Below ground, stormwater system raises bar
A GIANT 1.6 million-litre underground flood mitigation system is going in at the 200lot Wandana estate.
Villawood Properties and SPEL Stormwater are working on the project, one of three major stormwater systems on the estate.
The underground system involves the “three Es”: engineering, environment and education.
The first “E” is a truckload of structural engineering calculations, along with truckloads of elliptical, ribbed, arched tunnel sections, soil spoil, ballast, geo-fabric, mesh and concrete pits all bound for a subterranean address, that have gone into the project. The second E is all about safeguarding the environment from washaway rubbish — via sophisticated baffle-box filter cages.
The third is about education by “daylighting” processes normally consigned below a heavy manhole cover via a mesh steel viewer.
SPEL Stormwater senior tech consultant Kurt Jensen said the 5m deep system involved construction of 266 arches in eight 82m rows.
“You’ve got a sandwich of layers — geofabric, polymer liner and geofabric again — then a 250m layer of big rock through the bottom, rocks like rail ballast,” he said. “Then the arches go in with 300m rock over top.
“There’s been a lot of engineering gone into how they’re spaced apart to get the load bearing right. It is radical engineering.”
Gaps between the rocks, a 20mm-40mm mix, store water — up to 40 per cent of the volume they fill.
Combined, it makes for a meshed, fibred and stony, catacomb storage with a capacity to stagger the release of water for anything up to a one-in-100-year storm’s violent hydraulic excesses.
As it is all underground, the top of the system will be parkland.
The other stormwater systems at the Wandana estate involve more than 2km of gabion rock wall, lining several wetland retarding basins and a compact, highly engineered bioretention system with 200 cubic metres of underground storage, gross pollutant capture, and a 50sq m bioplanter to treat the water.