Pioneers thought outside the square

Simple but ingenious, the Waihao box is still helping prevent flooding of coastal Waimate after 100 years, as David Bruce reports.

The Waihao box is one of those "No 8 wire" solutions pioneers were good at, overcoming the vagaries of the new environment which they settled.

But they would never have dreamed that 100 years later, in a world so technologically removed from theirs as to be science fiction, it would still be standing and efficiently fulfilling its original role.

The structure, at the mouth of the Waihao River on the coast east of Waimate, helps prevent the flooding of thousands of hectares of rich and valuable farmland as well as providing a stable environment for the ecologically important Lake Wainono.

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It is the vital end of a drainage scheme, started in 1896, which protects coastal farmland from Makikihi in the north to south of the Waihao River.

The box works on a simple hydraulic principle. Under normal river flows, water from the Waihao River and Dead Arm, which links with Lake Wainono, flows through it, even though it may appear to be partially blocked by shingle, which can be metres deep on the top.

In times of flooding, the increasing water level on the land side increases the hydraulic pressure, clearing away shingle and then opening a channel through the bank next to the box.

The flow scours a channel from the lagoon to the sea. That, combined with stop banks and drainage ditches, protects the farmland surrounding the Waihao River and Lake Wainono.

The first Waihao box was built further north in 1896 but was destroyed by a huge flood in 1908.

The present box was built in 1910, further south at the mouth of the Waihao River, its piles sunk further into solid ground below the gravel bank.

Before the land was settled by Europeans in the 19th century, any flooding across the low-lying land that surrounded the Waihao River, the related Wainono Lagoon and wider catchment area was simply nature at work.

Michael Studholme, regarded as "the Father of Waimate", settled in the area in 1854, eventually establishing his Te Waimate Run which stretched from the Waihao River to Makikihi.

Since then, about 2500ha of fertile, valuable farmland has been developed in the Wainono catchment.

Flooding, as a result of the outflow from the lagoon and its river systems not being able to break through the gravel barrier to the sea, became a serious economic hazard, as well as a risk to the safety of residents, visitors and livestock.

It is believed the Waihao box is the only one of its kind still functioning in New Zealand and possibly in the world.