Longshore current

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Definition of Longshore current:
Current running parallel to the shore generated by obliquely incident waves.
This is the common definition for Longshore current, other definitions can be discussed in the article


Notes

The longshore current is mainly generated by the shore-parallel component of the stresses associated with the breaking process of obliquely incoming waves, the so-called radiation stresses. It is generally a fluctuating and meandering current that dominates in the surf zone. Other, generally smaller components of the longshore current are: currents driven by alongshore gradients in wave set-up and currents driven by tide and wind.


For further explanations, see Shallow-water wave theory#Longshore Currents and Littoral drift and shoreline modelling.

An empirical formula for the longshore current [math]V[/math] halfway the surf zone is[1]

[math]V = 1.17 \sqrt{g H_b} \sin \theta_b \cos \theta_b ,[/math]

where [math]g[/math] is the gravitational acceleration ([math]\approx 9.8 m^2/s[/math]), [math]H[/math] the root-mean-square wave height and [math]\theta[/math] the wave incidence angle. These quantities are evaluated at the depth of incipient wave breaking indicated by the subscript [math]_b[/math].


Recommended review article:

Hanes, D.M. 2022. Longshore Currents. Treatise on Geomorphology, 2nd edition, Chapter 8.04. Elsevier.

Recommended book:

Komar, P.D. 1998. Beach Processes and Sedimentation. Second edition. Prentice-Hall, 544pp.


References

  1. Komar, P.D. 1979. Beach-slope dependence of longshore currents. Journal of the Waterway, Port, Coastal and Ocean Division 105(4): 460–464