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Highway upgrades achieving flood immunity objectives

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Rodney Stevens, Clarence Valley Independent

A hydrological mitigation report on the Glenugie to Devlis Pulpit section of the Pacific Highway published by Transport for NSW shows the upgraded road is achieving its objectives of improving flood immunity.  

The report describes flood behaviour in the region, Transport for NSW’s flood management objectives, flood model outcomes, design and mitigation, and future planning.

To prepare the report Transport for NSW assessed the Woolgoolga to Ballina Pacific Highway upgrade’s impact on flood behaviour since 2012 and documented the results in a hydrological mitigation report for the Clarence River catchment available on the Pacific Highway website.

Independently reviewed flood models were also designed to predict flood impacts and direct floodplain management planning within the catchment.

“The Clarence River catchment is the largest on the east coast of NSW with a catchment area of about 22,700 kilometres,” the report states.

“Major flooding occurred as recently as 2021 and 2022 with floods in 1980, 1996, 2001 and 2013 some of the highest on record.

“To build our understanding of flood behaviour in the region we have collected and considered information provided by residents and property owners and used data from real events in 1980, 1996, 2001 and 2013.

“Flood management specialist consultants WMAwater was approved by the NSW Department of Planning, Industry and Environment (DPIE) to review the flood models and outputs developed by the project team.”

A common method used in construction to achieve the flood immunity objectives includes increasing the number and length of bridges over creeks, floodways and rivers, such as over the Clarence and Coldstream Rivers, Shark and Edwards Creeks, and replacing culverts with bridges.

“The Woolgoolga to Ballina upgrade has been built to meet its Minister’s Conditions of Approval (MCoA), improving highway flood immunity to between a one in 20 and one in 100-year flood event,” the report states.

“We mitigated the potential flood impacts of the upgrade by investigating many different design options to achieve better flooding outcomes, including changing the size and location of flood openings and increasing the overall length of floodway openings.

“Between Harwood and Chatsworth Island, the new northbound carriageway has been built to maintain flow of traffic during a one in 20-year flood. 

“The flood immunity of the southbound carriageway and local access to this carriageway has been improved to a one in five-year event.”

The Glenugie to Devils Pulpit section of the highway features 41 bridges over waterways with a combined length of 8.36km.

More than 200 configurations of bridge and flood relief culverts across the Clarence River floodplain were assessed, and bridge openings were increased by about 18 percent for improved flooding outcomes.

“The upgrade will increase flood levels at the Maclean levee by about 15 millimetres,” the report states. 

“To mitigate this, the low point of the Maclean levee was raised by about 100 millimetres for a distance of about 400 metres. 

“As a result, the flood management objectives were achieved, and the town now experiences an improved level of flood protection.”

As the construction of the highway changed the landscape which can impact flooding, the MCoA flood management objectives stipulated the maximum levels the project was permitted to increase flooding in residences or cane farmland was up to 50mm, with the duration of floods on these properties to experience no more than a 5 per cent increase.

An assessment of flooding on private properties found departures from the flood management objectives at four locations between Glenugie and Tyndale and at six locations between Maclean and Iluka Road.

“These departures are not predicted to have significant impacts on the use or productivity of the land and consultation has been carried out with all impacted stakeholders,” the report states.

The updated flood models and the hydrological mitigation report for Glenugie to Devils Pulpit will be submitted to the Minister for Planning and investigations into this year’s floods continue.

“Transport for NSW is committed to investigating the flood events of 2022 to understand how the water flows interacted with the highway,” the report states. 

“We expect this will take some time to complete. 

“We will continue to keep the community informed as the investigations progress.”

The full report is available online at www.pacifichighway.nsw.gov.au

Clarence Valley Independent 26 October 2022

This article appeared in the Clarence Valley Independent, 26 October 2022.

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