BREACH PROCESSES

Park. North- and south-directed longshore sediment transport rates are large along this coast, but

migration of a long spit extending north of the mouth of the Mad River located further to the

south (Komar, Borgeld, and Allen 2000) and other evidence indicates that the net direction of

transport is to the north (Costa and Glatzel 2002).

Fed by small streams, Big Lagoon and Stone Lagoon are prone to breach during or near the

end of the rainy season (Joseph 1958) that occurs from October to April, Figure 4, when the

water level in the enclosed lagoons can rise considerably above mean sea level in the Pacific

Ocean. Recent breaching of Stone Lagoon, believed to have occurred between March 13 and 15,

2002, is the subject of this paper, serving as a case study and focus for review of breach

processes.

BREACH PROCESSES

Breaching potential is achieved if the water level on one side of a narrow barrier spit or

island exceeds some critical elevation in relation to the crest of the barrier. Duration of higher

water level is also a contributing factor. Breaching can happen in two ways (Pierce 1970). A

breach may occur if running surface water scours a trough between the sea and the body of water

protected by the barrier. Such inundation can proceed from either the seaward side or the bay

(estuary, lagoon, river) side, and this process typically occurs from the sea side during times of

sustained high water levels (storms and storm surge). As another mechanism, if the barrier spit

is relatively narrow, seepage through the porous sediment driven by differences in water

elevation can liquefy the sediment-water mixture, allowing large volumes of material to be

transported quickly as slurry. This breaching is typically from the bay side, and it is not

necessary for the water level to have reached the top of the barrier spit or island.

Breaches opened artificially by digging a narrow channel across a barrier separating water

bodies of different levels will quickly deepen and widen, the water slicing through the barrier,

cutting steep side slopes. Growth and stability of the breach will depend on maintenance of a

sufficiently strong flow, as driven by a tidal exchange, river flows, or wind. Typically,

artificially opened breaches close naturally, such as at Stone Lagoon, and waves disperse the ebb

shoal while moving it onshore. The ebb shoal generated during a breach is an ephemeral feature,

unlike the flood shoals and flood wing spits that often become permanent and vegetated because

they are sheltered from sea waves.

Breaching potential is minimized if the barrier is high and wide, for which barrier elevation

and volume above mean sea level are key factors for resisting inundation and erosive wave

attack during times of the higher water level. On the Atlantic Ocean and Gulf of Mexico coasts

of the United States, breaching typically occurs from the seaward side through the combination

of elevated water level (storm surge is an increase in average water level above the predicted

astronomical tide) and large waves accompanying tropical storms, hurricanes, and northeasters.

Figures 1 and 2 are examples of breaching from sea to bay. At lower lying areas along the

Pacific Ocean coast, breaching of barriers from the seaward side also occurs, such as at the

mouths of small river and lagoon entrances that open and close periodically, possessing beaches

that do not reach adequate elevation to resist opening. Inman (1950) discusses the role of

extreme tides in influencing spit evolution at Mugu Lagoon, California.