Beaches are a major source of revenue for coastal towns in the United States, as flocks of tourists try to catch summer rays. A major threat to these coastal economies, however, is beach erosion. Waves near the shore can gradually pull large amounts of sand away from the coastline and deposit them in sandbars offshore, resulting in narrower beaches (and less room for sunbathing tourists to lay down their towels).
A recent study by García-Medina et al. deals with the physics of waves that cause beach erosion, something that has been studied since at least the 1950s. In particular, this team of researchers looked at what causes large wave run-up, which is the highest vertical point that a wave reaches once it hits the beach. The size of the wave run-up is critical to the likelihood of beach erosion, as large wave run-up can dislodge sediment high up on the shoreline and carry it back down to the ocean. Large wave run-up can also pull unsuspecting beachgoers into the water and cause fatalities. Such incidents are called “sneaker” waves on the West Coast of the United States and are the leading cause of drownings along the Oregon and northern California coasts.
The researchers examined how a phenomenon called bore-bore capture—when two or more breaking waves in the surf zone combine into one powerful wave rolling onshore—can lead to extreme wave run-up events. To evaluate the effects of bore-bore capture on wave run-up, they implemented a mathematical model with the ability to simulate the typical behavior of waves on a gently sloping beach. On beaches like this, a wave breaks near the shore, seawater washes up on land (that incoming water is called swash), and a mixture of seawater and sand retreats back into the sea (backwash).
Using their model, the researchers found that when two or more bores merge, the combination creates a wave run-up that could be more than 50% larger than the sum of the run-ups that would have been caused by the individual waves in isolation. So bore-bore capture allows waves to combine in a way that enables them to pack a much more powerful punch. But although bore-bore capture is a necessary component of large wave run-up, it is not the sole culprit. Much of the momentum propelling these waves up the shore comes from the interaction between bores and infragravity waves (smaller but longer waves that can be generated by wave groups).
Beach erosion is costly for communities in terms of lost income from tourism, but it also decreases property values, threatens infrastructure such as roads, destroys animal and plant habitats, and can increase the risk of flooding. Further, deaths due to large run-up events (associated with what are known as sneaker waves) are tragic and a major hazard. Knowing more about the mechanisms behind large wave run-up, as illustrated in this study, is an important part of understanding—and, ultimately, preventing—these harmful events. (Journal of Geophysical Research: Oceans, https://doi.org/10.1002/2017JC012862, 2017)
—Sarah Witman, Freelance Writer