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Undertow (water waves)
In physical oceanography, undertow is the steady current – below the splash zone – which is moving offshore for waves approaching a shore. The undertow is the return flow which compensates for the onshore-directed mean mass flow in 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). Undertow can be physically experienced as the offshore current pull that a person standing in the wave-breaking zone can feel most strongly near their feet, as each breaking wave advances towards them.
In popular usage, an undertow is often confused with a rip current.〔 While an undertow occurs everywhere under the shore-approaching waves, rip currents are strong and localized narrow currents occurring at certain locations along the coast.
== Oceanography ==
An "undertow" is a steady, offshore-directed compensation flow below the waves near the shore. Physically, nearshore, the wave-induced mass flux between wave crest and trough is onshore directed. So this mass transport is localized in the upper part of the water column, i.e. above the wave troughs. To compensate for the amount of water being transported towards the shore, a second-order (i.e. proportional to the wave height squared), offshore-directed mean current takes place in the lower section of the water column. This flow – the undertow – affects the nearshore waves everywhere, unlike rip currents localized at certain positions along the shore.
The term undertow is used in scientific coastal oceanography papers.〔〔〔 The distribution of flow velocities in the undertow over the water column is important in coastal sciences: it strongly influences the on- or offshore transport of sediment. Regularly, outside the surf zone there is a near-bed onshore-directed sediment transport (induced by Stokes drift and skewed-asymmetric wave transport). In the surf zone, the strong undertow generates a near-bed offshore sediment transport. These antagonistic flows may lead to sand bar formation where the flows converge near the wave breaking point, or in the wave breaking zone.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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