The Effects of Armoring Seacliffs on the Natural Sand Supply to the Beaches of California.

Seacliff erosion contributes to the natural sediment supply that provides California's beaches with sand. When an armoring structure (i.e. rip-rap, seawall) is built in front of a seacliff to hinder erosion and thus protect cliff-top development, the natural supply of sand from seacliff erosion is cut off. Thus, it is important to inventory the extent of eroding seacliffs along the coast of California and the degree to which they are armored to determine the human impact on natural sediment supply to the coast. The great majority (72% or 1275 km) of the coast of California consists of actively eroding seacliffs. More specifically, 13% of the coastline is resistant, high-relief, steep cliffs or mountains that contribute a minor amount of sand to the littoral budget, and 59% of the coastline is low relief (less than 100 m) wave-cut cliffs or terraces that, through erosion, produce a greater percent of sand-size material to the littoral budget than the high-relief, cliffed coastline. Using digital video footage of the California coast from 1998, it was determined that approximately 165 km (or 14%) of the state's more developed 1160 km coastline from Matin County to the US/Mexico border are presently armored; 93 km (56%) of this armoring protects back beach development, harbors, low bluffs, and dunes while the remaining 72 km (44%) of the armoring protects seacliffs. To assess the direct impact of coastal armoring on the contribution of littoral sediment from seacliff erosion, two littoral cells were chosen for detailed investigation. The Oceanside and Santa Barbara cells were selected to provide a littoral cell-specific sand budget analysis, including the pre-development budget and the extent of human impact on the budget. Overall, seacliff erosion plays an insignificant role as a source of sand for the Santa Barbara littoral cell in particular. The total amount of sand supplied to the beaches by seacliff erosion, whether under natural or actual conditions, is... [ABSTRACT FROM AUTHOR]