Multi-annual variability of storm events and morphological response in the SW Iceland: The Valahnúkamöl boulder barrier

While many studies have highlighted the high mobility of gravel/pebble beaches due to storm waves, further research is needed to improve understanding of the morphodynamic of coastal boulder accumulations. This paper provides original data about storm-induced morphogenetic processes for the Valahnúkamöl boulder barrier located in the Reykjanes Peninsula (SW Iceland). The study is based on annual topo-morphological surveys conducted from 2015 to 2023 and a shoreline change analysis for the period 1978–2023 using a series of aerial photos. In addition, hydrodynamic conditions (wave and water levels) were analysed using field records and model results. Results show a good correspondence of the morphological changes of the barrier to the frequency/energy of morphogenetic events. Interannual surveys (2015–2023) indicate a high mobility of boulders across the beachface regardless of the variability of hydrodynamic conditions. In contrast, boulders located on the crest and the back-barrier moved more episodically by overtopping and overwashing processes during extreme events combining storm waves and high spring tide levels. A massive landward transfer of boulders resulting in the most significant barrier retreat, −6.1 m, was recorded during the winter 2022. Between 1978 and 2023, the mesoscale shoreline changes of the barrier retreated landward by rollover. The pluri-decadal variability in shoreline retreat rates seemed to be strongly controlled by extreme hydrodynamic conditions. However, this cross-shore dynamic responses to episodic overwash depended on specific geological constraints. This survey provides a first quantitative assessment of the morphological impact of significant morphogenetic events within the highly energetic hydrodynamic context of SW Iceland. These data complement surveys undergone through a wide range of coastal morphologies elsewhere along the Western coasts of the North Atlantic basin. Such assessments may prove useful in anticipating the potential intensification of hydrodynamic conditions at mid-latitudes due to climate change.