Geomorphological control on boulder transport and coastal erosion before, during and after an extreme extra‐tropical cyclone

Abstract

Extreme wave events in coastal zones are principal drivers of geomorphic change. Evidence of boulder entrainment and erosional impact during storms is increasing. However, there is currently poor time coupling between pre-and post-storm measurements of coastal boulder deposits. Importantly there are no data reporting shore platform erosion, boulder entrainment and/or boulder transport during storm events -rock coast dynamics during storm events are currently unexplored. Here, we use high-resolution (daily) field data to measure and characterize coastal boulder transport before, during and after the extreme Northeast Atlantic extra-tropical cyclone Johanna in March 2008. Forty-eight limestone fine-medium boulders (n = 46) and coarse cobbles (n = 2) were tracked daily over a 0.1 km 2 intertidal area during this multi-day storm. Boulders were repeatedly entrained, transported and deposited, and in some cases broken down (n = 1) or quarried (n = 3), during the most intense days of the storm. Eighty-one percent (n = 39) of boulders were located at both the start and end of the storm. Of these, 92% were entrained where entrainment patterns were closely aligned to wave parameters. These data firmly demonstrate rock coasts are dynamic and vulnerable under storm conditions. No statistically significant relationship was found between boulder size (mass) and net transport distance. Graphical analyses suggest that boulder size limits the maximum longshore transport distance but that for the majority of boulders lying under this threshold, other factors influence transport distance. Paired analysis of 20 similar sized and shaped boulders in different morphogenic zones demonstrates that geomorphological control affects entrainment and transport distance -where net transport distances were up to 39 times less where geomorphological control was greatest. These results have important implications for understanding and for accurately measuring and modelling boulder entrainment and transport. Coastal managers require these data for assessing erosion risk.

Controls on the distribution of cosmogenic <sup>10</sup>Be across shore platforms

“…Several coastal platforms have been observed to erode due to quarrying and block removal (e.g. Dornbusch and Robinson, 2011;Naylor et al, 2016). In order to explore the potential implication of these erosion processes for the accumulation of 10 Be in a shore platform, we also evolved a series of stepped platforms by steady-state retreat.…”

“…Where available, information on the rate at which steps migrate and blocks are removed will help to inform sampling and interpretation (e.g. Dornbusch and Robinson, 2011;Naylor et al, 2016). Measuring 10 Be concentrations on shore platforms where block removal processes are dominant may allow the rate of step migration to be determined in the presence of large bedrock steps (Fig.…”

“…weathering and abrasion) or episodic (e.g. quarrying) processes (Dornbusch and Robinson, 2011;Naylor et al, 2016), and the relative influence of these processes on the distribution of 10 Be has yet to be explored. Similarly, the assumption that shore platforms evolve in morphological steady state (i.e.…”

Clast abrasion of a rock shore platform on the Atlantic coast of Ireland

“…Weiss and Diplas (2015) provided evidence that surface roughness in the vicinity of a clasts influences clast entrainment, while Naylor et al (2016) determined that topographic variability across a platform surface had a strong influence on entrainment and transport distance of clasts. Weiss and Diplas (2015) provided evidence that surface roughness in the vicinity of a clasts influences clast entrainment, while Naylor et al (2016) determined that topographic variability across a platform surface had a strong influence on entrainment and transport distance of clasts.…”

“…tidal inundation period and geological contingency), a trend that has also been noted by Stephenson and Naylor (2011b). Cruslock et al, 2010;Hall, 2011;Knight and Burningham, 2011;Stephenson and Naylor, 2011a;Naylor et al, 2016). Elucidating controls on clast production and clast transport dynamics are the dominant themes at this scale (e.g.…”