Fowler and Dalrymple (1990) examined the forcing of nearshore circulation
by incident wave groups. As incident wave trains propagate towards the shore
they alternatively reinforce and cancel each other out, creating periodic longshore
variations in wave height. Lines of cancellation, or nodal lines, can be seen in the
wave field. If incident wave trains are of the same frequency, rip currents
develop at the intersection of nodal lines. The spacing is predicted similar to
Dalrymple (1975).
2π
Yr =
(C4)
( k2 sin θ2 - k1 sin θ1 )
where
k = wave number (2π/L)
θ = wave angle of the incident wave trains
If the incident wave trains have slightly different frequencies, it was found that
rip currents could still be generated and that they migrate slowly along the beach
with the slowly moving nodal lines. The rip spacing was somewhat
overpredicted, and this was thought to be due to the use of linear theory and a
shallow-water assumption invoked to develop the theory.
Murray and Reydellet (2001) introduced a model in which rip currents are
self-organized, rather than being forced by bathymetric features or incident wave
patterns. Their model predicts that rip currents can arise from a wave-current
interaction that dissipates wave energy, onshore mass transport by waves,
offshore flow responding to imbalances between radiation-stress gradients and
setup in the surf zone, alongshore flow responding to alongshore surface slopes,
and alongshore dispersion of cross-shore momentum.
Cook (1970) made field observations of the occurrence of rip currents in
southern California. He concluded that the generation of the rips was influenced
by wave conditions, beach morphology, tidal level, and wind. The existence of
rip currents was highly dependent upon the height and period of waves reaching
the shoreline. The beach morphology was also found to affect rip currents in
several ways, including the steepness of the foreshore and the presence of cusps
and rip channels. Rip currents were poorly developed along steep beaches. The
existence of rip channels, formed during high-energy conditions, facilitates the
development of rips at lower energy conditions. Cook (1970) also found that
strong on- or offshore winds might influence rip current generation. An onshore
wind may hinder the development of rip currents by transporting surface water
toward the shore and may even eliminate rips entirely. Offshore winds reduce
wave drift and likewise may diminish the potential for rip current development.
Short and Brander (1999) investigated the relationship between rip current
spacing and the regional wave climate. They found a strong correlation between
rip spacing and levels of wave energy. They conclude, however, that there is still
no adequate explanation for the variation in rip spacing between different
beaches.
C4
Appendix C
Literature Review of Cross-Shore Transport by Rip Currents
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