Beach and Coastal Structure Impacts of Infragravity Waves During the Late December 2023 Swells in Southern California

The ocean is rarely still, even when the surface looks calm. While most people think of waves as the rhythmic, wind-driven rollers that crash on a beach, the sea also contains a hidden, slower motion that moves beneath the surface. Scientists call these long, rolling swells "infragravity waves." Unlike the familiar wind waves that break every few seconds, these slower waves can take anywhere from a few minutes to half an hour to pass a single point. They are often invisible to the naked eye because they do not look like crashing water; instead, they act like a slow, rising and falling floor that lifts the entire ocean surface. When a massive storm sends a powerful swell toward the coast, these slow waves can become trapped in bays and harbors, building up energy like water in a bathtub being shaken. This energy can suddenly push the water level much higher than expected, allowing the faster, crashing wind waves to ride on top of this elevated floor and reach far inland, causing flooding and erosion that standard forecasts might miss.

In late December 2023, a powerful storm in the North Pacific sent a massive swell toward Southern California, creating a perfect test case for understanding how these hidden waves behave. A team of researchers, including scientists from the U.S. Army Corps of Engineers and the University of Southern California, set out to investigate why this specific storm caused such dramatic and unexpected damage along the Santa Barbara Channel. They wanted to know if the slow, long-period waves were responsible for turning a large storm into a localized disaster. By combining data from offshore buoys, tide gauges, video footage, and detailed computer simulations, the team pieced together a story of how energy from the deep ocean traveled into the harbor, amplified by the shape of the coastline, and resulted in a sudden, rogue wave that flooded a street and reshaped the beach.

The storm, which hit from December 28 to 31, 2023, was part of a larger weather pattern associated with El Niño conditions. While the waves were large, they were not the largest ever recorded in the open ocean off the coast of California. However, once the swell reached the complex geography of the Santa Barbara Channel, the situation changed. The channel is a long, curved basin sheltered by islands and points of land, which causes waves to bend, reflect, and focus in specific areas. The researchers found that while the open ocean waves were significant, the real danger came from what happened next. As the storm waves entered the channel, they generated a strong pulse of infragravity energy. This energy did not arrive as a single wave but as a sustained, rhythmic oscillation of the water level that lasted for hours.

The most striking evidence of this phenomenon occurred in the city of Ventura on the morning of December 28. Around 10:50 AM, just as the tide was high, a massive "sneaker wave" suddenly surged over the beach and a low concrete seawall. This was not a typical breaking wave; it was a sudden, powerful rush of water that flooded a nearby parking lot and street, reaching depths of about one meter above the road surface. Eyewitness video and post-storm surveys showed that the water splashed up to a height of 5.5 meters above the normal water level. The researchers noted that this event was highly localized; while the entire coast was battered, this specific spot was hit with a force that seemed disproportionate to the offshore wave height. The timing of this event coincided with a period of intense, low-frequency energy in the harbor, suggesting that the slow waves had built up enough momentum to lift the water level, allowing the storm waves to ride over the seawall with devastating effect.

To understand how this happened, the team looked at data from tide gauges in both Santa Barbara and Ventura harbors. They used a method that breaks down the water's movement into different speeds, revealing that the harbors were vibrating at specific, natural rhythms. It is similar to how a specific note can make a glass shatter, but in this case, the ocean was resonating with the slow, long waves. The analysis showed that the harbors were amplifying waves with periods between 16 and 21 minutes. When the storm's energy matched these natural rhythms, the water level in the harbor began to oscillate with increasing intensity. In Ventura, this amplification created a "hotspot" where the water was constantly being pushed higher and higher, setting the stage for the sneaker wave.

The physical damage on the ground confirmed the power of these interactions. After the storm, the researchers surveyed the coastline and found that the beach had retreated by about 25 meters in some areas, with a steep, two-meter-high cliff of sand carved into the dunes. This erosion was not just from the wind waves crashing; it was driven by the sustained high water levels caused by the infragravity waves, which kept the water against the dunes for longer periods, washing away sand that would otherwise have been safe. The damage extended to the harbor structures as well. The offshore breakwater, a wall of large rocks designed to protect the harbor, suffered significant damage. Large stones were displaced, and the slope of the structure became dangerously steep. The researchers observed that while the infragravity waves themselves did not smash the rocks, they raised the water level enough to allow the smaller, faster storm waves to hit parts of the wall that are usually safe, wearing them down over time.

Using a sophisticated computer model, the team simulated the storm to see how the waves interacted with the harbor's shape. They tested waves of different lengths to see which ones would cause the most trouble. The model confirmed that the Santa Barbara Channel has specific resonant modes, or natural frequencies, that can trap and amplify long waves. When they simulated the actual conditions of December 28, the model showed that waves with a period of about 120 seconds were focusing energy right at the location of the sneaker wave. This simulation suggested that the sneaker wave was the result of a "perfect storm" of conditions: a high tide, a strong storm swell, and a specific, amplified long-wave rhythm that focused energy on that particular stretch of the coast.

The study also highlighted that not all parts of the coast react the same way. While the Santa Barbara Channel experienced record-breaking conditions, nearby areas like Santa Monica Bay were largely spared from the worst of the event because the shape of the coastline and the direction of the waves shielded them. This finding is crucial for coastal safety. It suggests that extreme wave events are not uniform; a storm that is dangerous in one harbor might be harmless in another just a few miles away. The researchers emphasized that relying solely on the height of the waves in the open ocean is not enough to predict coastal hazards. The slow, long-period waves can transform a manageable storm into a local disaster by lifting the water level and allowing waves to reach further inland than anyone would expect.

Ultimately, this research provides a clearer picture of how coastal communities can be vulnerable to forces they cannot always see. The infragravity waves acted as a hidden amplifier, turning a large swell into a localized flood and erosion event. By identifying the specific rhythms that cause harbors to resonate, scientists can better predict when and where these dangerous conditions might occur. The work does not claim to have solved the problem of coastal erosion, but it offers a vital piece of the puzzle: understanding that the ocean's slow, deep rhythms can be just as destructive as the visible, crashing waves. For the residents of Ventura and other coastal towns, this means that future safety plans must account for these long-period waves, ensuring that seawalls and beaches are designed to withstand not just the height of the storm, but the hidden, rising floor of the ocean beneath it.