of this type of situation may result in placement of a weir on the wrong side of the inlet.
It is recommended that the net drift direction at a project site be determined by using the following
approach:
1. Office examination of existing data on time history of a locality from photographs; charts;
maps.
2. Field visit, including aerial overflight.
3. Discussions with local specialists (coastal researchers, college professors, state or county
geologists).
4. Review of wave records or hindcast wave data (i.e. Wave Information Study (WIS))
5. Collection of supplemental field data, if existing data in inadequate to interpret the littoral
environment.
An historical perspective of coastal change is needed to understand prevailing coastal processes that
control a project area. Often historical data exist in the form of beach profiles, bathymetric surveys,
aerial photographic collections and shoreline change maps. Since these data sources may be dated,
procurement of recent data will greatly aid the investigation of present conditions. A field visit should
not be neglected because there is no better way for a researcher to get a feel for the setting and processes
which affect the region. From the air, many large-scale morphological features of the region become
clearly evident whereas from the ground they may be hard to detect. Engineers, scientists, and non-
technical shore users with local knowledge of a coastal area also provide a valuable source of information
on local coastal processes and shoreline response. Recognition of complex temporal and spatial drift
directions may require implementation of a limited field collection program to supplement analysis of
inadequately documented shoreline evolution and prevailing coastal processes. Data gaps can be
overcome with moderate field efforts that could include beach profile and bathymetric survey collection,
new aerial photography overflights and installation of short term (about 1 year) wave, tide and current
data collection equipment. Documentation of erosion and accretion trends and shoreline evolution can
be accomplished from profiles and aerial photography, while dominant forcing functions (ie. wave
climate, net drift directions) can be evaluated from physical measurements.
LONGSHORE DRIFT: Longshore (or littoral) drift is defined as: "Material (such as shingle, gravel,
sand, and shell fragments) that is moved along the shore by a littoral current" (Bates and Jackson, 1984).
Net longshore drift refers to the difference between volume of material moving in one direction along
the coast and that moving in the opposite direction (Bascom, 1964). Along most coasts, longshore
currents change directions throughout the year. In some areas, changes occur on cycles of a few days,
while in others the cycles may be seasonal. Therefore, one difficulty in determining the net drift
direction is defining a pertinent time frame. Net drift averaged over years or decades may conceal the
fact that significant amounts of material also flow in the opposite direction. In addition, variations in
meteorological conditions from year to year may result in changes in net drift. For example, storms may
cause large pulses of material to flow in one direction, while fair weather drift may normally be in the
opposite direction. Therefore, during especially stormy years, net drift may be significantly different than
during calmer years.
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