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Pavlock 2020

Drivers of Sub-Seasonal to Interannual Shoreline Change at Sunset State Beach in Monterey Bay, CA

Pavlock, M. (Spring 2020)

M.S. Thesis, Moss Landing Marine Laboratories and California State University Monterey Bay, 50pp.

ABSTRACT:

Expectations of future change call for a thorough understanding of short- and long-time scale processes that impact sandy beaches, as well as tests of coastal change models in a variety of coastal settings. However, existing shoreline change models have primarily been developed and tested in open coast environments. Therefore, this study takes place in the northern Monterey Bay where we investigate the effects of headland sheltering and complex inner shelf bathymetry on shoreline change at a sandy dune-backed beach, fronted by a submarine canyon system. Twenty months of half-hourly video imagery were used to build a high-resolution time series of shoreline and sandbar positions at Sunset State Beach from September 2017 to May 2019. Past studies have shown that high magnitudes of winter shoreline erosion in the Monterey Bay occur during El Niño periods, when storm tracks over the northeast Pacific Ocean shift southward. This motivated the assessment of interannual shoreline variability by extending the shoreline time series back to September 2014 with biannual in-situ surveys.

According to the video derived observations, the shoreline varied by approximately 60 meters while the sandbar varied by approximately 100 meters in the cross-shore direction. Winter shoreline erosion began when nearshore significant wave heights exceeded the 95th percentile (1.7m), and a greater magnitude of shoreline erosion occurred with higher average winter wave energy. Shoreline accretion appeared to be aided by the sandbar, which acted as a source of sediment in the early summer months of 2018. The influence of wave energy and direction on shoreline change was tested using an equilibrium shoreline change model and an alongshore sediment transport model. Shoreline change at Sunset State Beach depended primarily on wave energy, the root-mean-squared error (RMSE) of the equilibrium model alone was 6.4m. The addition of alongshore sediment transport to overall shoreline change resulted in a modest RMSE reduction to 5.6m, but equilibrium model parameters did not change significantly. According to the biannual time series of shoreline observations, high magnitudes of shoreline erosion can also occur during non- El Niño periods, due to westerly waves that bypass the Santa Cruz headlands and expose the northern Monterey Bay to wave attack. The accuracy of the shoreline change models used in this study was limited by annual variability in the summer shoreline position, motivating future investigations of temporally variable alongshore sediment supply. The results suggest that rather than relying on predictions of an El Niño index to predict shoreline change, predictions of the direction of storm tracks over the northeast Pacific Ocean could more accurately inform shoreline change predictions at the study site and in similar environments.