. The knowledge network of co-occurring keywords, depicted in Figure 6, consists of 297 nodes and 1346 connections. Within this network, the terms “pollution” and “heavy metal” stand out as prominent keywords. Two primary themes have emerged from the literature: (1) the factors influencing bio-swale efficacy and (2) soil pollution.
(1)
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 [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 [36]. Our analysis revealed that 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 [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 [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 [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 [52].
In a two-year investigation conducted in California, Barrett et al. [77] observed that pollution reductions were achieved through combinations of treatment width, slope, and vegetation cover in eight bio-swales. The study identified that treatment widths of 4, 5, and 9 m, respectively, were required for bio-swales with slopes of less than 10%, 10 to 35%, and 35 to 50% to achieve irreducible minimum concentrations for constituents impacted by vegetated treatment, provided that over 80% vegetation coverage was maintained. These findings underline the importance of carefully selecting treatment dimensions and vegetation coverage to ensure effective pollutant removal in bio-swales [76]. However, it is important to note that optimizing swale performance is not simply a matter of pursuing the gentlest possible slope. A comprehensive study conducted by Winston et al. [78] examined a broad spectrum of longitudinal slopes, ranging from 0.5 to 10%, and catchment areas, spanning 0.1 to 0.3 hectares (equivalent to 0.25 to 0.75 acres). This range closely mirrors the conditions prevalent in real-world urban settings. Their findings indicated that a swale length of 30 m could consistently achieve a significant reduction in silt content, exceeding 50%. Yet, their research also highlighted the diminishing returns associated with further extending the swale length. For instance, a study by Mohamed et al. [79] demonstrated that doubling the swale length to 60 m provided only a marginal gain in sediment reduction, a mere 10%. This raises questions about the cost-effectiveness of such a design modification, underscoring the need for strategic and efficient design decisions when implementing bio-swale systems in urban settings.
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 [47,77]. Yu et al. [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 [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 [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 [73]. Conversely, Barrett et al. [77] found rapid increases in effective solids removal with increased vegetation density above 80% coverage. Dense vegetation enhances the functions of filter strips and swales by increasing the storage, roughness, and blockage in the system, enabling plants to absorb pollutants and facilitating the precipitation of suspended pollutants [47,52,81].
(2)
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 [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 [33,51]. By promoting stormwater infiltration in swale channels, incoming pollutants are immobilized in swale channels or soils, thus reducing the conveyed pollution [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 [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 [62]. Little is currently known about the efficacy of bio-swales in treating bacteria. Filtration, soil adsorption, desiccation, and predation are some of the most common methods for removing bacteria [84], and bio-swales promote some of these pollutant removal mechanisms. Supporting the idea that bio-swales may be the optimal swale form when receiving waters that are contaminated by pathogens, Purvis et al. (2008) [85] found that a bioswale eliminated over 55% of fecal indicator bacteria from stormwater runoff. Additionally, bio-swales provide other benefits, such as reducing the pesticides and hydrocarbons that can be harmful to aquatic life in receiving waterways [86]. Organic contaminants, like polycyclic aromatic hydrocarbons, can be effectively removed from wet swales, similar to dry swales [87].

3.2.3. Biodiversity

Despite the established link between biodiversity and human health, limited research has been conducted on the impact of biodiversity on swales. In comparison to studies on hydrology and water quality, the literature shows a scarcity of research on grass swales’ biodiversity control. The countries that have contributed the most to this topic are the United States (16), China (7), Chile (7), and Germany (7), respectively. While Chinese researchers have been relatively late in this field, the ecological significance of bio-swales has attracted increasing attention from academics. In general, there is less collaboration between authors, institutions, and countries in this area of research. The co-occurring keyword network is illustrated in Figure 7, revealing 294 connections among its 88 nodes. Broadly speaking, the research is divided into macro and micro levels.
(1)
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 [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 [91]. Comprehending the complex interplay between plants, soil, and microbes in bio-swales is critical for improving their efficacy in mitigating stormwater runoff and minimizing the adverse effects of pollutants on urban ecosystems. Additional research is required to fully understand these relationships and develop more effective methods for managing bio-swale systems.
(2)
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 [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 [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 [80]. From a macro-ecological perspective, the adoption of bio-swamps in urban areas could have a positive impact on urban biodiversity. By improving water quality, providing green spaces, and mitigating the impacts of climate change, bio-swales could enhance urban biodiversity and contribute to the overall health of urban ecosystems.

3.3. Future Research Perspectives

Urban areas face numerous environmental challenges, including increasing stormwater runoff and declining water quality. Bio-swales, which are shallow, vegetated ditches that help manage stormwater, have emerged as a promising solution. However, to fully understand their potential as multifunctional green infrastructure, there is a need for additional research that explores how bio-swales can be optimized and integrated with other green infrastructure technologies. Despite previous research demonstrating their ability to provide various ecosystem services such as reducing stormwater runoff, enhancing water quality, and promoting biodiversity, additional research is needed to examine their potential to mitigate air pollution, reduce noise, and improve public health and well-being. Addressing these knowledge gaps is critical to maximizing the ecological and socioeconomic benefits of bio-swales and further advancing green infrastructure in urban areas.
Within the framework of the proven LID multi-objective optimization approach and multi-stage planning scenarios that help to maintain a balance between economy, reliability, and resilience throughout the life cycle of development [94], effective management of bio-swales is crucial for their long-term viability and ability to provide multiple ecosystem services in urban areas. Therefore, future research should focus on developing strategies to optimize their maintenance and management. This includes investigating the most effective management practices, such as the frequency of mowing, pruning, and weed control, to promote the ecological and socio-economic benefits of bio-swales. Moreover, exploring innovative technologies like remote sensing, drones, and machine learning algorithms could enhance the monitoring and evaluation of bio-swale performance, allowing for more precise and efficient management decisions. Such research endeavors could help urban planners and stakeholders to make informed decisions about bio-swale management, leading to improved environmental conditions and increased community well-being.
In addition to providing ecological benefits, bio-swales have significant potential to offer socio-economic advantages in urban areas. These benefits include increased property values, community cohesion, and educational opportunities. Therefore, it is imperative to explore the social and economic benefits of bio-swales, including urban residents’ attitudes and perceptions towards their role in enhancing urban resilience. Furthermore, bio-swales may offer a practical and environmentally friendly approach to address the challenges posed by climate change. Future research should investigate their capacity to sequester carbon, mitigate the urban heat island effect, and improve the resilience of urban communities to extreme weather events. Such studies could provide insights into how bio-swales can contribute to urban sustainability and offer new opportunities for green infrastructure development. By addressing these knowledge gaps, urban planners and policymakers can make informed decisions about the role of bio-swales in urban environments.

4. Conclusions

Our literature review of bio-swales indicates that this technology holds significant potential for providing ecosystem services in urban environments. Specifically, 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. For instance, interdisciplinary approaches are necessary to fully comprehend the social, economic, ecological, and engineering aspects of bio-swales. Moreover, specific experimental data and research on the synergies between bio-swales and other green infrastructure technologies are required. Additionally, biodiversity in bio-swales has been understudied, highlighting the need for diverse research methods to measure the application of ecosystem service benefits to bio-swale design features.

Author Contributions

Conceptualization, M.W. and J.S.; methodology, M.W.; software, T.C.; validation, J.L.; formal analysis, T.C.; investigation, T.C.; resources, M.W., J.S. and J.L.; data curation, T.C.; writing—original draft preparation, T.C.; visualization, T.C.; supervision, M.W., J.S. and J.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Natural Science Foundation of Guangdong Province, China [grant number 2023A1515030158], and the Science and Technology Program of Guangzhou, China [grant number 202201010431].

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

The study did not involve humans.

Data Availability Statement

The study did not report any publicly archived datasets.

Conflicts of Interest

Authors declare neither conflict of interest nor competing interest.

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Sustainability 15 08141 g001 550
Figure 1. Framework.
Figure 1. Framework.
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Figure 2. Number of published studies and cited frequency on bio-swales and change-points from 1995 to 2022.
Figure 2. Number of published studies and cited frequency on bio-swales and change-points from 1995 to 2022.
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Figure 3. (a) Countries’ productions and (b) country collaboration map.
Figure 3. (a) Countries’ productions and (b) country collaboration map.
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Figure 4. (a) Most locally cited authors and (b) most locally cited documents.
Figure 4. (a) Most locally cited authors and (b) most locally cited documents.
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Figure 5. (a) Keywords regarding bio-swales associated with hydrology and a (b) literature co-occurrence network of bio-swale research on hydrology hotspots between 1995 and 2022.
Figure 5. (a) Keywords regarding bio-swales associated with hydrology and a (b) literature co-occurrence network of bio-swale research on hydrology hotspots between 1995 and 2022.
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Figure 6. (a) Keywords in bio-swales associated with water quality and (b) a literature co-occurrence network of BS research on water quality hotspots between 1995 and 2022.
Figure 6. (a) Keywords in bio-swales associated with water quality and (b) a literature co-occurrence network of BS research on water quality hotspots between 1995 and 2022.
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Figure 7. (a) Keywords regarding bio-swales associated with biodiversity and (b) a literature co-occurrence network of BS research on biodiversity hotspots between 1995 and 2022.
Figure 7. (a) Keywords regarding bio-swales associated with biodiversity and (b) a literature co-occurrence network of BS research on biodiversity hotspots between 1995 and 2022.
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Chen, T.; Wang, M.; Su, J.; Li, J. Unlocking the Positive Impact of Bio-Swales on Hydrology, Water Quality, and Biodiversity: A Bibliometric Review. Sustainability 2023, 15, 8141. https://doi.org/10.3390/su15108141

AMA Style

Chen T, Wang M, Su J, Li J. Unlocking the Positive Impact of Bio-Swales on Hydrology, Water Quality, and Biodiversity: A Bibliometric Review. Sustainability. 2023; 15(10):8141. https://doi.org/10.3390/su15108141

Chicago/Turabian Style

Chen, Tong, Mo Wang, Jin Su, and Jianjun Li. 2023. "Unlocking the Positive Impact of Bio-Swales on Hydrology, Water Quality, and Biodiversity: A Bibliometric Review" Sustainability 15, no. 10: 8141. https://doi.org/10.3390/su15108141

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