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 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?
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.
Ground swell 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 towards the beach. If waves approach the shoreline at 45º, 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.