Commissioned by Altis Property Partners (developer), via Hansen Yuncken (project manager), Costin Roe Consulting undertook an overland flow (flood) assessment of 193 hectares of land in Orchard Hills, Sydney. The assessment was required to accompany an application for rezoning to facilitate the development of new industrial facilities on a 43.85-hectare subdivision.
2D TUFLOW hydrodynamic modelling by Costin Roe Consulting assisted in gaining the go-ahead for the proposed development – unlocking formerly unusable land to create business and employment opportunities, and improving the management of overland flow for the site and other land in the vicinity.The assessment prepared by Mark Wilson, B Eng (Civil) B Surv ME CPEng, Associate Director (Civil Engineering) of Costin Roe Consulting, first involved building two-dimensional TUFLOW hydrodynamic modelling for peer review by Worley Parsons, the engineers of a flood study of the area by Penrith City Council. Assisting Mark Wilson with the detailed programming and configuration of the TUFLOW modelling was Mitchell Cross, one of Costin Roe Consulting’s team of flood modelling engineers with TUFLOW modelling engine expertise.
To achieve peer-reviewed validation, Costin Roe Consulting’s TUFLOW modelling was used by Mark Wilson to simulate the occurrence of a range of probable flooding scenarios on the area – in its undeveloped state – to show the accuracy of the firm’s two-dimensional modelling performance against the accepted numerical data.
Next, Mark Wilson used the validated TUFLOW modelling-build to calculate and illustrate the differences in flood levels, velocity, and general hydraulics for the same range of probable flooding scenarios following construction of the proposed development, which would happen in two stages.
Finally, the two-part assessment Overland Flow Report Stage 1 and Stage 2 by Costin Roe Consulting successfully informed the NSW Department of Planning & Environment, Penrith City Council, development partners, and various stakeholders on the mitigation and management of land overflow at the site during and after construction of each stage of the project.
As a result of the scenarios produced by the 2D TUFLOW modelling in the Costin Roe Consulting assessment, it was demonstrated that a large percentage of this flood-affected land could be developed, and was suitable for rezoning, which would allow the Altis-proposed industrial development to proceed.
Costin Roe Consulting’s mastery of the TUFLOW modelling engine, and expertise in producing two-dimensional simulations of floodwater behaviour pre/post-development, enabled everyone – including the broader community following exposure of the proposal – to more easily appreciate the benefits offered by the development proposed by Altis, and more favourably consider the statutory changes needed to allow civil works and construction to begin.
Area formerly categorised as ‘high hazard’ due to overland flow
The site proposed by Altis for the development of major new warehousing and logistics facilities was the 43.85 hectare subdivision of a larger parcel of land zoned ‘Rural Residential’. For the proposed development to be given the go-ahead, the State Environmental Planning Policy (Western Sydney Employment Area) 2009 would need to be amended, and the land rezoned ‘General Industrial’.
Located at 585-649 Mamre Road, Orchard Hills, the site lay within what is known as the Mamre West Precinct. The site was identified by Penrith City Council in the report known as the South Creek Flood Study (Worley Parsons) on the Mamre West Precinct as being affected by overland flow associated with adjacent South Creek.
During overland flow events as detailed in the South Creek Flood Study, floodwater on the undeveloped site would be shallow and of low velocity except for the north-west corner, where slow-moving water could potentially reach 1.1m in depth. This area had been categorised by Penrith City Council as a ‘high hazard’ zone.
With a residential area to the north of the proposed development, and Erskine Park Employment Area on the eastern side, Altis and its project partners would be compelled to show conclusively that the civil works planned to make the 43.85-hectare site viable for development would not increase the risk or hazard of inundation for properties neighbouring or upstream/downstream of the development when completed and the overland flow from South Creek re-occurred.
Development would provide new access road and formal drainage system
Costin Roe Consulting’s easy-to-understand TUFLOW modelling successfully demonstrated how the Altis development would ultimately mitigate and improve the management of overland flow on the site and surrounding area. This would be achieved via engineered land-filling and the building of infrastructure, including a formal drainage system, where none had existed before.
A new access road would be constructed as part of the development project and handed over to the City upon completion. Previously unusable land in a district with prime accessibility to greater Sydney would be made serviceable. More jobs would be located in the far western suburbs of Sydney, handy to residential population centres where the types of land suitable for new industrial developments are in short supply.
Rigorous examinations and consultations involving two levels of government
However, with the Department of Planning & Environment (NSW) the consent authority for zoning amendments under the State Environmental Planning Policy (Western Sydney Employment Area) 2009, and Penrith City Council as a key authority to be consulted due to Section C3.5 of Penrith City Council Development Control Plan 2014 and its guidelines for flood-liable lands, the development proposal had to satisfy rigorous examinations and consultative processes involving these authorities. This is where the external scrutiny of the Costin Roe Consulting 2D TUFLOW modelling-build by Worley Parsons helped meet the high standards of diligence applied by the two levels of government.
Positive feedback throughout the modelling process
“The Mamre Road flood assessment for Altis and Hansen Yuncken involved a high level of consultation with Penrith Council, NSW Department of Planning, and peer review by Worley Parsons. We received positive feedback throughout the modelling process,” said Mark Wilson of the Overland Flow Report Stage 1 and Stage 2 by Costin Roe Consulting. “We will be completing more 2D flood assessments of this nature in future.”