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WILLIAMSTOWN, Mass., January 19, 2018—What is the largest rock that can be moved by storm waves breaking on the coast? New observations just published by a research group from Williams College show that it is at least 620 tons: the equivalent of about five and a half space shuttles, fully loaded for takeoff, or approximately three and a half blue whales.
Williams College Professor of Geosciences Rónadh Cox and her team of undergraduate researchers collected data showing that powerful waves from ocean storms on the west coast of Ireland had moved giant boulders. The team first collected photographs of boulder deposits along the coast, which they precisely positioned using GPS. Then, after hurricane-strength storms during the winter of 2013–2014, the team returned and painstakingly duplicated the photographs, showing which rocks had moved. The before-and-after comparisons showed that waves had shifted boulders weighing hundreds of tons, and had also created new boulders by ripping them straight out of the bedrock.
The largest boulders were found close to sea level, where the waves had shifted them just a few meters from their original locations. But the fact that they moved at all is remarkable since the boulders are more than three times larger than any known to have been moved by storms in the past.
The same storms dislocated boulders weighing tens of tons at the tops of high cliffs in some locations, and up to several hundred meters inland in other places: the combination of their weight and distance from the shoreline had previously led scientists to assume they could only be moved by tsunamis. The Williams College group’s work upsets that assumption, showing definitively that storm waves are powerful enough to transport objects weighing hundreds of tons.
The group used measurements of 1,152 boulders from many different locations on the Irish coast to show relationships between boulder size and topographic location (height above sea level and distance inland). These results can be used in many places around the globe where boulder movements have not been directly observed, to test whether other large boulders presumed to have been moved by tsunami may in fact have been created by storm waves.
The group’s work is attracting attention as a significant advance in understanding the full power of ocean waves. As the global climate changes and storm intensity increases, such research can help coastal engineers and policymakers who are trying to improve coastal resiliency. Adam Switzer, principal investigator at the Earth Observatory of Singapore, who was not involved in the study, said “We have long suspected that storm waves are capable of moving very large boulders and this study really adds to the body evidence for what storm waves can do at the coast. The work contains a very detailed review and an amazingly comprehensive dataset that will now allow engineers and modelers new insights into offshore wave power and rocky shoreline dynamics.”
Robert Weiss, associate professor of natural hazards at Virginia Tech, and who also was not involved in the project, agrees. “I always argue that we can only make progress in understanding hazards to prevent future disasters by bringing field-based and modeling-based science closer together,” Weiss says. “One of the ways to accomplish this goal is with carefully collected and large datasets. The presented research accomplished this with an impressive dataset that will give us work for the next decade.”
Although the new observations break all previous records for storm-wave boulder transport, Cox emphasizes that bigger rocks are likely to be moved by larger storms in the not-too-distant future. Switzer agrees, “We must now await the next super storm, and see what gets tossed. The next huge boulder may appear just around the corner.”
The study is published in Earth Science Reviews, and is available to read or download at https://www.sciencedirect.com/science/article/pii/S0012825217302350
Founded in 1793, Williams College is the second-oldest institution of higher learning in Massachusetts. The college’s 2,000 students are taught by a faculty noted for the quality of their teaching and research, and the achievement of academic goals includes active participation of students with faculty in their research. Students’ educational experience is enriched by the residential campus environment in Williamstown, Mass., which provides a host of opportunities for interaction with one another and with faculty beyond the classroom. Admission decisions on U.S. applicants are made regardless of a student’s financial ability, and the college provides grants and other assistance to meet the demonstrated needs of all who are admitted.