CBH 2
Qdrift =
(C9)
hb
As
Qroller =
(C10)
T
where
wave celerity
C=
wave profile coefficient (1/12 for saw-tooth bores)
B=
local wave height
H=
water depth over the bar crest away from rips
hb =
cross-sectional area of a surface roller (~0.9H2)
As =
Aagaard, Greenwood, and Nielsen (1997) found that, for normally incident
waves, rip current velocities were well correlated with velocities predicted from
the simple model.
Brander (1999a) found that rip current velocity was modulated by the tide,
experiencing maximums at low tide and minimums at high tide. He also found
that rip velocity decreases with cross-sectional area and is predicted by a linear
function (Equation C5), which provides an initial quantitative assessment of the
morphodynamic coadjustment between morphology and flow velocity. The
underlying hypothesis of Brander's (1999a) work is that rip current velocity is
maximized when morphological expression of the rip channel is amplified.
Rip current strength may also be altered by wave-current interaction
(LeBlond and Tang 1974; Noda et al. 1974). Noda et al. (1974) developed a
numerical model of wave transformation, nearshore circulation, and wave-current
interaction, and they found that the wave-current interaction altered incoming
wave characteristics in the nearshore and thus reduced the magnitude of
circulation velocities and horizontal circulation pattern. Similarly, the model of
Haas, Svendsen, and Haller (1998) predicts that the offshore extent of rip
currents is reduced when wave-current interaction is included. Yu and Slinn
(2003) also report that wave-current interaction reduces rip current strength and
restricts their offshore extent.
Zyserman, Fredsoe, and Deigaard (1990) presented a method to determine
the dimensions of rip current systems (including the width and depth of rip
channels). The method is based on an overall sediment balance in the nearshore
region and assumes equilibrium conditions exist and that no erosion or accretion
occurs.
Sediment Transport in Rip Currents
Rip currents have long been recognized as a mechanism for offshore
sediment transport (Shepard, Emery, and LaFond 1941). The first sediment
C7
Appendix C
Literature Review of Cross-Shore Sediment Transport by Rip Currents
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