Barrier spits enclosing lagoons can also breach by gradual rise in water level on the backside,
likely through seepage and failure by liquefaction. Previous work (Joseph 1958) found that a
critical water level in the enclosed Big Lagoon about 3-4 m above local mean sea level of the
Pacific Ocean triggered breaching from the lagoon side. Barrier spits across Big Lagoon, Stone
Lagoon, and Freshwater Lagoon are approximately of the same width and elevation, as all share
the same littoral system. Breaching of the barrier spit across Big Lagoon occurs more frequently
than at Stone Lagoon because Big Lagoon is fed by more streams. Rapid filling and more
frequent reaching of critical water elevation at Big Lagoon are responsible for more frequent
breaching there as compared to Stone Lagoon.
The March 2002 breach of Stone Lagoon closed to low tide water exchange within a week
after opening. Breach orientation appeared to shift substantially, caused by strong infilling by
longshore transport and encroachment of the associated spit that had formed on the south side of
the breach. Both banks of the interior of the initial breach were steep. The width of the
relatively narrow breach area greatly increased through welding of the ephemeral ebb shoal
deposit to the beach. Water level in the lagoon began rising after breach closure, and within a
month much of the flood shoal and breach spits were submerged.
ACKNOWLEDGEMENTS
This paper was produced as an activity of the Inlet Geomorphology and Channel Evolution
and the Inlet Engineering Investigations work units of the Coastal Inlets Research Program,
conducted at the U.S. Army Engineer Research and Development Center, Coastal and
Hydraulics Laboratory (CHL). Ms. Mary Claire Allison (CHL) rectified and interpreted the
vertical aerial photographs. Messrs. William Seabergh and Ty Wamsly, and Ms. Joan Pope of
CHL, and Dr. Gary Zarillo, Florida Institute of Technology, provided helpful reviews of this
paper. Permission was granted by Headquarters, U.S. Army Corps of Engineers, to publish this
information.
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