prediction of the response of the adjacent shoreline requires the ability to identify
the location, spacing, size, strength, and persistence of rip currents.
Mechanisms for Rip Current Generation and
Spacing
The current pattern that dominates the nearshore circulation partially depends
on the angle of wave approach. If waves break parallel to the shoreline trend,
generated currents will form a circulation cell. If waves break at large angles to
shore, the longshore current flows parallel to shore, confined between the
breakers and the shoreline. Circulation cells may also form if waves break at
small angles to the shoreline or if beach topography controls the pattern of
nearshore currents (Harris 1964; Komar 1998).
Shepard and Inman (1950) demonstrated that rip currents can be created by
longshore variations in wave height, are usually periodic in time and space, and
increase in velocity with increasing wave height. Several causes for the variation
in wave height have been proposed. Shepard and Inman (1950) identified wave
convergence or divergence over irregular offshore bathymetry as one
explanation. Also, in places where relatively straight beaches are terminated on
the downdrift side by an obstruction, a pronounced rip often extends seaward.
Generation mechanisms that require a longshore variability in the boundary (i.e.,
bottom topography or structures) have been termed structural interaction
mechanisms (Dalrymple 1978). Bowen (1969) applied the concept of radiation
stress, the excess momentum due to the presence of waves, to investigate how
circulation patterns are produced by the interaction of the wave field with
longshore variation in the nearshore region. The longshore variation can be
induced by changes in bathymetry. The theory showed that rip currents occur in
the surf zone where breakers are lowest, which is in agreement with field
observations. Noda (1974) developed an analytical model of wave-induced
circulation cells and rip currents that incorporated the interaction of incoming
waves with bottom topography as the driving mechanism, abstracting in part
work presented in Noda et al. (1974). More recently, Haas, Svendsen, and Haller
(1998) and Sorensen, Schaffer, and Madsen (1998) have also applied numerical
circulation models that produce rip currents driven by the wave-bottom
topography interaction.
Engineered and natural structures also influence the nearshore circulation.
Liu and Mei (1976) investigated rip current generation at groins. Numerical
results showed rip current cells with a spacing corresponding to L0/(2 sinθ0)
wave trains. Mei and Angelides (1977) examined the circulation around a
circular island and the formation of a single rip in the lee of the island.
Regular systems of rips are also found on natural beaches where there are no
regular variations in the bathymetry. Generation mechanisms that can occur on
uniformly planar beaches are termed wave interaction mechanisms (Dalrymple
1978). Bowen and Inman (1969) performed experiments in which the interaction
between edge waves and incident waves of the same frequency created
C2
Appendix C
Literature Review of Cross-Shore Transport by Rip Currents
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