Undertow
Definition of Undertow:
Undertow is the current flowing offshore near the seabed in the surf zone, mainly driven by wave set-up at the shoreline, and compensating for onshore mass transport by wave crests and wave bores.
This is the common definition for Undertow, other definitions can be discussed in the article
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Notes
The undertow is a net circulation in the cross-shore vertical plane representing a mechanism for maintaining the mass balance in the surf zone. Other possible mechanisms in the nearshore circulation is the three dimensional pattern of rip currents and the pattern of longshore currents in the case of oblique wave incidence. When standing just seaward of the shoreline in the surf zone, one can clearly feel the onshore surface current as a wave crest arrives, and the seaward current near the bottom that occurs beneath the next wave trough.
There is no generally applicable formula for the undertow velocity, as it depends on the particular shoreface morphology. Driving forces for the undertow are [1][2] (a) the gradient in the net onshore momentum flux (local radiation stress), which is stronger near the surface than near the bottom; (b) the net wave- and turbulence-induced vertical momentum flux towards the wave boundary layer (which is responsible for momentum dissipation and near-bed forward streaming); (c) the momentum flux associated with the surface roller of the spilling wave bore; (d) the pressure gradient related to the onshore slope of the mean water surface, the wave set-up.
The undertow current compensates for the onshore mass transport in the upper part of the vertical between wave trough and crest (Stokes drift and roller transport). The turbulent frictional dissipation of momentum by the undertow current is dynamically related to radiation stress decay.
Related articles
- Wave set-up
- Breaker index
- Radiation stress
- Wave transformation
- Shallow-water wave theory
- Shoreface profile
- Currents
References
- Jump up ↑ Deigaard, R., Justeen, P. and Fredsoe, J. 1991. Modelling of undertow by a one-equation turbulence model. Coastal Eng. 15: 431-459
- Jump up ↑ Zou, Q., Bowen, A.J. and Hay, A.E. 2006. Vertical distribution of wave shear stress in variable water depth: theory and observations. J. Geophys. Res. 111: 1–17
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