Wednesday, September 19, 2007

How does sand move on or off a beach? In ripples.

Here's a question that likely never occurred to you: What role to the ripples in the sand on the sea floor play in the transport of sand?

Folks actually study that, and seriously.

(Photo: Ripples [with paw prints] just above the waterline at Virginia's Assateague Island National Seashore, Photo by Capt. Albert E. Theberge, NoAA Corps [ret.])

And in a time when most Hawai'i beaches are eroding, ripple science is a field of study that could help understand the processes at work.

Recent research on the subject, “Video-based observations of nearshore sand ripples and ripple migration,” was published in the Journal of Geophysical Research Vol. 112. The authors include J.M. Becker, Y.L. Firing, J. Aucan, R. Holman, M. Merrifield and G. Pawlak, and they conducted their study at Waimea Bay on O'ahu..

In clear, shallow water, it's easy to walk out into the sea and see the long, sinuous humps like lines of dunes on the sandy bottom. What may not be clear unless you conduct measurements, is that those sand lines are moving.

At some beaches, in smaller wave conditions, the sand ripples seem to move toward the shore, and the research team is finding that as the ripples move, so does the sand itself. In fact, “ripple migration may be an important mechanism for onshore sand transport during accretionary phases at Waimea Bay,” the scientists wrote.

Lead author Janet Becker, of the University of Hawai'i's Department of Geology and Geophysics, said ripples are dynamic. They change shape, orientation and size with different conditions.

For example, when they team worked at Waimea Bay, they found that the nearshore ocean sand ripples generally were parallel to the shore, but she said “ripples do change orientation with the direction of forcing,” by which she meant that for ripples created by waves, they change their alignment depending on the direction from which the swell was coming.

On one April morning in Waimea Bay, the team measured ripples with a wavelength of .8 meters. That means that from the peak of one ripple to the peak of another, it was about 31 inches.

Becker said that the wavelength of ripples can be associated both with the size of the grains of sand on the ocean floor, and with the wavelength of the waves passing over the bottom.

The scientists found that on that day, the ripples were moving at about a quarter wavelength in an hour toward the shore. That meant that each ripple moved about 8 inches an hour. That works out to about 16 feet per day.

The ripple itself represents sand moving from one part of the sea floor to another.

Most of the time, sand ripples are moving toward shore. But during big winter surf at Waimea, they can move away from shore.

“Large swell events cause significant beach erosion,” Becker said in an email.

Many beachgoers notice that big winter storms can suck the sand away from the shore at some beaches, while quieter summer conditions tend to replenish the beach. The researchers are trying to learn more about these patterns.

“We are looking at the time periods just following these (large swell) events to determine whether the recovery of the beach is aided in part by the shoreward migration of sand waves,” Becker wrote in the email.

The paper says that the team's work to date suggests that, in fact, it seems as if sand is being transported through the migration of the ripples—that during low waves, as ripples on the sea floor move shoreward, the sand is migrating in the same direction.

“Our preliminary results support this hypothesis, but we need to make field measurements to confirm that the migrating sand ripples really transport a significant fraction of sand,” Becker said.

“We hope to conduct an experiment on this during the winter on the north shore.”

© 2007 Jan W. TenBruggencate

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