Hobie Surf: Surf Science 101: Wave Speed

Last week’s blog focused on the genesis of ocean waves. This weeks I will attempt to breakdown the often lengthy journey open ocean waves take on before reaching the coastline. We know that from low-pressure storm activity, strong winds can create waves that can exceed 50-feet in height. Waves of these immense heights are common within the fetch and just outside the area of storm activity but decay rapidly during the initial distance traveled. kind of like how a brand-new car driven off the car lot instantly loses value, waves generally lose between 80 – 90 per cent of their energy in the first 100 miles traveled away from the area of storm activity. So, an initial wave height of 30-feet will be reduced to 6-feet, soon after traveling away from the area of propagation.

Once waves travel out of the fetch, or area where the wind is the generating force, they are no longer wind waves and in-turn identified as free waves, or what we understand as ground swell.

Wave Speed

For me, the most intriguing characteristic of waves is wave speed. As waves travel away from the storm’s influence they begin to organize into groups of waves with similar period and wave length called wave trains or wave groups.

Hobie Surf: Surf Science 101: Genesis of Waves

Welcome to a new feature on our blog, think of it as your Sunday morning read. A chance to get up, sip your coffee and learn something new, or enrich your knowledge on something you already know a lot about. Gary Larson, will take you through the science behind the things we all enjoy, but aren’t really sure exactly why/how they happen in his course  Surf Science 101. Enjoy!!

Waves

Ladies and gentleman summer is upon us and that means south-swells are aplenty. You’re excited for the 18-20 second, New Zealand ground-swell from about 200° but not exactly sure why? Well, feel free to saddle-up for the inaugural weekly installment of Surf Science 101, where I will attempt to enlighten you on some of the perplexing phenomenon we, as surfers, often encounter in the ocean. This week I will be exploring the formation of ocean waves, the variables involved and how they can travel for thousands of miles before breaking along the coastline.

Contrary to the explanations I have heard, from middle America folk to the well seasoned Southern California surfer, waves that we surf on a daily basis are not caused by the moon, tides or the large freighters criss-crossing the ocean. The only variable that is absolutely essential for wave propagation is wind. That’s it. If the planet Earth ditched its moon, ceased all tidal fluctuations and sank every boat in all seven-seas, but wind still blew over the surface of the ocean, we would still have waves.

That being said, there are three important factors that govern wave heights; wind speed, wind duration and fetch. Fetch is understood as the distance over water that wind blows in approximately the same direction. If there is an increase or decrease in any of these three variables the wave height will either increase or decrease, respectively. For example, if a 40-knot wind blew for 24 hours over a 100 mile fetch the wave heights would be larger than if the wind speed was 30-knots and the fetch was 50 miles.

Wave Propagation Chart. Wave heights increase when the wind speed and/or fetch length increases.