MULTI-PRONGED INVESTIGATION USES COASTAL BOULDER DEPOSITS TO UNDERSTAND STORM WAVE AMPLIFICATION ALONG STEEP SHORELINES

Coastal boulder deposits (CBD) are supratidal clastic deposits that include isolated boulders, small clusters, and extensive ridge systems built of stacked, imbricated clasts. Individual boulders can weigh many 10s to 100s of tonnes. Emplaced by storm waves along steep rocky coasts, some are on cliffs as high as 50m above high water, and others are found at the back of shallowly dipping coastal ramps, up to 250 m inland of the tide line. They are found worldwide, but some of the best examples occur on the west coast of Ireland.

Measurements collected after severe storms in winter 2013-2014 yielded a catalogue of >1000 boulders that were moved by waves. The survey data—which link boulder masses to precise topographies—are a record of the energies transmitted by storm waves at specific elevations and distances inland. Comparison of these data to the marine buoy record for that winter shows that to move these rocks at the documented locations, storm waves must have been significantly amplified in the coastal zone.

Numerical modeling reveals that the required amplifications are likely when long waves traverse and shoal across irregular bathymetry, terminating in a steep cliff: these are the topographic signatures of CBD sites. Scaled wave tank experiments provide insights into the processes by which the collapse of amplified, cliff-overtopping waves generates platform-crossing bores capable of mobilising very large boulders; and ongoing drone-based monitoring allows us to track annual changes and relate them to seasonal storm data.

In aggregate, these studies allow us to link onshore energies (archived in the boulder masses activated by waves at known topographic locations) with offshore wave conditions. What we have learned so far is that given the correct topographic conditions, hurricane-force storms (with significant wave heights in the 10-12 m range), generate cliff-overtopping bores that can travel well inland, and are capable of moving megagravel weighing 100s of tonnes.