Lake Forsyth during heavy rain in Canterbury in March. Photo: Jan Daffin / Supplied
Christchurch city councillors have been presented with a plan to build a multi-million-dollar pump for Wairewa Lake Forsyth to improve water quality and reduce the risk of flooding.
The council currently opens the lake to the sea when the water reaches a certain level by digging a canal.
The city and regional councils were criticised earlier this year for not opening Lake Forsyth and nearby Te Waihora Lake Ellesmere during heavy rain and flooding.
Weeks earlier, thousands of migrating eels died at Te Waihora Lake Ellesmere trying to reach the sea.
Both lakes have to be opened mechanically. The responsibility for opening Lake Forsyth lay with the city council, while Lake Ellesmere openings were governed by a 1990 water conservation order and resource consents held jointly by the regional council and Te Rūnanga o Ngāi Tahu.
On Tuesday councillors were briefed about a proposal for a $2.6 million dollar barge and pump at Lake Forsyth, dubbed the Ocean Connection, that would allow a continual flow of water between the lake and sea.
Environment Canterbury councillor Iaen Cranwell said the current method of using heavy machinery to carve out an opening was expensive and risky, given the unstable shingle and proximity to the sea.
The city council's consent stated the lake should be opened to the sea when it reached 2.7 metres. During the flooding in Autumn, the lake peaked at 4.1 metres but severe weather prevented it being opened.
Engineer Wayne Alexander told councillors the flooding would not have been as bad if the pump had been in place.
The lake reached its highest level in 50 years following days of heavy rain. Had the pump been in place and running in the five weeks beforehand, the lake would have been 400mm lower, Alexander said.
He described the Ocean Connection as "a double-hulled barge with a sluice box on it and two pipes, 300 metres long and 600 millimetres in diameter. It will carry about 1.6 cubic metres of water a second in full flow, it's a lot of water, a lot of velocity".
Alexander said one of the pump's key benefits would be the ability to close the lake, which was currently prohibitively expensive.
Closing the lake cost around three times as much as opening it and had only happened once in the 16 years since the rūnanga built the canal from the lake to the sea, he said.
Alexander told the meeting the canal had gone a long way to improving the lake's health, taking it from one of the most degraded lakes in the country in 2005 to the most improved in 2022 but without the ability to close the lake, there was a real risk to the lake if it dropped to its natural level.
"Mid winter, it's no problem, but coming into summer, if you let it drop down you're likely to have to very low lake levels, high evaporation and percolation. The risks are high for the lake, potentially even tipping over, losing all its oxygen, and we've seen those [algal] blooms before," he said.
There was still uncertainty on the exact impact on migrating tuna (eels) at Lake Forsyth but the team had been consulting with the Department of Conservation.
"I don't think anyone's saying it's a given. There are aspects of the fish passage we don't know anything about but we're fairly confident that tuna would use this to migrate to the sea in March," he said.
Alexander said the "fish lift" technology to allow fish and tuna to pass through the pump was used on similar structures in other parts of the world, transporting salmon, for example.
The $2.6m cost estimate included a 30 percent contingency.
The project would bring savings, such as halving the number of times the lake would need to be mechanically opened, as well as biodiversity and community health benefits, Alexander said.
Lakes Ellesmere and Forsyth are among the most polluted bodies of water in the country.
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