c. Only monochromatic waves could be generated with the wave
generators, because the software for synchronizing the wave generators
for irregular waves was still in development.
As a result, strong adverse reflection patterns (from the vertical walls at the
two ends of the basin) and circulation cells developed throughout the facility
during the first 5 to 10 generated waves. In addition, no flow velocity
measurement sensors were available to accurately measure the wave-driven
longshore current in the surf zone.
Three conclusions were made based on the limited dye measurements. First,
NMLONG produced reasonable estimates of the peak longshore current
magnitude and cross-shore location, using the default bottom friction coefficient
of 0.01. However, insufficient data were collected to calibrate this coefficient.
Secondly, we were unable to verify the cross-shore distribution of the longshore
current predicted using NMLONG because of the physical limitations and
resulting adverse laboratory effects previously discussed. Therefore, the default
lateral mixing coefficient (0.30) was used. Thirdly, observations made during the
dye experiment strongly reinforced the expectation that a properly designed
external longshore current recirculation system would be required to maintain
longshore uniformity of waves and wave-driven currents in the facility,
especially for energetic wave conditions.
Influence of wave height
Figure 11 shows the calculated LSC distribution for Hs = 0.2, 0.3, and 0.4 m,
peak of the distribution significantly increases. The magnitude of the peak LSC
equals 0.21, 0.28 and 0.36 m/sec for Hs = 0.2, 0.3 and 0.4 m, respectively. This
is a relative increase of approximately 30 percent for each 0.1-m increase in wave
height in these cases. Secondly, the peak of the LSC distribution moves offshore
as Hs increases, because incident waves begin to break farther offshore. Thirdly,
the width of the LSC distribution increases as Hs increases, because the width of
the surf zone increases.
Influence of wave period
Figure 12 shows the LSC distribution for Tp = 1.0, 1.5, 2.0, and 2.5 sec with
Hs = 0.3 m and θ = 20 deg at the wave generator. Three general trends can be
the distribution increases slightly. The magnitude of the peak LSC equals 0.25,
0.26, 0.27, and 0.28 m/sec for Tp = 1.0, 1.5, 2.0, and 2.5 sec, respectively. This is
a relative increase of only 4 percent for each 0.5-sec increase in Tp for these
cases. Secondly, the peak of the LSC distribution moves slightly farther offshore
as Tp increases, because incipient breaking occurs slightly farther offshore.
Thirdly, the width of the LSC distribution increases slightly as Tp increases,
because the width of the surf-zone increases slightly.
19
Chapter 3
Longshore Current Recirculation System
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