The evolution of Jumpinpin Inlet - OzCoasts

The evolution of Jumpinpin Inlet
consistency results in a net northerly littoral transport, the rate of which varies with
wave duration and energy from 0.25 m/s to 0.5 m/s (COPE, 1985).
Inlets may have significant impacts on adjacent shores by interrupting the longshore
transport and trapping onshore–offshore moving sand. During flood tide, sand in the
inlet is carried a short distance into the bay and deposited as middle ground shoals.
The rate at which an inlet traps sand is higher immediately after the inlet opens. The
next ebb tide may return some of this material to the ocean in the form of ebb delta,
but some is always lost from the littoral system toward the downdrift side (SPM, 1984)
(loss in this manner is also considered a long-term change). Starvation of the
downdrift side combined with accretion on the updrift side can cause inlet migration or
even closure in the direction of transport.
4.1.4 Inlet migration and spit breaching
Normal spit accretion on the updrift side of a littoral drift tidal inlet (the recurved spit)
commonly induces longshore migration of the inlet mouth. As the updrift spit grows
the inlet narrows and current velocity in the throat increases. The opposite shore
preferentially scours to maintain the channel cross-sectional area. Spit growth and
downdrift bank retreat produces a net migration of the inlet in the direction of littoral
transport. The composition of the channel banks is an important parameter in inlet
migration. If the inlet becomes entrenched in resistant sediments further migration
may be impeded (FitzGerald et al., 1978).
Eventually, inlets may become overextended and recruit a new, more direct course to
the sea across the spit. In most circumstance the new inlet provides a more efficient
exchange with the ocean and will remain open, while the older, less efficient inlet
closes. The new inlet throat commences migration and the cycle begins again.
Spit breaching is facilitated when erosion has lowered and/or narrowed the barrier.
The end product of this process is bypassing of large quantities of sand from the
updrift side of the inlet to the downdrift shoreline.
4.1.5 Sand bypassing
Loss of sand to the system can be naturally or artificially prevented by the process of
sand bypassing. Natural inlets such as Jumpinpin have a well defined bar formation
on the seaward side. A part of the sediment transported alongshore moves across the
inlet mouth by way of this bar. Four methods of natural sand bypassing are: (i) ebb
delta breaching and bar migration; (ii) spit breaching; (iii) stable inlet processes; and
(iv) tidal current bypassing.
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