10 Longshore Sediment
Transport Experiments
Introduction
This chapter discusses the methods and procedures that were used to execute
movable-bed longshore sand transport (LST) experiments. The objective of the
experiments described here was to accurately measure the total LST rate, and its
cross-shore distribution, on a laboratory beach that is intended to represent an
infinitely long, natural beach having straight and parallel topographic contours,
i.e., water depths are invariant in the alongshore direction. For these idealized
beach conditions, along with wave and current forcing that has alongshore
uniformity, the LST rate also should be invariant in the alongshore direction.
Longshore uniformity of all processes represents the target conditions that are to
be created in the facility. This chapter presents results from initial experiments
performed in the facility, and provides information on how uniform the
conditions were.
Ideally, the sediment traps situated at the downdrift end of the beach should
be 100 percent efficient, i.e., completely trap the sand that the waves and currents
transport alongshore, no more, no less. However, the physical boundaries of the
facility, and imperfections in the systems and scheme used to control wave,
current, and sediment transport processes at the lateral boundaries, influence the
degree to which alongshore uniformity can be achieved. Chapter 9 addressed
how well uniformity was achieved, in terms of hydrodynamics, for a planar
concrete beach with straight and parallel contours. The situation becomes much
more complex for movable-bed experiments. With a mobile bed, sand begins to
immediately move in response to waves and currents and the beach topography
begins to evolve. Imperfections in lateral boundaries are expected to induce
anomalous beach response, loss of complete alongshore uniformity, and a less-
than-perfect trapping system. The degree of beach uniformity that is achieved in
the alongshore direction, and maintained during the course of experiments,
dictates the success of the experiments. The first few experiments were a learning
process. During the initial experiments, methods and procedures for conducting
movable bed experiments were developed and refined; and results and lessons-
learned are discussed here.
Results from two experiments are presented in this chapter. In the first,
predominantly spilling-type breaking waves were produced. These were created
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Chapter 10
Longshore Sediment Transport Experiments
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