Bio-retention systems / rainwater gardens

Bioretention / bioswale in median of Grange Avenue in Greendale, Wisconsin, 2010. Photo by Aaron Volkening.

Bioretention systems are a sustainable stormwater management strategy designed to mimic natural wetland processes and improve water quality. The key function of bioretention systems is to remove pollutants from stormwater. They achieve this by filtering the stormwater through a densely vegetated and typically, a biologically active sand and loam filter, sometimes incorporating biochar (Premarathna et al., 2023). Rainwater gardens, biofilters, bio-swales, tree pits, and other kinds of nature-based stormwater management can be thought of as types of bioretention systems. Originally introduced to treat highly polluted urban ‘first flush’ stormwater, and often placed along roads, bioretention systems can be integrated into larger systems like wetlands or serve as standalone treatment solutions depending on the environment (Osheen & Singh, 2019). See also sponge cities.

A bioretention system is typically a shallow depression or trench filled with a specially engineered soil mix, which promotes filtration and infiltration of stormwater. Native or adapted vegetation, such as grasses, shrubs, and trees, are planted within the bioretention area to enhance pollutant removal through processes like uptake and evapotranspiration. 

When rainfall occurs, runoff is directed into the bioretention area, where it is temporarily stored and treated by the soil and vegetation. The soil media acts as a filter, removing pollutants such as sediment, heavy metals, nutrients, and pathogens from the stormwater. Vegetation helps to stabilise the soil, increase infiltration rates, and provide habitat for wildlife.

As stormwater infiltrates into the soil, excess water is stored temporarily before slowly percolating into the groundwater or being directed into nearby water bodies, reducing the volume and peak flow of stormwater runoff. This process helps to recharge groundwater aquifers, prevent erosion, and minimise downstream flooding.

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Name of NbS

Bio-retention systems / rainwater gardens

Type of NbS

Hybrid living / engineered interventions

Location

Urban

Bioretention system in Milwaukee, Wisconsin, 2013. Photo by Aaron Volkening.

Relationship to Indigenous knowledge

Incorporating Indigenous knowledge into the design, implementation, and management of bioretention systems can enhance their effectiveness and in Te Moananui Oceania. Indigenous communities often have a deep understanding of local ecosystems, including native plant species, soil types, hydrological patterns, and seasonal cycles. This knowledge can inform the selection of appropriate vegetation and soil mixes for bioretention systems, ensuring they are well-suited to local conditions and capable of thriving in the long term.

Stormwater management techniques have been practiced for centuries in Te Moananui Oceania. Traditional methods, such as rainwater harvesting, terracing, and erosion control measures, can complement modern bioretention practices, providing holistic solutions that blend Indigenous wisdom with contemporary technology.

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Climate change benefits
  • Changes in rainfall
  • Flooding
  • Increased temperatures
  • Reduced water quality
  • Reduced air quality
  • Urban heat island effect
  • Reduced freshwater availability

Urban bioretention systems, if strategically placed in urban contexts, reduce flooding (Shafique, 2017). Vegetation in bioretention systems contributes to reducing temperatures and mitigating the urban heat island effect. The presence of vegetation improves air quality by filtering pollutants from the air (Biswal, 2021).

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Societal / socio-cultural benefits
  • Disaster risk reduction
  • Water security and quality
  • Waste management and hygiene

Bioretention systems offer a range of socio-cultural benefits including opportunities for community engagement, as residents can participate in planting and maintenance activities, fostering a sense of ownership and pride in local green spaces. 

Bioretention sites also serve as outdoor classrooms, providing educational opportunities for people of all ages to learn about water conservation, biodiversity, and environmental stewardship. By incorporating native vegetation and design elements inspired by local culture and heritage, bioretention systems help preserve Indigenous knowledge, promoting cultural diversity and celebrating local identity. 

Well-designed and maintained bioretention areas enhance the aesthetic appeal of urban landscapes, creating inviting gathering spaces for recreation and relaxation. Access to nature in these areas contributes to improved public health and well-being, fostering community cohesion and resilience (Soderlund and Newman, 2015). 

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

Bioretention systems offer multifaceted benefits beyond their primary function of removing pollutants from water and reducing flooding. They contribute to improving overall ecological health by positively impacting air, water, and soil quality. Bioretention systems are a form of urban greening, providing habitats for insects and other species, and potentially creating nesting sites for birds depending on planting palettes (Biswal, 2021). Studies have demonstrated that bioretention systems host a higher diversity of wildlife species compared to conventional lawn-type green spaces (Kazemi et al., 2011).

Bioretention systems facilitate an increase in pervious surfaces, allowing water to filtrate downwards instead of creating barriers like most surface mediums in urban contexts. Despite being small-scale, bioretention systems yield significant benefits by often being one of few impervious surfaces in urban networks.

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Technical requirements

The technical requirements of bioretention systems include the incorporation of various layers designed to capture pollutants effectively. At the top, vegetation must be carefully selected for its resilience to environmental stresses. Next is often mulch, which serves to retain solid waste and prevent it from infiltrating into the underlying soil filtration media. Finally, an underdrain is often situated at the bottom to prevent water from stagnating within the system for prolonged periods (Roy-Poirier, 2010).

Maintenance of bioretention systems involves regular monitoring of vegetation health and preservation of the top surface layer to ensure optimal water filtration capacity (de Macedo, et al., 2017). Water by Design (2014) provides further technical design considerations for bioretention systems.

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Issues and Barriers

In Te Moananui Oceania, several challenges and barriers exist regarding the implementation of bioretention systems. One significant issue is the lack of awareness and understanding among decision makers about the benefits and feasibility of bioretention techniques. Limited technical expertise and knowledge about appropriate plant species and design considerations may further hinder the widespread adoption of these systems.

The high initial costs associated with installing bioretention infrastructure may pose a barrier, particularly in regions with limited financial resources. The variability of climatic conditions and extreme weather events in the region present challenges in designing bioretention systems that can effectively manage stormwater runoff in extreme events. There can be issues with mosquitoes in bioretention systems if they are not properly designed, installed, and maintained. Stagnant water pools within bioretention basins can serve as breeding grounds for mosquitoes, especially in warmer climates or during periods of heavy rainfall. However, these issues can be mitigated through proper design such as incorporating slopes to promote drainage, ensuring proper grading to prevent water pooling, and incorporating mosquito larvae predators like fish or installing mosquito control devices.

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Opportunities

As urban areas an roading networks continue to expand in Te Moananui Oceania, there is a growing opportunity to incorporate bioretention systems, particularly in linking them to larger water management systems like sponge cities.

Bioretention systems can effectively act as initial filters for pollutants using the ‘first flush’ method, intercepting runoff from roads or other sources before it reaches wetlands or larger water-sensitive urban design systems. Bioretention systems also aid in preventing pollution from reaching coastal and reef environments when positioned between roads and buildings, and the ocean. Stormwater pollution causes reef sedimentation, excessive algal growth, coral bleaching, and coral disease (Hirschman & Collins, 2010). Such strategic placement not only mitigates the impact of pollutants but also enhances the overall effectiveness of water management strategies in urban areas.

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Financial case

Bioretention systems present a strong financial case in Oceania by reducing overall costs and offering long-term savings. Combining landscaping and stormwater management, these systems streamline design and construction processes, resulting in significant cost reductions (Jones & Jha, 2009).

Their low maintenance requirements lead to ongoing operational savings compared to traditional stormwater infrastructure, and they are easier to construct from local materials. By mitigating flood risks and improving water quality, bioretention systems help prevent potential damages to infrastructure, reducing the need for costly repairs or upgrades. 

The financial benefits of bioretention systems themselves extend beyond direct cost savings, contributing to sustainable development and improved resilience and economic prospects for communities in the highly interconnected ridge to reef ecologies of Te Moananui Oceania.

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Residential Bioretention Design guide. Diagram by Palmerston North City Council
References
  • Biswal, B.K (2021). Bioretention systems for stormwater management: Recent advances and future prospects. Journal of Environmental Management292, 112766.
  • de Macedo, M. B., Rosa, A., do Lago, C. A. F., Mendiondo, E. M., & de Souza, V. C. B. (2017). Learning from the operation, pathology and maintenance of a bioretention system to optimize urban drainage practices. Journal of Environmental Management204, 454-466.
  • Hirschman, D., & Collins, K. (2010). Adapting Stormwater BMPs for Tropical Watersheds and Coral Reef Protection. Water Resources IMPACT, 12(4), 11-14.
  • Jones, D., & Jha, M. K. (2009). Green infrastructure: Assessing the benefits of bioretention over traditional stormwater management. In Environmental Science and Sustainability. 2nd World Scientific and Engineering Academic and Society (WSEAS) International Conference on Natural Hazards, Morgan State University, Baltimore, MD (pp. 134-141).
  • Kazemi, F., Beecham, S., & Gibbs, J. (2011). Streetscape biodiversity and the role of bioretention swales in an Australian urban environment. Landscape and Urban Planning101(2), 139-148.
  • Muerdter, C. P., Wong, C. K., & LeFevre, G. H. (2018). Emerging investigator series: the role of vegetation in bioretention for stormwater treatment in the built environment: pollutant removal, hydrologic function, and ancillary benefits. Environmental Science: Water Research & Technology4(5), 592-612.
  • Premarathna, K. S. D., Biswas, J. K., Kumar, M., Varjani, S., Mickan, B., Show, P. L., … & Vithanage, M. (2023). Biofilters and bioretention systems: the role of biochar in the blue-green city concept for stormwater management. Environmental Science: Water Research & Technology, 9(12), 3103-3119.
  • Roy-Poirier, A., Champagne, P., & Filion, Y. (2010). Review of bioretention system research and design: past, present, and future. Journal of Environmental Engineering136(9), 878-889.
  • Singh, K. K. (2019). Rain Garden—A Solution to Urban Flooding: A Review. Sustainable Engineering, 27-35.
  • Shafique, M. (2016). A review of the bioretention system for sustainable storm water management in urban areas. Materials and Geoenvironment63(4), 227-236.
  • Soderlund, J., & Newman, P. (2015). Biophilic architecture: a review of the rationale and outcomes. AIMS environmental science, 2(4), 950-969.
  • Water by Design (2014). Bioretention Technical Design Guidelines (Version 1.1). Healthy Waterways: Brisbane. https://waterbydesign.com.au/download/bioretention-technical-design-guidelines 

Further resources: