Seafloor Shifts Can Displace Beaches

By Larry O'Hanlon, Discovery News
Discovery Channel

Stripped Irish Beach
June 18, 2004 —Rising sea levels from climate change are expected to wreck beaches and cause other coastal changes, but it may harder to be sure some troubles weren't already in the works, say Irish geologists.
Coastal researchers at the University of Ulster made the discovery while trying to figure out why a beach at an old seaside resort near Newcastle, Northern Ireland, lost its sand. It looked like the sort of loss that might be caused by sea levels rising and washing the sand out to sea, but it wasn't.


In a study published in the June issue of Geology, coastal geologists Andrew Cooper and Fatima Navas compared modern and century-old seafloor data of Dundrum Bay. By modeling the 1861 seafloor and comparing it with the 1968 seafloor, they were able to reproduce the way waves broke and pushed sand around the bay.

What the model revealed is that natural changes in the seafloor probably caused the loss of sand at the recreational beach, while at the same time piling up sand offshore and on the shores of a military installation on the other side of the bay, explained Cooper.

Such changes could happen along any coastline, but would be hard to separate from sea-level rise without very old and very reliable data.
"That sort of data is rare as hen's teeth," explained coastal geologist Orrin Pilkey of Duke University. Although the British Navy surveyed many coastal areas throughout the 19th century, many of the benchmarks they used to measure from are gone, so the data can't be used.

In the case of Dundrum Bay, however, the British Navy surveyed the seafloor in 1861 and the measurements were tied to some lasting onshore features. All Cooper and Navas had to do was feed the data into the computer model.
In 1861, the direction of waves as they neared shore and felt the steeper bottom was bent to direct surf energy straight into shore. That drove two sand-loaded currents in the bay, a northward one in the north and a southward one in the south.

By 1968, natural changes in the seafloor had altered the way waves break and move sand. The slope in the seafloor to the shore had become less steep, causing waves to break differently. The less- steep bay bottom caused the surf energy to move more northerly, shifting all the sand offshore and to the north onto the military beach that's closed to the public, Cooper said.
"It could be happening on many beaches around the world," agrees Pilkey.
But without reliable historic data, sea-level rise will likely take the rap.

Media contact: Tim Lucas, 919/613-8084 or tdlucas@duke.edu