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Storm in Iroise Sea: when the sea moves stone blocks

During extreme storm events, wave breaking can contain enough energy to quarry huge rocks off cliffs and transport them far from the shore. On 10 March 2008, 2008, a particularly high energy storm hit the French Atlantic coast in Brittany; the maximum recorded wave height in the Iroise Sea (Brittany) reached 18.15 m and the significant height (mean height of the highest third) 10.85 m. Because the storm occurred during an exceptional spring tide, the waves reached an even higher level when breaking on the shore. One month after the storm, the researchers undertook a detailed analysis of its impact on the stone blocks scattering the island of Banneg, in the north-west of Molène archipelago. Through a comparison with the situations observed prior to this storm and to the 1989-90 stormy season, their objective was to study the dynamics of quarrying, transport and deposition which lead to the particular block accumulations found on the island, and more generally to improve the understanding of the processes behind the construction of boulder ridges and clusters.

During the three-day long stormy episode, the maximum level reached by the sea at high tide regularly exceeded the top of the cliffs, sometime by a few meters. The water thus projected flowed southwards and eastwards, and the central part of the island (9 to 14 m high above sea level) was completely submerged by an extremely powerful surge locally up to 3 meters thick. The pressure exerted by wave breaking on the shore is such that blocks can be detached and displaced. But the blocks transported on the island all come from the cliff top and never from the intertidal zone.

Scientists studied the scars or sockets left by the removal of blocks from the cliffs and the displacement of isolated blocks. Modifications brought to clusters of blocks were studied with photographs taken before and after the storm, at the same places and from the same angles.

More than 180 scars of displaced or removed blocks were recorded, some of them at more than 10 meters above sea level. However the analysis of pictures shows that several hundreds of blocks were displaced. The exact trajectory of 63 of them (weighing on the average 1.9 tons) could be determined; the biggest weighed 42 tons and was quarried from the bedrock at 3 m above the highest spring tides and was deposited upside down 7 m away. Several clusters of blocks were deeply reworked.

In the central part of the island, the position and the morphology of the clusters (in lines parallel to the coast) reflect the slow evolution of the shoreline. When the cliff was eroded enough to let extreme water levels flow over its top, a first ridge of blocks formed behind it; then, as the slope of the cliff decreases with time, waves tend to progressively further to the rear of the cliff where they may develop a second or even a third series of clusters.

This study provides a general model making possible to explain the spatial distribution of these block clusters.

FS_fig8b

FS_fig8c

Reworking of a cluster of blocks by the March 2008 storm, shown by pictures taken before (top) and after (bottom): displaced blocks (1), deposited blocks (2), scars of removed blocks (3)

 

The paper

Fichaut B. et Suanez S., 2011. Quarrying, transport and deposition of cliff-top storm deposits during extreme events: Banneg Island, Brittany. Marine Geology, 283(1-4) : 36-55.

 

The journal

Since 1964, Marine Geology is an international journal devoted to marine earth sciences: geology, geochemistry and geophysics.

 

Further readings

Fichaut B. et Suanez S., 2008. Les blocs cyclopéens de l’île de Banneg (archipel de Molène, Finistère) : accumulations supratidales de forte énergie. Géomorphologie : relief, processus, environnement, 2008, n° 1

 

Contacts

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Communication and scientific mediation service: communication.iuem@univ-brest.fr


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