Abstract
Coastal boulder deposits (CBD) are wave-emplaced supratidal accumulations that record extreme inundation on rocky coasts. They are poorly understood but are of growing importance as we seek to better understand the extremes of wave power on coastlines. The Aran Islands, Ireland, host CBD in varying settings ranging from sheer cliff tops to wide shore platforms, and at elevations to about 40 m above sea level. Deposits are known to be active during strong storm events and provide a unique opportunity to examine relationships between wave energy, setting, and CBD occurrence. We use topographic elevation (Z) and offshore 100-years significant wave height (Hs,100) to calculate a dimensionless elevation Z* = Z/Hs,100 at 25 m intervals all along the Atlantic-facing coasts of the Aran Islands, and record whether CBD were present or absent at each location. The data reveal universal CBD presence at locations with low dimensionless elevations and near-monotonic decreasing frequency of CBD occurrence as Z* increases. On the Aran Islands, CBD are restricted to locations with Z*<3.13. For high elevation deposits it appears that unresolved local factors may be the major determinants in whether CBD will form. This approach can be applied at any CBD-bearing coastline and has the potential to change the way that we think about these deposits. Evaluation of dimensionless elevations at CBD locations around the world will help build broader understanding of the impact local shoreline conditions have on CBD formation. Determining these relationships contributes to the ongoing need to better understand interactions between extreme waves and rocky coasts.
Highlights
Wave-emplaced coastal boulder deposits (CBD) (Figures 1A,B) are enigmatic accumulations that occur above high tide as isolated boulders, disorganized clusters, or coast-parallel ridges (Williams and Hall, 2004; Cox et al, 2012; Bhatt et al, 2016; Lau and Autret, 2020)
Determining how wave climate and topography interact to influence Coastal boulder deposits (CBD) occurrence will lead to a better understanding of coastal geomorphology
Distributed along the ∼24 km of Atlantic-facing coastline are approximately 13 linear km of CBD (Cox et al, 2012). With their similar wave climates and geology, but varying coastline and nearshore conditions, the Aran Islands are an ideal and extend the survey along the entire length of the Atlanticfacing sides of the islands. This resulted in a dataset of 977 locations, with georeferencing, elevation, and CBD presence or absence recorded at each site (Supplementary Table S1)
Summary
Wave-emplaced coastal boulder deposits (CBD) (Figures 1A,B) are enigmatic accumulations that occur above high tide as isolated boulders, disorganized clusters, or coast-parallel ridges (Williams and Hall, 2004; Cox et al, 2012; Bhatt et al, 2016; Lau and Autret, 2020). Distributed along the ∼24 km of Atlantic-facing coastline are approximately 13 linear km of CBD (Cox et al, 2012) With their similar wave climates and geology, but varying coastline and nearshore conditions, the Aran Islands are an ideal and extend the survey along the entire length of the Atlanticfacing sides of the islands. This resulted in a dataset of 977 locations, with georeferencing, elevation, and CBD presence or absence recorded at each site (Supplementary Table S1). Location for studying the relationship between CBD occurrence, wave climate, and local geomorphology
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