suggested that Qp was slightly greater than Qpu. This slight discrepancy between
the two methods may be caused by the fact that, in the LSTF, Qp can vary by as
much as +20 percent of Qpu without a significant increase in Qr, as mentioned
previously. Therefore, it was concluded that Qp in Test 8E was essentially the
proper longshore current distribution for the irregular wave test series. A case
involving significant over-pumping was not conducted for the irregular wave
experiments.
Longshore Uniformity
This section quantifies the length of surf zone with the highest degree of
longshore uniformity of the hydrodynamic processes. In general, it can be
assumed that longshore uniformity should increase with increasing distance from
the lateral boundaries. However, from the perspective of measuring longshore
sediment transport in the LSTF, it is important to quantify the spatial limits of
this region; especially at the downstream end where sand traps will be located.
Longshore uniformity was quantified by an average value of the standard
deviation at each cross-shore position and at each transect within the length of
surf zone being evaluated. For both Test 6N and Test 8E, the standard deviation
was calculated independently for the wave height, mean water surface elevation,
and mean longshore current data sets. A new value of the average standard
deviation was calculated each time the representative beach length was
decreased, by excluding data from one or more transects from the calculation.
Transects at the upstream end of the facility were eliminated first, then transects
at the downstream end were eliminated. The length of the surf zone with the
highest degree of longshore uniformity is defined as the length at which a
minimum standard deviation is obtained.
Figure 65 shows results for the regular wave experiment. Longshore
variations in the average wave height measurements tend to decrease only
slightly as the length of beach being considered is decreased. In contrast,
longshore variations in the mean water surface elevation, and more importantly,
in the mean longshore current, decrease significantly with decreasing length of
testing region and approach a minimum asymptote at approximately 12 m. If a
shorter length of surf zone is considered, there is no significant increase in
uniformity. Therefore, it is concluded that the hydrodynamic measurements have
reached a minimum longshore variability if the length of surf zone being
considered is reduced to 12 m, starting at Y19 and extending upstream to Y31.
The high degree of longshore uniformity in this portion of the surf zone is
illustrated in Figure 66, a through c, which show the cross-shore distributions of
measured wave height, mean water surface elevation, and mean longshore
current, respectively, for transects Y19 through Y31. Figure 66a shows that the
greatest longshore variation in the measured wave height occurred at and
immediately offshore of the incipient breaker line. Wave breaking occurred
immediately shoreward of Wave Gauge 6. Deviations from the longshore
averaged wave height were as high as +8 percent. This is a laboratory effect
caused by generating regular waves in a wave basin with reflective boundaries.
However, the longshore variation in wave height measured in front of each of the
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Chapter 9
Establishing Uniform Longshore Currents
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