New research improves predictions of dangerous rip currents

New research about how the configuration of beaches and climate cycles affect rip currents will help improve the accuracy of forecasts of when and where dangerous rips occur on New Zealand beaches, potentially saving lives.

The research builds on a 10-year collaborative project between NIWA and the University of Waikato to study and forecast rip currents at Tairua Beach in the Coromandel.

Using images from a computer-controlled camera set up at the end of the beach, scientists have been looking at how rips develop, migrate and change over a 10-year period to build models to forecast rip occurrence.

A rip is an offshore current concentrated in a narrow channel along a beach, often catching swimmers unaware and pulling them out to sea.

Generally, the orientation, size and strength of rip currents depend on offshore wave conditions, including height and angle of the waves. By analysing wave characteristics against rip currents on the video images over a 5-year period, a predictive model to forecast rip occurrence was developed. Seventy-five percent of the time the model accurately predicted a rip current would occur.

Now new research is looking at how the how the shape and configuration of the beach – including the location of sandbars, channels, and the position and slope of the shoreline – can impact rip currents, to improve the accuracy of the forecasts.

University of Waikato senior lecturer Dr Karin Bryan says the new analysis provides exciting information about how stable rip currents are, where they might develop and how many there might be.

“Looking at the video you can clearly see some rip currents that did not move for nearly a year, especially near the headlands. At the central part of the beach, you might see up to 8 or 9 currents occurring. The central ones are really different, shifting and changing and even disappearing after a few weeks. We can extract precise measurements of these rips, which are used to fine-tune our model for forecasting rip currents, ultimately providing the level of accuracy needed to help surf lifesaving initiatives.”
Scientists have also been looking at how La Niña and El Niño climate cycles impact rip currents.
“With each climate cycle the beach rotates, pushing all the sand to one end of the beach. That movement is really controlling the size, stability and location of rip currents,” Dr Bryan says. “It’s the first time we have really analysed how these large-scale patterns control the smaller scale rip current patterns.”
NIWA coastal scientist Dr Giovanni Coco says the biggest gaps in global knowledge on rips is how currents react with the seabed.

“As a rule we know the larger the waves, the stronger the rip. But that’s not the only factor. We are now getting a much clearer idea of how beach morphology, shape and structure are related to rips developing. Because we have been collecting video data for so long at Tairua, we are in a unique position to be able to do this kind of work.”

“It’s a significant step forward internationally,” Dr Coco says.

Surf Lifesaving New Zealand Life Saving Services and Education Manager, Brett Sullivan, says a high proportion of drownings that occur on the coast are directly related to rips. They are also responsible for the majority of rescues performed by lifeguards.

“Rips are a significant hazard on New Zealand beaches and Lifeguards spend many hours working to reduce the risk they pose to the public. People drown fast so any new research that helps Lifeguards to focus their attention and lifesaving equipment to where it is needed most, certainly has the potential to save lives.”

The research is a long-term collaborative project between NIWA and the University of Waikato, assisted by Environment Waikato.