A view of Lake Waikare from the leaking control gate. Photo / Supplied
A view of Lake Waikare from the leaking control gate. Photo / Supplied
Waikato Regional Council is due to start work this week to remove and repair a leaking flood control gate that holds back the flow from a lake containing about 45 million cubic metres of water.
The council faces a balancing act as the lake must not fall below the level required by its resource consent and water flowing out of the lake must not be allowed to flood the wetland downstream.
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.
The steel gate was installed in 1965 by the Waikato Valley Authority. Photo / Supplied
Regional Resilience principal technical adviser 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 12m X 4m high control gate, installed in 1965 by the Waikato Valley Authority, is made of steel.
Its seals are worn so it is 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 April 1 to September 30. 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.
The channel control gate discharges into a stream that leads to the Whangamarino Wetland. Photo / Supplied
"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," said 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 lake bed), 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 60m X 3m high and is coming 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 last week, recognising the cultural significance of the site.
