Figure 10. Calculated breach depth compared to measurements.
The model underestimates breach width at Moriches Inlet. This is
attributed to the aerial photographs giving a maximum width that includes a
broad shallow area on the east side of the breach that could not be reached by
survey boat. The simulation better reproduces maximum breach depth than the
average value, which again contains contributions that had to be estimated for a
large reach of shallow water not surveyed.
Simulations were also performed for larger and smaller longshore sediment
transport rates QL than the 250,000 m3/year specified as a representative value.
These sensitivity tests indicated that the breach would not have closed even
under unusually large potential infilling by longshore transport.
4.
CONCLUDING DISCUSSION
A morphologic model of coastal barrier breaching was developed and shown to
reproduce trends in breach width documented in a dike-burst laboratory
experiment and in measured width and depth of the 1980 breach, Moriches
Inlet, NY. For the Moriches Inlet breach, hydrodynamics in both the existing
inlet and the breach opening were simulated simultaneously with evolution of
the breach.
The model balances erosion produced by tidal (and other) flow through a
breach with input by longshore transport, which would tend to close the breach.
In the present version of the model, closure occurs only by depth infilling and
not by spit encroachment. The breach model runs rapidly, and numerous tests
indicated the model to be robust in numerical stability and in producing
reasonable results.
Breach opening depends directly on accuracy of data on the initial
condition of the barrier island, in particular, about the lowest and narrowest
section, and on the water level (tide and surge) and wave conditions. It was
found that wave setup during storms can significantly increase water level at the
site of a potential breach and exert strong control over breach opening and
development.
Acknowledgments
This paper was prepared as an activity of the Coastal Inlets Research
Program, Inlet Channels and Morphology Work Unit, U.S. Army Corps of
Engineers (USACE). We would like to express appreciation to Dr. Henk
Steetzel of Alkyon Hydraulic Consultancy & Research and to Dr. Paul Visser of
the Delft University of Technology for providing supplementary information on
the physical model dike experiments. Mr. Gil Nersesian, formerly of the
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