The measured cross-shore distribution of the mean longshore current at
transects Y19 through Y31, and the longshore-averaged values of the mean
longshore current for these four transects, are provided in Appendix A. The
longshore averaged values of wave height and mean water surface elevation,
from Y19 through Y31, are also provided.
Longshore Current Steadiness and Repeatability
Individual fixed-bed hydrodynamic experiments lasted from 2 to 3 hr,
depending on the number of transects measured. Moveable-bed longshore
sediment transport experiments also will be conducted continuously for a several-
hour duration, perhaps longer. Therefore, the issues of current steadiness and
repeatability over this time scale are important. Experiments were conducted to
investigate the time required for the mean currents in the wave basin to reach
steady-state conditions. Results from Test 8E, the irregular wave case, are shown
in Figure 69. The data represent the mean longshore current distribution
measured at transect Y27 at three different times: 20, 110, and 150 min after the
start of the experiment. Pumps were started at time zero. Wave generation
commenced at the 10-min mark, after the 20 pumps had been adjusted to within
1 percent of the target discharge rates. Results showed that mean currents
reached steady state within 10 min of starting the wave generators. The standard
deviation of the mean longshore current, averaged for all cross-shore positions
along Y27, was 0.0022 m/sec and 0.0017 m/sec for Test 8E and Test 6N,
respectively.
0.50
Y27A: T=20 min
0.45
Y27: T=110 min
0.40
Y27B: T=150 min
0.35
0.30
Pump Settings
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
-0.10
0
2
4
6
8
10 12 14
16
18
20
Cross-shore Location (m)
Figure 69. Irregular wave Test 8E: Sequential measurements of mean longshore
current at Y = 27 m
119
Chapter 9
Establishing Uniform Longshore Currents
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