CLAST-SIZE DISTRIBUTION ANALYSIS OF BOULDER BEACHES IN WESTERN IRELAND

Boulder beaches occur worldwide but are understudied, and fundamental sedimentary characteristics remain poorly understood. To better characterize boulder beach clast populations, we studied eighteen boulder beaches in western Ireland using a combination of field and digital methods. Understanding basic characteristics of boulder beaches is a necessary first step toward asking questions about human interactions with these high-energy coastal environments. We used drone photogrammetry and Agisoft Metashape to generate orthoimages for digitally measuring clast sizes via PebbleCounts (Purinton and Bookhagen, 2019, Earth Surface Dynamics). Image preprocessing in GIMP and Adobe Lightroom enhanced grain boundaries, and we made adjustments to PebbleCounts’ built-in parameters (originally developed for fluvial gravels) to optimize clast recognition. Manual grid counts (315 clasts per beach) provided direct field data to validate the results. We found that PebbleCounts clast size distributions were highly comparable to manual grid counts after correcting for a systematic ellipse-fitting underestimate, and applying a standard area- to grid-count conversion. We also conducted visual analysis of roundness patterns.

We found substantial variability from site to site. Whereas some beaches showed strong and progressive up-beach fining, our analysis shows it to be less general than had formerly been proposed for boulder beaches. Up-beach fining was strongly developed only on wide beaches, which also tended to have substantially larger median clast sizes overall. Narrower beaches had smaller clasts, and tended to show little change in clast size across the beach, some showed initial up-beach coarsening followed by fining, and one coarsened up overall. We interpret this to mean that boulder beaches are much more dynamically diverse than has previously been recognised.

Similarly, despite prior interpretations that boulder beaches are characterized by up-beach decrease in roundness, we found little difference across most beaches. These preliminary results will be augmented with data from additional sites as well as year-to-year comparisons that will contribute to understanding the long-term development of these environments.