D.G. Hamilton, B.A. Ebersole r Coastal Engineering 42 (2001) 199218
201
ies used passive recirculation systems, which means
Mizuguchi and Horikawa Z1978. were the first to
that the longshore current was allowed to passively
measure the vertical structure of the longshore cur-
flow back to the upstream end of the basin, either
rent in the laboratory. A passive recirculation scheme
inside or outside of the waveguides.
was used allowing the longshore current to recircu-
Dalrymple and Dean Z1972. developed a circular
late outside of the downstream waveguide and re-en-
wave basin with a spiral wave generator and a
ter into the testing region beneath the wave genera-
circumferential beach to eliminate the boundary ef-
tors. They found the mean longshore current to be
fects caused by the upstream and downstream
relatively uniform with depth. Visser Z1991. ob-
boundaries of the facility. Potential methods to deal
served the same. Simons et al. Z1995. concluded
with the problem of wave-induced circulation in
that the vertical velocity structure of the longshore
shallow wave basins continued to be addressed in
current followed a logarithmic profile during prelim-
subsequent years, see Dalrymple et al. Z1977. and
inary evaluation tests in the large-scale Coastal Re-
Kamphuis Z1977..
search Facility at HR Wallingford. The active recir-
Visser Z1980, 1982, 1984, 1991. documented the
culation system used in the Coastal Research Facility
most comprehensive laboratory study of longshore
is a further improvement upon the system used by
currents previously conducted for a relatively wide
Visser Z1991. in that adjustable weirs are used to
range of test conditions. Visser measured the three-
control the cross-shore distribution of longshore cur-
dimensional structure of the longshore current and
rent at both the upstream and downstream bound-
developed a method to maximize longshore uniform-
aries of the facility ZHR Wallingford, 1994.. Hamil-
ity by utilizing an active external recirculation sys-
ton et al. Z1997. summarizes the seven types of
tem driven by a pump. A distribution system was
longshore current recirculation systems used in labo-
used at the upstream boundary to distribute the long-
ratory investigations of longshore currents.
shore flux across the surf zone. At the downstream
boundary, however, a distribution system was not
used, but the width of the opening in the waveguide
3. Design of the LSTF
was optimized based on the width of the surf zone.
The longshore current then flowed further down-
Fig. 1 is a photograph of the LSTF taken during a
stream toward a single point of intake to the external
regular wave experiment. The LSTF has dimensions
recirculation system. This laboratory study sets a
of approximately 30-m cross-shore by 50-m long-
high standard for experimental design and attention
shore by 1.4-m deep, see Fig. 2. Unidirectional,
to detail. The experiments were conducted on rela-
long-crested waves are generated with four piston-
tively steep beach slopes Z1:10 and 1:20. and were
type wave generators. The concrete beach has a
restricted to longshore currents generated by regular
longshore dimension of 31 m and a cross-shore
waves.
dimension of 21 m, with a plane slope of 1:30. Small
Reniers and Battjes Z1997. measured the cross-
longshore variations in beach elevation can have a
shore distribution of longshore current for regular
significant influence on longshore current patterns
and irregular waves, and they used a similar method
ZPutrevu et al., 1995.. Therefore, the beach was
of external recirculation as that used by Visser. Tests
designed and carefully constructed with straight and
where conducted on two types of beaches; one with
parallel contours and a high degree of accuracy
Zvertical variation of "2 mm..
a constant 1:20 beach slope and one with an offshore
bar. In the case of the barred beach, measurements
The coordinate system in the facility is right-
revealed a bimodal longshore current distribution
handed, with the origin at the downstream, shore-
with one peak near the crest of the offshore bar and
ward end of the wave basin Zsee Fig. 2.. The positive
the second peak near the shoreline. For the case of
x-axis is directed offshore and is measured relative
the constant beach slope, the limited geometric scale
to the upper edge of the concrete beach slope. The
of the experiments made it impossible to measure the
positive y-axis is directed upstream. The z-axis is
peak longshore current, due to inadequate water
measured positively upwards with the origin at the
depth in the surf zone.
still water level. The coordinate system was chosen
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