11 Conclusions
A large-scale facility for investigating nearshore and surf zone sediment
transport processes has been constructed and successfully applied. The facility
provides the Corps of Engineers with a capability for studying longshore sand
transport at scales that are much larger than the scales at which previous
laboratory research has been conducted to examine these processes. The LSTF is
state-of-the-art in terms of the systems that have been built for generating
relatively large-scale laboratory waves (four phase-synchronized electric-motor-
driven wave generators), pumping and recirculating the longshore current (20
independent and separately controlled pump and piping networks for accurate
control of the cross-shore distribution of the current), measuring the quantity of
sand that moves along the beach (20 instrumented sediment traps), and a fully
programmable instrumentation bridge and data acquisition system that includes
10 ADV current sensors, 10 capacitance-type wave gauges, four FOBS arrays,
and an automated beach profiling system. All facility components were designed
to maximize accuracy of data acquired in movable-bed experiments and
minimize the amount of human labor (and time) required to perform them.
Two comprehensive test series were conducted on an planar concrete beach,
one involving regular waves and the other irregular waves. These data are
valuable for several reasons: (a) the geometric scale of the experiments is
significantly larger than the scale of laboratory experiments conducted in the
past; (b) the concrete beach has a relatively mild 1:30 slope, unlike previous
laboratory experiments; (c) both regular and irregular wave forcing were used;
and (d) the vertical velocity structure of the longshore current was accurately
measured using ADVs. These data are provided for use in theoretical studies and
for calibration and verification of analytical and numerical models.
The pump settings that produced the proper longshore current distribution in
the LSTF were determined through an iterative process based on mean longshore
current measurements in the surf zone and offshore in the internal recirculation
zone. This procedure worked well. When the proper distribution is pumped, the
degree of longshore uniformity in mean longshore current increases noticeably,
and the magnitude of the current offshore in the recirculation zone approaches a
minimum value. Internal recirculation cannot be completely eliminated,
however, and its magnitude was about 10 percent of the total flow rate in the surf
zone for the two wave cases examined on the concrete beach.
The final selection of proper pump settings was confirmed by the insightful
criteria developed by Visser (1991). But unlike the findings of Visser for the
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Chapter 11
Conclusions
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