by specifying incident irregular waves that had a significant wave height at the
wave generator of 0.25 m (in 0.9 m of water) and a peak spectral wave period of
1.5 sec. Incident waves for the second experiment were also irregular, but had a
significant wave height of 0.23 m and a peak spectral period of 3.0 sec. These
lower-steepness waves produced predominantly plunging-type wave breaking.
The incident wave direction for both experiments was 10 deg, relative to shore
normal, at the wave generator. These two wave cases represent the upper
magnitude of incident wave energy, in terms of significant wave heights, for
which LST experiments can be effectively performed in the facility.
Issue of Sand Recharging
As waves and currents begin to transport sand along the beach, a deficit of
sand is created at the updrift boundary if no sand is introduced. Originally, a
system was envisioned that would actively recirculate sand during an experiment,
and introduce it at the updrift end of the beach in a prescribed cross-shore
distribution that was to be determined from trap measurements made at the
downdrift end. The system would recirculate a water-sand slurry in the same
manner as the pumps recirculate water (Chapter 3) and continuously discharge it
along the updrift boundary of the beach in a controlled manner. A sizable effort
was expended on both conceptual and more detailed engineering designs of such
a system. Both hydraulic and mechanical systems were considered for
introducing sand at the upstream end, and several design concepts for each type
of system were evaluated. However, in light of uncertainties associated with
each design, it appeared that even though considerable resources could be spent
on a sand-recharging system, performance of the system might not meet
requirements and expectations.
Concurrent with work to design an automated recharge system, preliminary
experiments were conducted with the mobile sand beach. Results suggested that
alongshore uniformity over the downdrift two-thirds of the beach was reasonably
well maintained during what was anticipated at the time to be a typical
experiment duration for moderate to high LST conditions (1 to 4 hr). As
anticipated, a beach discontinuity formed at the updrift end of the facility, but the
erosion-induced perturbation did not rapidly propagate down the beach during a
several-hour period, even under rather high longshore transport conditions (some
of the largest that can be produced in the facility). Perturbations were mostly
confined within a 5-m zone immediately adjacent to the updrift boundary. The
duration of experiments is also limited by the capacity of the traps (Chapter 4).
The total amount of sand that was transported alongshore during a several-hour-
long experiment for high-transport conditions is generally less than 1 percent of
the 125 m3 of sand that comprises the artificial beach (D50 = 0.15 mm).
After considering a number of factors, including technical difficulty and
inherent limitations, time-to-construct, and monetary constraints, a decision was
made not to proceed with construction of an active sand recharge system.
Instead, a system of dredging and recirculating sand off-line after an experiment
was completed was adopted (Chapter 4). The system involves use of a trash-
pump to recirculate the sand-water slurry and positioning of the discharge point
to roughly fill in the areas where the sand deficit is greatest. The purposes of the
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Chapter 10
Longshore Sediment Transport Experiments
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