1 : a turbulent surface of water caused by strong currents setting over submerged ridges or shoals or by winds opposing a current —usually used in plural. 2 obsolete : cataract, waterfall. 3 : a place provided for the overflow of surplus water (as from a canal or lock)

What is an Overfall at sea?

Whenever the tide turns, water heads from one area to another. ... Tidal currents moving over a rough seabed make the surface of the sea turbulent. This effect is known as 'overfalls'. A shallow sea, fast currents and more dramatic submarine landscape… all make the water rougher.

Why are overfalls dangerous?

Overfalls are most hazardous when their waves are high enough to capsize a small boat or to break heavily over its deck and cockpit. For waves to become high before they break they must also be fairly long and this occurs during storms, when big waves are both high and long.