ERDC/CHL CHETN-IV-12 (Revised)
December 2000
planform shoaling patterns associated with each channel shoaling mechanism presented here are
demonstrated schematically in Figure 1.
a. Channel migration. The lateral movement of a channel is due to the natural meandering
trend of a turbulent flow regime. One side of the channel or thalweg shoals as the other side of
the channel erodes. Channel cross-sectional area is approximately conserved during migration.
If the eroding side of the migrating channels meets a resistant surface such as a jetty, bulkhead,
rock outcrop, or consolidated clays, the channel may deepen and narrow, but retain the capability
to accommodate the same hydraulic discharge. Migrating channels may meander back and forth
within a limited area or exhibit a continuous net translation of their position. The development
of shoals in the deltas and spits at the inlet help promote channel migration.
b. Morphodynamic pathways. Higher energy environments, particularly within the littoral
zone, often include dynamic morphological features. These features may migrate or serve as a
pathway where sediment moves along the top of them. Such features include sandbars and
migrating shoals. They may even be generated by aeolian processes as wind-driven sands and
migrating dune fields cross the land and deposit in adjoining waters. Sand moves along or with
these features until it is deposited within a more protected area such as behind a structure or in
deeper water where there is less flow. As sand moves along the coast and around the tips of
navigation structures, the material will cascade into the dredged channel depression, causing a
localized and distinctive feature that frequently reappears after removal. Material carried into a
channel along a sediment transport pathway may in turn be redistributed along the channel length
as a result of tidal and river currents.
sediment entrained in the water column will be deposited. This frequently occurs where the
channel or confining banks widen or when the current field meets some energy-dissipating force
such as a stagnant water body or a wave field. The river or tidal currents lose energy and are no
longer capable of carrying their sediment load. The result is usually a localized area of rapid
deposition where a recognizable shoaling feature evolves. This is the primary mechanism behind
the growth of ebb, flood, and river deltas.
d. Abandonment. Natural inlet evolution frequently involves the formation of new channels
and the abandonment of old ones. Delta growth and shoal migration can cut off the flow that has
caused one channel to be maintained and redirect the flow to another location. Locally, channel
relocation may occur within the flood or ebb delta as shoals shift and channels through the delta
are cut off. On a more dramatic scale, an entire inlet might be relocated as the river takes a
different course to the sea or as a breach forms across the barrier beach, capturing the tidal prism.
Double inlet systems are usually not stable and eventually one inlet will close.
e. Bed form regimes. Water flowing over the bottom can generate bed forms that will vary
according to flow velocity, depth of water, and sediment grain size. For a given grain size,
planar beds, ripples, and dunes (i.e., sand waves) will each be the stable phase under a specific
flow regime. For example, with a mean grain size of 0.3 mm, the channel bed would be rela-
tively flat with sand ripples at a flow velocity of 0.4 m/s, have sand waves at 0.7 m/s, and
become planar at 1.5 m/s. Sand waves are linear shoals that usually transit the channel
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