ERDC/CHL CHETN-IV-30
December 2000
c.
Models 7-9
Figure 1.
(Concluded)
Model 2. Ebb-Tidal Delta Breaching. Ebb-tidal delta breaching occurs at tidal inlets that
have stable throat positions, but whose main ebb channels cyclically migrate downdrift
(Figure 1a). The dominant direction of longshore transport at these sites produces a preferential
accumulation of sediment on the updrift side of the ebb-tidal delta. The sediment accumulation
causes a downdrift deflection of the main ebb channel, which at some inlets may ultimately
impinge against the downdrift inlet shoreline. This pattern of channel migration commonly
induces erosion along the adjacent beach. A severe deflection of the main channel produces flow
at the inlet that is hydraulically inefficient. Eventually, this condition results in the ebb discharge
being diverted to a more direct seaward pathway through the ebb-tidal delta.
The breaching process can occur gradually over a period of 6 to 12 months or catastrophically
during a single storm when discharge of floodwaters increases the scouring of the ebb currents.
Once formation of the new channel is completed, it will convey most of the inlet tidal prism.
Thus, the abandoned channel gradually fills with sediment deposited by both tidal and wave-
generated currents. The breaching process commonly results in the bypassing of a large portion
of the ebb delta sand. Some of this sand fills the old channel, while the rest forms a subtidal or
intertidal bar complex that migrates onshore ultimately attaching to the landward beach. These
bars are identical to the ones previously described for the stable inlet processes and contain equal
or greater volumes of sand. The entire developmental process takes from 5 to 10 years to
complete.
Model 3. Inlet Migration Spit Breaching. Sand transported along the beach and deposited
in a tidal inlet constricts the inlet throat decreasing the flow area. As Escoffier's (1940) stability
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