Hervé Regnauld

Geological interpretation of combined SEABEAM, GLORIA and seismic data from Porto and Vigo Seamounts, Iberian continental margin

Long-range sidescan sonar (GLORIA) data over Porto and Vigo Seamounts collected in 1978 has been re-interpreted in the light of SEABEAM bathymetric surveys conducted in 1982. The application of quantitative bathymetric information enables the interpreter to allow for artefacts inherent in the GLORIA data and to separate topography-related primary backscattering variations on the sonographs from more subtle changes that result from textural, slope and outcrop effects. The distinctions are made easier when slant-range corrected GLORIA data are available.Use of the combined survey data to precisely locate seismic profiling tracks and to identify likely areas of outcrop has allowed refined geological maps of the seamounts to be drawn and regional fault trends detected. The overall outline of the seamounts appears strongly fault-controlled.Porto and Vigo Seamounts are made up of the same geological formations and have had a similar structural history since their uplift as continental fault blocks in the Late Cretaceous to Middle Eocene period. Ravines that dissect the presumably lithified scarps bounding the seamounts may be relict features but still appear to control sediment input to gulley and channel systems in the surrounding topography. Sedimentary ridges associated with the seamounts represent anomalously thick sequences of post-Eocene material and probably result from interaction of downslope sedimentary processes and contour-following boundary currents.

Coastal Changes and Cultural Heritage (1): Assessment of the Vulnerability of the Coastal Heritage in Western France

ABSTRACT Present climatic change and anthropogenic pressure increasingly affect the coastal zone. The Channel and Atlantic seaboards of France are no exception, with hundreds of archaeological sites currently threatened by the accelerated relative rise in sea level, erosion, and various anthropogenic modifications to the environment. In this article we propose a multidisciplinary approach to assess the vulnerability of archaeological heritage in the West of France. We also aim to establish an original methodology (and specific tools) for the observation of risks and the definition of impacts they may have on heritage sites. This research opens up a theoretical perspective for building a vulnerability model for archaeological heritage that takes into account the evolution of the environment and the specificity of each site over the course of the next 20 to 30 years.

Variability of sediment transport in beach and coastal dune environments, Brittany, France

Abstract On the coasts of Brittany (English Channel and Bay of Biscay), barrier systems were surveyed between 1995 and 2000. The beach profiles have a very high variability, which cannot be statistically explained by linear correlation with the wind, the waves, or the tides. The behaviour of the profile is represented on a phase diagram (speed of profile change: Y-axis, thickness of the profile: X-axis). The points in the center of the profile “rotate” around an average equilibrium which is seldom measured in the field. The seaward edge of the beaches oscillates between loss and gain, but with a net positive budget. The landward top of the beach displays a range of oscillations. The dunes always have a positive budget. The whole behaviour of the system is explained if the precise succession of anticyclonic and cyclonic winds is taken into account. Long periods of easterly winds (offshorewards) tend to produce a calm sea and to increase tidal sediment settling on the seafloor. If an onshore westerly storm occurs just after such a period, it hits a sediment-rich environment and produces a net accumulation on both the beach and the dunes. Periods of westerly calm to moderate winds do not help accumulation: a full going storm will hit a depleted environment and produce erosion. The speed of dune accretion and the budget of the beaches seem to partly depend on the ratio of cyclonic to anticyclonic conditions.

Coastal Changes and Cultural Heritage (2): An Experiment in the Vilaine Estuary (Brittany, France)

ABSTRACT Coastal archaeological heritage is threatened by a variety of environmental changes and anthropogenic activities. To address how coastal sites may be affected by these processes in western France, we have developed specific and testable methods which have been implemented to assess their relative vulnerability within the framework of the ALERT project (Daire et al. 2012 in the current issue). This has been tested in the Vilaine estuary area (Southern Brittany) where 22 coastal archaeological sites were investigated using one of the tools referred to as the VEF = Vulnerability Evaluation Form. The results of the test indicate which archaeological sites are in the greatest danger of being destroyed, leading to a preliminary effort to map vulnerability in the Vilaine estuary area. The results demonstrate that this approach provides a technique for assessing heritage vulnerability and can be used in a broader context as a benchmark for the management of archaeological sites.

The last interglacial shoreline in northern Brittany, western France

Abstract Shorelines of the last interglacial highstand in western France are mainly represented by rocky abrasion platforms on which fossil gravel barriers are located. These coastal accumulations are scattered around islands (Ouessant, Belle-Ile) and on the mainland. They are buried by periglacial deposits, consisting of successions of sandy/gravelly units, then heads, loess and sands. OSL dating has allowed the first sandy/gravelly layers to be dated to 90 ka and the loess and sands to 6.4 ka and 0.47 ka. The comparison of the precise stratigraphy of several cross-sections enables the reconstruction of the evolution of the landscape and the description of the configuration of the coastline at the end of the last interglacial.

Storms impact on morphodynamics of human controlled coastal features in western France: the prevailing role of local management practices

On the western coast of France some storms have a dramatic impact while others have almost none. Some sites were insensitive to storms until the 50ies and are, today, retreating. This paper tries to explain why. First we sort out which storms are responsible for retreat and second we search for any change in management practices on the sites. The erosive storms are not always the most powerful ones (high wind speeds or high waves) but the ones which occur during the peak of high spring tides and have long period waves. The sites which retreat are the ones which, during the 20th Century; were maintained and rebuild, for economic reasons after each small storm event. Today they are left to themselves as they have no more economic activities and are turned into “natural areas”. Therefore the sensitivity of these sites to storm is human-induced. Though, they may be even more sensitive if the observed evolution in storminess (namely the increasing frequency of clusters of storms) appears to be a real trend.

Evolution of a Coastal Beach/Barrier/Marsh System in Response to Sea Level Rise, Storm Events and Human Impacts: A Case Study of Trunvel Marsh, Western Brittany

The evolution of coastal sites such as beach/barrier/marsh systems is known to be strongly forced by sea level rise and controlled by storms, sediment input and human impacts. The relative weight of each may vary in time. However, it is difficult to determine the relative importance of these forcing controls and, therefore, how coastal systems evolve through time. In order to study this evolution we have selected the case study of Trunvel marsh, western Brittany, France, which is directly exposed to the most violent storms and has been extensively depleted of sediment during and since WW2. The relative balance of anthropogenic and meteorological controls and relative sea level rise is compared. Sediment cores have been obtained from within the marsh, cross sections of the barrier have been studied and air photos and old maps have been analysed. From 4000 BP to recent times the system has behaved in a simple way: the beach and the barrier accumulated sand and gravel, seeming to migrate inland with relative sea level rise and the marsh was alternatively eroded by the local river or fed by aeolian drifted sands. Very occasional storms may have breached the barrier and temporarily invaded (flooded) the marsh. Conversely, large events of river discharge may have breached the barrier, although there appears to be some natural resilience and the barrier rebuilds itself after each storm and the marsh is, once again isolated from the sea. At the beginning of the Roman period land use change appears to have modified the river discharge, following which the marsh seems to have been in its natural condition again until WW2, although some dykes were built and channels excavated. During WW2 the gravel was almost totally removed and used for concrete to build fortifications along the coast. After WW2, the system was totally controlled by management practices, the aim of which was to recreate a “natural” environment so that today this is a “human made natural landscape” and is now classified as a nature reserve. The barrier is no longer able to withstand storms and the river discharge does not always reach the sea. Therefore human management of water level in the marsh is today the main morpho-dynamic control for the whole system.

Spits on the French Atlantic and Channel Coasts: Morphological Behaviour and Present Management Policies

Spits are present all along the Channel and Atlantic coasts of France. They belong to different types, depending on their composition and genesis. Some of them were initiated when RSL stabilized about its present level, 2,500 years ago. Many others are much more recent and were created when major land-use changes took place in mainland watersheds. Soil erosion increased the fluvial sediment supply and many river mouths were stabilized. All these spits continue to evolve under variable degrees of human control (mainly on sediment delivery). This paper present three examples (Arcay, Talbert, Cayeux) which illustrate the main types of issues faced by coastal planners dealing with spits in western France.