CETN II-30
(3/92)
Coastal Engineering
Technical Note
USING MORPHOLOGY TO DETERMINE
NET LITTORAL DRIFT DIRECTIONS IN COMPLEX COASTAL SYSTEMS
P U R P O S E : This Technical Note provides guidance in use of morphologic indicators to determine the
net littoral drift direction along coastal areas. Determination of Iongshore transport and deposition of
sediment is important in the design of coastal shore protection and navigation projects. Along coas
complex circulation and sediment transport, use of relatively simple techniques to determine drift
directions using coastal landform shape can usually provide design guidance. Relative magnitude and
variability of longshore transport also can be determined via morphologic indicators. This CETN
provides general guidance, but care must be used as there are always exceptions.
B A C K G R O U N D : Determination of the net or predominant longshore drift direction along a segment of
coastline is an important aspect of most coastal engineering projects. Often, drift direction in the vicinity
of a project can be determined by analyzing morphology of the adjacent shorelines and coastal region.
Such an analysis can be made at reasonable cost by using existing data sources such as aerial
photographs, topographic and hydrographic maps, historical shoreline change maps and project notes.
Complex sand circulation systems, that vary on both temporal and spatial scales, exist along many coasts.
Assumptions of net drift directions based on large regional scale indicators may give false information
on the local project scale. It is important to keep the following points in mind:
a. Littoral transport is spatially varied and can occur on a mega- or mesoscale. Temporal
variation also occurs, with changes in meteorology or current circulation patterns.
b. Morphologic indicators can help define circulation cells, sediment transport trends, and
longshore variability in littoral circulation patterns.
c. Key morphologic indicators can be shoreline trends, offshore bathymetric features, and
microscale features (such as variations in sediment characteristics or localized deposition at
jetties or other structures).
d. Morphologic response to changes in coastal processes provides the key to using coastal
features in identifying process and response mechanisms in complex coastal systems.
Identification of temporal responses in morphology can provide a picture of the dynamic
equilibrium of changing coastal processes active along the coast.
e. Care must be used in identifying all morphologic indicators in any given coastal area, since
use of only one morphologic indicator (such as shoreline trend) may be misleading. A prime
example is in the case of a downdrift offset inlet landform, where wave refraction around
an ebb tidal delta sets up a localized drift reversal and sediment deposition occurs on the
downdrift shoreline adjacent to the inlet (see for example Hubbard, 1975). Misinterpretation
1
US Army Engineer Waterways Experiment Station, Coastal Engineering Research Center
3909 Halls Ferry Road, Vicksburg, Mississippi 39180-6199
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