Aerial footage captures opening of lake to sea

Dramatic aerial footage has been released of the recent opening of Te Waihora / Lake Ellesmere to the sea.

Lake Ellesmere is the largest lake in Canterbury and has no natural outlet to the sea. The first written record of it being artificially opened to the sea was in 1852. It has happened more than 300 times since then for cultural and environmental reasons.

The latest opening was primarily to allow inward spring fish migration. 

Leigh Griffiths, Environment Canterbury Manager River Engineering, said the earthworks to open the lake started on September 23 and the lake was successfully opened five days later.

“Generally it takes anywhere from three days to a week to open, but if the sea conditions have been rough it has taken up to six weeks in the past,” she said. “Getting the weather window right for the physical works and the forward forecast is critical for maintaining the opening.”

For an opening to be successful, the lake needs to have reached minimum levels required in the Water Conservation Order, the sea needs to be relatively calm and machinery needs to be mobilised and able to gain safe access to the site, which can be restricted by heavy seas.

“During this opening the sea was relatively calm; you can imagine the risks when the waves are a little bigger,” Leigh said. “It’s dangerous work and we are fortunate that our staff and contractors are so experienced.

“Our site supervisor, Mike Hyett, has opened it over 100 times so he knows the conditions and how to get the job done safely. During large seas the waves crash over the top of the beach crest and into the lake - unless you’ve seen it, it’s hard to appreciate. You can’t open it in those conditions as the cut gets filled back in by gravel movement anyway and safety always comes first.”

In the first stage of the opening, machinery needs to be able to work safely within and adjacent to the lake; in the final stage machinery needs to able to work safely in the surf zone. Once the cut is opened, it needs time to develop and remain open which relies on relatively calm sea conditions.

“On this occasion a large volume of shingle washed into the lake so the cut to open was 290 metres long,” Leigh Griffiths said. “On average about 30,000 cubic metres of material has to be shifted so it’s no small job.”

Once the lake is opened, there is very limited ability to control how long it stays that way. The length is determined by weather and sea conditions and how quickly gravel is deposited by the sea to close the cut.

“Artificial closure may be desirable closer to summer if lake levels get very low and calm sea conditions mean there is no natural closure of the cut,” Leigh said. “Artificial closure has not been attempted to date.”