Ericeira: shoaling and refraction doing a good wave job

Shoaling and refraction are two of the most important variables when it comes to wave height forecast. Wind produces large ground swells which travel thousands of miles until they hit the coastlines. The face of the wave can quickly change, though.

The truth is that these factors - shoaling and refraction - can easily and dramatically change the face height of waves in two close surf spots separated by 100 meters or miles. So, how big will waves be?

Shoaling has a determining influence in waves and wave height. When waves reach shallow water areas or seaside regions where the bottom seafloor offers obstacle, then these waves tend to rise up.

Ground swell waves accumulate energy in longer periods and start to shoal sooner. The result is bigger waves than short period waves. Also, we know from wave physics that a wavelength of 7 meters produces waves which break as soon as they only have one meter of height and a total depth of 1.3 times the wave height.

So, the wave steepness concept relates to shoaling, too. If a seven-meter wavelength produces one-meter waves reaching 1.3 meters of depth, they will break. That is the ration, says physics.Ocean floor is not constant, stable and flat. A radical slope will produce different surfing conditions from a gradual rise in another seafloor.

Refraction also occurs in shallow water. When waves reach the coastline, they encounter and adapt to different water depths as they move towards the beach. If waves approach the shoreline at 45º, for example, and hit a reef, they will bend, change direction and speed.

Steepness, height, angle, energy, wavelength, shoaling and refraction are concepts that travel together when planning an accurate surf forecast for you local spot.

Learn how waves are formed.
Track your own waves and swell.
Know the difference between ground swells and wind swells.
Read the best surf forecasting books.