Lakebed map data updated

A diver inspects hessian matting on the lakebed in Paddock Bay, Lake Wanaka. Native plants can be...
A diver inspects hessian matting on the lakebed in Paddock Bay, Lake Wanaka. Native plants can be seen growing through the matting. Nutrient loads, the volume of water, and the amount of sediment going into Lake Wānaka have all had an effect on water quality. PHOTOS: NIWA
After an active fault was discovered running under Lake Wānaka three years ago, Niwa scientists have mapped the lakebed to better understand its underwater structure and potential for earthquakes and tsunamis.

Niwa marine geology technician Sam Davidson said Lake Wānaka was one of the most photographed locations in the country, but until now, its depths had remained a mystery.

"We discovered some really interesting features, including complex channel systems from the rivers that feed into it.

"We also saw dramatic steep slopes that plummet to the deepest parts of the lake."

Because the lake was located on an active fault and had steep underwater slopes that could create a landslip, Mr Davidson said Lake Wānaka was at risk of a "tsunami-like event" which could threaten lakefront communities.

"Now we have a clear picture of the lake’s structure we can better inform hazard modellers and councils to better prepare for these events."

Wānaka sits in a U-shaped valley which was carved by an ancient glacier during the last Ice Age, more than 10,000 years ago.

Mr Davidson said he and his team scanned the lakebed by retrofitting a multibeam echosounder to a small dive vessel, which they steered up and down the lake over several weeks.

It calculated the water depth by bouncing soundwaves off the lakebed at a rate of 10 per second, and timing how long they took to return.

From this data, they were able to build a full 3D model of the entire area.

A 3D graphic of high-resolution bathymetry data retrieved from the lakebed mapping project.
A 3D graphic of high-resolution bathymetry data retrieved from the lakebed mapping project.
The map also allowed Niwa hydrodynamics scientist Dr David Plew to study the lake’s water quality.

"New Zealand has experienced dramatic changes in land use over the past century, which has had big impacts on our freshwater systems.

"Nutrient loads, the volume of water, and the amount of sediment going into Lake Wānaka have all been impacted.

"And climate change is also affecting our lakes and will do so even more in the future", he said.

To predict how the lake would change, Niwa was developing computer models of the catchment and lake.

These need accurate bathymetry data, and up until now, Niwa was working from charts created in the 1970s.

"This new depth data that we've captured is more accurate and detailed", Dr Plew said.

"This is especially important in shallow parts of the lake like Roys Bay and Stevensons Inlet, but also where rivers flow into the lake, because these areas have seen some of the biggest impacts and changes since the 1970s."

Otago Regional Council natural hazards manager Dr Jean-Luc Payan was delighted with the new map.

"The new data is invaluable to inform natural hazards investigations and to understand the consequences of natural hazards events on people and infrastructure in the Lake Wānaka area."

john.lewis@odt.co.nz

 

 

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