ERDC/CHL CHETN-IV-30
December 2000
semi-indurated sediments. As an inlet migrates, it leaves behind a series of curved beach ridges
that define the updrift spit. Inlet migration commonly lengthens the inlet channel, which
connects the open ocean to the backbarrier bay, lagoon, or marsh and tidal creek system.
Elongation of the inlet channel increases frictional resistance of tidal flow thereby reducing tidal
range in the bay.
Differences in tidal phase and tidal range between the ocean and backbarrier can augment the
breaching of the spit and formation of a new (relocated) tidal inlet (Figure 1a). Spits are usually
breached during storms when waves erode the beach and dune system, reducing the width of the
barrier. Storm surges produce washovers (precursors of inlets) and increase tidal flow. The new
inlet is commonly located along the updrift spit at a position where the barrier is narrow and the
backbarrier tidal prism is easily accessed. The hydraulically favorable position of the new inlet
promotes capture of the old inlet's tidal prism and its eventual closure. The end product of the
spit breaching process is that a large quantity of sand is transferred from the updrift to the
downdrift inlet shoreline.
Model 4. Outer Channel Shifting. This mechanism of inlet sediment bypassing is similar
to ebb-tidal delta breaching, but is limited to the seaward end of the main ebb channel and
involves smaller volumes of sand (Figure 1b). In this process, the inner portion of the main
channel remains in a fixed position while the outer channel is deflected downdrift because of
preferential accumulation of sand on the seaward, updrift side of the swash platform. As the
outer portion of the channel becomes more deflected, which at some inlets can produce a right-
angle bend, flow through the outer portion of the channel becomes increasingly less efficient.
Eventually, a new channel is cut through the distal portion of the ebb delta that shortens the
pathway of flow. Cutting of a new channel is commonly initiated during high spring tides when
peak flows occur in the channel. The sand that had been located on the updrift side of the outer
channel and is now on the downdrift side has bypassed the inlet.
As the bypassed sand moves onshore by flood-tidal and wave-generated currents, it is commonly
molded into a large swash bar that migrates landward and attaches to the downdrift beach. These
bars are usually much smaller than those produced through ebb-tidal breaching processes but
may still contain 5,000 to 50,000 m3 of sand. At some inlets, the bypassing of sand by outer
channel shifting may occur between the longer episodes of major ebb-tidal delta breaching. At
other inlets, particularly those having deep channels, outer channel shifting may be the dominant
mode of sediment bypassing. At these sites the inner channel may be entrenched in resistant
sediments precluding channel migration. Willapa Bay Inlet in southwest Washington is an
example of this type of channel system, although the bypassed sand does not form a bar complex
that migrates onshore to the downdrift shoreline (Hands and Shepsis 1999).
Model 5. Spit Platform Breaching. At most migrating inlets, the updrift barrier spit is
fronted by a large intertidal spit platform. The platform may extend from 100 to more than
1,000 m into the inlet producing a highly asymmetric channel configuration. In Model 5, large
quantities of sand are bypassed when a new channel is breached through the spit platform
(Figure 1b). The major channel in the backbarrier usually runs parallel to the rear of the spit as it
nears the inlet and then turns to flow around the spit platform. This pattern is analogous to flow
through a river meander bend consisting of a point bar (spit platform) and channel cut bank
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