spilling-breaker wave case. This wave case was expected to produce a slightly
erosive condition, characterized by loss of sand from the beach face and offshore
movement of that sand, which did occur. At the beginning of this 6-hr wave run,
the beach had already been subjected to numerous hours of wave action, had
essentially reached its equilibrium profile shape, and had been remolded to the
equilibrium profile shape with straight and parallel contours. The upstream
boundary is located on the right hand side of the figure; and the sediment traps
are located along the left side of the figure. Sand is transported alongshore, from
right to left. Beach contour elevations are reported in tenths of meters, relative to
the still-water line, which has an elevation of 0.0 m.
Alongshore locations (m)
Figure 79. Beach morphology after 6 hr of wave activity for the spilling-breaker
case (elevations are in meters)
The effect of not introducing sand to the updrift boundary is clearly evident
in Figure 79. Along the offshore portion of the updrift boundary, where the
magnitude of longshore transport was rather low, the perturbation is very abrupt
and is only evident within a distance of 1 to 2 m from the boundary. However,
the perturbation extends a distance of 5 to 8 m from the boundary in the middle
and inner regions of the surf zone, where the LST rates were higher. Where the
deficit-induced perturbation transitions into the region of beach where straight
and parallel contours are maintained, several undulations in beach morphology
are evident. The undulations developed during every wave run following
remolding of the beach. The reason for their occurrence is not definitely known.
However, the difference in orientation between the updrift wave-guide (15 deg)
and the angle of wave incidence at the wave generators (10 deg) is suspected to
be a contributing factor. The difference in angle was imposed in an attempt to
compensate for wave energy that is lost through the updrift wave guide, thereby
minimizing the alongshore extent of the current recirculation area that formed
near the shoreline due to this energy loss.
134
Chapter 10
Longshore Sediment Transport Experiments
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