Australia adopts sensitive stormwater reuse
Suburban expansion in Melbourne is putting added pressure on water supplies and watercourses. Phil Charlton, technical director of UK consultancy Hyder in Australia, reports on an environmentally sensitive stormwater reuse project, born of a joint effort between the public and private sectors, that has even managed to preserve the habitat of a rare species of butterfly.
Altona Meadows, to the west of Melbourne, Australia, is similar to many expanding city suburbs in that the demand for new houses is inevitably coupled with demand for water. Water-sensitive urban design (WSUD) has been promoted by Melbourne Water for some time and at the heart of a recently completed project in Altona is a stormwater reuse system that latest estimates show will save around 2.5GL/y of potable water.
Built adjacent to a local school, the development comprises around three hectares of new private housing and over five hectares of sports and recreational facilities for use by the school and the community. As a joint commitment, Hobsons Bay City Council (HBCC); Altona Green Primary School; the developer, Rogers Milne & Associates; and the consultant, Hyder Consulting set out to adopt WSUD best practice.
The system is already demonstrating reductions in pollutants within receiving drainage watercourses too (see Table 1). It consists of a system of swale drains that collects and filters the stormwater runoff from residential, educational and recreational zones into a 400,000L underground cellular tank for reuse in the irrigation system of the two sporting ovals (see Figure 1).
Swales are grassland depressions, which lead water overland from the drained surface to a storage or discharge system, providing temporary storage for stormwater and reducing peak flows. In this case, the swale system acts as a retention system to minimise impact on the existing drainage system which outfalls to Port Phillip Bay.
The stormwater system collects the water from the residential lots at the legal point of stormwater discharge and directs it via underground drains to the filter system beneath the swale drain in the park. The main road has a one-way cross-fall towards the park that allows collection of the stormwater in the channel on the low side of the road and discharges the water into grassed swale drains.
The two grassed sporting ovals are also graded towards the grassed swales, they are situated on clay, which does not allow the irrigation water to penetrate deep into the ground but only within the root zone. Excess surface water from irrigation and rainfall will flow off the surface and into the swale drains for collection.
Therefore, all stormwater that is deposited on the subdivision and parkland finally enters the slotted underground drain below the grass swales. The water is then pumped from the underground drain to a 400,000L underground cellular storage tank for reuse in the irrigation system servicing the two sports fields.
The grassed swales also allow short-term ponding of surface water runoff from rainfall events. The choice of a high permeability soil infiltration medium in the under-swale collection drains ensures that ponding times are kept to a minimum.
Design also ensures that water depths are only of the order of 200-300mm at most during the more frequent storm event (5-year Average Recurrence Interval design storm). Keeping velocities to a minimum along the swale drains during the major 100-year storm event, together with the installation of grates on all inlets, ensures that a safe environment is maintained at all times.
The system is provided with overflow protection in the event of failure so that excess stormwater will discharge along the roads in accordance with conventional drainage under surcharge conditions. A major constraint within the site was its inherent flatness.
As a result the swale drains could not be designed to drain on one continuous grade without individual segments grading to local low points. Overall the system allows for the 1-in-100 year flow to drain longitudinally along the swales.
Elimination of grated entry pits, with stormwater discharging either directly from roof drainage or by chutes into the open swale drain, means that gross pollutants such as cans, paper and organic material from gardens are deposited on the grassed areas rather than collected in the underground drainage system. The visual impact of rubbish on open grassed areas is a further incentive for residents to monitor and limit accidental deposits of such gross pollutants.
As well as all the environmental benefits, there are a number of positive economic outcomes. Using the proposed sporting fields to attenuate stormwater discharge and removing pollutants with the bioretention filter enabled an exemption to be given to the normal drainage contribution charges.
This resulted in significant savings on the total development costs, which were in turn passed on to potential property owners and to the broader community. Altona Green Park has certainly proved that urbanisation and environmental gain can be compatible with real outcomes.
Ray Baker of HBCC said, “Collectively, we recognised that this would be environmentally positive, would provide an excellent opportunity to allow the public and local industry to see theory in action, was likely to provide the best return on allotment sales, and would reduce long-term maintenance costs”.
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