D.G. Hamilton, B.A. Ebersole r Coastal Engineering 42 (2001) 199218
213
steadiness and repeatability over this time scale are
a new elevation in the water column, prior to each
important. Experiments were conducted to investi-
subsequent set of measurements. The vertical struc-
gate the time required for the mean currents in the
ture of the mean longshore current for Tests 6N and
wave basin to reach steady-state conditions. Results
8E are given in Appendix B.
from Test 8E, the irregular wave case, are shown in
Fig. 15. The data represent the mean longshore
current distribution measured at transect Y27 at three
9. Application to sediment transport studies
different times: 20, 110, and 150 min after the start
of the experiment. Pumps were started at time zero.
As discussed in the previous section, the mean
Wave generation commenced at the 10-min mark,
currents in the wave basin were found to reach
after all 20 pumps had been adjusted to within 1% of
steady state within 10 min of starting the wave
the target discharge rates. Results showed that mean
generators. In addition, with experience, only ap-
currents reached steady state within 10 min of start-
proximately five iterations are required to converge
ing the wave generators. The standard deviation of
on the proper pump settings. These are two very
the mean longshore current, averaged for all cross-
positive conclusions with respect to future sediment
shore positions along Y27, was 0.0022 and 0.0017
transport experiments in the LSTF, since the time
mrs for Tests 8E and 6N, respectively.
will be minimized during which sediment will be
A related issue is measurement repeatability. Cur-
moving in response to improper longshore currents.
rent measurements at Y27 were repeated five times
As shown, the high cross-shore resolution of the
during Test 6N, one immediately after the other, to
longshore current recirculation system in the LSTF
quantify the repeatability of the mean longshore
allows very accurate control of the longshore current
current measurements. Each set of measurements
distribution. Since the pumps are digitally controlled,
was sampled for 500 s. The standard deviation of the
and the in-line flow sensors allow for real-time data
five mean longshore current measurements, averaged
collection, it typically only takes 3060 min to
for all cross-shore positions, was 0.0015 mrs. This
re-adjust the pump settings for a new iteration.
confirmed that the repeatability of ADV measure-
Therefore, in future sediment transport experiments,
ments was quite good.
the pump settings can be easily adjusted in response
The steadiness of the current regime and the
to changing beach morphology.
repeatability of the ADV measurements allowed the
Most of the longshore sediment transport experi-
vertical mean current structure to be measured with a
ments will be conducted using irregular waves. For
high degree of confidence. Measurements were made
the irregular wave case, the degree of longshore
by accurately repositioning all of the ADV sensors at
uniformity in mean longshore current and wave
height is quite good downstream to Y14, the closest
transect to the downstream boundary. As shown in
Fig. 16, there were small decreases in longshore
current speed near the downstream boundary, on the
order of 10% at the peak, relative to the average
current for the 12-m region of the beach with the
highest degree of longshore uniformity ZY19 through
Y31.. The reasonably high degree of longshore uni-
formity at the downstream end of the beach is a very
positive result from the standpoint of conducting
longshore sediment transport experiments in the
LSTF, since the sand traps will be located at the
downstream end of the beach. Nonetheless, some
inefficiency is expected at the sand traps due to the
slight reduction in current magnitude and wave en-
Fig. 15. Test 8E: sequential measurements of mean longshore
ergy at the downstream boundary.
current at Y27.
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