NEWS

The Earthmover & Civil Contractor

September 2005

Treating stormwater with dignity: a case study

The University of South Australia sold its Underdale campus last year, ending a 28-year presence in the western suburbs of Adelaide. The sale, which raised more than $30m, will help pay for Blueprint 2005, a $135m capital development program (Earthmover, February 2004).

DEVELOPER Urban Pacific Ltd is turning the 13 ha site into Promenade, a housing estate of 145 allotments with prices starting in the high $200,000s. Priority has been given to reducing the amount and frequency of stormwater discharging from the site into the River Torrens that runs alongside.

A soakage trench 150m long, 3m wide and 2.5m deep, was installed for the first stage of the project. It has the capacity to filter out pollutants and to retain 150,000 litres of runoff from 45 properties and 400m of new road, allowing the filtered stormwater to be slowly absorbed into the surrounding ground.

Plastic Crates

The soakage trench is constructed of 1500 AUSDRAIN EnviroModules made from recycled polypropylene placed together. The modules sit on a bed of coarse river sand over which is laid Geotex 401 filter fabric which is a non-woven PP needle-punched, heat-set geotextile. Once in position, the modules are totally encased in the geotextile and backfilled with more sand. The combination of sand and geotextile forms a filter membrane that traps particulate containing nutrients and heavy metal contaminants while allowing the stormwater to travel through.

The system is designed to treat a storm of magnitude up to one-in-three-months, according to civil engineer James Pannell. Rainfall events of up to this magnitude account for 95% of total urban stormwater runoff, so the filter system will effectively clean and dispose of the majority of stormwater generated from the development, reducing the pollutant load into the River Torrens.

Pannell says it is a cost-effective way of retaining and infiltrating stormwater on site and a joint initiative between West Torrens City Council and Urban Pacific. “What we have done is to divert small flows into this device until it fills up,” he adds. “When the capacity of the trench is exceeded, the stormwater continues on by pipe to the river.

“You get the best benefits in sandy soils, which you don’t often find in Adelaide except along the coast. In this case we are near the river and the soils are quite loamy with areas of sand.”

Civil works next month

Civil works for the second and final stage is anticipated to start in October. This stage will use space next to the river so stormwater can be piped from the roadways into vegetated swales and from there into a similar bio-retention device. This will comprise plastic modules, also in a deep trench, and a sand filtration system so that only clean water comes out and discharges into the river.

Houses built in Promenade will have five-star energy rating, rainwater tanks – they become mandatory in SA for new houses from July 2006 – and all major stormwater drain discharges on the estate will have gross-pollutant traps.

Torrens Catchment Waste Management Board project manager is pleased West Torrens City Council and Urban Pacific “have embraced the concept of water sensitive urban design. The board has always been a strong advocate that new land development should not have an adverse impact on catchment water quality and should treat stormwater with the dignity it deserves as a precious resource.”