Man-made rip current tames elusive, wild waters for research

By on July 16, 2013

Rip currents are dangerous to beach goers, but difficult to track down and monitor for research.  That’s why, last summer, a group of researchers from the Woods Hole Oceanographic Institution built their own.

Steve Elgar, senior scientist at the Woods Hole Oceanographic Institution specializing in ocean physics who led the experiment, said the currents are hard to find. What’s more, they are commonly found in places to where it’s hard to collect data, like near a wave-battered pier.

Rip currents result from a combination of inbound waves and underwater channels in the sand, which create an avenue for strong outbound currents. Elgar said its still not clear which element of the currents is the initiator, though he suspects that it varies. Melissa Moulton, a graduate student at the Massachusetts Institute of Technology has been working with Elgar to figure that question out.

The bigger picture for the researchers is creating a model that will give people the opportunity to incorporate rip currents into research from a desk, instead of a trip to the beach. “I’m trying to put myself out of business, in a sense,” said Elgar. “Our goal is to figure out the physics so you don’t need us anymore.”

It’s a noble goal, but it won’t come easy. Elgar described measuring rip currents among the constant rumbling surf with words like “turbulent, non-linear and chaotic.”

To create a rip current, the team traveled to the U.S. Army Corps of Engineers Field Research Facility in Duck, N.C. There the researchers dug a large trench in the surf zone, measuring 30 meters wide by 100 meters long by 2 to 3 meters deep. The idea was to funnel water away from shore, creating a giant rip current.

The military landing vehicle the researchers used to dig a trench that would create a rip current (Credit: Steve Elgar)

The team used a Vietnam War-era military landing craft to dig the massive trenches.  They created the channel with the 75-foot-long aquatic vehicle’s guarded 3-foot propellers. “We just drove it right on the beach and wiggled its butt back and forth to make a trench,” Elgar said.

The team tried to hire a dredge rig to dig the trench, but no dredging company would consider getting their crafts that close to shore.  The landing craft ended up being a more economical option anyway, as a free military surplus vehicle.

It took three attempts to successfully create the current. “We dug one of the trenches and the waves were just small,” Elgar said. “It filled in and we never made a rip current.”

The second try ended the same way.  The third attempt worked.  The team created a rip current flowing an average speed of 2 knots out to sea, which lasted for nearly 36 hours.

Once the current was in motion, the team started collecting data. They wanted to understand how sea floor sand, waves, currents and the beach all interact during a rip current to determine the tugging tides physical properties.

Elgar said he expected the trench fill quickly,  like a sand castle moat after one wave passes over. He hurried to deploy their array of sensors, but he found the current and trench were relatively stable with average sized waves passing over.

A host of sensors were used during the experiment to determine current movement and trench depth.  Acoustic current meters tracked wave and rip velocities. A GPS-guided jet ski driver took bathymetric surveys of the trench below with a sonar mapping instrument.  WHOI-designed altimeters gave depth readings at fixed points during the experiment.

The result of the research team’s manmade rip currents, they hope, will be an equation. “We would like to be able to understand what’s going on in a sense that a physicist can write an equation on what’s going on,” said Elgar.

He said the man-made rip current makes for a great test for their developing model. “The rip current and the subsequent filling in of the trench is providing a wonderful test bed for model development, model calibration and model verification,” Elgar said. “The extreme perturbations to the seafloor, and the corresponding strong changes in the waves and currents are a challenge for models, and thus a nice testbed.”

The research is ongoing, but it has many potential benefits. Elgar said an accurate rip current model could be used when planning  new beach structures like piers. It may help municipalities keep beaches safer. The model would also lessen the cost of related research.

The research team’s manmade rip current was funded by the Office of the Assistant Secretary of Defense for Research and Engineering. People from WHOI, the U.S. Army Corps of Engineers and MIT all contributed to the project.

Top image: A rip current forms along trench dug with a military landing vehicle (Credit: Steve Elgar)

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