Quarrying, transport and deposition of cliff-top storm deposits during extreme events: Banneg Island, Brittany
Résumé
On 10 March 2008, a particularly high energy storm hit the French Atlantic coast and the western part of the English Channel. This storm did not generate exceptionally high waves: significant and maximum wave heights recorded in the Iroise Sea (Brittany) reached 10.85 and 18.17 m respectively, whereas during the 1989-90 winter storms, they had reached 12 and 20 m, respectively. The exceptional character of the March 2008 storm event arises from the fact that it occurred during an exceptional spring tide. From a morphogenetic point of view, the effects of this storm in terms of block quarrying, transport and deposition on Banneg Island (Brittany) were significant. This study shows that the weight of the blocks displaced during the event was between 0.07 and 42.64 tons, with a median value of 0.72 tons. More than 60% of the blocks were quarried from the wave-scoured cliff-top platform, demonstrating that the favoured zone for supplying material was located inland of the cliff. Two dominant modes of transport were involved depending on the relationship between extreme water levels and cliff height. At the centre of the island, the height of the waves breaking over the top of the cliffs on the western coast resulted in a torrential surge that flowed out towards the eastern coast of the island over a landward-sloping platform. On the western coast of the island, blocks weighing between 0.3 and 1.4 tons were displaced between 50 and 90 m from the cliff edge by this flow. At the flow outlet on the eastern coast (90 m from the western cliff edge), a pit 1.6 m deep was excavated at Porz ar Bagou cove and some of the mobilised blocks were deposited in two parallel lobes about 40 m seaward of the eastern shoreline. Elsewhere on the island, block transport occurred by airborne projection although wave heights were lower than the altitude of the cliffs. The pressure exerted by breaking waves on the bedrock was sufficient to quarry and displace blocks. A temporal comparison of the changes recorded in the double and triple boulder ridges showed that the most seaward ridges were practically untouched with the most important changes occurring in the second and third ridges. These observations allow us to propose a spatio-temporal model for the accretion of cliff-top storm deposits (CTSDs), with the various stages of CTSD formation being directly related to the morphological evolution of the cliff.