Undertow (water waves)
In
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]
Oceanography
An "undertow" is a steady, offshore-directed compensation flow, which occurs below waves near the shore. Physically, nearshore, the wave-induced
The term undertow is used in scientific coastal oceanography papers.
Seaward mass flux
An exact relation for the mass flux of a
Similarly,
with the mean water depth and the fluid density. The positive flow direction of is in the wave propagation direction.
For small-amplitude waves, there is equipartition of kinetic () and potential energy ():
with the total energy density of the wave, integrated over depth and averaged over horizontal space. Since in general the potential energy is much easier to measure than the kinetic energy, the wave energy is approximately (with the wave height). So
For irregular waves the required wave height is the
The distribution of the undertow velocity over the water depth is a topic of ongoing research.[5][6][7]
Confusion with rip currents
In contrast to undertow, rip currents are responsible for the great majority of drownings close to beaches. When a swimmer enters a rip current, it starts to carry them offshore. The swimmer can exit the rip current by swimming at right angles to the flow, parallel to the shore, or by simply treading water or floating until the rip releases them. However, drowning can occur when swimmers exhaust themselves by trying unsuccessfully to swim directly against the flow of a rip.
On the United States Lifesaving Association website, it is noted that some uses of the word "undertow" are incorrect:
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.[2]
See also
- Longshore current– A current parallel to the shoreline caused by waves approaching at an angle to the shoreline
References
Notes
- .
- ^ a b United States Lifesaving Association Rip Current Survival Guide, United States Lifesaving Association, archived from the original on 2014-01-02, retrieved 2014-01-02
- ^
MacMahan, J. H.; Thornton, E. B.; Reniers, A. J. H. M. (2006). "Rip current review". S2CID 14128900.
- hdl:1912/4067
- ^
- ^
- ^
- S2CID 109502295
- from the original on 2015-06-15
- S2CID 123723040
Other
- Buhr Hansen, J.; Svendsen, I.A. (1984), "A theoretical and experimental study of undertow", in Edge, B.L. (ed.), Proceedings 19th International Conference on Coastal Engineering, Houston: ASCE, pp. 2246–2262
- Okayasu, A.; Shibayama, T.; Mimura, N. (1986), "Velocity field under plunging waves", in Edge, B.L. (ed.), Proceedings of the 20th International Conference on Coastal Engineering, vol. 1, Taipei: ASCE, pp. 660–674
External links
- Tatiana Morales (2004-05-27), Watch out for rip tides, CBS News, retrieved 2015-06-24