The ADVs were positioned at elevations approximately one-third of the water
depth above the bed.
At the beginning of each experiment, the instrumentation bridge was
positioned at Y27, and the elevation of the still-water level was measured with
the 14 wave gauges. The pumps were started and set to prescribed discharges to
create the desired longshore current distribution. Data collected with the in-line
flow sensors (one in each of the 20 pump-and-piping systems) were analyzed to
ensure the pumps were operating at the proper flow rates. The wave generators
were started and waves were generated continuously throughout the experiment.
After 10 min of wave generation (20 min after the pumps were started), data
were collected at the Y27 transect. All sensors were sampled at 20 Hz for 500
sec during both the regular and irregular wave test series. The process of
repositioning the instrumentation bridge and acquiring 500 sec of data was
repeated at transects Y15, Y19, Y23, Y27, Y31, Y35, and Y39. After the last
transect was completed at Y39, a third set of data was acquired at Y27. Multiple
measurements at transect Y27 were collected to assess repeatability of the
measurements and the steadiness of the hydrodynamic conditions.
The time series were visually inspected during experiments to assess data
quality. In very shallow water, air bubbles from breaking waves penetrated into
the water column, to the depth of the ADV 1 and 2 sensors, causing undesirable
spikes in the velocity time series. These spikes were removed during post-
processing with a filtering routine developed and discussed in Chapter 8. The
ADV measurements further offshore did not need to be filtered.
Detailed dye measurements were performed during each experiment to
inspect patterns in the flow streamlines by injecting the dye into the water at
discrete points. Dye observations focused on: (a) uniformity of flow streamlines
along the beach; (b) streamline patterns of flow exiting the upstream flow
channels and approaching the downstream flow channels; and (c) streamlines in
the offshore region of the basin where internal recirculation occurred. Dye also
was used to obtain qualitative information on the longshore current in very
shallow water, shoreward of ADV 1.
Procedure for Tuning the Longshore Current
This section contains three subsections. The first describes the iterative
process that was used to establish the proper magnitude and cross-shore
distribution of the longshore current along the beach by adjusting the pump
settings for the external recirculation system. To verify these results, the two
subsequent subsections describe two criteria proposed by Visser to confirm that
the proper total longshore flow rate was recirculated. However, these two criteria
consider only the magnitude of the total longshore flow rate. They do not help
determine the proper cross-shore distribution of the longshore current that should
be recirculated.
Figure 56 conceptually illustrates the flow conditions in the LSTF during the
experiments. The quantity Qs is the total longshore flow rate in the surf zone
between the wave setup limit and the point of transition where the mean
104
Chapter 9
Establishing Uniform Longshore Currents
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