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Observations of coastal cliff base waves, sand levels, and cliff top shaking

Wave heights and beach elevations, programmatically extracted from cliff base pressure sensor observations, are used to explore wave–cliff interaction and develop wave–cliff impact metrics. Spectra of cliff base water levels were elevated at sea swell and infragravity energies. Cliff shaking signals between 20–45 Hz were above noise levels only during periods of wave–cliff interaction and correlate well with several wave–cliff impact metrics. Cliff motions at site specific frequencies appear to provide a proxy for wave–cliff interaction. Impact metrics of hourly cliff base significant wave height, maximum cliff base water depth, and duration of wave attack were best correlated with cliff shaking (r2 = 0.73–0.75). Maximum hourly cliff base water depth provides a robust wave-based metric for marine forcing and can be modeled with runup equations and beach elevations. Cliff shaking frequencies most correlated with cliff base wave metrics differed from peak frequencies in the seismic signals that are generated by incident wave loading and distant ocean waves. The methods presented here are applicable to a wide range of settings beyond rock coasts, and can provide novel real-time time series of beach sand levels and hydrodynamic conditions.

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