Channel widening near Alexandra Canal outfall (artist’s impression)

GREEN SQUARE: PROTECTING COMMUNITIES AND ENABLING URBAN RENEWAL THROUGH EFFECTIVE FLOOD RISK MANAGEMENT
An outline of an innovative design and construction solution, including a new 2.5km underground stormwater drain.
N Taylor, D Kent, K Woodbury, M Lewis, S Muralitharan
Publication Date (Web): 22 September 2016
DOI: https://doi.org/10.21139/wej.2016.034


Located between Sydney Airport and the city, Green Square is Australia’s largest urban renewal project. Until recently the recurring likelihood of major flooding during torrential rain hindered the development of this precinct. Now work is well underway to significantly reduce the flooding risk and create the catalyst for other liveability improvements such as road upgrades, enhanced amenities and infrastructure for this flourishing community. 

Overall, the $16 billion redevelopment will transform Green Square from old industrial land into a modern, high-density and sustainable hub, suitable for a population of 60,000 people and forecast to provide 20,000 permanent jobs. As the area sits on a major floodplain, once known as the Waterloo Swamp, periods of heavy rainfall present a significant safety risk to the public and community. To address the flooding risk, the City of Sydney and Sydney Water defined the Green Square Stormwater Drain Project and, after a competitive tender process, formed the Drying Green (DG) Alliance with United Group Engineering, Seymour Whyte Constructions, WSP/Parsons Brinckerhoff, and RPS Manidis Roberts. 

The DG Alliance team united with a common goal to develop a solution that was sustainable, innovative and respectful to the community. In a busy urban environment the team faced technical, logistical and community-related challenges to make this happen, but the alliance delivery method allowed the wider team to be involved in evaluating the options and determining infrastructure with the greatest benefit and value to the community. 

Using a thorough planning process and rigorous modelling, the DG Alliance developed an innovative design and construction solution, which includes a new 2.5 kilometre underground stormwater drain from Link Road, Zetland to the Alexandra Canal. The aim is to reduce high hazard flooding to low hazard flooding in the one per cent Annual Exceedance Probability event without adversely impacting the surrounding areas.  

While the client’s reference design consisted of box culverts installed by trenching, to reduce the construction footprint the Alliance design used mainly pipes (twin and triple DN1800 RCPs) installed by a combination of open trenching and microtunnelling. Microtunnelling involves using a tunnelling machine to install pipes underground without disturbing the surface.  Microtunnelling is being used for more than half the drain and resulted in:

  • Minimum impact on the more than 120 underground utilities that cross the GSSD alignment;
  • Minimum impact on existing roads and developments (no open cut);
  • Minimum environmental impact (spoil and dewatering)
  • Minimum community and traffic impact (less open cut)


The hydraulics also differed from the reference design, allowing for a pressurised system that varies with flow and tail-water conditions. Extensive modelling was undertaken to design the system and prove that it would meet the required flood mitigation criteria. The modelling used both a 2D surface model (TUFLOW) to determine flood impacts and a 1D trunk drain model (XPSWMM) to assess the pipe hydraulics. Critical structures were also modelled using 3D computation fluid dynamics (CFD). Physical modelling was also undertaken on three of the most critical ore representative structures to confirm the CFD results. 

 
Flood hazard before GSSD:
         
Flood hazard after GSSD:
Flood hazard before GSSD   Flood hazard after GSSD

The resulting innovative design will achieve the required reduction in flood levels at a fraction of the cost of the reference design. Overall construction of the Green Square Stormwater Drain Project began in May 2015 and is progressing well. It is expected to be finished by the end of 2017 with minimal impact on residents, businesses and the community. 


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