When a river meets the sea

When you fly across New Zealand, you can often see plumes of sediment flowing into the ocean from some of our mightiest rivers.

The mixing of the two bodies of water is incredibly complex, but NIWA oceanographers have found a spot that provides them with a simpler system to study – Deep Cove, in Doubtful Sound, where the tailrace from the Manapouri underground power station flows into the sea.

Deploying near-surface instruments designed to measure currents right at the surface. Image: Craig Stevens

In the lab at Deep Cove, preparing instrumentation. Image: Craig Stevens

Testing the autonomous research kayak, designed by the team from Oregon State University. Image: Craig Stevens

Craig Stevens controlling the autonomous kayak. Image: Rebecca McPherson

Looking down Doubtful Sound, showing the steep walls of the fjord. Image: Craig Stevens

Tyler Hughen from Scripps Institute of Oceanography deploying an autonomous drifting profiler. Image: Craig Stevens

Rebecca McPherson from the University of Auckland with a turbulence profiler. Image: Craig Stevens

Craig Stevens and Joe O’Callaghan, and a team of their international collaborators from Oregon State University and Scripps Institution of Oceanography, took high-tech gear to Deep Cove to track what is going on as the two waters meet and mingle.

“The Manapouri tailrace outflow into Deep Cove creates an amazing natural laboratory to study how rivers enter the sea and how water bodies mix and entrain,” says Craig Stevens. “It generates flows of several metres per second and creates a sharp interface between fresh and salty water, as well as a tightly focused plume structure that persists down the fjord for many kilometres.”

At flows of eight to 10 km/h, the discharge is comparable to New Zealand’s largest rivers. “It doesn’t sound fast but from an ocean perspective, that’s rocketing along.”

The team deployed moorings and used torpedo-shaped turbulence profilers and autonomous vehicles to monitor the complex processes and interactions.

Back in the office, the number crunching has only just begun, but Joe O’Callaghan says the Manapouri tailrace is a great proxy for a natural river.

“It changes like you turn a tap on or off. It mimics the things you see at the coast, when a river discharges and rushes out.”

The team says the Marsden-funded research will improve understanding about how material that flows off the land ends up in the ocean. That understanding will also result in better predictive tools and computer models.