At times of intense precipitation by coastal storms or through rainfall in the watershed,
breaching can occur from the landward side because of unusually high bay, estuary, lagoon, or
river water level, causing either seepage and failure of the barrier or scour by water flow over the
barrier toward the sea. For the barrier spits of the Humboldt County lagoons, small streams and
runoff during the rainy season gradually raise the water level and cause breaching from lagoon to
ocean by seepage and failure.
BREACH SUSCEPTIBILITY INDEX
Natural breaching from the seaward side depends on elevation and width of the land barrier,
elevation of the surge, duration of the surge, and wave height and period, as leading factors
among others. Here we introduce a quantity termed a "breach susceptibility index" that explains
in part the tendency for barriers to breach from the seaward side. The breach susceptibility index
B is defined as:
B = S10/R
(1)
where S10 is the effective surge level for the 10-year storm (as defined by water level), and R is
the diurnal tidal range. The logic behind the index is that barrier islands and spits will build to at
least a minimal elevation given by the highest regular tide, wave and wind set up, and wave
runup. If the surge of a representative storm (also accompanied by set up and runup) approaches
the diurnal tide range, then breaching is likely. The parameter S10 refers to an effective 10-year
storm surge, because definition of such a quantity for the Pacific coast of the United States is
difficult. On the northern Pacific coast, tropical storms are absent, but there are several
independent contributions to extreme water levels from ENSO (El Nio-Southern Oscillation)
events, infragravity waves or surf beat long-period surface waves (Inman and Jenkins 1989
who also discuss wave runup and set up for the Pacific coast; Komar, Diaz-Mendez, and Marra
2001), tsunami (seismic waves), and local and distant storms. The steep continental slope of the
Pacific coast suppresses storm surge, and a 0.3- to 0.9-m surge is considered large (e.g., Komar
1986; Flick 1998; Komar, Diaz-Mendez, and Marra 2001). In contrast, on the Atlantic and Gulf
coasts, storm surges can exceed 4 m, typically exceeding 1 m each year. In defining the breach
susceptibility index, the 10-year storm surge is employed as a quantity that is statistically stable
yet describes a relatively frequent occurrence that might cause breaching. The index is intended
as a regional overview and does not include specifics of the barrier island or spit configuration,
which would need to be considered at the next step of quantification. Similarly, duration of the
storm surge is not explicitly included, although it is accounted for implicitly in the 10-year surge.
Through consultation of a number of references, representative values of the breach
susceptibility index were compiled as listed in Table 1. The index varies over more than an
order of magnitude, with smaller values associated with the northwest Pacific and larger values
with the Louisiana and Texas coasts. The relatively small tidal range on the Louisiana and Texas
coasts indicates that sand-deficient parts of those coasts, having low barrier islands and spits
where wind cannot readily build dunes, will be susceptible to inundation and breaching during
storms. Breaching and overwash of Padre Island, Texas, the longest continuous barrier island in
the world, is well known. Price (1947) reports as many as 45 temporary breaches in the island
after passage of a hurricane in 1933, and Hayes (1967) mentions "numerous hurricane channels"
4
Previous Page
