ERDC/CHL CHETN-IV-30
December 2000
coasts, sand enters the main inlet channel along the beach by wave action or through marginal
channels by tidal currents. Additional sand is transported into the inlet by flood-tidal and wave-
generated currents over the shallow swash platform that flanks both sides of the main channel.
At jettied entrances, sand may enter the inlet directly from the beach and/or shoreface in
situations where the updrift jetty is short or where the beach has accreted to approach the
seaward end of the jetty. In other cases, rip currents move sand toward the seaward end of jetties
where flood currents can transport sediment into the inlet. The net movement of sand into the
backbarrier or out the main channel or jettied channel is controlled by the dominance of the
flood- versus ebb-tidal currents, respectively.
At most inlets, sand is transported into the bay during storms, when large waves increase the
delivery of sand to the inlet and the storm surge produces strong flood currents. Under normal or
non-storm conditions, sand in the inlet channel is moved in a net seaward direction and is
ultimately deposited on the terminal lobe (outer bar). Waves shoaling and breaking on the
terminal lobe generate currents, which augment flood-tidal currents and retard ebb-tidal currents.
Because of the wave current interaction, sand on the terminal lobe is transported landward across
the swash platform or along the periphery of the delta toward the adjacent beaches. Sediment
movement onshore typically takes place in the form of large, landward migrating swash bars
hundreds to thousands of meters long, 50 to 100 m wide, and 1 to 3 m in height. Along the
downdrift beach, sand may be recirculated back toward the inlet or transferred farther down the
barrier depending upon the morphology of the ebb-tidal delta and wave approach. These general
patterns of sand transport result in sediment bypassing at inlets and are described in the
following section.
MECHANISMS OF INLET SEDIMENT BYPASSING: The following examples demonstrate
mechanisms by which sand is transferred to the downdrift shoreline. These conceptual models
build on the pioneering work of Bruun and Gerritsen (1959) and Bruun (1966) and later research
by FitzGerald (1982, 1988). Additional models are presented here based on more recent tidal
inlet investigations. Both natural and structured inlets are considered.
Model 1. Stable Inlet Processes. A stable inlet is one that has a stable inlet throat (non-
migrating) and a stable main ebb channel position through the ebb-tidal delta. The stability of
the inlet is usually related to the channel being anchored in a substrate resistant to erosion.
Bypassing at these inlets occurs through the formation, landward migration, and attachment of
large bar complexes to the downdrift shoreline (Figure 1a). The development of bar complexes
results from the stacking and coalescence of swash bars on the delta platform. Wave-built swash
bars move onshore due to the dominance of landward flow created by wave swash. Their
stacking results from a decrease in the rate of onshore migration. As the bars migrate up the
shoreface, they gain a greater and greater intertidal exposure. Consequently, wave swash, which
causes their onshore movement, operates over an increasingly shorter period of the tidal cycle.
This developmental process at Price Inlet, SC (Figure 2) was responsible for delivering 100,000
m3 of sand to the downdrift beach. The time frame for these bars to form and migrate onshore
is highly variable but usually takes from 4 to 10 years. The size of bars and the volume of
sand moved onshore generally increases with increasing inlet size. Bars migrating onshore from
ebb-tidal deltas of Friesian Inlets along the German North Sea coast commonly break up and lose
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