Annual Mobility of Coastal Boulder Deposits During Storm Events: Inishmaan, Ireland

Coastal boulder deposits (CBD) are geomorphic signatures of past extreme wave events (EWEs) such as storms and tsunamis. CBD can be used to reconstruct EWEs if the link between local wave climatology and boulder transport is understood. This proxy of wave data is important for building records in regions where wave data are sparse and for coastal hazard management. This study is focused on storm-driven coastal boulder transport along the west coast of Ireland.

We monitored coastal boulder motion at four sites on Inishmaan, Ireland over two winters (2022/23 and 2023/24). We manually relocated most study boulders to supra-tidal positions closer to the shoreline where they were more likely to be mobilized during storms. The boulders were instrumented with accelerometers to capture the timing of orientation changes, while pressure sensors were deployed on the supratidal platforms to record onshore wave conditions. Site characterization and distance measurements were conducted using uncrewed aerial vehicles (UAVs) for Structure-from-Motion (SfM) processing to create Digital Terrain Models (DTMs). Additionally, traditional ground surveys—measuring boulder axes and GPS locations—were augmented with iPhone LiDAR to estimate boulder volumes. Other methods included, 3D printing for sensor housing and Apple Airtags for boulder tracking.

Our results indicate that boulder transport occurs almost exclusively during storm events coinciding with high tide. We also found that transport was more likely for boulders on low-roughness platforms compared to those constrained by geomorphic features such as grykes, boulder ridges, or platform steps.

Our findings suggest that some boulders are mobilized annually, with isolated boulders being particularly susceptible to storm-driven transport. These results have implications for interpreting preservation potential in the context of coastal hazard assessments. Ongoing numerical modeling will enhance our understanding of regional wave climatology associated with these transport events.