P. Wang et al. / Coastal Engineering 46 (2002) 175211
203
from the spilling case, and the lower panel shows
4.5.1. Longshore sediment flux
results from the plunging case. Several representative
Due to limitations of the data acquisition systems,
locations, horizontally and vertically in the water
synchronized simultaneous measurements of instan-
column, are shown in each panel. Results at other
taneous sediment concentration and velocity through-
horizontal and vertical locations are comparable. Over-
out the water column and at different cross-shore
all, the differences were small, mostly within F 8% for
locations could not be achieved. Therefore, true
the spilling case. For the plunging case, the differences
values of instantaneous longshore sediment flux
were slightly greater although still mostly within
could not be computed directly from the product of
F 12%, with a few points exceeded 15%. These results
velocity and concentration (Eq. (1)). In addition,
suggest that longshore sediment flux obtained from the
longshore currents were sampled at 20 Hz, while
product of time-averaged longshore current and time-
concentrations were sampled at 16 Hz. In order to
averaged sediment concentration provides a good esti-
adopt Eq. (1) to estimate instantaneous sediment
mate of the temporal average of instantaneous long-
flux, the velocity and concentration data were
shore sediment flux. The maximum uncertainty caused
resampled at 4 Hz, which was chosen to avoid data
by neglecting the time-variant contributions should
interpolation. Different artificial instantaneous sedi-
generally be within F 12% for both cases. The rela-
ment flux time series were computed in which the
tively small contribution from the time-variant portions
first point of the longshore current record was first
to the averaging is probably because temporal varia-
matched with the second point of the concentration
tions of longshore current are fairly small relative to the
record, then matched with the third and fourth point
average values (Fig. 9) and also because the time
of the concentration record, etc. In this way, the
variations of longshore current and sediment concen-
concentration time series were shifted relative to the
tration were largely independent, which should result in
velocity record by a constant time of 0.25 s at each
a near-zero average product. It is worth noting that the
step. A set of 48 artificial instantaneous longshore
above conclusion was reached in light of the fact that
sediment flux calculations spanning a total time shift
longshore current was not significantly influenced by
of 12 s was obtained. Because the velocity measure-
individual wave motions. Under larger breaker angles,
ment was started within 3 F 2 s after the concen-
this may not be the case.
tration sampling, one of the shifted flux values
The product of time-averaged profiles of long-
should closely represent the product of the simulta-
shore current and sediment concentration yields an
neous u and c. Each of the 48 instantaneous flux
estimate of the suspended sediment-flux profile
time series was averaged over the 10-min sampling
through the water column. Within 3 cm from bed,
interval and then compared with the flux obtained
measurements of sediment concentration and long-
from the product of time-averaged velocity and time-
shore current were conducted at the same elevation,
averaged concentration. If the averages of all 48
at 1-cm intervals. Minor differences (less than 3 cm)
artificial instantaneous fluxes (with one representing
in measurement elevations existed in the upper
the true value) are reasonably constant and compare
portion of the water column. Because the vertical
well with the flux obtained from time-averaged
gradient of sediment concentration was much greater
longshore current and time-averaged concentration,
than that of longshore current, the longshore currents
it is then reasonable to believe that the first term in
were linearly interpolated to match the levels of the
Eq. (5) provides an acceptable estimate of longshore
sediment concentration measurements where neces-
sediment flux at a specific elevation in the water
sary.
column. Partitioning the current and concentration
The resultant sediment-flux profiles exhibit a rap-
(Eqs. (3) and (4)) into high- and low-frequency
idly upward-decreasing trend, except at the plunging
components is a fairly complicated procedure and
breaker line, where a relatively uniform flux was
is being examined in a separate study.
obtained throughout the water column above 4 cm
from bed. In the breaker zone, both the shapes and
percentage difference from the flux obtained from the
the magnitudes of the sediment-flux profile were
product of time-averaged longshore current and time-
Previous Page
