Treatment MeasuresSwales : Overview
Swales are linear depression of channels that provide for stormwater collection and conveyance. Swales may simply be grass-lined or more densely vegetated and/or landscaped. While swales provide for stormwater conveyance, they also lend to the screening or removal of gross pollutants, such as litter and coarse sediment, from stormwater runoff.
In urban areas, swales may be used as an alternative to the conventional street naturestrip or in central median strips of roads, through to runoff collection points in carpark areas. Hydraulically swales can reduce run-off volumes and peak flows1. Current designs involve the use of grass or other vegetation (such as rushes) to carry out this function. Examples of their current use are in road medians and verges, car park run-off areas, parks and recreation areas.
Swales initially immobilise pollutants, by binding them to organic matter and soil particles. Settling, filtration and infiltration into the subsoil achieve ultimate pollutant removal. Certain pollutants, such as hydrocarbons, may be digested and processed by the soil microorganisms in the filter strip. Consequently, adequate contact time between the run-off and the vegetation and soil surface is required to optimise pollutant removal.
Advantages of Swales
- can reduce and delay storm run-off,
- retains particulate pollutants close to source,
- more aesthetically appealing than kerb and gutter,
- Relatively inexpensive to construct.
Limitations of Swales
- limited removal of fine sediment and dissolved pollutants,
- requires larger land area than kerb and gutter, with certain activities restricted (for example, car parking),
- sunny aspect is required for plant growth, limiting its application in shaded areas,
- only suitable for gentle slopes (less than five per cent),
- Regular inspections are required.
In the case example in the diagram below, all stormwater runoff originating from the "high side" allotments of the streetscape (including roof runoff), drains to a 6 m wide swale, via a one-way crossfall road surface (standard road width of 6 m). Allotments on the "low side" of the streetscape contribute roof stormwater runoff only, this may be piped against the grade to the swale for treatment and conveyance.
Swales design purpose is to carry varying flows. The aspects of these flows are discussed in the following.
Minor Storm Flows
Minor stormwater flows shed by the "low side" roofs pass directly to the trench in the above system. Roof stormwater runoff undergoes a first flush into small "leaky wells" located in the swale, to remove any leaf litter and sediment loads. This first flush action provides protection to the integrity and longevity of the swale infiltration media.
Bores or aquifers may also be set at regular intervals along the trench. A portion of the filtered stormwater may be conveyed to these aquifers and stored beneath the site for subsequent reuse for landscape irrigation. Runoff which is not conveyed to the aquifer in small storms, and is not "lost" to soil moisture replenishment along the length of the trench, passes from the catchment as treated stormwater into a downstream receiving waterway environment, in an improved water quality state.
Major Storm Flows
Large storm flows will cause the trench to operate at capacity and the surplus to appear as surface-moving flow in the swale channel. Such flows should occur about once per year or less, depending on the design philosophy employed, and may, following extreme rainfall, fill the swale and overflow to the roadway as normal overland flows.
Although flow penetrating the "perched" filter media during large storm events will emerge from the trench as filtered flow, further downstream of the system, surface-moving flow will carry some gross pollutants (eg sediments and litter) directly from the streetscape. Pollutant concentrations in these outflows should be relatively low within these flow volumes.
There is opportunity, however, to integrate swale and infiltration systems at the streetscape level with other downstream treatment measures at the precinct or regional scale. The downstream measures include inlet zones, sediment traps, wetlands etc - such a combination can account for any "untreated" or bypassed flows. This practice is referred to as a "stormwater treatment train".
References for Further Information
- Horner R.R., Skupien J.J., Livingston E.H. & Shaver H.E., 1994, Fundamentals of Urban Runoff Management.
- Schueler T.R., Kumble P.A. & Heraty M.A., 1992, A Current Assessment of Urban Best Management Practices: Techniques for Reducing Non-Point Source Pollution in the Coastal Zone.
- Schueler T.R., 1987, Controlling Urban Runoff-A Practical Manual for Planning and Designing Urban BMPs.
- Camp Dresser & McKee, 1993, California Storm Water Best Management Practice Handbooks: Municipal.
- Ontario Ministry of Environment and Energy, 1994, Stormwater Management Practices Planning and Design Manual.
- Whelans, Halpern Glick Maunsell, Thompson Palmer and Murdoch University, 1994, Planning and Management Guidelines for Water Sensitive Urban (Residential) Design.
- Auckland Residential Council, 1992, Design Guideline Manual: Stormwater Treatment Devices.