facility he used, for the LSTF, Qp can vary by as much as +20 percent of Qpu
process is placed on examination of the measured data, taking advantage of the
high degree of cross-shore resolution in the recirculation system itself. However,
even for this range of Qp the inshore two-thirds of the mean longshore current
distribution was relatively unaffected by Qr. The effect of internal recirculation,
Qr manifested itself primarily on the offshore tail of the measured longshore
current distribution. Therefore, tuning of the longshore current focused on the
inshore portion of the distribution first, saving the offshore tail for last.
Measured currents in the offshore tail were always higher than the equivalent
pump settings, regardless of the magnitude because of the internal recirculation
in the region offshore of the surf zone.
The active external longshore current recirculation system and the
operational procedure used to pump the proper current, led to a 12-m long region
of the beach that was characterized by a very high degree of longshore
uniformity in waves, currents, and mean water levels, between Y (longshore)
coordinates of 19 and 31 m. The standard deviation of the longshore variation in
longshore current is still relatively small downstream to Y = 14 m, at the
downdrift boundary.
Two experiments that involved a movable-bed quartz-sand beach, which had
a very narrow distribution of grain sizes and a median grain diameter of 0.15
mm, were successfully conducted. Irregular wave conditions were generated in
each experiment. The first involved wave conditions that produced
predominantly spilling-type wave breaking. In the second experiment
predominantly plunging breakers were generated. For both experiments, waves
having a similar significant wave height were adopted. The type of wave
breaking was controlled by use of a different wave period.
Each experiment was comprised of a series of tests, or runs. Initial tests were
conducted to iterate toward the proper longshore current distribution in the
presence of a movable bed and to allow the beach profile shape to reach an
equilibrium, or near-equilibrium, condition. Iterative procedures that were
determined from the concrete-beach experiments were successfully applied to the
movable-bed experiments. Subsequent wave runs involved acquisition of data to
examine: alongshore variability in processes, steadiness, and repeatability of
measurements and estimates of local and total LST rates, and investigate the
vertical structure of currents and sediment concentration.
Results suggest that breaker-type and beach morphology exert a strong
influence on the total LST rate and its cross-shore distribution. More LST
occurred for the plunging wave case than for the spilling wave case, despite both
cases having approximately the same incident wave height and direction (the
former LST rate was 2.6 to 2.7 times greater than the latter). The cross-shore
distribution of LST for each case was different. For the spilling-breaker case, the
magnitude of the LST rate gradually increased with increasing proximity to the
shoreline, and a local peak in the distribution existed in the swash zone. For the
plunging-breaker case, a local peak in the LST distribution existed at the
breakpoint bar that was produced. A second peak existed in the swash zone, as
was found for the spilling-breaker case, but the magnitude of the swash zone
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Chapter 11
Conclusions
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