D.G. Hamilton, B.A. Ebersole r Coastal Engineering 42 (2001) 199218
200
1. Introduction
the LSTF recirculation system to meet these objec-
tives is the subject of this paper. Two comprehensive
Waves breaking at oblique angles to the coastline
series of longshore current experiments were per-
generate a mean current in the surf zone that flows
formed on a fixed concrete beach to facilitate evalua-
parallel to the coast. Breaking waves and the long-
tion of the system. The experiments were done as a
shore current are capable of transporting hundreds of
precursor to a more complex series of moveable-bed
thousands of cubic meters of sand along the coastline
longshore sediment transport experiments.
during a typical year. One motivation for studying
The paper is organized as follows. Section 2
the longshore current and longshore sand transport is
provides a brief overview of previous laboratory
to improve existing methods for calculating nearshore
investigations of longshore currents, with emphasis
sediment transport rates, which can then be applied
on the method used to create the proper longshore
to practical engineering applications such as: predict-
current. Section 3 provides a brief description of the
ing beach response in the vicinity of coastal struc-
layout of the LSTF, as well as the design of the
tures, beach-fill project evolution and renourishment
external recirculation system and the lateral bound-
requirements, and sedimentation rates in navigation
aries. Section 4 gives an overview of the experimen-
channels.
tal program along with the incident wave conditions
The Large-scale Sediment Transport Facility
generated for both test series. Section 5 describes the
ZLSTF. has been constructed at the U.S. Army Engi-
methodology used to acquire data, and the type of
neer Research and Development Center's Coastal
data collected during each experiment. Section 6
and Hydraulics Laboratory Zsee Rosati et al. 1995..
describes the procedure used to establish the proper
The intent for the facility is to reproduce, in a
longshore currents. Section 7 quantifies the long-
finite-length wave basin, certain surf zone processes
shore uniformity of the waves, currents and mean
found on a long straight natural beach, including
water surface elevation, as a function of beach length.
sand transport and beach change processes. Success-
Section 8 discusses the steadiness of the flow rates in
ful execution of sediment transport experiments re-
the basin and the repeatability of the flow measure-
quires a method for establishing the proper longshore
ments. Section 9 discusses the results from the stand-
current, or at least a reasonable representation of the
point of conducting longshore sediment transport
proper longshore current recognizing that all pro-
experiments in the LSTF in the near future. Section
cesses that occur on real beaches are not simulated in
10 provides conclusions.
the facility. In the context of this paper, the term
AproperB longshore current is used to describe the
longshore current that would be generated along an
2. Previous laboratory investigations of longshore
infinitely long beach having a cross-section and inci-
currents
dent wave forcing that are invariant in the longshore
direction. An active pumping and re-circulation sys-
tem is used in the LSTF to establish the proper
Relatively few carefully controlled laboratory
longshore current Zsee Hamilton et al. 1996, 1997..
studies, which involved the study of longshore cur-
As a design objective, the external recirculation
rents, have been conducted. In the earliest investiga-
system and procedures for operating it should maxi-
tions, only the average longshore current velocity,
mize the length of beach for which waves, currents,
across the width of the surf zone, or the maximum
sediment transport, and beach morphology are nearly
velocity, was measured ZPutnam et al., 1949; Breb-
uniform in the alongshore direction. Longshore uni-
ner and Kamphuis, 1963.. Galvin and Eagleson
formity is important both in regions where wave,
Z1965. were the first to measure the cross-shore
current, and sediment concentration measurements
distribution of the longshore current at several
are made and at the downdrift boundary of the beach
shore-normal transects along the beach. This in-
where sand traps are located. Creating the proper
creased detail, although very valuable, revealed some
longshore current and a high degree of longshore
of the challenges associated with obtaining longshore
uniformity are difficult to achieve. The capability of
uniformity in the laboratory. All three of these stud-
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