Boulder Dislodgement By Storms: What can we learn from the boulder fields Central-Western Ireland?

Coastal boulder deposits of western Ireland include over 1200 boulders and megagravel documented to have moved during storms in the winter of 2013 - 2014. The data come from seven areas on the mainland and outlying islands. The mass spectrum for the transported clasts ranges from 0.1 tonnes for the lightest boulder to 620 tonnes for the heaviest megagravel. This is a unique dataset that provides the opportunity for detailed quantitative analysis of dislodgement criteria.

We employ the Boulder Dislodgement and Sliding Model of Weiss et al (2022) to calculate dislodgement flow velocities for each boulder. In each of the seven areas, we derive the dislodgement flow velocity to mass (DV-M) relationships for each area and perform hypothesis tests to decern if the DV-M relationships in the different areas are governed by (a) the characteristics of the geological formation and topography from which the boulders are quarried, (b) the characteristics of the storm events, such as significant wave heights and period, or (c) a interactsions/combined influence of (a) and (b).

Interrogating the boulder data and model results objectively, with statistical and other quantitative methods, will help to shed light on the question of whether coastal boulder deposits are useful event records in the context of non-linear wave-wave interactions nearshore that may result in significant under- and overestimations of the causative event if nonlinear wave-wave interactions are ignored. For example, constructive wave-wave interactions can result in a wave condition that is much larger and dislodges a significantly heavier boulder than the individual waves could. Similarly, destructive wave-wave interaction can also cause much smaller wave conditions than the individual waves indicate and cause only the dislodgment of much lighter boulders than the individual waves would.

The wide range in boulder masses and topographic settings n the dataset can help to establish a better understanding of potential uncertainties for dislodgement velocities in general; and this approach should be valuable for application in other situations, for example, for isolated boulders in coastal settings or for cases in the stratigraphic record.