Type 6: Visser (1982 and 1991) was the first to use an active recirculation
system driven by a pump. As with the Brebner and Kamphuis (1963)
experiments, the wave guides were open at both ends of the beach. However, in
this case, 12 independent weirs are used to input the required cross-shore
distribution of longshore current at the updrift boundary.
Type 7: H. R.Wallingford (1994) and Simons et al. (1995) provide a
description of the Coastal Research Facility at H.R.Wallingford, United
Kingdom. This facility is a further improvement of Type 6 in that adjustable
weirs are used at both ends of the facility. Therefore, the cross-shore distribution
of longshore current can be controlled at both lateral boundaries.
Based on an investigation of the various techniques used to recirculate the
longshore current in a wave basin with a finite length, a Type 7 recirculation
system was developed for the LSTF. A Type 7 recirculation system should
maximize the length of surf zone in which the hydrodynamics are essentially
uniform in the alongshore direction. Furthermore, the large geometric scale of
the wave conditions that can be generated in the LSTF will produce a significant
amount of suspended sediment transport. It is therefore necessary to maintain
relatively straight and parallel current streamlines at the downstream boundary,
so that the cross-shore distribution of the total longshore sediment transport rate,
(measured using sand traps at the downstream boundary) is not skewed. For this
reason, the recirculation system in the LSTF was developed with the capability of
controlling the cross-shore distribution of longshore current at both the upstream
and downstream lateral boundaries.
System Requirements
Five primary requirements were identified for the longshore current
recirculation system:
a. Must have a pumping capacity to recirculate the longshore flux
associated with a wide range of wave, water level, and bathymetric
conditions.
b. Must have the flexibility to recirculate relatively low longshore current
magnitudes associated with, for example, the offshore tail of the
longshore current distribution. Therefore, the system should have the
capability to recirculate between 10 and 100 percent of the maximum
design pumping capacity at each cross-shore location.
c. Should be capable of maintaining constant discharge rates for several
hours of continuous operation to accommodate the time scales associated
with conducting moveable-bed experiments at this relatively large
geometric scale.
cross-shore distribution of the longshore current being recirculated. This
is of paramount importance for moveable-bed experiments because the
cross-shore distribution of recirculated longshore current must be
adjusted in response to the changing beach profile.
16
Chapter 3
Longshore Current Recirculation System
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