Swales are linear, depressed channels that collect and transfer stormwater. They can be lined with grass or more densely vegetated and landscaped.
Treatments and contaminants
Primary and secondary treatment includes:
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physical screening of sedimentation (coarse and fine)
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infiltration of stormwater into soils
Typical retained contaminants include:
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gross pollutants
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coarse sediments
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some nutrient removal
How swales work
Swales can convey stormwater and screen and remove gross pollutants, such as litter and coarse sediment.
Swales initially immobilise pollutants by binding them to organic matter and soil particles, then remove them by settling, filtration and infiltration into the subsoil.
Certain pollutants, such as hydrocarbons, may be digested and processed by soil microorganisms in the filter strip. To optimise pollutant removal, swales need adequate contact time between the run-off and the vegetation and soil surface.
When to use swales
In urban areas, swales may be used as an alternative to:
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conventional street nature strips
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centre median strips of roads
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run-off collection points in car park areas
In rural areas with enough space and slope, swales may be used with the sealing of rural roads to reduce negative impacts from increased stormwater run-off.
Advantages and limitations
Advantages of swales are that they:
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reduce and delay stormwater run-off
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retain particulate pollutants close to source
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are more aesthetically appealing than kerb and gutter
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are relatively inexpensive to build
Limitations of swales are that they:
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have limited removal of fine sediment and dissolved pollutants
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use more land area than kerb and gutter, and restrict certain activities like car parking
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require a sunny aspect for plant growth, which limits their application in shaded areas
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are only suitable for gentle slopes of less than 5% gradient
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require regular inspections
Tips and advice
Infiltration and treatment performance can be improved by:
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elevating the outlet slightly to facilitate detention and infiltration
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using of gentle slopes
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areas of slight depression
Check if dams can be used to retard and slow the flow along longer sloping sections.
For more information on the design of swales, refer to Chapter 8 'Swales and buffer strips' in the WSUD Engineering Procedures available for purchase from CSIRO Publishing.
WSUD Engineering Procedures - CSIRO Publishing