Waikato Regional Council is embarking on a project to remove a flood control gate that holds back the flow from a lake with about 45 million cubic metres of water in it.

The northern outlet control gate at Lake Waikare has to be taken out for inspection and repairs, but the council needs to hold back the water in the lake during works so it doesn’t flood the Whangamarino Wetland, which is an international Ramsar site in recognition of its outstanding biodiversity.

Regional Resilience Principal Technical Advisor Ghassan Basheer says the $1.3 million project to repair the gate and hold back the water “is quite a complex exercise because of the consequences if suitable and robust controls are not in place”.

“There is about 45 million cubic metres of water in Lake Waikare, which essentially would overwhelm the Whangamarino Wetland and raise its levels by at least 600-700 millimetres, impacting on the prized fauna and flora that live there.”

The 12 metre long by 4-metre-high control gate, installed in 1965 by the Waikato Valley Authority, is made of steel and very heavy.

It needs to be repaired because its seals are worn so it’s leaking, which impacts the control of the level of Lake Waikare as part of the Waikato Flood Protection Scheme and as required by its resource consent.

The consent allows for a targeted minimum water level of 5.4 metres above sea level during the period 1 April to 30 September. However, due to evaporation, dry weather and leakage through the gate the level could drop up to 10 centimetres below the targeted minimum level. Fixing the leakage is expected to significantly improve the control over the lake level.

“Behind the gate is the whole lake. Obviously, we can’t just remove the gate as the lake will discharge into the Whangamarino Wetland and that would be an environmental catastrophe,” says Mr Basheer.

To be able to completely remove the control gate, the lake will first be dammed using an Aquadam, which is basically a heavy-duty plastic pipe filled with water (the weight will keep it on the lakebed), and then another steel and timber gate (a stoplog) will be fixed to the wing walls of the existing control gate structure. Once the stoplog is installed the Aquadam will be removed.

The Aquadam will be about 60 metres long and 3 metres high and is being exported from the United States.

Mr Basheer says the control gate will be removed and transported to a workshop where it will be repaired and painted.

“That in itself is like moving a house on a trailer and will take about 24 hours to get it on, so it’s all taken a lot of careful planning and organising.

“We’ll also be repairing the concrete floor on which the control gate sits, and the rock lining at the entrance to the entire structure.”

Council staff have engaged with iwi and key stakeholders and worked closely with consultants and contractors on the project.

A blessing was undertaken this week, recognising the cultural significance of the site.

Physical works are expected to start in the first week of March.

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