Storm-related change of the northern San Mateo County coast, California

Storms induce changes in both the beaches (reversibly) and seacliffs (irreversibly) along the northern San Mateo County coast in central California. Repeated profiling of three beaches documented erosion during the onset of the 1997-98 El Nino storm season, then accretion after the largest storms passed, but there were significant differences among beaches. LIDAR surveys before and after the El Nino storm season showed a significant spatial redistribution of sand. It was preferentially transported to the north on individual pocket beaches, building the beach seaward and providing more protection to the coastal cliffs. In contrast, sand was eroded from the southern end, exposing the cliffs to the direct attack of waves. Weakly lithified seacliffs retreat landward up to several meters during a storm season, and retreat of some cliffs was amplified during 1997-98 El Nino winter. During the period of our observations, retreat was primarily the consequence of slope failure driven by freshwater processes such as heavy rainfall and groundwater flow; waves apparently played a secondary role.