Wave Characteristics
In Heavy Seas and Surf Rescue Techniques – General, we emphasized the importance of anticipation—specifically, understanding how the environment will move the rescue swimmer (“RS”) and survivor. This awareness allows the flight mechanic and flying pilot to work in concert, positioning the helicopter and managing the cable to keep the RS and survivor in the optimal location for recovery. In dynamic hoisting environments, the crew must hoist to where the RS and survivor will be, not where they are in the moment.We discussed how different wave types affect swimmer and survivor movement:
- Rolling waves create an orbital motion, moving the RS and survivor in a circular pattern. The larger the wave, the greater the orbital motion.
- Spilling waves push the RS and survivor laterally with moderate energy as the wave gradually breaks.
- Plunging waves deliver a more forceful lateral push as the crest collapses abruptly with increased energy, often displacing the RS and survivor a greater distance.
In this post, we will take a closer look at the factors that influence when a wave will break and whether it takes on spilling or plunging characteristics. We will examine how wavelength and wave period affect swell size and speed as well as explore how currents impact wave behavior. Finally, we will discuss how understanding currents and identifying lulls can improve RS and survivor recovery during helicopter rescue operations.
Water Depth – While several factors contribute, a wave typically breaks when the water depth is approximately half its wavelength. As a general rule, the larger the wave, the greater the wave length, and the deeper the wave energy. As the wave energy reaches the sea floor, the bottom of the wave slows causing the crest of the wave to spill or plunge. Therefore, typically larger waves break farther offshore because they encounter enough bottom friction to cause them to spill and plunge in deeper water.
Wavelength and Period – While longer wavelength and period often correlate with larger offshore wave heights, this is not always the case. However, longer waves travel faster and carry energy deeper below the surface, which consistently produces more powerful surf. From a helicopter crew’s perspective, these long-period swells—though more powerful—can actually make deployments and recoveries easier than short, steep waves, at a shorter interval. Even if smaller in size, short-period swells allow for less time to deploy and recover RSs between waves.
Slope of Underwater Terrain – The steeper the underwater slope, the more quickly a wave will rise and break, resulting in a more abrupt and forceful lateral energy transfer. Gradual slopes tend to produce spilling breakers, where the wave energy is released over more time and distance. In contrast, steeper slopes generate plunging waves, which break with greater force as the crest collapses. An extreme example is the steep shoreline, where surging waves crash directly onto the beach. These powerful waves can quickly sweep people off their feet and into the surf, as the steep terrain compresses the wave energy into a short, intense burst of lateral motion.
Wave Sets and Lulls – Waves typically arrive in groups known as “sets,” often consisting of 3 to 7 waves that can increase in size as the set progresses. Between these sets, there are lulls—periods of relative calm marked by smaller, less powerful waves. A classic Coast Guard rescue scenario involves a vessel disabled and drifting near the edge of the surf zone. While conditions may initially seem manageable, a large set can suddenly extend the surf zone farther offshore, quickly turning rolling seas into violent surf capable of capsizing the vessel. When a rescue is in progress, it can be safer and more effective to wait for a lull to execute the recovery, rather than attempting a hoist in the middle of a large, powerful set.
Currents – In heavy seas, large volumes of water are driven toward shore and must return to the ocean. This return flow often forms rip currents, which typically channel through deeper sections of the beach or near points, rocks, or jetties. While rip currents pose a serious hazard to swimmers trying to stay nearshore, they can actually work to a rescuer’s advantage. If a RS and survivor enter a rip current, it can help carry them out of the surf zone into deeper water—where waves are less likely to break and conditions tend to be more manageable for recovery.
River outflows and large-scale ocean currents can significantly influence wave behavior. When currents flow with the swell or wave direction, they tend to flatten and space the waves out. In contrast, when currents oppose the swell, they compress the wave energy—causing the waves to steepen, stack up, and crumble and break earlier.
A notable example is the mouth of the Columbia River, where tidal flow dramatically affects sea conditions. During a flood tide (when water is pushing upriver), waves often increase in speed but decrease in height. During an ebb tide (when the river is flowing out to sea), waves tend to grow larger and bunch closer together.
The dynamic of the outflowing river, combined with the nearby Astoria Canyon, where the sea floor rises rapidly standing up already large waves that have grown across the Pacific, contribute to the region’s notorious reputation as the “Graveyard of the Pacific.”
Similarly, George’s Bank, located between Cape Cod and Nova Scotia, features a sharply rising seabed that, under the right conditions, produces very large and steep waves—conditions that have tested Coast Guard Air Station Cape Cod crews during some of their most notable rescues.
Farther south, off the mid-Atlantic coast, the fast-moving Gulf Stream can collide with large swells generated by large low-pressure systems, hurricanes, and tropical storms. This interaction can produce towering, steep waves that spill even in deep water—without needing the influence of shoaling terrain. A similar phenomenon contributes to the large seas commonly encountered in the Bering Sea and leads to high seas rescues by Kodiak and Elizabeth City crews.
