Open Water Hazards: Surf – Waves, Sets & Lulls, Undertow


From small lake ripples to huge ocean swells, waves are usually caused by the wind. It should be noted that while wave motions travel, the water does not. Large steep waves breaking close to shore are particularly dangerous to young children and the elderly, since the rushing falling water can knock them from their feet and roll them about under the surface.

Types of Waves

 There are four basic types of breaking water waves:

  • Spilling – are much gentler with the crest of the wave gently spilling down the front face of the wave as it breaks. They occur when waves travel from deep water to shallow water over a wide, gentle sloping bottom. These are the safest type of wave to swim in.
  • Plunging/Dumping – have the classic tube or barrel shape and are often called dumping waves. They occur when waves travel from deep water to shallow water very quickly and have to slow down rapidly. That’s why you get plunging waves on steep beaches (shore-dump!), on sandbars, and at the outside edge of rock and coral reefs. Be very respectful of plunging waves!


  • Collapsing – Collapsing waves are a cross between plunging and surging, in which the crest never fully breaks, yet the bottom face of the wave gets steeper and collapses, resulting in foam.
  • Surging – do not plunge or spill, but bulge up near the shoreline and then rush up the beach very quickly. The backrush is equally quick and is often mistaken as undertow. They occur on most beaches that have a moderate slope usually during smaller swell.

(Source: Science of the Surf)


Sets & Lulls

A set is a series of large breaking waves. A lull is a relative calm between breakers which enables swimmers/surfers to swim/paddle out from shore.

Set of Waves.
A set of Waves.


Also called backwash. Prevalent on steep beaches at high or near high tide. The returning wave gains speed because of the steep incline, returning to the surf with great force. The flow is beneath the surface, swimmers feel as if their feet are being pulled out from under them and are being pulled out to sea. The waves moving towards shore on the surface combined with the undertow can knock the swimmer over. Undertow is particularly dangerous to children or non-swimmers at the water’s edge. The swimmer can allow himself to be carried out and then return with the next wave.


The term “undertow” is often used incorrectly in the mistaken belief that on some beaches there is a water flow or current that can pull a person down vertically and hold them underwater until they drown. The United States Lifesaving Association explains this further:

“A rip current is a horizontal current. Rip currents do not pull people under the water–-they pull people away from shore. Drowning deaths occur when people pulled offshore are unable to keep themselves afloat and swim to shore. This may be due to any combination of fear, panic, exhaustion, or lack of swimming skills.
In some regions rip currents are referred to by other, incorrect terms such as ‘rip tides’ and ‘undertow’. We encourage exclusive use of the correct term – rip currents. Use of other terms may confuse people and negatively impact public education efforts.”