D.G. Hamilton, B.A. Ebersole r Coastal Engineering 42 (2001) 199218
214
tion and verification of analytical and numerical
models.
The pump settings that produced the proper long-
shore current distribution in the LSTF were deter-
mined through an iterative process based on mean
longshore current measurements in the surf zone and
offshore in the internal recirculation zone. This pro-
cedure was found to work well. When the proper
distribution is pumped, the degree of longshore uni-
formity in mean longshore current increases notice-
ably, and the magnitude of the current offshore in the
recirculation zone approaches a minimum value. In-
ternal recirculation cannot be eliminated, however,
and its magnitude was found to be about 10% of the
Fig. 16. Test 8E: distribution of mean longshore current at down-
stream boundary.
total flow rate in the surf zone for the two wave
cases examined.
The final selection of proper pump settings were
Finally, the large geometric scale of the wave
confirmed by the insightful criteria developed by
conditions tested in the LSTF will generate a signifi-
Visser Z1991.. But unlike the findings of Visser for
cant amount of suspended sediment transport. It is
the facility he used, for the LSTF, Qp can vary by as
much as "20% of Qpu without a significant in-
therefore necessary to maintain relatively straight
and parallel current streamlines at the downstream
boundary, so that the cross-shore distribution of the
iterative process is placed on examination of the
total longshore sediment transport rate, measured at
measured data, taking advantage of the high degree
the downstream boundary, is not skewed. For this
of cross-shore resolution in the recirculation system
reason, the external recirculation system was devel-
itself. However, even for this range of Qp the in-
oped with the capability of controlling the cross-shore
shore two-thirds of the mean longshore current dis-
distribution of longshore current at both the upstream
tribution was relatively unaffected by Qr . The effect
and downstream lateral boundaries.
of internal recirculation, Qr manifested itself primar-
ily on the offshore tail of the measured longshore
current distribution. Therefore, the tuning focussed
10. Conclusions
on the inshore portion of the distribution first, saving
the offshore tail for last. Measured currents in the
Two comprehensive test series were conducted on
offshore tail were always higher than the equivalent
an idealized concrete beach with straight and parallel
pump settings, regardless of the magnitude.
contours, one involving regular waves and the other
The active external longshore current recirculation
irregular waves. Both test series were conducted with
system and the operational procedure used to pump
long-crested, unidirectional waves. These data are
the proper current, led to a 12-m long region of the
valuable for several reasons: Z1. the geometric scale
beach that was characterized by a very high degree
of the experiments is significantly larger than the
of longshore uniformity in waves, currents, and mean
scale of laboratory experiments conducted in the
water levels, between Y Zlongshore. coordinates of
past, Z2. the concrete beach has a relatively mild
19 and 31 m. Furthermore, as discussed in a previous
1:30 slope, unlike previous laboratory experiments,
section, the standard deviation of the longshore vari-
Z3. both regular and irregular wave forcing were
ation in longshore current is still relatively small
used, and Z4. the vertical velocity structure of the
downstream to Y s 14 m.
longshore current was accurately measured using
The steadiness of mean flow conditions, quality
acoustic-doppler velocitimeters. The data are pro-
of the sensors, and repeatability of the measurements
vided for use in theoretical studies and for calibra-
allow accurate data sets to be acquired, not only of
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