ERDC/CHL CHETN-IV-33
June 2001
ASYMMETRIES IN TIDAL CHANNELS: Asymmetries in the ebb shoal and channel
alignment hold implications for operation and maintenance of inlet navigation projects.
Placement of navigation channels in the preferred location and alignment of the natural channel
may reduce dredging costs, a concept introduced by Price (1951), if navigability can be
maintained.
Asymmetry in ebb-tidal shoal shape may also depend in part on the orientation of the entrance
channel, and vice versa. Vincent, Corson, and Gingerich (1991) classified types of channel
instabilities based on certain channel instability indicators. They noted that channel instabilities
could cause asymmetries in the ebb shoal. FitzGerald, Kraus, and Hands (2001) noted the
mechanism of sand transport at tidal inlets termed ebb-tidal delta breaching occurs at inlets
where the main ebb channel is migratory to a downdrift orientation. Severe deflection of the
channel causes a hydraulically inefficient situation, and the inlet discharge eventually forms a
more efficient path through the old ebb delta. An orientation of the channel away from shore-
normal may result from many factors. Accumulation or intrusion of sediment on one side of the
ebb shoal can move a channel. Asymmetry of the flood shoal and branching ebb channels in the
back bay can orient the jet at an angle. For example, at Ponce Inlet and at Murrells Inlet, SC, the
main ebb channel has been deflected by the action of both processes, accumulation of sediment
on one side of the channel and an asymmetrical orientation of the back bay channels (FitzGerald
1982). Recently, it was found that at Shinnecock Inlet, NY, eddies formed at jetties during ebb
flow can migrate and redirect the jet, hence channel, persistently in one direction (Militello and
Kraus 2001).
Although wave height is usually the main wave parameter determining the critical sea state for
navigation (Demirbilek and Sargent 1999), the direction of waves with respect to vessel motion
is also a controlling factor, even in milder wave conditions. In propagating toward an inlet on
ebb tide, waves can steepen (Smith 1999; Larson and Kraus 2000), so it is safest for a vessel to
meet the waves at a small angle, often on a sinuous course between wave crests and troughs. If
the alignment of the navigation channel from shore-normal tends to direct vessels into the
predominant waves, it may bring favorable navigation conditions, whereas if it places the vessels
broadside to waves, the channel alignment would be unfavorable.
FitzGerald (1984) described a process of inlet migration at unjettied inlets that causes the ebb
shoal to become asymmetrical. One side of an inlet may experience accretion caused by a
differential in longshore transport. At the same time the other side of the inlet begins to erode
owing to a decrease in sediment supply. Resultant changes in inlet entrance morphology alter
the orientation of the ebb current. A reorientation of the main ebb channel occurs, caused by this
new current pattern, and the ebb shoal planform is altered. FitzGerald, Kraus, and Hands (2001)
introduce examples of sediment bypassing at natural, sandy inlets. The modes of bypassing
found to occur often impact the main ebb channel at an inlet and create changes in the shape of
the ebb shoal often causing a previously symmetric ebb shoal to take on an asymmetric shape
even if this change is temporary. Mining of ebb shoals and possible modifications to the
morphology have been discussed by Cialone and Stauble (1998).
ADDITIONAL INFORMATION: This note was produced under the Coastal Inlets Research
Program (CIRP) by Ms. Erica Eva Carr, Coastal Engineering Department, University of Florida,
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