parallel contours throughout the beach. Beaches having three-dimensionality affect incident waves
and, subsequently, the longshore currents and sediment transport associated with the waves.
To minimize adverse laboratory effects created by the boundaries of the finite-length beach,
wave-driven currents were supplemented by an external recirculation system (Hamilton et al., 2001,
Hamilton and Ebersole, 2001, and Visser, 1991). The recirculation system consisted of 20
independent vertical turbine pumps placed in the cross-shore direction at the downdrift boundary.
Flow channels placed upstream of each pump were used to direct flow to the pump, which externally
re-circulated water to the upstream end of the facility where it was discharged through flow channels
onto the beach. The objective of this system is to maximize the length of beach over which waves
and wave-driven longshore currents are uniform by continually re-circulating currents of the same
magnitude as the mean wave-driven longshore current through the lateral boundaries of the facility.
Each pump includes a variable speed motor to control discharge rates, which allows variation in the
cross-shore distribution of longshore current.
The facility includes a 21-m-long instrumentation bridge that spans the entire cross-shore of the
beach. The bridge serves as a rigid platform to mount instruments and observe experiments. Each
end of the bridge is independently driven on support rails by drive motors, which allows it to travel
the entire length of the wave basin. The bridge can be driven either at the bridge or remotely by
entering the desired longshore (Y) location on a PC in the LSTF control room.
Time series of water surface elevations were measured using single-wire capacitance-type wave
gauges. Ten gauges mounted on the instrumentation bridge measured wave heights as they transform
from offshore to nearshore. Additionally, a gauge was placed in front of each wave generator to
measure offshore wave characteristics.
orbital velocities and unidirectional longshore currents). The ADVs were positioned at the same
cross-shore position on the bridge with the wave gauges, but separated by approximately 40 cm in
the longshore direction to prevent interference between the two instruments. All of the ADVs sample
at a point, but were mounted on vertical supports so that the vertical position of the sampling volume
to be adjusted. Typically, the ADVs were positioned vertically to sample at a location that gives the
approximate average velocity in the water column (an elevation equal to one third of the water depth
from the bottom, Hamilton et al., 2001).
Sediment flux was obtained from twenty 0.75-m wide traps located at the downdrift boundary,
which encompassed the entire surf zone. Eighteen traps were installed in the downdrift flow
channels and two additional traps were located landward of the first flow channel to quantify
longshore sediment transport rate near the still-water shoreline and in the swash zone. Each sand trap
is equipped with three load cells to weigh the amount of trapped sand, to determine the cross-shore
distribution of longshore sediment transport.
Smith et al
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