Undertow (water waves)

This article is about undertow beneath water waves. For other uses, see Undertow (disambiguation).
This article is about an offshore current under all near-shore waves. Not to be confused with Rip current.
A sketch of the undertow (below the wave troughs) and the shore-directed wave-induced mass transport (above the troughs) in a vertical cross-section across (a part of) the surf zone. Sketch from: Buhr Hansen & Svendsen (1984); MWS = mean water surface.

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.[1]

In popular usage, the word undertow is often misapplied to rip currents.[2] An undertow occurs everywhere underneath shore-approaching waves, whereas rip currents are localized narrow offshore currents occurring at certain locations along the coast.[3]

  1. ^ Svendsen, I. A. (1984). "Mass flux and undertow in a surf zone". Coastal Engineering Journal. 8 (4): 347–365. doi:10.1016/0378-3839(84)90030-9.
  2. ^ Cite error: The named reference USLA was invoked but never defined (see the help page).
  3. ^ MacMahan, J. H.; Thornton, E. B.; Reniers, A. J. H. M. (2006). "Rip current review". Coastal Engineering Journal. 53 (2): 191–208. doi:10.1016/j.coastaleng.2005.10.009. hdl:10945/45734. S2CID 14128900.