oblique angle from shore normal, trending northwest to southeast, as evidenced by the shear
bank on the south side. Within a few days (March 18), a broad spit had formed on the south
side, and a narrower wing spit had also formed on the north side, redirecting the flow along a
southwest-to-northeast axis. The minimum width of the breach was 91 m +/-5 m as determined
from the March 18 photograph. An ebb shoal deposit is observed on March 27 that is located
slightly to the north with respect to the breach, a location consistent with the inferred initial
orientation of the breach. By April 6, the breach had closed at low tide, but was observed to
have water in it on higher tide. The ebb shoal had begun welding to shore, and it evidently
spread north and south to form a wide deposit across the breach by April 18. The flood shoal
appears to diminish in area; however, there is no transporting force to remove sediment from this
shoal. The apparent diminution in areal extent of the flood shoal and breach margins is an
artifact of water level in the lagoon that rose after the breach closed.
The sequence of four photographs was rectified by reference to the log on the south side of
the breach and other features common to all photographs. The result of tracing the interpreted
dry beach perimeter is shown in Figure 12. The interpreted dry shorelines on the northeastern
side of the barrier spit and on the straight southeastern bank do not show significant change in
location, owing to their relatively steep slopes. The perimeter of the flood shoal and large spit
located on the southeastern side of the breach decreases each week of sampling because of the
mild slopes of these deposits. According to Figure 3, infilling of the lagoon by creeks and runoff
from the bluffs can be expected to overcome evaporation for the months of March and April.
CONCLUDING DISCUSSION
Review of the literature revealed that there is a paucity of information on the physical
processes associated with breaching of barrier islands and spits beyond qualitative reporting of
case studies, despite the significant potential environmental and societal consequences that
unintended breaches can bring. In fact, the word "breach" does not appear in the indexes of most
references and textbooks on coastal geomorphology and coastal engineering. Breaching can
occur from either the seaward side or the bay (estuary, lagoon, river) side, depending on relative
water elevation. The breaching process can occur either by scour produced by surface water
flow or by seepage and liquefaction. Narrow breaches widen and deepen quickly, and the side
banks of a breach are initially very steep after breaching. Depending on the magnitude of water
exchange between the sea and the bay, a breach can quickly close, as at Stone Lagoon, or remain
open and perhaps become a dominant inlet such as at Little Pikes Inlet, thereby jeopardizing the
stability of existing inlets and jetties.
A susceptibility index was introduced to estimate the tendency of barrier islands and spits to
breach from inundation from the seaward side. Defined as the ratio of the effective 10-year
storm surge and the diurnal tidal range, a breaching susceptibility index value greater than unity
indicates greater possibility for breaching than a value less than unity. The breaching
susceptibility index explains in part why breaching and inundation are commonly observed
processes on the Texas and Louisiana coasts, and why they are less so on the Pacific coast, with
barrier islands and spits on the Atlantic coast intermediate in potential to breach.
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