January 13, 2004
14:37
WSPC/101-CEJ
00097
529
Analytical Model of Incipient Breaching of Coastal Barriers
those for the base rate and double the base rate. The characteristic morphology
growth time is τ = Ve/Q = 900, 000/30, 000 = 30 days.
5. Discussion and Conclusions
This paper has presented an analytical morphologic model of incipient breaching
of alluvial barriers. Closed-form solutions were obtained through simplifying as-
sumptions about breach configuration and approach to equilibrium. Processes as-
sociated with the driving hydrodynamics of sediment transport and bank erosion
and collapse are represented implicitly through specification of the maximum sed-
iment transport rates at the bed and the sides of the breach at initial breaching,
and by the equilibrium values of the breach width and depth. Additional or micro-
scale physical processes could be represented in numerical solution of the governing
Eqs. (4) and (5), for which time-dependent sediment transport rates and bank fail-
ure could be calculated while maintaining a known, geometry. Seven variables are
found to control breach growth in such a macroscale description: initial width and
depth of the breach, equilibrium width and depth of the breach, width of the bar-
rier island, and maximum or initial net sediment transport rates at the bottom and
sides of the breach. Breach growth follows an exponential behavior governed by a
characteristic time scale τ = Ve/Q, where Ve is the volume of the breach at equilib-
rium, and Q is a representative maximum net sediment transport rate through the
breach.
Sensitivity testing of the morphologic model and comparison with observations of
the 1980 breach at Moriches Inlet, New York, demonstrated validity of the model in
capturing the general qualitative and quantitative features of coastal barrier breach-
ing. Data-based refinements can be made by the inclusion of empirical predictive
formulas for the equilibrium depth and width. Most of these extensions would re-
quire information on the hydrodynamics and sediment transport at the breach,
which would need to be supplied by hydrodynamic and sediment transport models
coupled to the morphologic breaching model. However, estimates of the basic gov-
erning variables drawn from engineering judgment and observations at the site and
at nearby inlets may be suitable for obtaining a reconnaissance-level prediction of
coastal breach growth without recourse to sophisticated models.
Acknowledgements
This study was conducted under the Inlet Geomorphology and Channels Work
Unit of the Coastal Inlets Research Program (CIRP), US Army Corps of En-
gineers (USACE). Review by colleagues Ty Wamsley and William Seabergh are
appreciated. Permission was granted by Headquarters, USACE, to publish this
information.
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