ERDC/CHL CHETN-IV-12 (Revised)
December 2000
protruding above the authorized channel depth, even though the average channel depth is
sufficient for navigation. Sand waves will migrate and their amplitude and period may change as
conditions change. However, a particular section of channel that is characterized by sand waves
will usually experience their return within a short interval after dredging.
f. Regional siltation. Fine-grained sediment, usually originating from outside the inlet, will
gradually deposit and blanket the area. These deposits are usually thicker within the deeper
dredged navigation channels, which are not naturally maintained by strong currents. These
sediments are eroded and transported from upland or offshore sources as suspended material, as
a result of either high rainfall within the watershed basin or coastal storms. Regional siltation
rates are usually cyclic and are higher during periods of high river discharge, increased
biological growth, and poststorm adjustment. Regional flocculation might also be promoted by
changes in water chemistry. For example, increased salinity can cause clays to flocculate and
precipitate out of the water column.
g. Geotechnical. Geotechnical shoaling is caused by a slumping of channel sidewalls. Such
shoaling may be due to the channel walls being overly steep relative to the slope stability angle
for the sediment or may even be the result of loadings such as storm waves or earthquakes.
Geotechnical failure of channel walls and the resultant channel infilling most typically occurs
after a major channel-deepening operation. In these situations, the channel crest may widen and
the channel side slopes flatten.
INLET CHANNEL ZONES: Several inlet classification schemes have been developed and
presented in the professional literature. U.S. Army Corps of Engineers (1995) summarizes some
of these. However, the following classification and discussion are specifically designed to
address those inlet geometric and functional elements that are important relative to the process of
channel shoaling.
Typical inlets have four major regions: the offshore, the nearshore, the inlet proper, and the
inland water body (i.e., a river, estuary, or bay/lagoon). Inlets may be jettied or unstructured,
display various degrees of distinguishable tidal (i.e., ebb and/or flood) or river deltas, have an
open water back bay, or be a part of a river system. They may have varying configurations of
multiple natural inlet and/or navigation channels. The coastal inlet system may, in turn, connect
to an inland waterway.
At the simplest level, channel shoaling is the result of a balance between riverine processes and
sediment supply and littoral processes (i.e., waves or tidal) and sediment supply. For each zone
of the inlet, the interactions of the different sediment transport processes conspire to produce
shoaling trends that are similar at inlet after inlet.
A geomorphic framework based upon the balance between riverine and coastal contributions has
been used to develop three conceptual models of inlets. These models are based upon the zones
and processes associated with channel shoaling. The difference between each inlet type depends
on the relative dominance of the sediment transport and shoaling processes.
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