Lake opening captured on film


Dramatic aerial footage of the recent opening of Te Waihora / Lake Ellesmere to the sea illustrates the scale and challenge of this important task.

Te Waihora is the largest lake in Canterbury and has no natural outlet to the sea. The lake was opened by generations of Ngāi Tahu living at Taumutu prior to pakeha arrival. The first settler written record of an artificial opening to the sea was in 1852 and it has been opened over 300 times since.

There is a large range of cultural and environmental reasons for opening the lake. In this instance it was primarily to allow inward spring fish migration.

Leigh Griffiths, Environment Canterbury Manager River Engineering, said the earthworks to open the lake started on 23 September 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.”RIMG1038-lake-opening-150x150

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 Griffiths said. “Artificial closure has not been attempted to date.”

Background

The opening of Te Waihora/Lake Ellesmere is governed by a National Water Conservation Order and a suite of resource consents held jointly by Te Rūnanga o Ngāi Tahu and Environment Canterbury who make the final decision to open after consultation with key stakeholders.

The stakeholders make up a group that operates under the lake opening “protocol”, which is a guide for collaborative and transparent decision making. Under the protocol, every group has an opportunity to make their view known. These views are all taken into account before a final opening decision is made by the joint consent holders.

Experts are engaged as required to provide technical advice and forecast weather and sea conditions are also considered. When and at what level the lake is opened to the sea requires consideration of a number of matters including wildlife, wetland vegetation, fish habitat, mahinga kai, customary fisheries, water quality, summer levels, land inundation, waterway networks and infrastructure, and fish migration for a variety of species in autumn and spring.