cut after the 1961 Hurricane Carla. The northwest Pacific coast is relatively robust in resisting
breaching from the seaward side, because the large tide range allows waves to create high and
wide beach berms (if sediment is available), and the surge is relatively weak. In summary, it
appears that for areas with a breaching susceptibility index greater than a critical value around
unity are more prone to breach from the seaward side than those that have a value much less than
unity.
HUMBOLDT LAGOONS
The barriers enclosing the lagoons of the Humboldt Lagoons State Park are spits that
emanate from the rocky headlands and are composed mainly of medium sand, pebbles, and some
cobble carried by the longshore current. Spits are narrow sand bodies that grow toward open
water from a sand source. The longshore spits fronting the lagoons of Humboldt County are
remarkable in being straight and of near-constant width. Spit recurving is a commonly observed
morphologic feature at natural tidal inlets (those unmodified by structures), whereby the
terminus of the spit will curve bayward. Schwartz (1972) compiled classic papers on spit
geomorphic processes; Kraus (1999) presents simple mathematical models describing spit
elongation; and Kraus and Seabergh (2002) describe physical and mathematical modeling of
spits at inlets. Growth of the spits in a straight line from headland to headland, as well as their
relatively constant width, indicates that surge and tidal processes, which cause spit recurving,
overwash, and breaching, are weak compared to longshore processes. Pierce (1970), Leatherman
(1981), and Byrnes and Gingerich (1987), among others, describe overwash processes of barrier
islands and spits. The Humboldt lagoon barrier spits are similar in uniformity to spits observed
in the interior of bays and lakes, where the tidal range is moderate and storm waves attenuated.
The competing transporting mechanisms at a breach or inlet on a tidal coast are depicted in
Figure 5. Positional and locational stability of a breach or inlet, as well as cross-sectional area of
the channel through the opening, are determined by a balance of the sweeping action of the tidal
(and river) current and the delivery of sediment by the longshore current (Bruun and Gerritsen
1959, 1960; Kraus 1998). Although breaches of the Humboldt County lagoons are initiated
during higher elevations of lagoonal water, discharge of the excess water to achieve approximate
equality with sea level occurs within a day or so. After that, tidal action controls the sweeping
force, and the lagoonal surface areas are too small to maintain the necessary velocity for the
breach gorge to be self-scouring.
Table 2 lists various factors and their associated time scales that influence development of
spits and elongation of barrier islands. An alongshore-directed current is required to transport
sediment from its source. As the subaqueous platform of the spit is established (Meistrell 1972),
the spit emerges, and onshore-directed motions associated with wave run up raise its elevation.
The maximum elevation achieved by this process occurs at high tide. During times of high mean
water level, wave action can wash sediment landward as sheets. The overwash process (the
hydraulic action) produces the geomorphologic feature of washover fans of sediment deposits.
No dedicated study exists of the physical processes of breaching of the Humboldt Lagoons
State Park barrier spits, the few studies available primarily concern biology and chemistry.
Residents and park officials are well aware of breaching in Big Lagoon, which occurs annually,
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