Pluri-decadal to annual storm-induced geomorphic processes on the Valahnúkur coastal boulder ridge, SW Iceland - Archive ouverte HAL
Communication Dans Un Congrès Année : 2019

Pluri-decadal to annual storm-induced geomorphic processes on the Valahnúkur coastal boulder ridge, SW Iceland

Résumé

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 and to better interpret fossil boulders deposits. This paper provides original data about storm-induced geomorphic processes and their impact on the Valahnúkur boulder ridge (figure 1). The study site is located west of the Reykjanes Peninsula, in the south-west of Iceland. Firstly, the historical evolution of the boulder ridge was reconstructed from a set of aerial photographs between 1978 and 2010. Photographs were geometrically corrected and georeferenced according to the standard procedure to quantified the shoreline changes. Secondly, four kite and drone surveys were realized yearly between May 2015 and May 2018. The Digital Elevation Models (DEMs) produced using Structure-from-Motion photogrammetry were compared to deduce the morphological changes. Four orthophotographs were analyzed to quantified the movement of boulders. Hydrodynamic conditions were reconstructed using wave buoy and tide gauge measurements. Storminess was first assessed using a POT method. The extreme morphogenetic events were recognized using the 98th percentile of the significant wave height. The storm duration was estimated using the 75th percentile to define the start-time and the end-time of storm events. Wave runup were calibrated from the analysis of a wave/swash motion data set acquired by video monitoring. The historical analysis of shoreline change shows a significant landward retreat of the ridge during the last 40 years. The Highest rates of retreat are recorded in the northern part of the barrier where a large washover fan was formed during the 1990s and reactivated in the 2000s. This suggests a set of overwash events and the episodic flooding of the boulder ridge, probably during major storm events. The southern part of the barrier also shows high migration rates suggesting sediment transfers from the beachface to the back-barrier. The annual topo-morphological survey indicates various morphological responses according to the frequency and magnitude of winter storm events. From May 2015 to May 2016, the swash processes were dominant. The mobility of boulders was moderate (N=557) and restricted to the beachface (figure 2). These changes were attributed to fair weather conditions during the winter. The period from May 2016 to May 2017 was dominated by overwash processes. The results highlighted a crest reworking and a landward projection of high number of boulders (N= 6971) due to several intense winter storm events associated with high water levels. From May 2017 to May 2018, the morphodynamic regime was dominated by overtopping processes. Despite the large number of boulders moved (N=4057), the sediment transfer was limited to the crest line. These changes can be attributed to three storm events associated to moderate water levels.
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Dates et versions

hal-04670838 , version 1 (13-08-2024)

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  • HAL Id : hal-04670838 , version 1

Citer

Pierre Stéphan, David Didier, Ronan Autret, Samuel Etienne, Bernard Fichaut, et al.. Pluri-decadal to annual storm-induced geomorphic processes on the Valahnúkur coastal boulder ridge, SW Iceland. 20th Congress of the International Union for Quaternary Research (INQUA), INQUA, Jul 2019, Dublin, Ireland. ⟨hal-04670838⟩
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