Extraordinary boulder transport by storm waves (west of Ireland, winter 2013–2014), and criteria for analysing coastal boulder deposits

Abstract

Before-and-after photos of supratidal coastal boulder deposits (CBD) in the west of Ireland show that storms in the winter of 2013–2014 transported boulders at elevations up to 29m above high water, and at inland distances up to 222m. Among the clasts transported are eighteen weighing more than 50t, six of which exceed 100t. The largest boulder moved during those storms weighs a fairly astonishing 620t.The boulders moved in these recent storms provide pinning points for mapping storm-wave energies on coasts: their topographic positions mark elevations and distances inland reached by wave energies sufficient to dislocate those specific masses. Taken together, the CBD data reveal general relationships that shed light on storm-wave hydrodynamics. These include a robust correlation (inverse exponential) between maximum boulder mass transported and emplacement height above high water: the greater the elevation, the smaller the maximum boulder size, with a dependency exponent of about -0.2 times the elevation (in metres). There is a similar relationship, although with a much smaller rate-of-change (exponent -0.02), between boulder mass and distance inland, which holds from the shoreline in to about 120m. Coastal steepness (calculated as the ratio of elevation to inland distance) seems to exert the strongest control, with an inverse power-law relationship between maximum boulder mass and slope ratio: the more gentle the topography, the larger the moved boulders.Quantifying CBD dynamics helps us understand the transmission of wave energies inshore during high-energy storm events. The transported boulders documented here are larger than many of those interpreted to have been moved by tsunami in other locations, which means that boulder size alone cannot be used as a criterion for distinguishing between tsunami and storm emplacement of CBD. The biggest blocks—up to 620t—are new maxima for boulder mass transported by storm waves. We predict, however, that this record will not last long: the 2013–2014 storms were strong but not extreme, and there are larger boulders in these deposits that didn't move on this occasion. Bigger storms will surely move larger clasts, and clasts at greater distances from the shoreline. These measurements and relationships emphasise the extreme power of storm waves impacting exposed coastlines, and require us to rethink the upper limits of storm wave energy at coasts.