Abstract
AbstractExtreme 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 km2 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. © 2016 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.
Highlights
There is mounting evidence for storm induced boulder movement in a range of geomorphological settings (Paris et al, 2011)
SWAN model outputs show that waves reached a maximum height of 5 m at the model output site 1 km offshore during the most intense part of the storm between 10 and 12 March 2008 (Figures 1A, 3A and 3C); these data were closely correlated with wave statistics from the nearest buoy (Figure 3B)
Our results show that position relative to the boulder trap seems to affect the amount of transport during a multi-day storm event, where loosing packed boulders on the edge exposed to incoming waves were transported further than similar sized and shaped boulders further into the boulder trap, suggesting that position and packing are further controls within trap transport dynamics
Summary
There is mounting (but still limited) evidence for storm induced boulder movement in a range of geomorphological settings (Paris et al, 2011). Recent research has documented stormdriven, bi-monthly to century timescale changes in coastal boulder deposits in subtidal (Mastronuzzi and Sanso, 2004), intertidal (Chen et al, 2011; Knight and Burningham, 2011; Peréz-Alberti and Trenhaile, 2015), supratidal (Noormets et al, 2004; Goto et al, 2013) and cliff-top (Hansom et al, 2008; Fichaut and Suanez, 2011) settings These typically report movement of boulders months to years before and after a storm and reflect poor time-coupling between field observations and actual storm events. In addition to advancing our understanding of current process dynamics, these data can aid sedimentological studies of tsunamis and storms by providing evidence of how extreme storms alter rocky coasts and how rock coast geomorphology meditates these processes These data can help shed light on the critical factors controlling entrainment and sediment transport under storm conditions, potentially leading
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