13.5: The Timing of Waves

The time element of progressive waves turns out to be quite important for surf prediction. When watching waves from the end of a pier, you are likely to notice wave crests passing the end of the pier one after another. If you watch the water level along one of the pier pilings, you’ll see the water rise as the crest approaches and fall as the crest passes. After a few to several seconds, it happens again. Seconds later, it happens again, and so on. The time between successive crests is the wave period.

One way to understand wave periods is to consider a freeway full of cars barreling down the road. Each car maintains a certain time between it and the car ahead of it. You can measure the time between you and the car ahead of you by asking a passenger in your car to start a stopwatch at the moment the car ahead of you crosses under an overpass. Stop the stopwatch right when you pass under the overpass and you have the time that separates you and the car ahead of you. Think of cars as wave crests, and you have the idea.

Of course, wave periods can be measured using more sophisticated equipment. Ocean-deployed weather buoys digitally record the rise and fall of a float as waves pass by. These real-time wave-monitoring systems are critical for alerting scientists and authorities when potentially destructive big waves are approaching a shoreline. Of course, surfers take an interest in these buoy measurements as well.

Wave periods serve as the basis for a more formal classification system of waves. Capillary waves have the shortest periods, less than 0.1 seconds. Waves with periods of 0.1 to 1 second—what we might call ripples—are classsified as ultragravity waves. The kind of waves we experience most often on the ocean—ordinary gravity waves—vary from 1 to 30 seconds. Waves with periods from 30 seconds to 5 minutes—infragravity waves—originate from interactions between shorter period waves in coastal waters. These waves likely play a role in sediment transport and other nearshore processes (Cook 2018). Long-period waves—waves with periods from 5 minutes to 12 hours—include storm waves and tsunami. Beyond that, we venture into tides and other kinds of very long period waves.

Wave periods serve as a proxy for surf conditions. In general, the longer the wave period, the faster and more energetic the wave. Savvy surfers check surf reports, internet, app- or even phone-based tools that report information on wave periods, weather, and other surf-related factors. In Southern California wave periods of 10 seconds or more are considered pretty rocking. Wave periods of more than 20 seconds are insane! Some of the most epic surf days have wave periods of more than 20 seconds. Fortunately, such long-period waves are rare. They create very dangerous conditions along the coast and can tear up a coastline and damage property in just a few hours.

Though not wind-driven, tsunami—long-period waves generated by vertical motions of the seafloor during shallow earthquakes—have very long periods, on the order of five minutes to an hour. That’s why it’s important to remain out of harm’s way after the first arrival of a tsunami. There are more waves to come! Tsunami may travel the ocean for days.