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.

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

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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