EM 1110-2-1100 (Part III)
30 Apr 02
Chapter III-2
Longshore Sediment Transport
III-2-1. Introduction
a. Overview.
(1) The breaking waves and surf in the nearshore combine with various horizontal and vertical patterns
of nearshore currents to transport beach sediments. Sometimes this transport results only in a local
rearrangement of sand into bars and troughs, or into a series of rhythmic embayments cut into the beach. At
other times there are extensive longshore displacements of sediments, possibly moving hundreds of thousands
of cubic meters of sand along the coast each year. The objective of this chapter is to examine techniques that
have been developed to evaluate the longshore sediment transport rate, which is defined to occur primarily
within the surf zone, directed parallel to the coast. This transport is among the most important nearshore
processes that control the beach morphology, and determines in large part whether shores erode, accrete, or
remain stable. An understanding of longshore sediment transport is essential to sound coastal engineering
design practice.
(2) Currents associated with nearshore cell circulation generally act to produce only a local
rearrangement of beach sediments. The rip currents of the circulation can be important in the cross-shore
transport of sand, but there is minimal net displacement of beach sediments along the coast. More important
to the longshore movement of sediments are waves breaking obliquely to the coast and the longshore currents
they generate, which may flow along an extended length of beach (Part II-4). The resulting movement of
beach sediment along the coast is referred to as littoral transport or longshore sediment transport, whereas
the actual volumes of sand involved in the transport are termed the littoral drift. This longshore movement
of beach sediments is of particular importance in that the transport can either be interrupted by the
construction of jetties and breakwaters (structures which block all or a portion of the longshore sediment
transport), or can be captured by inlets and submarine canyons. In the case of a jetty, the result is a buildup
of the beach along the updrift side of the structure and an erosion of the beach downdrift of the structure. The
impacts pose problems to the adjacent beach communities, as well as threaten the usefulness of the adjacent
navigable waterways (channels, harbors, etc.) (Figure III-2-1).
(3) Littoral transport can also result from the currents generated by alongshore gradients in breaking
wave height, commonly called diffraction currents (Part II-4). This transport is manifest as a movement of
beach sediments toward the structures which create these diffraction currents (such as jetties, long groins, and
headlands). The result is transport in the "upwave" direction on the downdrift side of the structure. This, in
turn, can create a buildup of sediment on the immediate, downdrift side of the structure or contribute to the
creation of a crenulate-shaped shoreline on the downdrift side of a headland.
b. Scope of chapter. This chapter defines terms associated with the longshore transport of littoral
material, presents relationships for the longshore sediment transport rate as a function of breaking waves and
longshore currents, discusses the dependence of longshore transport relationships on sediment grain size, pre-
sents a method for calculating the cross-shore distribution of longshore sand transport, and overviews analyti-
cal and numerical models for shoreline changes which include longshore sediment transport relationships.
III-2-2. Longshore Sediment Transport Processes
a. Definitions. On most coasts, waves reach the beach from different quadrants, producing day-to-day
and seasonal reversals in transport direction. At a particular beach site, transport may be to the right (looking
Longshore Sediment Transport
III-2-1
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