longshore; however, the middle portion of the beach remained uniform and the higher
resolution was not required. Higher irregularity in the bathymetry occurred near the
upstream and downstream boundaries, and denser profile lines were required.
Twenty traps were installed in the downdrift flow channels to collect sand transported
through the downdrift boundary. Seventeen traps were placed in the flow channels of the
first 17 pumps, and one trap was placed in the flow channel of pump 19. Traps were
omitted from channels 18 and 20 because sediment transport was expected to be low in the
offshore region. The remaining two traps were placed in the swash zone.
To determine the cross-shore distribution of longshore sediment transport, each sand trap
was equipped with three load cells to weigh the amount of trapped sand. The total capacity
of the 20 traps is 2500 kg. As experiments progressed, the updrift end of the beach became
depleted of sand. It was necessary to dredge the traps to replenish the updrift portion of the
beach, and rebuild the beach to uniform and parallel contours.
LONGSHORE TRANSPORT EXPERIMENTS
Longshore transport experiments were conducted for two breaker types with the wave
conditions given in Table 1. Zero-moment wave height, Hmo, water depth, h, and incident
wave angle, θ, were the same in both cases, and only the peak wave period, Tp, varied.
Table 1. Longshore Sediment Transport Experiment Wave Conditions
θ
Hmo
Tp
h
Breaker Type
m
deg
m
sec
Spilling
0.247
1.5
0.9
10
Plunging
0.247
3.0
0.9
10
Uniformity of Longshore Currents
The first step of the experiments was to determine the distribution of wave-induced
longshore current. Visser (1991) determined from laboratory experiments that if the re-
circulated currents either exceeded or were less than the wave-driven currents, an internal
current would develop and re-circulate within the offshore portion of the basin. Visser also
found that as the pumped currents approached the proper current, the internally re-
circulated current was minimized. Therefore, it was desired to match wave-driven currents
with pumped currents. Initial pumps were based on results of the numerical model
NMLONG (Kraus and Larson 1991), and the iterative approach described by Hamilton and
Ebersole (2001) and Hamilton, et al. (2001) was used to determine the optimum pump
settings. After the beach had reached an equilibrium or quasi-equilibrium profile, and the
pumped currents matched measured velocities, experiments on longshore transport rate
were initiated. Spilling breakers required 1,330 minutes to reach equilibrium in the model
and plunging breakers reached equilibrium after 280 minutes of wave action.
Smith and Wang
1221
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