OREGON STATE UNIVERSITY

Rip currents could play role in increased coastal erosion

03/11/2009

CORVALLIS, Ore. – Amid growing concern about rising sea levels triggered by global warming, Oregon Sea Grant researchers at Oregon State University are discovering that rip currents might play a role in coastal erosion because they create rip “embayments” – or low areas on sandy beaches – that expose nearby land to higher rates of erosion by wave activity.

“There is now evidence that we’re experiencing larger coastal storms and increased wave heights that result in larger waves along shorelines,” said Merrick Haller, a coastal engineering professor at OSU who led recent research on these embayments.

“When rip currents pull sand offshore, they leave behind rip embayments, which become ‘erosional hot spots’ where the beach is much thinner, making the sea cliffs or land beyond these embayments more vulnerable to erosion caused by larger waves.”

Rip currents form in places where the water that is driven ashore with the waves drains back out to sea forming a current perpendicular to the coastline. Swimmers can be caught in these currents and pulled offshore. The Pacific Northwest is known for having strong rip currents, probably due to large swells offshore, said Haller, whose research focused on several beaches along the Oregon coast.

Using funding provided by Oregon Sea Grant, and building on earlier research conducted by OSU’s Paul Komar, an emeritus professor in the College of Oceanic and Atmospheric Sciences, Haller and his research team studied the morphological characteristics of these embayments to better determine how rip currents form embayments. The team also studied whether the locations of embayments can be predicted.

“A major challenge is to predict where rip currents will appear, because if we knew how to predict them, we could not only warn swimmers, we could also predict where erosion will likely occur farther inland from the embayments they form,” Haller said.

This knowledge would be helpful for coastal development. Many structures sited along the coast were built before it was known that rip embayments influence erosion.

In 2006, Jonathan Allan, a coastal geomorphologist with the Oregon Department of Geology and Mineral Industries who helped Haller with the embayment research study, documented a sea cliff that eroded approximately 20 feet in a single weekend to within a few feet of an existing home near Gleneden beach.

Haller catalogued existing embayments using several years of LIDAR data collected by the National Oceanic and Atmospheric Administration. The data were used to find the locations of embayments as well as the topography of the exposed beaches near the embayments.

“We wanted to know if these embayments show up in the same places, or if they migrate north or south in a certain pattern depending on wave conditions,” Haller said. “What we found is that where they form seems to be random; they showed no tendency to always show up at the same spots, nor did they appear to migrate, and they tended to disappear in less than six months.”

But Haller’s team wanted to learn how rip embayments form and under what conditions. So they ran computer model simulations, the findings of which suggest that embayments might preferentially form during moderate storms, instead of large storms as previously thought.

“When waves are really big, they start breaking way offshore, so by the time they arrive onshore, the energy is dissipated and erosion is spread uniformly along the beach,” Haller said. “But our findings suggest that there appears to be a middle range of wave heights that lead to a strong feedback between wave breaking and the shape of an incipient embayment. This feedback can drive a strong rip current and further embayment formation. Hence, embayment formation may be more prevalent during moderate storms.”

But Haller is quick to point out that more research is needed before concrete conclusions can be reached. Many other factors may play a role in embayment formation, including sand grain size, antecedent wave conditions and rock outcrops in shallow areas of the ocean. This near shore area is also challenging to study because it is shallow, waves are constantly breaking and LIDAR cannot penetrate murky water.

Recently, Haller and Peter Ruggiero, an OSU professor of geosciences, assembled a specialized personal watercraft equipped with echo sounders, computers, and a GPS system to collect bathymetry data in these shallow areas near shore.

This data will add to their understanding of how rip embayments form, helping the researchers eventually find ways to predict how and where rip currents and their embayments develop.

Oregon Sea Grant, founded in 1968 and based at Oregon State University, supports research, education, and public outreach to help people understand, responsibly use, and conserve ocean and coastal resources.