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Bioswales

Objective: 
Sectors: 
Organisation: 
UNEP-DHI Partnership – Centre on Water and Environment
Technology group: 
Definition: 
Bioswales are landscape elements designed to remove silt and pollution from surface runoff water.
Body: 

Description

Bioswales are strips of vegetated areas that redirect and filter storm water. A typical bioswale is a long, linear strip of vegetation in an urban setting used to collect runoff water from large impermeable surfaces such as roads and parking lots. Bioswales serve a similar purpose to that of gutters. The advantage of using bioswales is that the vegetation and soil in them slows down and collects water, allowing it to infiltrate soil, in addition to filtering pollutants. The current increase in storm frequency and scale can result in sewage or other polluted water overflow, making bioswales important for climate change adaptation. Bioswales are often found parallel to long roads as they require a long and straight area to increase retention and filtration time. Bioswales’ slanted walls direct rain into the vegetated depressions. In some cases, check dams are built in the bioswale to further reduce water flow velocity.

Implementation

An analysis of local rainfall distribution and evapotranspiration patterns and existing absorption capacity of drainage systems informs bioswale siting and capacity. Soil and plants that improve infiltration capacity should be selected. Using native species helps avoid potential negative environmental consequences related to introduction of non-native species. While there are various bioswale building methods, typical construction activities would include digging a linear depression with slanted walls, adding a layer of gravel for stabilization and drainage, adding an additional layer of soil/compost mixture on top of the gravel, planting vegetation in the soil, and building check dams if necessary. Regular monitoring to ensure efficiency is necessary. Additional maintenance such as removal of non-native plant species, system redesign to improve direction of storm water into it, and cleaning to prevent clogging, may also be required. 

Technological maturity:                4-5
Initial investment:                           1-3
Operational costs:                           1-2
Implementation timeframe:         1-3

* This adaptation technology brief includes a general assessment of four dimensions relating to implementation of the technology. It represents an indicative assessment scale of 1-5 as follows:
Technological maturity: 1 - in early stages of research and development, to 5 – fully mature and widely used
Initial investment: 1 – very low cost, to 5 – very high cost investment needed to implement technology
Operational costs: 1 – very low/no cost, to 5 – very high costs of operation and maintenance
Implementation timeframe: 1 – very quick to implement and reach desired capacity, to 5 – significant time investments needed to establish and/or reach full capacity
This assessment is to be used as an indication only and is to be seen as relative to the other technologies included in this guide. More specific costs and timelines are to be identified as relevant for the specific technology and geography.

Technological Maturity

Widely used

Technology Owners

  • Communities and land owners
  • Implementing and support agencies e.g. NGO, local government

Needs Addressed

  • Conservation of water resources
  • Traditional methods of water management

Adaptation Effects

  • Increases water security and provides and additional source of water for agricultural purposes
  • Increases agricultural production and therefore food security and economic resilience
  • Reduces extent of storm water, thereby reducing flood impacts
  • Contributes to groundwater recharge
  • Reduces reliance on contaminated ground water sources

Cost

Dependent on materials used

Energy Source

Human resources to install and maintain

Ease of Maintenance

  • Requires maintenance and monitoring to ensure maximised operation is maintained
  • Maintenance includes vegetation management, removal of sediment accumulation, damage reparation and replacement of rocks

Technology Performance

  • Very effective for treating storm water when properly maintained
  • A 4m swale can reduce rainwater runoff on a road by 25 per cent

Opportunities and barriers

Opportunities:

  • Low cost technology
  • Relatively simple and quick implementation. Can be encouraged through appropriate training and awareness raising
  • Once bioswale vegetation has been established, little maintenance is required
  • Multifunctional technology: it has aesthetic/recreational value as well as functional value
  • Can benefit from collaboration between communities, designers, climate scientists and governments to ensure sustainability in design and management - should be designed in consideration of local climatic context and probable changes
  • Government incentives can encourage land owners to install bioswales on their land

Barriers:

  • Requires above ground space, which may be a barrier in densely populated areas
  • Vegetation may fall prey to disease, invasive species or insects, which may require re-establishment#Trained designers needed to design bioswales according to local context and geography
  • Requires complex engineering and design knowhow

Co-benefits, Suitability for Developing Countries

Environmental benefits

  • Reduced carbon emissions in comparison to alternative storm water treatment methods and centralised management systems
  • Limit flow of water into centralised management systems thereby saving energy
  • Ecologically sustainable and beneficial approach. Results in a higher biodiversity value than alternative solutions such as gutters. Vegetation provides a diversity of flora that serves as a habitat for fauna. 
  • Can be constructed using locally available materials
  • Removes silt, heavy metals and other pollutants from storm water. This is important in urban settings, where concentrations of pollutants are high, particularly in proximity of roads.
  • Provides water absorption and infiltration that improves recharge and helps avoid polluted water from entering groundwater.
  • Promotes evaporation and does not absorb as much heat as paved surfaces reducing the urban heat island effect* in cities.

*Urban heat island effect is when cities are significantly warmer than their surroundings due to heat produced from human activity and technologies (cars, factories, appliances etc.), and the high concentration of buildings, which absorb heat much more than e.g. vegetation.

Socio-economic benefits

  • Purifies water, which in turn decreases polluted water volume entering the drainage and sewer systems, subsequently reducing the costs of transporting and treating the water. Reduced loads on conventional storm water management systems may also help avoid extra expansion costs.
  • Adds aesthetic and recreational value, improving quality of life for local communities. It may also help reduce sound pollution, if expanded to wider areas.
  • Less costly than centralised treatment systems. Provides multiple benefits as a simple and low cost solutions. Affordable to construct and maintain.
  • Can be implemented, managed and maintained by communities
  • Requires land ownership/ management authorisation
  • Not widely used as a technology in Asia-Pacific therefore will require comprehensive awareness raising, communication, training and marketing plans as well as financial support to introduce to developing countries

Information Resources