traces in the cross-shore direction. Hand shovels and rakes are used to cut and fill
in these areas. In addition, it is difficult to mold the beach in some regions to the
desired elevation using only the tractor. Shovels and rakes are used to mold the
steep foreshore slope or, if present, an offshore bar (Figure 31). The system of
bridge-mounted rods used with the tractor and hand tools allows the elevation to
be controlled in the longshore direction within 1 cm. Figure 32 is a bathymetric
plot of the beach after it had been remolded.
Figure 31. Fine-tuning of the initial beach profile is achieved using a hand held
shovel and rake, especially in regions where it is difficult to use the
tractor, such as the steep front slope of the foreshore berm
During the course of an experiment it becomes necessary to replenish the
beach. As sand is trapped downdrift during an experiment, the updrift boundary
becomes depleted of sand causing non-uniformity in the longshore direction
(Figure 33). The purposes of beach replenishment are to recharge the sediment
supply at the updrift end of the beach and to restore the beach to one with straight
and parallel contours. Because the total amount of longshore sediment transport
during individual wave runs is only a small fraction of the total amount of sand
on the beach (typically less than 1 percent), continuous sand recharging during
wave experiments is not necessary. The traps are dredged as described in Chapter
4 with sand being placed on the updrift portion of the beach. The main portion of
the beach in the middle of the basin requires little attention in part to the uniform
condition maintained by the longshore current recirculation system. The beach is
rebuilt following the previously described method to the equilibrium, or quasi-
equilibrium, profile. During initial experiments, the beach evolves as it is
subjected to waves until an equilibrium, or quasi-equilibrium, condition is
established. It is desired to rebuild the beach to the profile the beach is seeking;
therefore, the average beach profile measured along the uniform portion in the
middle of the beach is used for rebuilding an evolving beach.
50
Chapter 5
Beach Morphology Control
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