Shoalon

Meet Shoalon, an ocean wave who slows down and stands up tall as the sea floor rises to meet him.

When a wave's length is much longer than the water is deep, the whole column of water from surface to sea floor moves with it. The floor then controls the wave's speed. Deeper water lets it race, and shallow water holds it back.

The speed of such a wave is v = √(gh). In the open ocean, 4,000 m deep, v ≈ √(9.81 × 4000) ≈ 198 m/s, about 710 km/h. At a depth of 1 m it is only about 3.1 m/s.

As a wave moves into shallower water its front slows first, and the water behind catches up and piles higher. That is why waves rise steeply before they break on a beach.

Tsunamis are the most dramatic case. Their wavelengths are hundreds of kilometres, so even the deep ocean counts as shallow, and they cross whole oceans in less than a day.