12 December 2001
INTRODUCTION. This chapter describes the important physical
processes related to the movement of sediment around and in harbors and coastal
facilities. An understanding of sediment movement is required for the proper design
and maintenance of coastal facilities, such as siltation in harbors, shoreline erosion
near structures, scour and burial of cables and pipelines, or anchoring in the nearshore.
Much of the information related to these topics is contained in the CEM and the draft
American Society of Civil Engineers (ASCE) "Standard for Shore Protection Systems".
These and other applicable references are cross-referenced below by subject. The
development of local criteria is essential in many cases due to the variation in
meteorological and geological conditions at different geographical sites. If there are
specific additional criteria to be considered, recommendations should be provided to
Naval Facilities Engineering Service Center, Code 51, Port Hueneme, CA 93043,
telephone (805) 982-1170.
SEDIMENT TRANSPORT PROCESSES. Sediment transport in the
nearshore zone is generally a result of the combination of breaking waves and various
patterns of nearshore currents, characterized as a vector with both longshore and
cross-shore components. Analysis of these two components has historically been
performed separately because one or the other tends to dominate in a particular
scenario. A discussion of the components of sediment transport and methods of
analysis is found in Section III of the CEM, starting with a discussion of sediment
classification by size and properties, followed by transport processes of cohesionless
and cohesive sediments, and concluding with a discussion of sediment transport
outside the surf zone.
Sediment Transport Rates. Estimates of sediment transport rates can
be derived either from calculations or through analysis of historical data. Although
analysis of historical shoreline changes may provide a higher level of confidence,
underestimation of the transport rate has not been uncommon in past practice. Where
accuracy is critical to project development, construction and monitoring of a test groin to
verify the estimate should be considered. However, the test groin must extend seaward
far enough to trap all the littoral material. Representative examples of historical data for
various coastal locations are shown in Table 3-1.
Sediment transport and deposition occur on open coasts, in tidal inlets,
estuaries, harbors, and rivers. Sedimentation problems occurring in locations such as
these are a function of soil type, continuity of materials, and the potential for fluid
motion to transport material.
Harbor Siting. Assess sedimentation processes when siting a harbor or
an open-coastal littoral system, in an inlet system, or in a river-mouth estuary system.
In each of these systems, transport capacity and sediment supply factors must be taken
into account. A state of natural equilibrium may be due to unchanging channel depths