Ar
αr =
(C7)
ArHT
with
mean, depth-averaged flow velocity in the rip channel
ur =
peak wave period
T=
= root-mean-square wave height
Hrms =
Ar =
average cross-sectional area available for rip flow
ArHT =
cross-sectional area available for rip flow at high tide
Observations and analysis made by Ranasinghe et al. (2000) suggest that rip
flows are bathymetrically controlled once the rip channels are formed. They
found that rips are fairly persistent in time and migrate in the longshore direction
under obliquely incident waves. Sufficiently strong longshore currents cause rip
channel migration in the direction of the longshore current. Ranasinghe et al.
(2000) also found that rip channels disappear when storms rework the nearshore
morphology and then reappear in a different location when the storms subside.
Rip Current Strength and Rip Channel Size
Shepard, Emery, and LaFond (1941) correlated rip current strength with
increasing wave height. McKenzie (1958) and Harris (1961, 1964) observed in
field studies that heavy seas produced strong rips and smaller waves produced
weaker, more numerous rips. Cook (1970) also observed that the largest rips are
formed during periods of intense wave activity and found the influence of the
tidal cycle on rip size and strength to be ambiguous. Sonu (1972), however,
observed that the intensity of wave breaking, controlled by the tide, corresponded
with a proportionally stronger circulation. Thus, rip currents velocities were
generally stronger at low tide and weaker at high tide. Using a two-dimensional
(2-D) depth-averaged hydrodynamic model, Ranasinghe et al. (2000) found that
rip velocity increased with increasing wave height. The model results also
predicted increased rip velocity with falling tide levels.
Aagaard, Greenwood, and Nielsen (1997) suggest that the amount of water
returned seaward by any rip should depend on the onshore discharge of water by
waves breaking across the bar and on the spacing between rip channels.
Therefore,
Yr
ur = ( Qdrift + Qroller )
(C8)
Ar
Qdrift = onshore discharge of water between rips due to mass transport in waves
(Stokes drift) and Qroller = the mass transport in wave rollers.
C6
Appendix C
Literature Review of Cross-Shore Transport by Rip Currents
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