transport study relating to rip currents was made by Ingle (1966), who found that
dyed sand grains being moved by longshore currents were intercepted and
transported offshore by rips. Cook (1970) noted that rips can transport large
volumes of sand to the inner shelf and can cause beach erosion.
Based on field data from the North Sea, Aagaard, Greenwood, and Nielsen
(1997) found that sediment flux in the rip neck channel depends on the tidal
stage. Large offshore transport due to a rip current occurred at low tide. Smaller
onshore transport due to oscillatory incident waves and weak mean currents
occurred at high tide. Sediment concentrations were moderately dependent on
tidal stage, with higher concentrations occurring at low tide.
Few attempts have been made to quantify the sediment transport of rip
currents. The number of direct measurements of sediment transport in rips is
limited and little is known about these processes. Dolan et al. (1987) estimated
the sediment lost from a littoral cell by rip currents using data from the Littoral
Environmental Observation (LEO) program and plausible assumptions. The
transport rate in each rip (Qr) is computed as:
Ar ur cr
Qr =
(C11)
ρs
where cr = suspended load concentration, and ρs = density of sand. The volume
of sand being transported for a particular stretch of beach was then calculated by
multiplying the transport rate times the number of rips and the percent chance of
rip occurrence.
Brander (1999b) conducted the most comprehensive study on sediment
transport processes in rip currents. He found that the vertical distribution of
sediment flux in the rip neck exhibited an exponential decrease in flux away from
the bed. The magnitude of the flux was strongly influenced by the velocity of the
flow. Therefore, the bulk of transported sediment moves close to the bed with
approximately 50 percent of total sediment flux in the rip channel occurring in
the bottom 10 percent of flow, whereas 15 percent occurs in the upper half of the
water column.
Another finding by Brander (1999b) is the strong functional relationship
between the transport rate and the velocity cubed. Through linear least squares
regression, Brander (1999b) developed the following expression for rip current
transport, qr, in units of kg/min.
qr = 27.6ur3 + 0.92
(C12)
This relationship indicates that sediment transport in rips increases as rip current
velocity cubed. Sediment transport, however, is complicated by sediment
entrainment, tidal modulation, and the need to consider the combined effects of
waves and currents. Equation C12 was developed for a low-energy rip current
system. However, Brander and Short (2000) presented evidence to suggest that a
C8
Appendix C
Literature Review of Cross-Shore Transport by Rip Currents
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