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Kempsey Bypass detailed design and flood mitigation

Roads and Traffic Authority, Kempsey Bypass Alliance, Abigroup

Improving flood immunity for a community.


The Kempsey Bypass is the initial stage of the Kempsey to Eungai Upgrade project, involving a 14.5 km, four-lane divided highway which crosses the Macleay River floodplain to the east of Kempsey. The 3.2 km bridge across the Macleay River and floodplain is the longest in Australia. The project involved extensive mitigation works to compensate landholders for impacts on flood behaviour resulting from the bridge construction.

In order to progress the project, assessments of flood impacts and possible mitigation measures were required, as well as consultation with the community and affected stakeholders.


WMAwater was involved in all phases of the project including route selection, feasibility environmental impact assessment, concept design and construction. We undertook more than 20 detailed hydraulic modelling assessments for various aspects of the project including:

  • Hydrologic and hydraulic modelling for route assessment;
  • Impact assessment of various bridge configuration options and mitigation works;
  • Community liaison to negotiate flood mitigation works;
  • Assessment and advice to the construction team relating to flood impacts of the construction platform in the Macleay River and other temporary works;
  • Detailed modelling of specific areas to identify impacts of mitigation and localised construction works on individual properties; and
  • Consultation with a wide variety of stakeholders including Council, SES, and local community regarding the impact of the bypass.

As part of the work, we determined the criteria to identify if mitigation measures were required based on duration of flooding, afflux, evacuation routes and type of property. Modelling was undertaken to confirm requirements, and to test mitigation measures.

We negotiated mitigation measures on behalf of RMS, including the raising of 20 houses and construction of 30 stock mounds and a levee. We assisted with the negotiations for the proposed levee, which was to be located on several commercial and residential properties, by explaining the benefits of the levee, and offering alternatives and advice on how to incorporate the levee into the property in the least obtrusive way. We also met with concerned residents, Council, SES and OEH to discuss their concerns.

As part of this project a detailed report on mitigation measures was prepared for the Department of Planning and a summary of changes to flood behaviour as a result of the Bypass was prepared for SES.


We delivered for the client a compliant design that met all the flood objectives. We developed mitigation measures that improved the flood immunity of the community.