Multidecadal-Scale Beach Changes in the Zuma Littoral Cell, California

Abstract

This paper describes multidecadal-scale beach changes in the 27-km long Zuma littoral cell, southern California, over 75 years (1928-2002) and suggests explanations based on ocean-climate forcing and other factors. Over this period, beaches within the cell, between Point Mugu and Point Dume, have experienced little human interference compared with other beaches in the region. The methods involve selection of eight target beaches and measurement of changes in beach width from vertical aerial photographs obtained at irregular intervals between 1928 and 2002, supported by archival studies, repeat field surveys and statistical analysis. The photogrammetric data show considerable seasonal and annual changes in width within and between beaches, but also significant trends at longer time-scales. In temporal terms, beach behavior throughout the cell is characterized by short-term episodes of erosion related to seasonal storms and recurrent El Niño events, and longer-term changes that appear to operate in a cyclic manner over decades. The latter we correlate with the Pacific Decadal Oscillation (PDO), as reflected in greater or lesser storminess related to sea surface temperatures across the northeast Pacific Ocean. For most beaches, net erosion coincides in varying degrees with PDO warm phases from 1925 to 1946, and again from 1977 to 2002, whereas net accretion coincides with the PDO cool phase from the 1947 to 1976. The beach-sediment regime is complicated by lag effects, sediment delivered by local streams as storms move onshore, and by reduced seacliff erosion following coastal highway construction. These findings have important implications for coastal management at decadal time-scales.