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Abstract Extreme wave impacts on coastal infrastructure, including bridges, is a growing problem. Highest energy events are difficult to study because direct observations in real time are almost impossible. Also the non‐linear dynamics are difficult to model or to reproduce in scaled experiments. Coastal boulder deposits incorporating megagravel (y axis >4.1 m) provide natural analogues of infrastructure components such as bridge deck slabs; and field measurements of these wave‐transported blocks can address this knowledge gap. In this contribution we present geoscience data relating coastal topography, wave climate, and dislodgement of megagravel up to 620 tonnes. We hope that this will generate new synergies among coastal geocientists and engineers. 

Coastal boulder deposits (CBD) are wave-emplaced supratidal accumulations that record extreme inundation on rocky coasts. They are poorly understood but are of growing importance as we seek to better understand the extremes of wave power on coastlines. The Aran Islands, Ireland, host CBD in varying settings ranging from sheer cliff tops to wide shore platforms, and at elevations to about 40 m above sea level. Deposits are known to be active during strong storm events and provide a unique opportunity to examine relationships between wave energy, setting, and CBD occurrence. We use topographic elevation (Z) and offshore 100-years significant wave height (H s,100 ) to calculate a dimensionless elevation Z* = Z/H s,100  at 25 m intervals all along the Atlantic-facing coasts of the Aran Islands, and record whether CBD were present or absent at each location. The data reveal universal CBD presence at locations with low dimensionless elevations and near-monotonic decreasing frequency of CBD occurrence as Z* increases. On the Aran Islands, CBD are restricted to locations with Z*<3.13. For high elevation deposits it appears that unresolved local factors may be the major determinants in whether CBD will form. This approach can be applied at any CBD-bearing coastline and has the potential to change the way that we think about these deposits. Evaluation of dimensionless elevations at CBD locations around the world will help build broader understanding of the impact local shoreline conditions have on CBD formation. Determining these relationships contributes to the ongoing need to better understand interactions between extreme waves and rocky coasts. 

Abstract Coastal boulder deposits (CBD) provide what are sometimes the only remaining signatures of wave inundation on rocky coastlines; in recent decades, CBD combined with initiation of motion (IoM) analyses have repeatedly been used as primary evidence to infer the existence of ancient tsunamis. However, IoM storm wave heights inferred by these studies have been shown to be highly inaccurate, bringing some inferences into question. This work develops a dimensionless framework to relate CBD properties with storm‐wave hindcasts and measurements, producing data‐driven relations between wave climate and boulder properties. We present an elevation‐density‐size‐inland distance‐wave height analysis for individual storm‐transported boulders which delineates the dynamic space where storm‐wave CBD occur. Testing these new relations against presumed tsunami CBD demonstrates that some fall well within the capabilities of storm events, suggesting that some previous studies might be fruitfully reexamined within the context of this new framework.