One of the most extensive reconstructions in the more-than-100-year history of the Waihao Box on the Waimate coast will close a popular reserve because of the hazards involved.
From Monday, weather and sea permitting, Environment Canterbury (ECan)
will start spending an estimated $400,000 on the damaged sea end, most coming from a loan repaid by ratepayers who benefit from the drainage scheme.
Rotten and worn timbers have stopped the box from working properly and it needs to be fixed to remain effective.
During the work, the Waimate District Council has agreed to close its reserve on the end of Byrne Rd at the river mouth, unless there are special circumstances, and access along the coast around the box will also be closed off.
ECan senior engineering adviser Leigh Gillies said yesterday work on the box would take about six weeks, again dependent on weather and sea conditions.
Since it was constructed in 1910 as part of a drainage system for the Waihao River and Lake Wainono catchments, it has been rebuilt four times.
The latest reconstruction work is because of deterioration in the structure.
Two contractors will use cranes and diggers to remove timber and gravel, then drive piles. The 20m damaged portion, including some original timber from 1910, will be then replaced with 15m of new piles and timber.
The whole box will be moved several metres inland to compensate for erosion.
Construction would add to what was already a hazardous area because of frequent heavy seas and shifting gravels around the box, she said.
''The safety and health risks associated with the site are considered significant; it's particularly hazardous work,'' she said.
A temporary river mouth north of the structure would be opened.
Wainono Drainage Group chairman Mick Laming said the repairs would mean better drainage and provide environmental benefits.
''After more than a century, the box has become woven into the overall ecology of the river, providing a vital link between it and the open sea for whitebait, eels and other water life.
''The river and lagoon is also important for recreation,'' he said.
In the late 1800s as the area was settled, flooding became a serious economic hazard when the outflow from the lagoon and its river systems was not able to break through the gravel barrier to the sea, as well as a risk to the safety of residents, visitors and livestock.
Part of an overall drainage scheme, the Waihao Box, while looking like a simple hardwood construction, was a complex solution by early pioneers to overcome periodic flooding of thousands of hectares of valuable and rich farmland from Makikihi south to Studholme on the coast.
It also provided a stable environment for the ecologically and culturally important Lake Wainono.
The Waihao Box is believed to be the only one of its kind still functioning in New Zealand and possibly in the world.
Because of its historic importance, a photographic record of the work will be compiled for the New Zealand Historic Places Trust.
Waihao Box
• Designed by pioneers, built from hardwood in 1910, at the Waihao River mouth and Lake Wainono outlet.
• Part of a drainage system to stop flooding of farmland from Makikihi south to Studholme.
• Latest work estimated at $400,000.
• One of the biggest jobs in box's history.
HOW IT WORKS
• Its north side is open at the river end so flooding pressure forces water out the sides. This speeds up erosion of gravel alongside, creating a natural channel to the sea. A ''nose'' prevents the sea from flowing in and blocking the outflow.
