EM 1110-2-1100 (Part III)
30 Apr 02
volume is called a source. Alternately, any process that results in a net loss of sediment from a control
volume is considered a sink. Some processes can function as sources and sinks for the same control volume
(e.g., longshore sediment transport). The balance of sediment between losses and gains is reflected in
localized erosion and deposition. In general, longshore movement of sediment into a coastal compartment,
onshore transport of sediment, additions from fluvial transport, and dune/bluff/cliff erosion provide the major
sources of sediment. Longshore movement of sediment out of a coastal compartment, offshore transport of
sediment, and aeolian transport and washover that increase beach/island elevation produce losses from a
control volume. Sediment budgets, including the types and importance of sources and sinks, are discussed
in detail in Part IV-5.
(b) The appropriate level of detail for a littoral budget is a function of the intended uses of the littoral
budget, and the available resources to complete the project. The essential components of a littoral budget
include a site description, background, and examination of previous analyses. Past and present conditions,
and the results of other studies, must be examined before initiating a new budget analysis.
(c) The longshore sediment transport rate must be determined next. This requires data on wave
conditions over as long a time period as possible. These waves are propagated to and transformed in the surf
zone. Appropriate sediment transport equations must be applied, ideally using historical shoreline positions
and wave conditions for the same time period to, in effect, "calibrate" the transport equations for the study
site. Very often, shoreline change models, which use the sediment transport equations, are applied. Boundary
conditions defined at the start of the analysis are changed, so that sensitivity of the budget to these conditions
may be evaluated.
(d) The actual sediment budget may then be determined. Usually there are poorly quantified components
remaining in the analysis, such as offshore gains and losses. These must be estimated using any available
data, engineering judgment, and the requirement that the budget close. Although a significant effort goes into
the development of a littoral budget, it must be remembered that it is an estimate and may easily be in error.
In addition, the budget is usually calibrated with shoreline positions over a number of years, and therefore
indicates long-term average rates of change. It may not be indicative of the changes in any one year.
(2) Variations in longshore sediment transport along the coast.
(a) As described in the next section, noncohesive shorelines are not typically straight. The shoreline
orientation and the degree to which waves refract, shoal, and converge or diverge along the shoreline
determine variations in the potential transport rate along the coast. These variations are important
determinants to shoreline change along the coast. For a nonuniform coastline, the potential longshore
sediment transport rate is computed at discrete points along the coastline using values of the local breaking
wave height and angle, where the latter is expressed relative to the local shoreline orientation. A ray-tracing
or grid-based wave refraction analysis is typically employed to determine these values.
(b) Shoreline change may be related to the computed gradients in transport rate along the coast. For
example, areas of convergent transport may correspond to a sediment sink (or deposition). Areas of divergent
transport may correspond to a sediment source (or erosion if the area is not a source). As long as there is an
unlimited sediment source, a shoreline's response to longshore sediment transport should be dependent on
gradients in transport along the coast rather than magnitudes of transport.
III-2-48
Longshore Sediment Transport
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