In: Proc. 29th Coastal Engineering Conference, World Scientific Press, 2005, pp. 2120-2132.
NUMERICAL MORPHOLOGIC MODEL
OF BARRIER ISLAND BREACHING
NICHOLAS C. KRAUS
U.S. Army Engineer Research and Development Center
3909 Halls Ferry Road, Vicksburg, MS 39180, USA
Alpha Hydraulic Engineering Consultants Co., Ltd.
516-336, 9-14 Hassamu, Nishi-Ku
Sapporo, Hokkaido 063-0829, Japan
A numerical model of barrier island breaching is presented that is based on an assumed
rectangular breach form. Sediment transport is calculated at the bottom and sides of the
breach as driven by the Keulegan one-dimensional inlet hydrodynamics equations.
Longshore transport is included and can close a breach. Multiple inlets to the same bay
can be represented. The model is validated by comparison to laboratory data and to
measurements of the 1980 breach at Moriches Inlet, Long Island, New York.
Coastal barrier islands can breach during storms and times of elevated water
level. Breaching is expected to become more prevalent with rise in sea level,
erosion of the coast, and continued length of service of jetties that cause erosion
along the adjacent beaches. Quantitative predictive tools are needed to assess
vulnerability of coastal barriers, design breach prevention, develop beach-
closure plans, estimate the fate of a breach, and evaluate the consequences of a
breach to the neighboring beach, bay, and inlets sharing the same bay system.
Coastal breaching processes and its engineering implications are reviewed by
Kraus et al. (2001), Kraus and Wamsley (2002), and Kraus (2003).
A coastal breach is a new opening in a narrow landmass such as a barrier
island or barrier spit that allows flow between the water bodies on each side.
Initial breaching is characterized by strong cross-barrier (cross-shore) transport
that creates the new opening. During initial breach growth, which is usually
rapid because of disequilibrium in water level or flow rate, the cross-barrier
sediment transport rate is expected to be much greater than the longshore
transport rate. As the breach depth and width approach equilibrium, or the
causative mechanism (storm surge, elevated bay water level, strong river flow)