conditions. The model is analogous to the Wright and Short (1984) model of
beach states. Under decreasing energy conditions, the rip system evolution was
characterized by a narrowing and deepening of the channel, a gradual reduction
of Ar. The reduced energy levels allow onshore transport of sediments that
contribute to the constriction of the rip channel. Brander (1999a) also developed
a function relating Ar to average rip current velocities. The function is a
quantitative assessment of the morphodynamic coadjustment between
morphology and flow velocity. An average size of a rip may be identifiable from
bathymetric data or aerial photography.
Rip current velocities appear to be determined by incident wave heights and
modulated by tidal level. Several investigators have correlated rip current
velocity with increasing wave height (Shepard, Emery, and LaFond 1941;
McKenzie 1958; Harris 1961; Cook 1970; Sonu 1972; Ranasinghe et al. 2000).
Some of these researchers (Sonu 1972; Ranasinghe et al. 2000) and others
(Brander 1999a; Aagaard, Greenwood, and Nielsen 1997) have also recognized
the influence of the tidal cycle. Brander (1999a) relates velocity to the cross-
sectional area of the rip channel. Aagaard Greenwood, and Nielsen (1997) were
able to predict velocities for normally incident waves by means of a simple
model based on onshore discharge of water by breaking waves. Using this
method and assuming depth-limited breaking, it may be possible to estimate
changes in rip flow velocity with changes in tide level. The estimation of rip
velocities may also be obtained from field measurements or physical and
numerical circulation model results.
Potential offshore transport of rip currents
Sediment transport in rip currents is determined by a combination of several
processes that change in time and space. Despite the complexities and given the
current state of knowledge, a simple approach similar to Equation C10 may be
appropriate for estimating transport in cases where sufficient data are available.
Equation C10 requires a cross-sectional area of flow, a rip current velocity, and
sediment concentration in the rip channel and gives and estimate of potential
transport for a single rip current. The formulation is complicated by the fact that
the velocity and sediment concentration of rip flow are tidally modulated.
Estimates of the number of rips and persistence (also tidally modulated) are
required to obtain an offshore transport estimate for an entire system.
Sediment removed from littoral system
The portion of rip transport actually removed from the littoral zone
determines the response of the shoreline. Sand removal from the active nearshore
zone can occur as an "overshooting" of sediment by strong rips, as described by
Dolan et al. (1987). Sediment can also be removed by rip transport that carries
sediment out past a jetty that is then carried by the longshore current into an inlet.
Dolan et al. (1987) estimated an "overshooting" rate of 15 percent at Oceanside,
CA. Such an estimate would need to be made based on all available data for a
specific site. For the case of transport around a jetty, one possible method would
C11
Appendix C
Literature Review of Cross-Shore Sediment Transport by Rip Currents
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