D.G. Hamilton, B.A. Ebersole r Coastal Engineering 42 (2001) 199218
205
beach., and Y35 Zcenter of the upstream half of the
spikes were removed during post-processing with a
beach.. During Tests 6N and 8E, and several others,
filtering routine developed specifically to handle the
measurements were made at all seven transects.
characteristics of the spikes. The ADV measure-
Ten capacitance-type wave gauges and 10 acous-
ments further offshore did not need to be filtered.
tic-doppler velocitimeters ZADV. were co-located in
Detailed dye measurements were performed dur-
a cross-shore array on the instrumentation bridge.
ing each experiment to inspect patterns in the flow
Wave and current sensors are numbered in ascending
streamlines by injecting dye into the water at discrete
order, starting with number 1 and moving offshore to
points. Dye observations focused on: Za. straightness
number 10. Four other wave gauges were fixed in an
of the flow streamlines along the beach, Zb. stream-
array along the x s 18 m contour line, one centered
line patterns of flow exiting the upstream flow chan-
in front of each wave generator. Wave set-up and
nels and approaching the downstream flow channels,
set-down were obtained by using the wave gauges to
and Zc. streamlines in the offshore region of the
measure the elevation of the still water level prior to
basin where internal recirculation occurred. Dye was
each experiment and then subtracting that elevation
also used to obtain qualitative information on the
from the mean water surface elevation measured
longshore current in very shallow water, shoreward
during the experiment. The ADV's were set at eleva-
of ADV 1.
tions approximately one third of the water depth
above the bed.
At the beginning of each experiment, the instru-
6. Procedure for tuning the longshore current
mentation bridge was positioned at Y27 and the
elevation of the still water level was measured with
all 14 wave gauges. Then the pumps were turned on
Section 6.1 describes the iterative process that
and set to prescribed discharges Zsee Section 6. to
was used to establish the proper magnitude and
create the desired longshore current distribution. Data
cross-shore distribution of the longshore current along
collected with the in-line flow sensors Zone in each
the beach by adjusting the pump settings for the
of the 20 pump-and-piping systems. were analyzed
external recirculation system. To verify these results,
to ensure the pumps were operating at the proper
Sections 6.2 and 6.3 describe two criteria proposed
flow rates. The wave generators were then turned on
by Visser to confirm that the proper total longshore
and run continuously throughout the experiment. Af-
flow rate was being recirculated. However, these two
ter 10 min of wave generation Z20 min since the time
criteria consider only the magnitude of the total
the pumps were started. data collection began at the
longshore flow rate. They do not help determine the
Y27 transect. All sensors were sampled at 20 Hz for
proper cross-shore distribution of the longshore cur-
500 s during both the regular and irregular wave test
rent that needs to be recirculated.
series. The process of repositioning the instrumenta-
Fig. 2 illustrates conceptually the flow conditions
tion bridge and acquiring 500 s of data was repeated
in the LSTF during the experiments. The quantity Qs
at transects Y15, Y19, Y23, Y27, Y31, Y35, and
is the total longshore flow rate in the surf zone
Y39. After the last transect was completed at Y39, a
between the wave set-up limit and the point of
third set of data was acquired at Y27. Redundant
transition where the mean longshore current reverses
measurements at transect Y27 were collected to as-
direction; Qp is the total longshore flow rate actively
sess repeatability of the measurements and the
pumped through the external recirculation system; Qr
steadiness of the hydrodynamic conditions Zsee re-
is the total longshore flow rate that internally recircu-
sults in Section 8..
lates in the offshore region; and Qc is a secondary
The time series were visually inspected while the
offshore circulation cell limited to the length of each
experiments were conducted to assess data quality.
wave board, between two adjacent baffles. In this
In very shallow water ZADV 1 and 2., air bubbles
facility, Qc develops as a result of the baffles that
from breaking waves penetrated into the water col-
extend shoreward of the wave boards. In general, it
umn, to the depth of the ADV sensors, causing
was found that Qc decreases as Qr decreases, be-
undesirable spikes in the velocity time series. These
cause Qr drives Qc . At a transect midway along the
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