Undertow (water waves)

In physical oceanography, undertow is the undercurrent that moves offshore while waves approach the shore.

Undertow is a natural and universal feature for almost any large body of water; it is a return flow compensating for the onshore-directed average transport of water by the waves in the zone above the wave troughs.

The undertow's flow velocities are generally strongest in the surf zone, where the water is shallow and the waves are high due to shoaling.

[3] An "undertow" is a steady, offshore-directed compensation flow, which occurs below waves near the shore.

Physically, nearshore, the wave-induced mass flux between wave crest and trough is onshore directed.

This mass transport is localized in the upper part of the water column, i.e. above the wave troughs.

This flow – the undertow – affects the nearshore waves everywhere, unlike rip currents localized at certain positions along the shore.

[5][6][7] The distribution of flow velocities in the undertow over the water column is important as it strongly influences the on- or offshore transport of sediment.

In the surf zone, strong undertow generates a near-bed offshore sediment transport.

[5][6][7][8] An exact relation for the mass flux of a nonlinear periodic wave on an inviscid fluid layer was established by Levi-Civita in 1924.

[9] In a frame of reference according to Stokes' first definition of wave celerity, the mass flux

through: Similarly, Longuet Higgins showed in 1975 that – for the common situation of zero mass flux towards the shore (i.e. Stokes' second definition of wave celerity) – normal-incident periodic waves produce a depth- and time-averaged undertow velocity:[10] with

the total energy density of the wave, integrated over depth and averaged over horizontal space.

The distribution of the undertow velocity over the water depth is a topic of ongoing research.

[5][6][7] In contrast to undertow, rip currents are responsible for the great majority of drownings close to beaches.

However, drowning can occur when swimmers exhaust themselves by trying unsuccessfully to swim directly against the flow of a rip.

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.

Use of other terms may confuse people and negatively impact public education efforts.