Hydrodynamics, sediment transport, and El Nino-induced seacliff erosion along the coastline of central California

Records from Peru documenting the effects of one side of the Southern Oscillation, 'El Nino-Southern Oscillation' (ENSO), have stretched back for centuries: however, there is little evidence that this large-scale oscillation has historically affected the coast of central California. It is shown that ENSOs, especially intense ENSO events, produce higher waves and sea level, heavier precipitation, and more cyclones and local storms than normal along the central coast. The Southern Oscillation, thought to only affect low latitudes, significantly influences the timing and magnitude of storms that strike central California. It appears that the frequency and intensity of ENSO events exert primary control on the geomorphologic evolution of central California's shoreline.;It was shown that the geology and structure are the main controls on the distribution of sediment in rocky nearshores as they define the patterns through which rivers and stream incise over the course of sea level fluctuations. Paleo-stream channels incised on the shoreface are depressed below the surrounding bedrock; thus, headlands extend offshore as bedrock ridges and stream channels are fronted by sediment-tilled depressions. This high bathymetric relief suggests that large waves are needed to suspended sediment high enough off the bed to be transported over the bedrock ridges. Thus, timing and magnitude of sediment transport along rocky shorelines is strongly dependent on the frequency and magnitude of energetic storms, much more so than along the planar, sandy shorelines.;An instrument was deployed in a paleo-stream channel off a pocket beach north of Santa Cruz, California. This environment was energetic and wave-orbital motions dominated the near-bed flow. Intermittent, intense periods of sediment suspension were correlated with large waves and wave groups. Over 9500 sediment suspension events (SSEs) occurred. SSEs are shown to contribute almost 100% of the total sediment suspended during storms, demonstrating the importance of these events to sediment transport. Shelf sediment transport models cannot accurately model the magnitude and direction of sediment transport on this shoreface due to the lack of information regarding the instantaneous coupling between fluid flow and sediment suspension. Time-variant models must be developed to accurately model sediment transport in this type of environment.