Observations and modeling of San Diego beaches during El Nino

摘要

Subaerial sand levels were observed at five southern California beaches for 16 years, including notable El Ninos in 1997-98 and 2009-10. An existing, empirical shoreline equilibrium model, driven with wave conditions estimated using a regional buoy network, simulates well the seasonal changes in subaerial beach width (e.g. the cross-shore location of the MSL contour) during non-El Nino years, similar to previous results with a 5-year time series lacking an El Nino winter. The existing model correctly identifies the 1997-98 El Nino winter conditions as more erosive than 2009-10, but overestimates shoreline erosion during both El Ninos. The good skill of the existing equilibrium model in typical conditions does not necessarily extrapolate to extreme erosion on these beaches where a few meters thick sand layer often overlies more resistant layers. The modest over-prediction of the 2009-10 El Nino is reduced by gradually decreasing the model mobility of highly eroded shorelines (simulating cobbles, kelp wrack, shell hash, or other stabilizing layers). Over prediction during the more severe 1997-98 El Nino is corrected by stopping model erosion when resilient surfaces (identified with aerial imagery) are reached. The trained model provides a computationally simple (e.g. nonlinear first order differential equation) representation of the observed relationship between incident waves and shoreline change. (C) 2016 Elsevier Ltd. All rights reserved.