BREACHING ADJACENT TO JETTIES: Jetties interrupt the natural pathway of sediment
that is transported alongshore by obliquely incident waves and the associated longshore current.
As one geomorphic response, the shoreline adjusts through the redistribution of sediment both
near the inlet and, typically, for a considerable distance updrift and downdrift. The distance
depends on the length of the jetties and dredged channel, wave height, and balance of net and
gross longshore sediment transport, among other factors. Recession of the shoreline adjacent to
a jetty, whether on the ocean side (Kraus and Wamsley 2003) or the bay side (Seabergh 2002),
weakens the barrier island and increases breaching susceptibility.
A breach located near an inlet will increase the effective channel cross-sectional area of the
combined opening to the ocean, reducing the tidal current through the inlet and its scouring
action. Reduction of the tidal current in the inlet is an indirect cause of channel shoaling and
increased channel maintenance dredging. Material entering the bay through the breach may be
transported into the navigation channel, a direct cause of channel shoaling.
Breaching adjacent to a jetty isolates the structure from land. In addition to exposing the jetty to
a potential scouring current and waves that can undermine the structure at the landward end,
similar to tip scour common at the seaward ends of jetties, landward access to the structure is lost
for inspection and maintenance. Breaching produces an environmental change both locally and
regionally through alteration of the horizontal pattern of the tidal and wave-induced current,
which can change sedimentation patterns and, possibly, salinity in the bay or lagoon.
Breaching of narrow barrier islands or barrier spits is possible at either the downdrift or updrift
beach adjacent to jetties, and selected processes are discussed here, with examples.
Breaching downdrift of jetties. Chronic erosion is commonly observed on the downdrift side
of stabilized inlets on coasts where there is a strong net direction of longshore sediment
transport. The beach between the downdrift jetty and downdrift attachment bar can become
isolated from sediment sources in severe cases (Hanson and Kraus 2001). Such is the situation at
Shinnecock Inlet, a federally maintained inlet on the eastern end of Long Island, NY, where
potential breaching of the downdrift (west) beach has been imminent several times since jetty
construction by the county in the 1950s.
Figure 1 shows Shinnecock inlet in October 1996, with the western beach (left side of jetties)
eroded to endanger the public road and the marina complex on the north (bay) side of the barrier
island. Emergency measures have been taken by the county and town during severe storms to
prevent breaching. The U.S. Army Engineer District, New York, places material dredged from
the Federal channel onto the eroding beach segment as a least-cost disposal alternative.
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