Complexities of pumping the proper longshore current
The process of determining proper pump settings for movable-bed LST
experiments is much more complex than that for experiments involving a
concrete beach. In the latter case, the bottom does not respond during the
iterative process in which incorrect pump settings are gradually improved to
converge on the proper settings. The iterative process on a concrete beach is
much more "forgiving," unlike that for movable-bed experiments, in which the
bed immediately begins to respond to wave forcing and incorrect pump settings.
The recommended iterative procedure involves approaching the proper settings
from below. In this situation, the discharge pumped is less than the proper
amount, and return flows tend to transport sand offshore along the downdrift
boundary. This creates an undesirable beach response that is nonuniform in the
alongshore direction and occurs directly in front of the traps, which in turn alters
wave breaking and current patterns near the downdrift end of the beach. Also,
during each iteration, measurements must be made at multiple transects to assess
the degree of alongshore uniformity that has been achieved. More transects (i.e.,
more data) are desirable to ascertain the degree of alongshore uniformity that
exists. But more time is required to make them, and during this time the beach is
responding to improper flow conditions. Typically the process to converge on
the proper pump settings will require from 8 to 12 hr of run time. Depending on
the longshore transport rate, this may require remolding of the beach anywhere
from one to three times. Beach remolding is a very time-consuming process, and
one to be minimized.
Equilibration of beach profile shape
Evolution of the beach to a near-equilibrium profile shape also requires time.
The time frame to reach a near-equilibrium beach profile condition depends on
the degree to which the preexperiment profile is out of equilibrium with the
incident wave conditions and the rate of sediment transport. The goals of
arriving at an equilibrium beach profile shape and of establishing the proper
pump settings are accomplished concurrently. Usually, the beach must be
remolded several times during this process.
Figure 71 illustrates profile evolution for the irregular wave case that
produced plunging-type breakers. Profile measurements were made near the
center of the beach. A number of profiles are shown. The different curves
shown in the figure correspond to different elapsed times relative to the starting
time. Included in the figure are the pretest profile, profiles measured at the
conclusion of numerous test runs that varied in duration from 40 to 100 min,
profiles measured before and after beach rebuilding, and the profile shape after
10.5 hr (630 min) of wave activity. After approximately 180 min, the beach had
evolved to its near-equilibrium state for these incident wave conditions.
126
Chapter 10
Longshore Sediment Transport Experiments
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