Barrier beach breaching from the lagoon side, with reference to Northern California

摘要

During the dry season in California, when storm action is limited and river flow is weak, the mouths of many estuaries close, creating barrier beaches and ponding water in the backing lagoons. If these barrier beaches do not breach naturally or are not manually breached, flooding hazards can develop in adjacent low-lying areas. Many barrier beaches are breached manually to facilitate migration of salmon or the threatened steel-head trout. Natural and manual breaching of barrier beach lagoons holds consequences for species transiting or inhabiting such freshwater lagoons. This paper discusses the breaching processes of barrier breaches on the coast of northern Califor- nia, for which a new breaching susceptibility index is introduced. Susceptibility of breaching from the lagoon side is found to be related to the ratio formed as the water-head difference between the lagoon and ocean divided by the width of the barrier beach. The index indicates that a barrier beach will tend to breach where it is most narrow, which is commonly observed. The head difference represents the destructive force promoting breaching, and the barrier width represents the constructive force resisting breaching. The susceptibility index is tested successfully through case studies of the Carmel River Lagoon, Lake Earl, Redwood Creek, Russian River Estuary, and Stone Lagoon.