EXPLAINED: Bridge design changes to improve flood mitigation

ROADS and Maritime Services have released a report explaining changes in the design of the proposed Grafton Bridge Project that will improve upstream flood mitigation.

The report is backed up by a visual explanation of the changes to the bridge design and improvements to the levee system,which says upstream floods will only increase by 3cm rather than the original calculation of 9cm in one in 100 year floods.

Click on the Vimeo link below to watch the RMS video 'Grafton Bridge Developing the new flood model'.

The original design of the bridge has eight supports on the piles. However with improvements to the design, there are now two supports meaning water will flow around the supports with less obstructions.

Detailed river bed surveys were carried out in 2015 to give more accurate measurements of the depth of the river. The last survey of this type was conducted in 1963.

The new survey found the river is deeper in some areas particularly around the existing bridge and in the channel on the the southern side of Susan Island. This means the river is now holding more water and therefore over topping of the levee is lower than the estimations made during the environmental assessment.

The RMS have also consulted property owners who are not protected by the levee system and are discussing adjustments to help reduce the impact of the new bridge on their property.

As part of their levee system investigations, the RMS considered dredging the river to help reduce the flood impact, however as dredging could not provide a long term solution, the idea was shelved.

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