Morphodynamic behavior of the Valahnúkur boulder barrier, SW Iceland

While recent studies highlighted the great mobility of boulder beaches related to the impact of storm waves, numerous researches are still needed to better understand the morphodynamic of coastal boulder accumulations. This paper provides original data about storm-induced geomorphic processes and their impact on the Valahnukur coastal barrier. First described by Etienne & Paris (2010), this massive accumulation of boulders is located between the Valahnukur hyaloclastite cone and prehistoric lava flows of the Reykjanesta Point, in the south-west of Iceland. Numerous evidences of barrier flooding and intense boulder mobility were observed in the field. Between May 2015 and May 2017, three kite surveys were realized over the Valahnukur boulder barrier and surrounding areas. The Digital Elevation Models (DEMs) produced using Structure-from-Motion photogrammetry were compared to deduce the morphological changes. Three orthophotographs were analyzed to estimate the changes in the spatial distribution of boulder sizes. Hydrodynamic conditions were reconstructed using wave buoy and tide gauge measurements. Wave runup were calibrated from the analysis of a wave/swash motion data set acquired by video monitoring related to different hydrodynamic conditions. The chronology of extreme water levels has been reconstructed on this high refective beach to estimate the elevation of the wave action during the survey period. Results indicate a high mobility of the Valahnukur boulder beach. From May 2015 to May 2016, the clast mobility was moderate and was restricted to the sea face of the barrier. These changes were attributed to fair weather conditions during the winter. From May 2016 to May 2017, the results highlighted a strong alongshore sediment transfer, a crest reworking and a landward projection of high number of boulders due to several intense storm events associated with high water levels during the winter.