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

Infragravity band and (b) cliff impact band seismic energy density.

TitleObservations of coastal cliff base waves, sand levels, and cliff top shaking
Publication TypeJournal Article
Year of Publication2016
AuthorsYoung A.P, Guza RT, O'Reilly W.C, Burvingt O., Flick RE
JournalEarth Surface Processes and Landforms
Date Published2016/09
Type of ArticleArticle
ISBN Number0197-9337
Accession NumberWOS:000383615900008
Keywordsbluff recession; california; cliff shaking; Coastal cliff; erosion rates; ground motions; microseismic; model; new-zealand; ocean waves; seismic noise; shore platforms; soft rock cliffs; wave impact; wave-cliff interaction

Concurrent observations of waves at the base of a southern California coastal cliff and seismic cliff motion were used to explore wave-cliff interaction and test proxies for wave forcing on coastal cliffs. Time series of waves and sand levels at the cliff base were extracted from pressure sensor observations programmatically and used to compute various wave impact metrics (e.g. significant cliff base wave height). Wave-cliff interaction was controlled by tide, incident waves, and beach sand levels, and varied from low tides with no wave-cliff impacts, to high tides with continuous wave-cliff interaction. Observed cliff base wave heights differed from standard Normal and Rayleigh distributions. Cliff base wave spectra levels were elevated at sea swell and infragravity frequencies. Coastal cliff top response to wave impacts was characterized using microseismic shaking in a frequency band (20-45Hz) sensitive to wave breaking and cliff impacts. Response in the 20-45Hz band was well correlated with wave-cliff impact metrics including cliff base significant wave height and hourly maximum water depth at the cliff base (r(2) = 0.75). With site-specific calibration relating wave impacts and shaking, and acceptable anthropogenic (traffic) noise levels, cliff top seismic observations are a viable proxy for cliff base wave conditions. The methods presented here are applicable to other coastal settings and can provide coastal managers with real time coastal conditions. Copyright (C) 2016 John Wiley & Sons, Ltd.

Short TitleEarth Surf. Process. Landf.

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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