P. Wang et al. / Coastal Engineering 46 (2002) 175211
209
vertical structure of the time-averaged cross-shore
Osborne and Greenwood (1992) performed a
current demonstrates the commonly observed struc-
detailed field study of sand transport across a barred
ture of undertow, with onshore flow in the upper water
coast. They found that all three, i.e., average, high-, and
column and offshore flow in the lower water column,
low-frequency components (Eqs. (2) (5)), were
divided at the level of the wave trough. The shape of
important in contributing to cross-shore flux, and a
the mean cross-shore current profile can be reasonably
different term tends to dominate at different cross-shore
approximated with parabolic curves although the
locations. Generally, the time-averaged terms (first
goodness-of-fit was better for the spilling than the
term on the right-hand side of Eq. (5)) have significant
plunging case.
influence on offshore-directed transport. For onshore-
Sediment concentration decreases rapidly, over
directed transport, the frequency-dependent terms (sec-
nearly four orders of magnitude, upward through the
ond and third terms in Eq. (5)) play significant and
water column across most of the surf zone. An
complicated roles. Van Rijn and Havinga (1995) and
exception occurred at the breaker line for the plunging
case, where relatively homogeneous concentration
from a series of laboratory studies.
was measured throughout the water column above 4
cm from the bed. Temporal variations of sediment
suspension were dominated by events of large con-
5. Concluding summary
centration. Qualitatively, the sediment-suspension
events were controlled by local patterns of wave
In general, surf-zone currents and sediment concen-
breaking. This is observed at the plunging breaker
trations are characterized by rapid and substantial
line, where local sediment-suspension events are
temporal and spatial variations. Patterns of temporal
strongly influenced by the location of the plunging
and spatial variations of longshore current, cross-shore
point. The point of plunging varied considerably for
current, and sediment concentration are significantly
the irregular wave cases. Temporal variations at differ-
different. Caution should be exercised when averaging
ent elevations in the water column were not exactly in
these parameters over time and space and using the
phase although a similar general trend was observed at
averaged values to calculate mean sediment flux.
times, indicating that horizontal advection may not be
Compared to the magnitude of temporal variations
neglected at small temporal scales.
of cross-shore current and sediment concentration,
Nearly identical sediment concentrations were
longshore current remained relatively steady over time
measured within 3 cm from the bed near the breaker
and did not seem to be influenced significantly by
line for both the spilling and the plunging cases,
individual wave motions for the present small breaker
despite the significant differences in bed conditions,
angle of 6.5j. The vertical structure of time-averaged
near-bottom velocities, and turbulence intensities.
longshore current exhibited a logarithmic upward
While high in the water column, approximately one
increasing trend, as opposed to a general comprehen-
order of magnitude greater sediment concentration
sion of homogeneous longshore current profiles. The
was measured at the plunging breaker line than at
time- and depth-averaged longshore current, not
the spilling breaker line. Across most of the surf-bore-
counting the portion of the water column above the
dominated mid-surf zone, suspended sediment con-
wave trough, matched well with the time-averaged
centrations were rather similar for both the plunging
current measured at an elevation from the bed equal to
and the spilling cases.
1/3 still-water depth. For the two irregular wave cases
The product of time-averaged profiles of longshore
characterized by different breaker types, the cross-
current and sediment concentration, and their integra-
shore distribution of depth-averaged longshore current
tion over depth, yields a reasonable estimate of long-
remained relatively uniform over much of the mid-
shore sediment flux, as compared to the measured
surf zone, with a minor peak measured just seaward of
amount at the downdrift traps. This is attributable to
the swash zone and a decreasing trend through the
the relatively steady longshore current and possibly to
incipient breaker zone.
the cancellation of the time-varying contributions
Cross-shore current varies greatly and rapidly with
during averaging. Uncertainties of up to 30% to
time, dominated by individual wave motions. The
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