Pierre Stéphan

Long-, Mid- and Short-Term Evolution of Coastal Gravel Spits of Brittany, France

Gravel spits of Brittany have experimented a long morphosedimentary evolution over the last millenia. Based on analysis of several back-barrier holocene sediment stratigraphies, distinct phases of construction and barrier breakdown were recognized, indicating the role played by storminess and sediment supply during the late-holocene period. Over the last centuries and decades, a deficit of sediment bubget affecting several gravel spits is highlighted. Therefore, actual coastal evolution of most of them is mainly dominated by cannibalization, landward retreat by rollover and complete destruction of the spits in some places. This coastal erosion is related to the lack of significant offshore sediment input or from the erosion of unconsolided cliffs. Locally, anthropogenic forcing have axacerbated the erosion processes by sediment minings and/or the construction of hard defense structures. For the swash-aligned gravel spits, frequency and magnitude of overwash processes is controlling the rate of landward retreat by rollover. This morphodynamic behaviour is illustrated by topo-morphological surveys realised between 2002 and 2012 on Sillon de Talbert spit which has experienced a complete crestal removal during the 10 March 2008 Johanna storm. Although this event have a 50-100yrs occurence, the barrier has exhibited a rapid crestal rebuilding by overtopping processes, illustrating the great resilience of the spit. Thus, coastal erosion management strategies mainly based on hard defense structures are gradually abandoned for new management policies based on soft operations. Nowadays, gravel spits of Brittany are also considered as a geological heritage and management plans are establishing by local authorities. 1. GENERAL SETTING In Brittany, gravel beaches are located mainly on the northern and western coast (fig. 1). Their construction is mainly due to the shoreward removing of periglacial deposits initially accumulated on the coastal shelf during the post-glacial marine transgression. The southern part of Brittany is characterized by sandy beaches and coastal dunes, except locally where a coarse material is provided by erosion of cliffs formed by Pleistocene deposits (head). Nowadays, unconsolidated cliffs are considered as the most significant source of coarse sediments in Brittany (Guilcher et al. 1957, 1990). In the Bay of Brest, the highly weathered of shale cliffs also contribute locally to feed the gravel barriers. In the Bay of Brest, the indentations of the jagged coastline were favorable to the construction of a numerous smale-scale barriers and spits with a high morphological diversity (fig. 2A to 2D). The lenght and the volume of gravel spits in the bay of Brest never exceed 700 m and 100.000 m 3 respectively. Nevertheless, the Sillon de Talbert spit studied in the north of Brittany forms the bigger accumulation reaching 3.2 km long and sediment volume estimated at 1.23 x 10 6 m

Mobilité des cordons littoraux et submersions marines. Les flèches de galets de la rade de Brest (Bretagne, France)

ABSTRACT Firstly, the morphosedimentary changes of ten gravel barriers located in the bay of Brest are analysed using aerial photographs taken between 1952 and 2004 and DGPS surveys (2004–2005). An analysis of meteorological and oceanographic conditions during this period is then carried out using tide, wind speed and direction data. Finally, the synthesis of these data shows three morphological responses of the gravel spits which allows to define a sensitivity scale for the evaluation of coastal damage.

Chronology of Holocene storm events along the European Atlantic coast: New data from the Island of Yeu, France

This paper reviews the reconstruction of European Atlantic storm events with the contribution of a new stormy reconstruction in its central part. Three marsh environments on the island of Yeu were chosen to identify disturbing storm events from the Mid- to Late Holocene with vibracore sampling, radiocarbon dating and sedimentary analysis. Nine probable intervals of high energy deposition in these low-transport-activity environments are estimated: 600–500, near 1590, 2100–1950, 2850–2350, 3500–3270, 5400–5370, 6650–6510, near 7000 and between 7670 and 7470 calibrated years before present (cal y BP). By comparison with sedimentological paleostorm studies, we confirm six European Atlantic storm events estimated at near 600–300, 1700–1100, 2900–2500, 3500–3300, 5500–5100 and 7700–7100 cal y BP, corresponding to worldwide Holocene cooling climatic periods. A comparison with other storminess reviews of worldwide main stormy coasts shows that Holocene storms can increase during global cooling periods in the northern hemisphere.

The Stone Tidal Fish Weirs of the Mol`ene Archipelago, Iroise Sea, Brittany, Western France: a long-term tradition with early megalithic origins

This reports on a project that combined evidence gleaned from aerial photographs, place-names, interviews, topography, LIDAR data, and sonar bathymetry to locate stone tidal fish weirs in the Mol`ene Archipelago. The results were verified by diver and pedestrian visual surveys. Models of Holocene sea-level change allowed a group of possibly Late Mesolithic–Early Neolithic weirs to be recognized, with a second group broadly dated to the later Neolithic–Early Bronze Age. The construction of these long megalithic structures is compared to the funerary monuments for which the Mol`ene Archipelago is well known, in terms of technique, cost, and societal organization.