New culvert will super-size flood protection

Publish Date : 11 Sep 2023
Paerata Culvert
The capacity of the Paerata culvert will increase massively after an upgrade project.

The race is on to replace the Paerata culvert before any more adverse weather events.

The existing 1.8m x 1.8m box culvert carries the Whangapouri creek and can manage a one-in-five-year deluge, but more than 25 properties, both homes and commercial premises, were flooded in this year’s cyclone and heavy rains when it failed to cope.

Franklin Local Board chair Angela Fulljames says the flooding also caused widespread erosion of downstream stream banks and left some slopes unstable, and the culvert is part of a series of projects proposed to improve flooding in the catchment.

“There are also issues with the road over-topping and that can’t be left unaddressed when you consider about 17,000 vehicles cross the culvert every day.”

Investigatory drilling work began in the area at the end of 2021 and Fulljames says a plan is now in place to massively increase the capacity of the culvert.

Neighbouring properties have already been communicated with and will continue to be updated, while the traffic plan maintains road access that includes keeping two lanes open whenever possible, and some overnight closures during weekends.

Board deputy chair Alan Cole says when work is complete the culvert will cope with what is known as a one-in-100 ARI – annual recurrence interval - 20 times its current capacity.

“The work will also stabilise the stream banks to protect private structures but all while retaining as many of the stream’s natural features as possible and include measures to treat any new stormwater,” he says.

The existing culvert will become a bridge, work that will inevitably cause traffic management to be put in place during the building project.

“With 17,000 vehicle movements carrying who knows how many passengers, it’s going to be critical that sound traffic management is in place.”

Fulljames says converting the culvert into a bridge removes structures from the stream and despite the size of the project, carries the smallest construction impact.

Engineering is underway and construction is expected to start in November and take a year.

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