The evolution of Jumpinpin Inlet - OzCoasts

The evolution of Jumpinpin Inlet
4.1.6 Inlet stability and closure
Some inlets are comparatively permanent features being maintained by tidal
exchange supplemented by storm generated currents, whereas other inlets tend to
close. The tendency toward closure is related to the ratio between littoral drift and
tidal flow bypassing of sand past the inlet (Stanley & Swift, 1974). When, as the result
of a storm the cross-sectional area of an inlet is reduced below a certain value, the
tidal currents could become too small to flush the sediment coming into the inlet by
the longshore current. Longshore processes will dominate over tidal flow and the inlet
will close. The importance of inlet width has been explored by Van De Kreeke (1985;
1990a,b) and is described in terms of stability analysis and closure curves.
Stability of single inlets deals with the equilibrium between gorge cross-sectional area
and inlet hydrodynamics. From Van De Kreeke (1990a), the parameters of stability
are actual tidal maximum of the bottom shear stress and the equilibrium shear stress.
The equilibrium shear stress is defined as the bottom stress induced by tidal currents
that is required to flush sediments carried into the inlet by longshore currents.
When actual shear stress equals the equilibrium shear stress, the inlet is in
equilibrium with the hydraulic environment. If the actual shear stress becomes larger,
the inlet goes into scouring mode. If equilibrium shear stress is larger the inlet is in
shoaling mode. The equilibrium is considered stable if, after changes such as storms,
the inlet cross-sectional area unconditionally returns to its equilibrium value.
For two-inlet bay systems Van De Kreeke (1990a) provides a possible configuration
for equilibrium flow curves (see Figure 4.2). The open and closed circles imply four
sets of values for which both inlets have cross-sectional areas that are in equilibrium
with the hydraulic environment. Results of a stability analysis on a two-inlet bay
system conducted by Van De Kreeke (1985) showed that at best two sets of
equilibrium flow area can exist. Based on these results, a set of stable cross-sectional
areas cannot exist and ultimately one or both the inlets will close.
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