The proportionality of the transport rate to the velocity cubed provides
evidence that perhaps a Bagnold-type approach may be suitable for modeling
sediment transport in rip currents. Conceptually, the Bagnold model is based on
the concept that the waves act to entrain sediments while a current transports the
sediment. The data in the Brander (1999b) study support this concept.
Estimating Sand Transport by Rip Currents
Rip currents are an integral component of the nearshore circulation system
along most of the world's beaches and a major mechanism for the offshore
transport of water and sediments. Estimating the transport by rip currents and the
resulting shoreline change requires prediction of transport in time and space. The
volume of the sediment actually removed from the active littoral zone must also
be determined. The complexities of prediction and possible mechanisms to make
first-order estimates are discussed in the following paragraphs.
Spacing, persistence, size, and strength of rip currents
Predicting the response of the shoreline and sediment budget to rip currents
requires specifying the location, spacing, persistence, size, and strength, of the rip
currents. Numerous studies have been conducted to explain the generation and
spacing of rips along the beach. To date, no single theory adequately accounts for
the spacing of rip currents on different beaches. The observed rip current patterns
on natural beaches are probably the product of a combination of driving
mechanisms and physical boundary conditions (Short and Brander 1999).
Bathymetric features are likely the dominant controlling mechanism once rip
currents are formed. Therefore, observation and bathymetric data may be
valuable sources for specifying locations. Numerical circulation models may also
provide guidance on where rips may form.
The generation and persistence of rip currents depends on several factors.
First, in the case of shore-perpendicular structures, rip formation is governed by
the direction of the longshore current. The structure redirects the feeding
longshore current offshore. In the absence of structures, the generation of rip
currents is influenced in large part to the angle of wave approach. For waves
approaching the shoreline at a large angle, rip currents will typically not develop
or be weak. Waves approaching perpendicular to or at small angles promote the
generation of rips. Rip persistence is also strongly influenced by bathymetry.
Aagaard, Greenwood, and Nielsen (1997) found that the development of rip
flows depends on wave dissipation in the rip neck, which is suggested by the
ratio of significant wave height to water depth, γs. This concept is consistent with
the underlying indication of research that rip flows are sensitive to the degree of
morphological expression at the rip channel. The determination of critical values
of wave angle approach and/or γs for rip current development may provide a
mechanism for defining rip persistence.
Rip channel size is primarily determined by wave energy conditions. Brander
(1999a) developed a model of rip channel evolution under decreasing energy
C10
Appendix C
Literature Review of Cross-Shore Transport by Rip Currents
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