Shifting Tactics in Shifting Shoals

The best-laid plans of scientists often go awry when they actually get into the field.

“That’s when designing an experiment becomes adapting an experiment,” said Peter Traykovski, an oceanographer at Woods Hole Oceanographic Institution (WHOI).

Traykovski had to adapt earlier this year when he confronted one of the most dynamic places on Earth—the New River Inlet in North Carolina. The inlet is a narrow, meandering chokepoint where the river and the Atlantic Ocean collide, where waves, winds, tides, and currents constantly jostle one another in unpredictable ways.

Traykovski and WHOI colleague Rocky Geyer had planned on using an underwater robotic vehicle to survey the sandy seabed. But shoals in the inlet proved too shallow and dynamic at low tides to accommodate the vehicle, so Traykovski had to improvise. He bought a commercial catamaran kayak, rigged scientific gear onto it, and navigated himself into the inlet to get detailed sonar images of the rippling sands.

Traykovski was among researchers from several institutions who have converged on New River Inlet in a five-year project funded by the Office of Naval Research to study the complex dynamics that move water and sand in inlets and river mouths. The inlet, just downstream of the Marine Corps Base Camp Lejeune, is a classic testbed to begin to understand similar places around the world where the Navy might have to navigate amphibious landings, dredge shipping channels, and make plans to mitigate potential pollution spills.

To this environmental battlefield, the scientists brought an arsenal of instruments, including ground-based, airborne, and satellite cameras and radars; boats; and amphibious and autonomous underwater vehicles. WHOI scientists Britt Raubenheimer and Steve Elgar joined colleagues from Scripps Institution of Oceanography to deploy dozens of underwater metal frames throughout the inlet, each equipped with instruments to measure waves, currents, the water’s turbidity, and other features over a full 30-day tidal cycle. Scripps scientists dispensed harmless pink dye into the inlet and tracked how it flowed and dispersed.

Time and tide wait for no scientist

Traykovski studied how currents and tides move sand on the seabed. While others pursued bigger pictures, Traykovski focused on the small scale—sand ripples that shift 3 to 13 feet. To that end, he deployed underwater devices he called “quadpods” in the inlet. The tops of these 7-foot-tall, four-footed frames were submerged by 2 feet at high tide and exposed by 2 feet at low tide. They were equipped with current meters to measure water flow and turbulence and downward-aimed sonars trained on 108 square feet of sand. Sonar images, taken every 30 minutes, showed details of how and why sand moves back and forth as tides and currents move over it (see video).

“Then we needed to know how this representative small scale fits into the bigger picture,” Traykovski said—that is, how these small-scale sand shifts result in large-scale changes that move and shape sandbars and beaches, which, in turn, can redirect currents and waves.

That’s where Traykovski’s one-of-a-kind, makeshift vehicle came in. He equipped his catamaran kayak with two types of sonar and a high-resolution GPS and traversed the inlet channel, grabbing precisely located sonar images of seabed sand patterns as he went. With its two-horsepower motor and catamaran design, he could operate if breaking waves did not exceed two feet.

But in the roiling inlet, 100 yards wide and 3 to 20 feet deep, waves sometimes rose 3 to 6 feet.

“The wave energy coming in against strong outgoing tides creates very rough and turbulent conditions,” Traykovski said. “Every tidal cycle—every six hours—I found sand movement equivalent to what I’ve seen during big storms off Martha’s Vineyard. The water flowing through the narrow inlet creates strong currents, about 3 knots, and the potential for rapid sand movement. That explains why they have to dredge the inlet constantly.”