Ericeira: a surf spot where you can see shoaling and refraction doing an excellent job on waves

Shoaling and refraction are two of the most important variables when it comes to wave height forecasts.

Wind produces large ground swells, which travel thousands of miles until they hit the coastlines. The faces of such waves can change quickly.

Shoaling and refraction are factors that can easily and dramatically change the face height of waves, say, between two nearby surf spots separated by 100 meters or miles.

So, how can we predict the sizes of the waves at the two spots? 

Rise and Bend

Shoaling has a determining influence on wave height.

When waves reach shallow water areas or seaside regions where the seafloor offers an obstacle, the waves tend to rise up.

Groundswell-generated waves accumulate energy over longer periods and begin to shoal sooner. The result is bigger waves than we see with short-period waves.

Also, we know from wave physics that a wavelength of seven meters produces waves that break as soon as they only have one meter of height and a total depth of 1.3 times that wave height.

Refraction also occurs in shallow water.

When waves reach the coastline, they encounter and adapt to different water depths as they move toward the beach.

If waves approach the shoreline at 45 degrees, for example, and hit a reef, they will bend, changing direction and speed.

Steepness, height, angle, energy, wavelength, shoaling, and refraction are concepts that belong together when planning accurate surf forecasts for your local spot.

Learn how waves are formed and the difference between ground swells and wind swells.

Discover the best surf forecasting books.

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