Ranasinghe et al. (2000) analyzed daily time exposure video images to
examine the effect of incident waves on the spacing and persistence of rip
channels at Palm Beach, Australia. Results indicated that rip channels do not
have a preferred location along the beach and that rip spacing did not increase
with increase in wave height. These results suggest that bathymetric features are
the dominant controlling mechanism once rip currents are formed.
Morphological Feedback and Rip Current
Persistence
McKenzie (1958) was the first to suggest that nearshore circulation may be
dominated by storm-induced bathymetry for long periods after a storm. Cook
(1970) found that rip channels are cut during prolonged periods of high waves
and then gradually fill under calmer conditions. But, once the high-energy
bathymetry is established, the rip currents are able to persist over periods of time
with varying wave conditions. Sonu (1972) found a correlation between
circulation patterns and surf zone bathymetry. Longshore currents moved across
the undulatory bathymetry from shallow to deeper regions, with rips at the
depressions.
Aagaard, Greenwood, and Nielsen (1997) found that rip persistence depends
on the extent of wave energy dissipation in the rip neck. At low tide, when wave
energy dissipation was intense and occurred across a wide zone, the rip was
active. At high tide, the reduced wave dissipation and a restricted zone of wave
breaking usually resulted in the rip being inactive. Aagaard, Greenwood, and
Nielsen (1997) determined that the tidally dependent rip behavior is suggested by
the ratio γs of significant wave height to water depth in the rip neck. Based on
field data from the North Sea coast at Jutland, Denmark, rip activity developed
when γs reached a critical value of approximately 0.35 in the rip neck.
Brander (1999a) concludes that morphologic control, modulated by
hydrodynamic forcing, plays an important role in the temporal behavior of rip
flow. Rip circulation is driven by longshore and cross-shore pressure gradients
within the surf zone and is maintained by the bathymetric feedback. Therefore,
rip current behavior and form is dictated by the nature and degree of the
coadjustments between morphology, hydrodynamics, and sediment transport.
Brander (1999a) introduced a function to provide an initial quantitative
assessment of the morphodynamic coadjustment between morphology and flow
velocity in an evolving low-energy rip:
r = -18.6αr + 17.8
(C5)
where
urT
r =
(C6)
H rms
C5
Appendix C
Literature Review of Cross-Shore Sediment Transport by Rip Currents
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