Along the shoreline, between alongshore coordinates of 33 and 39 m, the
foreshore beach slope is much narrower than along the rest of the shoreline
(evident by more closely-spaced contours). A scarp formed along the seaward
face of the beach berm in this region. No scarp was created along the rest of the
shoreline. This anomalous erosion zone is also caused by the lack of a sediment
supply at the updrift end of the beach. A peak in distribution of LST rate
occurred in the swash zone.
Bathymetric contours remained reasonably straight and parallel between the
alongshore coordinates of 15 and 30 m. This is the same region that had the
highest degree of alongshore uniformity in terms of longshore currents for the
concrete beach tests. These two observations are probably related.
Along the downdrift boundary, slight changes in contour orientation are
evident within several meters of the boundary. This is evidence that there is
some anomalous erosion and/or accretion along this lateral boundary. The traps
are not 100 percent efficient. These anomalies must be accounted for in
estimates of LST rates that are derived from the trap weight measurements. The
process for doing so is discussed in more detail later in this chapter.
In the offshore region, where the magnitude of the LST rate was low, the
contours remained straight and parallel up to a point about 1 m away from the
downdrift boundary. There, localized erosive perturbations are evident.
In the midsurf zone accretion fillets are evident, and the alongshore distance
from the boundary that is affected increases with increasing proximity to the
shoreline. The trap measurements indicated that the local LST rate increased
with increasing proximity to the shoreline. This suggests that the magnitude of
the accretive perturbation increases with increasing longshore sand transport rate.
The same observation was noticed at the updrift boundary.
At the shoreline, an accretionary fillet is evident, but superimposed on it is a
slight localized erosive area directly adjacent to the downdrift boundary. This is
attributed to interactions of wave uprush with the bricks that comprise the
adjustable downdrift beach template. The brick template holds the sand beach in
place, preventing it from sloughing off into the sediment traps. In the foreshore
zone, slight erosional perturbations are also evident. These seem to form for the
same reason, interaction of wave uprush with the lateral boundary, which is
discussed in more detail later in this chapter.
Figure 80 shows the condition of the same beach after exposure to nearly
four additional hours of essentially the same wave and current conditions. The
beach morphology seen in Figures 79 and 80 is quite similar. The erosion
perturbations at the updrift end of the beach grew in severity as would be
expected, since no sand was supplied; but the alongshore extent of the
perturbation did not change appreciably. At the downdrift end of the beach, the
regions of localized accretion and erosion did not change much during the
additional 4 hr. The perturbations seemed to reach a near-equilibrium condition,
presumably with sand being transported into the traps more efficiently once this
condition is reached.
135
Chapter 10
Longshore Sediment Transport Experiments
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