COASTAL MEGAGRAVEL AND CLIFF-TOP BOULDERS RECORD EXTREME OVERLAND FLOW VELOCITIES DURING STORM WAVE INUNDATION: RELEVANCE FOR COASTAL HERITAGE MANAGEMENT (Invited Presentation)

Coastal megagravel and cliff-top boulders record extreme overland flow velocities during storm wave inundation: relevance for long-term coastal management

Boulders and megagravel accumulate above the highest tide line on high-energy rocky coasts worldwide. Collectively referred to as coastal boulder deposits, they are sometimes interpreted as ancient tsunami deposits, because hydrodynamic calculations initially suggested that the scale of the boulders and their distance from the shoreline excluded storm wave emplacement. Increasingly, however, observations are revealing that many of these deposits are storm activated. This shift in understanding requires that we examine more closely how storm waves behave at coasts, and attempt to quantify the power that they can exert. Open questions include: what are the largest masses that can be moved by storm waves? How strong are the horizontal flow forces exerted by inland-rushing wave-derived bores? Can we estimate a likely return time for the strongest events? Although the sites where coastal boulder deposits occur are generally remote and inhospitable, documenting overland flow conditions in these places cab have relevance for coastal management more broadly, as they provide markers for upper limits of storm wave power.

As one case-study example, at an exposed promontory site on the island of Inishmore, Ireland, wave-driven boulders are over-riding and obscuring parts of a prehistoric fort (Dún Dúchathair, probable Iron Age) that sits 25 m vertically above high tide. Hydrodynamic equivalencies based on boulder dimensions indicates that cliff-top horizontal flow velocities in this area could approach 4 m/s during extreme wave overtopping. Analysis of meteorological buoy wave data suggests approximately 20-year return times for events of this magnitude. These data are valuable for heritage managers seeking to understand and quantify the effects of natural processes on ancient structures.

Coastal boulder deposit observations can greatly increase our understanding of the effects of storms on rocky coasts. Annual monitoring of sites, so that movements can be detected and related to storm climatology, is particularly valuable in that regard.