If you’ve driven any length of the Sonoma Coast and northward, you’ve probably seen these signs:
These warnings are almost always accompanied by a family or group of tourists doing precisely what the sign says not to do. And every year, people drown after being caught unawares by sleeper waves.
It’s the deceptive nature of sleeper waves, sneaker waves, rogue waves — all names for the same phenomenon — that people unfamiliar with the California coast are lulled into a false sense of security. They are rogue or sleeper precisely because they strike randomly, out of the great blue, often with force that can draw people who are standing below the berm into the current, or knock bystanders off cliffs or rocks. They can stretch 100 yards beyond the regular foam line.
The Hows of Sleeper Waves
From what I’ve read, the mechanism of rogue waves isn’t 100 percent understood. But a rogue can be a convergence of sorts, of two wave trains meeting and cresting at the same point, creating one huge sleeper. (The whole expanse of wave sequence is called a wave train.)
Normal wave motion:
Sleeper waves tend to occur along rocky shorelines where there’s a steep and deep drop off from the shoreline. That is, a good part of the Northern California coast.
Sleeper waves differ from tsunamis in that sleepers are wind-driven waves. Tsunamis result from seismic action.
This weekend, we’ve had high swell warnings with predictions of 30-foot swells in some areas. Swells form far from the beaches where they break, out at sea in storm or high-gale conditions. They can travel immense distances. We saw the beginnings of this high-swell warning in the form of intense breakers yesterday along the beaches. The pounding of the surf is enough to keep your attention on the water line, with an eye to higher ground.
For a more detailed look at wave dynamics, Stormsurf has a great explanation of both wind waves and swells.