NEWS

On January 20, 2024, a series of waves washed over Roi-Namur, the second-largest island on Kwajalein Atoll in the Republic of the Marshall Islands (RMI). According to a statement by the U.S. Army, the event caused substantial flooding in the northern portions of the island, which is home to the Army’s Ronald Reagan Ballistic Missile Test Site.

While an extraordinary event, it was not the first time this had happened to islands in the Pacific Ocean; in recent history, similar incidents occurred in 2008, 2014, and 2020. U.S. Geological Survey (USGS) and Pacific Islands Climate Adaptation Science Center (PI-CASC) funded researcher Curt Storlazzi conducted studies in Roi-Namur after it endured its largest overwash event in recent history in 2008, observed another on Roi-Namur in 2014, and observed a similar event on Ofu, American Samoa, in 2020.

What Happened? Not a Rogue Wave?

 So, what causes such a phenomenon? “It was not a ʻrogue wave,’ ʻtsunami,’ or ʻwave group,’” explains Storlazzi. “It was the result of a culmination of interactions between swell, infragravity waves, and resonance.”

To understand what happened on Roi-Namur, it is important to first understand simple wave mechanics.

If you have ever been to the beach, you may have noticed that the peaks in waves pass a given point every 5-25 seconds–the time between two successive wave crests is termed the “wave period.” Waves arrive in “sets” or groupings of waves of variable height. The time between wave sets is on the order of 5-10 waves, therefore, sets come in periods on the order of minutes. A “swell wave” is a long-period wave that is generated by far-away storms with periods that are greater than 7 or 8 seconds.

A wave breaks when the water is about as deep as the wave is tall, so wave height determines the depth at which a wave breaks. Therefore, smaller waves break in shallower water, closer to shore, and taller waves break in deeper water, further from shore.

Most island reefs are characterized by a wide, relatively horizontal “reef flat” that extends from shore seaward to a point, termed the “reef crest,” which is where incoming waves break due to its shallow water depth. Offshore of the reef crest is the “fore reef,” which slopes seaward to depths of 30-40 meters. The reef crest typically dissipates 70-95% of the incoming ocean energy by wave breaking, resulting in a basin of calmer waters over the reef flat up to the shoreline.

In the January event at Roi-Namur, swell waves with 16-18 second periods were arriving in sets on the order of 1-3 minutes. Because the waves in the sets were not all the same height, they broke at different points (depths) on the fore reef (larger waves) and reef crest (smaller waves). The location where the waves were breaking shifted back and forth–closer then farther, then closer, then farther from shore, forcing water onto the reef flat by pumping it like a paddle with alternating long and short strokes. This alternating pattern, in turn, created “infragravity waves,” which are long-period waves with periods similar to that of the wave sets (1-3 min) on the reef flat.

“Think of the reef crest, reef flat, and the shoreline like an enclosed basin,” explains Storlazzi. “In this case, the swell waves and infragravity waves caused the water levels on the reef flat to resonate or oscillate at periods of 10-15 minutes, similar to a jump rope oscillating between two people at fixed endpoints. These oscillations made portions of the enclosed basin deeper (for a little while), allowing for taller swell waves and infragravity waves to propagate across the reef flat and strike the coast without breaking on the reef flat and dissipating their energy there,” he said.

Wave run-up, or how far breaking waves wash up the beach, directly relates to the wave’s period, making resonant oscillations on the order of 10-15 minutes pivotal to such an event occurring. Compared to waves with shorter periods (typically on the order of seconds) that you would see wash up and down the beach face, very long-period waves have the potential for very long run-up and thus inland flooding potential.

“When swell waves, infragravity waves, and very long-period resonance culminate and positively reinforce themselves, they cause massive run-up, so infrequent but large like a tsunami wave, that runs far up the beach and inland – this is what was observed on Roi Namur,” said Storlazzi.

Long Term Effects

Most low-lying islands, such as those in RMI, are often subjected to flooding and overwash, especially during large wave events. January’s incident in RMI illustrates the compounding effects that the nation is struggling with, not just inundation due to sea-level rise, but also the synergistic effects of sea-level rise on the impact of storm waves. The maximum elevations of these islands are about 2 meters above sea level, and thus making them more susceptible to flooding as sea level increases.

According to a 2018 study conducted by the USGS, National Oceanographic and Atmospheric Administration, Deltares, and the University of Hawaiʻi, RMI is projected to be uninhabitable by the mid-21st century because of increasingly frequent flooding. The combination of sea-level rise inundation and wave-driven flooding is projected to cause damage to infrastructure and salinization of the islands’ freshwater aquifers too frequently, preventing natural recovery between overwash events.

In addition to the dire impacts on infrastructure and water security, increased overwash events will create losses to biodiversity, cultural sites and place-based relationships, and subsistence food security on the island and in the nearshore environments.

Storlazzi goes on to state, “Our increased understanding of these wave and water level dynamics and the resulting flooding and island overwash processes allows us to not only better forecast them to reduce risk over the short term, but also develop adaptation plans to reduce their impacts and thus increase the resiliency of the islands’ communities in the future.”

Higher sea level makes the threshold for flooding lower and thus smaller waves can cause the same amount of flooding at higher sea levels than larger waves at lower sea levels. And smaller waves occur more frequently than larger waves, so sea level rise makes flooding events (that require smaller waves at higher sea levels) more likely to occur.