January 13, 2004
14:37
WSPC/101-CEJ
00097
517
Analytical Model of Incipient Breaching of Coastal Barriers
breaching from the bay or lagoon side), presence of waves, gross longshore trans-
port rate, hydraulic efficiency according to the location of the breach within the
bay to other inlets, sedimentary composition of the site, and presence of barriers
such as a jetties, groins, and headlands. In the absence of erosion-resistant bound-
aries, coastal breaches appear to open rapidly, then gradually widen and deepen
to some equilibrium dimensions. The rapid-growth stage is referred here as incipi-
ent breaching, occurring prior to strong influence of longshore sediment transport.
Ephemeral breaches tend to open widely and deepen only to about mean sea level,
whereas breaches that become more permanent tend to increase in depth to that
of neighboring natural inlets or to depths of neighboring inlets before stabilization.
If a breach tends to become stable, known empirical relations for tidal inlets would
then govern the morphology (see Kraus, 2001 for a summary).
Breaching occurs at lower areas in the coastal barrier, and the side slopes of
the initial breach tend to be steep, being eroded by strong flow, notching, and
collapse in a type of avalanching. If erosion-resistant materials are not present, a
breach typically widens and deepens rapidly to approach equilibrium dimensions
that depend on the factors described above. Models of coastal breaching are still at
a rudimentary stage. The model formulated here is based on observations of general
tendencies of breaches.
3. Analytical Model of Incipient Breaching
A breach through a barrier is idealized as shown in the rectangular section in Fig. 4.
To derive a closed-form solution, the governing equations describing breaching are
∆x
∆x
x
y
z
z
L
QS
QB
∆z
x
Fig. 4. Definition sketch for breaching model.
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