9
Establishing Uniform
Longshore Currents
Introduction
Successful execution of sediment transport experiments requires a method
for establishing the proper longshore current for a given incident wave condition.
The term "proper" longshore current is used to describe the longshore current
that is generated along an infinitely long beach having a cross section and an
incident wave forcing that are invariant in the longshore direction. As discussed
in Chapter 3, an active pumping and recirculation system was developed for the
LSTF to establish the proper longshore current.
The external recirculation system and procedures for operating it should
maximize the length of beach for which waves, currents, sediment transport, and
beach morphology are nearly uniform in the alongshore direction. Longshore
uniformity is important both in regions where wave, current, and sediment
concentration data are collected, and at the downdrift boundary of the beach
where the sediment traps are located. Creating the proper longshore current with
a high degree of longshore uniformity is difficult to achieve. The capability of
the LSTF recirculation system to meet these objectives is the subject of this
chapter. Two comprehensive series of longshore current experiments were
performed on the fixed concrete beach to facilitate evaluation of the system. The
experiments were conducted as a precursor to a more complex series of
moveable-bed longshore sediment transport experiments. The majority of the
material discussed in this chapter was published by Hamilton and Ebersole
(2001).
Experiment Program
A number of preliminary experiments were conducted to investigate:
(a) long-term oscillations in pump discharge rates (found to be negligible);
(b) flow patterns created by pumping only (i.e., no wave forcing); (c) flow
patterns with waves only (i.e., no external current recirculation); and (d) the time
required for mean velocities in the wave basin to reach steady state.
After these preliminary experiments were completed, two comprehensive test
series were conducted, one using regular waves and the other using irregular
101
Chapter 9
Establishing Uniform Longshore Currents
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