A related issue is measurement repeatability. Current measurements at Y27
were repeated five times during Test 6N, one immediately after the other, to
quantify the repeatability of the mean longshore current measurements. Each set
of measurements was sampled for 500 sec. The standard deviation of the five
mean longshore current measurements, averaged for all cross-shore positions,
was 0.0015 m/sec. This confirmed that the repeatability of ADV measurements
was quite good.
The steadiness of the current regime and the repeatability of the ADV
measurements allowed the vertical mean current structure to be measured with a
high degree of confidence. Measurements were made by accurately
repositioning all of the ADV sensors at a new elevation in the water column,
prior to each subsequent set of measurements. The vertical structure of the mean
longshore current for Test 6N and Test 8E are given in Figures A-1 and A-2,
Appendix A.
Application to Sediment Transport Experiments
As discussed in the previous section, the mean currents in the wave basin
reached steady state within 10 min of starting the wave generators. In addition,
with experience, approximately five iterations are required to converge on the
proper pump settings. These are two very positive conclusions with respect to
future sediment transport experiments in the LSTF, because the time will be
minimized during which sediment will be moving in response to improper
longshore currents.
As shown, the high cross-shore resolution of the longshore current
recirculation system in the LSTF allows very accurate control of the longshore
current distribution. Because the pumps are digitally controlled, and the in-line
flow sensors allow for real-time data collection, it typically only takes 30 to 60
minutes to readjust the pump settings for a new iteration. Therefore, during the
sediment transport experiments, the pump settings can be easily adjusted in
response to changing beach morphology.
Most of the longshore sediment transport experiments will be conducted
using irregular waves. For the irregular wave case, the degree of longshore
uniformity in mean longshore current and wave height is quite good downstream
to Y14, the closest transect to the downstream boundary. As shown in Figure 70,
there were small decreases in longshore current speed near the downstream
boundary, on the order of 10 percent at the peak, relative to the average current
for the 12-m region of the beach with the highest degree of longshore uniformity
(Y19 through Y31). The reasonably high degree of longshore uniformity 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 because of the slight reduction in
current magnitude and wave energy at the downstream boundary.
120
Chapter 9
Establishing Uniform Longshore Currents
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