ERDC/CHL CHETN-IV-33
June 2001
Morphologic Asymmetries at
Entrances to Tidal Inlets
by Erica Eva Carr and Nicholas C. Kraus
discusses selected morphologic symmetries of ebb shoals and channels at tidal inlets with
implications for maintenance of navigation channels and sediment bypassing to the adjacent
beaches. Much of the information contained in this Technical Note was developed from a
database created by the Coastal Inlets Research Program (CIRP) and may be found at
BACKGROUND: Navigational improvements will alter the morphology of an inlet and may
have unintended consequences for channel maintenance, integrity of the jetties, and natural
bypassing to the adjacent beaches. This Technical Note examines the characteristics of selected
symmetries in morphological forms at inlet entrances and presents empirical quantitative
relationships for their prediction. Possible applications of this information include:
a. Formulation of sediment budgets at inlets, where detailed sediment pathways are
required.
b. Determination of the predominant (net) direction of longshore sediment transport.
c. Determination of the natural causes of entrance channel migration and realignment (both
for maintenance of existing channels and for modification of channel alignment).
d. Consequences of construction of or modifications to jetties, such as alteration of sediment
pathways.
e. Understanding and estimation of the locations areas of erosion and accretion near inlets.
Guidance on effective areas of placement of dredged material for benefit of the down-
f.
drift beaches.
Asymmetries in the morphology of ebb-tidal shoals (also termed ebb-tidal deltas or entrance bar)
and orientation of the entrance channel are produced by both dynamic and static factors.
Dynamic factors include the magnitude and direction of net longshore sediment transport, tidal
prism, relict ebb shoal, offshore extent of the ebb jet, riverine sediment supply, flood shoal
evolution, dredging of the channel, and wave refraction and diffraction over the offshore
bathymetry and ebb shoal. Static factors include the locations and configurations of jetties,
offshore and nearshore bathymetry, size and shape of the back bay, and constraints as imposed
by the local geologic structure such as hard bottom.
The asymmetric inlet morphology at East Pass, FL, is shown in Figure 1, as inferred by the
pattern of breaking waves. The ebb shoal in the broad sense is comprised of the ebb shoal
proper, the updrift and downdrift bypassing bars, and the attachment bars. The ebb shoal proper
forms primarily in the stream of the ebb jet, whereas formation of the bypassing bars owes more
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