Treatment MeasuresSwales : Maintenance & Costs
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The most important maintenance consideration is to preserve the dense, healthy vegetation cover of a filter strip or swale. Maintenance activities can include watering, reseeding, weeding or fertilising the area, similar to that done for standard grassed nature strips. Regular inspections are required to assess the vegetation cover, the presence of any channelisation and weed problems. Installation and maintenance costs for grass swales are low; and typically lower than kerbs or underground pipes.
US EPA Stormwater Technology Fact Sheet 'Vegetated Swales' quotes figures for a 4.5 metre wide swale with a depth of 0.5 metres;
Installation~ A$30 to 57 per linear metre.
Maintenance ~ A$3.70 per linear metre.
The maintenance figure above is based on 8 mowing cycles per annum, plus 4 inspections and general revegetation work (estimated at 1% of swale area).
Case Study: Kinfauns Estate - Hastings
(Source Lloyd, S. et al 2002; CRC for Catchment Hydrology: Industry Report)
Vegetated swale conveying road runoff.
This swale was constructed covering and area of 2700m² at a total cost of installation of $41,750. This translates to about $15.50 per square metre.
The maintenance of swales is rough at best to economically quantify. The installation of swales has a scheduled regime of maintenance in order to have the swale operating at maximum efficiency. The following information is provided as a case study into the maintenance regime of a typical swale over a six-year time span.
(Diagram source: Maintenance records supplied by Indigenous Gardens Pty Ltd, 2002)
First three years (after installation)
- Catchment development phase - significantly more weeding due to planted vegetation trying to compete with invasive weeds
- More litter due to housing construction in adjacent properties
Last three years
- Developed Catchment: maintenance costs are significantly reduced. It is not clear if maintenance costs will be reduced further over time.
- Based on the above figures; maintenance of the system over the last three years places the cost at $2.35 per m²/yr. If the cost is based on last years (6th yr) maintenance cost is reduced to $1.55 per m²/yr.
Maintenance costs for grassed swales (based on figures provided by VicRoads) is $2.50 per m²/yr. Mowing can be a costly exercise given the frequency of grass cutting per year. Appropriate selection of grass or vegetation for swales will provide more effective management costs.