The perfect solution to this university’s stormwater management challenges arrived with the delivery of one of the world’s largest percolation tanks, designed to send water deep into the ground.
As the lowest lying area of UNSW Sydney’s Kensington campus, the Village Green was previously an area of turf traditionally used as a cricket pitch.
However, during times of high water-flow after particularly large storms, the oval would be used for an altogether different sport: kayaking. “The area collects water from about 70 per cent of the campus,” says Russell Druce, Hydraulic Engineer at UNSW Estate Management.
“The old system contained a small percolation trench, about 1300 m3, just below the pitch surface. When that tank was overwhelmed with high flows, the pitch surface would fill up — there was sometimes water coming up to the edge of the picket fence all around the oval.”
“That was actually part of the design. The oval collected water – it did the job of a detention basin. We have photos of people kayaking and using it as a water park, during some of those events.”
UNSW Estate Management commenced the project in June 2021. It would replace the 1300 m3 percolation trench with a new percolation tank with a volume of 17,000 m3. The engineering challenges of such a project, at the centre of a live university environment, were numerous.
The top three challenges were: removing pollutants from the inflow to prevent siltation; protecting the construction activities from stormwater run-off; and working next to the vibration-sensitive Newton Building.
Time to percolate
The new percolation tank at UNSW, which Druce describes as being “like a massive underground carpark, only without the line markings for car spaces”, is a concrete cavern with an open base.
“We’re talking about a tank that is designed to detain water, and also facilitate percolation into the ground, below,” says Duncan Crook, the Arup civil engineer who developed the original concept for UNSW.
“We say percolate because we’re talking about stormwater seeping into the ground below the tank and, ultimately, into the Botany Sands aquifer below.
“The concrete roof slab is supported by the external walls of the tank as well as intermediate columns founded on concrete footings to minimise settlement, which is why it looks a little bit like an underground carpark when you’re inside it.”
UNSW extracts borewater from the aquifer underlying the tank and has recently upgraded the borewater treatment plant on campus. This allows extracted treated borewater to be used for irrigation and other uses, including air conditioning.
“The new system is much larger than the one we had before,” Druce says. “It improves our capacity. We have had a few shortfalls, particularly in 2019 when we had several very hot days and had to isolate some of our buildings because we couldn’t supply them with non-potable water.
“We had to revert to a potable system to make up the shortfall. It’s good we have that capability, but we’re much better off running on non-potable water or bore water.”
An important design consideration was minimising the risk of sediment entering the percolation tank.
Sediment flushing into the tank could lead to progressive siltation of the base of the tank and reduce the rate of percolation over the longer term.
To reduce this risk the project team implemented a stormwater quality improvement device (SQID). Stormwater drains were connected together and a SQID positioned immediately upstream of the tank. The SQID is a large, concrete chamber containing several internal weirs that progressively slow down the flow, trapping sediment, while floating debris is caught in baskets.
After the treated water exits the SQID, it enters a scupper chamber, which sits along the entire eastern flank of the percolation tank. This chamber is slightly deeper than the rest of the tank and once again slows flows and collects any residual sediment not contained in the SQID.
“That scupper chamber does all the heavy lifting,” Druce says. “Any additional flows from a higher intensity storm flood out into the larger tank. As a whole the tank accommodates the one per cent annual exceedance probability — AEP — storm, which will fill the tank to about 95 per cent.”
A supplementary measure is a permeable geofabric placed over the earth floor, with a thin layer of washed river gravel on top. This is to allow further filtration of the water before it enters the ground. The SQID, scupper chamber and base protection measures work together to help prevent siltation.
The ultimate destination for the stormwater is the Botany Sands aquifer which UNSW, as a major land manager in the area, has a responsibility to preserve.
“At the university we draw bore water to provide about 40 per cent of our non-potable water needs. The aquifer is a massive resource, not only for cost but also from a sustainability standpoint. So the more water we put into the aquifer, the more we’re ensuring that the aquifer is going to provide that resource in the future,” Druce says.
Flood-proof construction
Ironically, the very event the infrastructure was being created to handle — a major storm — was also the event that could slow or disrupt the project during construction.
“We put a lot of thought and planning into managing water away from where the construction was happening,” Crook says. “We put up several lines of fences and barriers, such as earth mounds, to stop stormwater from barrelling into our site, as well as temporary overflow measures.”
On paper, the solution looks like several lines of defence on a battlefield, against attacks from the north and south.
“We had to make sure there was low chance of stormwater spilling into the oval,” Crook says. “Unless it’s extreme rain, in which case there’s nothing we could do. The only effective way to deal with this risk was to get on with the job as fast as we could, to get the tank built before the rain came.”
One interesting challenge, says Geoffrey Lim, UNSW Estate Management’s Project Manager, was that the site was adjacent to the Newton Building.
“This building houses some of the top research projects in the University, several of which also happen to be highly vibration sensitive,” Lim says.
“One of our challenges during construction was to dig a giant hole and construct a massive tank while sensitive research was being carried out on our doorstep.”
Several vibration monitors were installed within the Newton Building and vibration was monitored and regularly reported throughout the project.
Vibration reports and timings were sent to all of the building occupants on a fortnightly basis, so they could be compared against any research anomalies occurring during those times.
“We held fortnightly stakeholder engagement meetings, not just with those in the Newton Building but also in other surrounding colleges, just to make sure they all had a chance to voice any concerns and remain informed,” Lim says.
Project pluses
The university expects numerous benefits now the percolation tank is up and running. These include:
- Water and environmental
- The natural water cycle benefits as stormwater is directed into the aquifer.
- The additional groundwater could reduce UNSW’s dependency on the public water supply.
- Proactive treatment of stormwater removes pollutants, maximising the quality of water draining into the aquifer.
- Downstream flood risk is reduced as greater volumes of stormwater are captured, also satisfying Randwick City Council for onsite detention.
- Students and staff
- All-weather sports pitches and other modern facilities have now been established within the Village Green area.
- The new sports pitches are flood resilient, minimising downtime after rainfall.
- The infrastructure is below ground, so is unobtrusive.
- Telemetry equipment within the tank monitors performance and provides a teaching aid for students.
- The project details are now available to students of civil engineering and geotechnical engineering as a study resource.
- University operation and maintenance
- Designed to be low maintenance, the new stormwater system minimises operational costs and disruption.
- The tank is engineered to eliminate the risk of settlement and maximise design life.
- Regular cleaning and periodic inspection are able to be conducted to optimise long-term performance.
- The tank size design helps future-proof campus developments.