2021 CSDMS meeting-014

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Self-organization of coastal barrier systems during the Holocene

Giulio Mariotti, Louisiana State University Baton Rouge Louisiana, United States. gmariotti@lsu.edu

Understanding the response of coastal barrier systems to sea level rise is a crucial societal need. Despite the problem having been studied extensively, major knowledge gaps remain. For example, neither the sedimentary record nor existing numerical models have been conclusive in explaining the formation of barrier islands. Here I present a comprehensive 2D model that seamlessly couples cross-shore and along-shore transport, tidal transport, storm surges, and wind waves, and use it to simulate an idealized passive margin during the last 7,000 years. In the early Holocene, when sea level was rising ~20 mm/yr, shoals and ephemeral barrier islands formed, periodically drowned, and then formed again at a landward location. Shoal emergence was triggered by the disequilibrium of the recently submerged shelf, especially for large waves and mild shelf slopes. About 5000 years ago, as sea level rise slowed down to ~1 mm/yr, barriers stabilized and even prograded seaward. The combination of excess sediment in the nearshore and storm surges allowed barriers to accrete above mean high water. When barriers eventually equilibrated to the new sea level rise rate and started to retreat, their retreat rate was highly variable in space and time due to autogenic processes such as inlet formation and backbarrier channel interception. This variability also included multi-decadal periods of localized progradation. Both lag dynamics and autogenic processes confound the relationship between barrier retreat and sea level rise rate.

Comparison of barrier morphology 5000 years after the sea level deceleration, approximately representing present day conditions. Different combinations of swell waves, tidal range, and shelf slope are considered.

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Barrier island formation during the last 7000 years