Bioswales/ swales

Bioswale in Portland, USA. Photo by NACTO.

Bioswales can be in urban areas, specifically near roads to filter polluted runoff and absorb carbon. They are most efficient in dense urban areas where there is green canopy and pervious surfaces.

Bioswales are landscape features designed to manage stormwater runoff and improve water quality in urban areas. They are typically shallow, vegetated channels or depressions filled with native plants, gravel, and soil to capture, slow, and filter stormwater. They emulate wetlands to a certain extent.

There are three different types of bioswale systems: vegetated, mulched and xeriscape. Bioswales are an effective and sustainable stormwater management solution that integrates natural processes into urban infrastructure, promoting water conservation, biodiversity, and environmental resilience.

Name of NbS

Bioswales/ swales

Type of NbS

Hybrid living/ engineered intervention

Location

  • Urban
  • periurban
  • rural

Case Study

Long Bay Residential streets

Long Bay residential development swales. Photo from Auckland Design Manual (2024)

Relationship to Indigenous knowledge

Water is a vital element in many Te Moananui Oceania cultures, and is regarded as having a living essence. This impacts how stormwater should be managed in the region. Stormwater discharge deposited directly into water without being filtering it through the earth does not follow Māori values in Aotearoa New Zealand for example (Simcock, 2017). Bioswales capture runoff water and filter it through the earth rather than directly into oceans or rivers. 

Bioretention / bioswale in median of Grange Avenue in Greendale, Wisconsin, 2010. Photo by Aaron Volkening.
Bioswale. Photo by Guzzardo Partnership.
Climate change benefits
  • Biomass cover loss
  • Changes in rainfall
  • Increased temperatures
  • Reduced air quality
  • Sea level rise
  • Urban heat island effect.

Bioswales are designed to handle the first stream of water and therefore run-off pollutants when it rains. They can be sized and designed to be adaptable to climate change with changes in rainfall. Bioswales sequester carbon through vegetation (Chikeluba, 2022). 

Bioswales act as a rainwater catchment which can be a technique to mitigate the impacts of sea level rise (Newman & Qiao, 2022).

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Societal / socio-cultural benefits
  • Disaster risk reduction
  • Waste management and sanitation
  • Water security.

The increased population density in urban areas has exerted significant pressures, particularly on the biodiversity of plants and animals, water security, and vulnerability to extreme disasters. Bioswales mitigate local flooding and mitigate the impacts of storm events occurring at medium or frequent intervals (Kabisch et.al, 2017). 

Designating green spaces within cities provides opportunities to conserve biodiversity, benefiting human populations through improved stormwater management, urban cooling, and air purification (Filazzola, 2019). Additionally, bioswales contribute to the visual appeal of streetscapes and public areas, augmenting the overall quality of urban life.

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Ecological and biodiversity benefits
  • Climate regulation
  • Disturbance prevention
  • Habitat provision
  • Purification

Bioswales have capacity to not only absorb water and filter pollutants but also to create vital habitats for birds and insects. These features play a pivotal role in bolstering biodiversity and providing urban wildlife with essential habitats, especially as linear connectors between habitat areas. The incorporation of native plant species into bioswales contributes further to biodiversity preservation and conservation efforts in urban settings.

The design of bioswales is geared towards facilitating the natural filtration of pollutants and sediments from surface runoff, thus diminishing the volume of contaminants entering waterways. This process not only aids in replenishing groundwater reserves but also helps mitigate flooding risks by effectively absorbing excess water during intense rainfall events. Bioswales efficiently capture and isolate a spectrum of pollutants, including sediments, nutrients, metals, synthetic organics, pathogens, and hydrocarbons (Mazer et al. 2001).

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Technical requirements

The dimensions including the length and width and shape of bioswales can be determined by the amount of rainfall predicted in a certain area and site specific climate (Chikeluba, 2022).

Bioswales perform best when placed next to roadsides as a strip of either vegeated or xeriscaped landscape to capture runoff from the road. Slopes within the swale are required to be under 5% to achieve maximum filtration. Vegetation is used to slow down the water. The more dense the vegetation, the more effective it is at slowing down the water and preventing erosion. Swales need to be maintained to avoid too much vegetation from growing and preventing the movement of water however.

Issues and Barriers

When compared to wetlands, studies of bioswales have shown that they tend to have higher abundance of mosquitos (Filazzola, 2019). Being such a small-scale green infrastructure system, bioswales also have less benefit when it comes to large-scale catastrophic rain events (Kabisch et.al, 2017). In events such as river flooding, seaside flooding or cloud bursts larger scale systems like wetlands will be more effective.

Opportunities

In Oceania, bioswales present a valuable opportunity for sustainable stormwater management. Due to their small-scale nature, bioswales are ideally suited for locations where the volume of runoff is not excessively high. 

By connecting bioswales with other strategies such as constructed wetlands, pervious surfaces, and sponge city techniques as part of a larger urban water management system,  they can effectively contribute to filtrating pollutants and reducing flooding risk. This interconnected approach optimises stormwater treatment efficiency while minimising environmental impacts. 

In Te Moananui Oceania’s diverse ecosystems, bioswales can be tailored to incorporate native plant species, enhancing their effectiveness in preserving local biodiversity and ecosystem health. Overall, the strategic implementation of bioswales offers a sustainable and adaptable solution for mitigating stormwater pollution and enhancing urban water management practices.

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Financial case

Articles have suggested that bioswales are cost-effective in terms of flood containment capacity, and reduction of pollutant outflow off-site when compared to other green infrastructure like wetlands which are more expensive and harder to implement and maintain (Grove et.al, 1999).

Bioswale diagram. Image by The Constructor.
References
  • Simcock, R. (2017). Water sensitive Design in Auckland, New Zealand. In Susanne M. Charlesworth, Colin A. Booth (Eds.). Sustainable surface water management: A handbook for SuDS (380 – 392). https://doi.org/10.1002/9781118897690.ch28
  • Filazzola, A. (2019). The contribution of constructed green infrastructure to urban biodiversity: A synthesis and meta-analysis. Journal of Applied Ecology, 56 (9), 2131 – 2143.  https://doi.org/10.1111/1365-2664.13475
  • Mazer, G. (2001). Limitations to vegetation establishment and growth in biofiltration swales. Ecological engineering, 17 (4), 429 – 443). https://doi.org/10.1016/S0925-8574(00)00173-7
  • Chikeluba, I.J. (2022) Urban green infrastructure and its effects on climate change – a review. Available at SSRN 4190459. http://dx.doi.org/10.2139/ssrn.4190459
  • Newman, G.D & Qiao, Z. (2022) Landscape Architecture for Sea Level Rise: Innovative Global Solutions. Routledge. doi: 10.4324/9781003183419
  • Kabisch, N., Korn, H., Stadler, J., & Bonn, A. (2017). Nature-based solutions to climate change adaptation in urban areas: Linkages between science, policy and practice. Springer Nature.
  • Groves, W. W., & Hammer, P. E. (1999). Analysis of bioswale efficiency for treating surface runoff (Doctoral dissertation), University of California, Santa Barbara.

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