CETN II-30
(3/92)
Longshore drift was traditionally conceptualized as a "river of sand" flowing uniformly along the shore.
Recent research has shown'that, instead, the coastline can be divided into littoral cells of varying length
(Carter, 1988). Cell boundaries may be well-defined or ephemeral. Well-defined boundaries tend to be
coastal structures or morphological features such as headlands, shoals, river mouths, or inlets which
exercise major control on the wave refraction pattern and/or inhibit longshore sediment transport.
Ephemeral or transitory boundaries are much harder to locate because they are caused by cyclic changes
in net drift direction generated by variability in the incident wave climate. Each combination of
deepwater wave height, wave period, and approach direction causes a unique cell structure. As the waves
change, the cell boundaries move, enabling sediment to be passed alongshore. This concept helps explain
why even subtle meteorological variations may affect net drift.
The predominant megascale drift direction is north to south along most north to south trending coasts of
the U.S. (ie. Atlantic, Pacific, peninsular Gulf Coast of Florida, Texas Gulf Coast, east and west side
of Lake Michigan) due mostly to prevailing storm tracks and meteorology. There are, however, many
macroscale local variations in this pattern due to local morphologic controls. Littoral cells have been
defined as areas of coast where no inflow or outflow of sediment occurs (Smith and Sayao, 1989). Little
interaction usually occurs between cells and coastal processes may be quite different between adjacent
alongshore cells. Investigators are discovering localized circulation cells on all coasts. The southern
California coast can be divided into several cells based on river locations, headlands and submarine
canyons as seen in Figure 1 (Inman and Frautschy, 1966; Kadib, 1989). The Atlantic coast of Florida
north of Cape Canaveral may have several local cells based on inlet locations, offshore bathymetry and
wave refraction patterns (Stapor and May, 1983; Stauble and Da Costa, 1987).
EXAMPLES OF MORPHOLOGIC DRlFT INDICATORS: Examples of various coastal features and their
interaction with longshore currents are summarized in Figure 2. In all the examples, predominant drif:
direction is from left to right, and land is in the upper part of the image.
A. A rocky headland has interrupted longshore drift by projecting farther seaward than the adjacent
beaches. In addition, the wave field has probably been affected by refraction around the promontory.
Sand has accumulated on the updrift (left) side of the headland, while the downdrift side is exposed and
has suffered more erosion.
B. A-sand spit is growing from left to right across the mouth of a stream or inlet where it enters the sea.
Recurved beach ridges are formed as the spit grows. If the sediment supply is adequate, the spit may
completely block the stream periodically. After storms, increased river flow or overwash and storm
surge volumes may break through the spit at a location updrift of its previous opening. Note that
although the bend in the stream and the spit's projection to the right in this example normally indicate
that drift is from left to right, there are locations where updrift inlet migration and spit growth have been
documented.
C. Recurved ridges, which are convex to the right, and the convex shape of the entire spit indicate
growth to the right.
D. Tapering beach ridge sets may represent locations where boundaries of circulation cells have become
well established (Carter, 1988). There may be only minor sediment transfer between cells. Transfer
occurs when the ridge sets on the updrift end of a cell are eroded. Sand is then carried by littoral
currents to the downdrift end of the cell, where it feeds growth of more beach ridges.
3
Previous Page
