off-line beach replenishment are twofold: recharge the sediment supply at the
updrift end of the beach, and restore the beach to one with straight and parallel
contours. Following sand recirculation, the wave basin is drained, and remolding
of the beach to the desired shape is accomplished (Chapter 5 for more details).
The middle two-thirds of the beach generally requires little, if any, remolding due
to the alongshore-uniform condition that is maintained in this region. Typically,
there also are slight perturbations very close to the downdrift boundary, primarily
accretion. Beach response at the downdrift end is discussed in more detail
further into this chapter.
Procedures for Executing Experiments
For each experiment (an experiment is characterized by use of a single-
incident wave condition), process measurements were made for a series of test
segments, or wave runs. Each segment lasted from approximately 40 to 200 min
in duration, depending on the quantity of sand that was transported along the
beach. Shorter-duration tests were adopted for higher, longshore, transport
conditions. Duration also was dictated by the purpose of the test segment. Each
segment was designed to focus on one of the following progressive goals, with
the final goal being acquisition of accurate and comprehensive measurements of
the LST rate, and its cross-shore and vertical distribution patterns. The
progressive goals, listed in sequential order, include:
a. Obtain optimal settings for the pump circulation system, i.e., pump the
proper longshore current, to minimize boundary influences and minimize
recirculation of the longshore current.
b. Allow the beach
to reach equilibrium,
or near-equilibrium, conditions
terms of beach profile shape.
c. Provide adequate measurement coverage in the alongshore and cross-
d. Provide adequate measurement coverage throughout the water column.
e. Repeat key measurements to ensure data quality and repeatability.
Each individual test segment followed a set procedure to ensure data
comparability. The procedure adopted for each test segment was as follows:
a. Conduct a complete pretest beach survey.
b. Perform pretest sediment trap sampling (under quiescent conditions to
"zero" the traps).
c. Check instruments, data acquisition systems, and initialize sensors.
d. Start sampling of sediment trap weight.
e. Start longshore current circulation system (sequential start-up of pumps,
working onshore to offshore).
Start wave generators
(synchronous start-up of all four).
g. Begin sampling of wave, current, and sediment concentration.
i. Stop the longshore current circulation system.
Longshore Sediment Transport Experiments