Jean-Paul Barusseau

Sediment Entrainment on a Wave-Graded Shelf, Roussillon, France

Abstract The sands of the inner shelf off the Roussillon coast of the microtidal Golfe du Lion display a seaward-fining texture and merge, in water deeper than 20–30 m, with midshelf muds. There is a qualitative correspondence between the textural gradient and the seaward-attenuating, annual (surface gravity) wave power expended by tangential stress on the shelf surface. Various equations, used to quantify the relation of sediment texture to fluid energy, predict the trend of the observed textural gradient, but overestimate its value. An empirical threshold equation (Komar and Miller, 1975a, b) and a theoretical null point equation (Johnson and Eagleson, 1966) give comparable results, implying that both may be describing the same state. There is a disparity between the predictions of an incipient motion equation (Johnson and Eagleson, 1966) and the threshold equation, probably because static instability of grains occurs prior to dynamic instability of grains in an increasing fluid velocity field. Theory forecasts that, in water shallower than 30 m, the sands are frequently entrained by shoaling waves, particularly during the winter months; the threshold of movement of sands in 5 m of water is exceeded for more than 40% of the year. Hence the grain size of the shallow-water sands does not reflect the maximum energy level of the environment.

Morphodynamique d'une plage macrotidale à barres

The topography, currents, waves, and sediment dynamics of a macrotidal ridge and runnel beach were monitored from April 24 to May 4, 1997 in order to characterise the morphodynamics of this beach type under fair-weather and storm conditions. Sand tracer experiments show moderate longshore and cross-shore transport components while profile surveying highlights a remarkably stable system. These trends are not in agreement with the current pattern, which is dominated by an essentially tidally driven longshore component. The stability of the ridges reflects the high rates of migration of the breaker/swash zone in response to the important vertical tidal excursion across a wide, low-gradient beach.

QUATERNARY ACCUMULATION RATES BY HEMIPELAGIC AND GRAVITY CURRENT SEDIMENTATION ON THE ATLANTIC MARGIN OF AFRICA: CONTROL FACTORS OF ADVECTIVE AND VERTICAL FLOWS

Abstract Particulate fluxes of terrigenous non-carbonate material have been assessed on the Atlantic continental margin off Africa, from 25°N to 15°S. Overall, during the Quaternary intertropical regions between 10°N and 15°S saw deposition isorates (between 1 and 4 g cm −2 10 −3 yr) in a more remote position from the coastline than in the subtropical and arid zones. By comparing the accumulation rhythms during the last two Ericson biozones, Z and Y , an increase by a factor of 2 or 5 is shown during Z on most shelves and slopes; the larger the volume of seasonal runoff, the greater the difference (Senegal). A reverse ratio is observed off Morocco because of the lithometeoric (aeolian) fluxes during Y . On the deep-sea bottom the ratio of Z Y rates is generally close to 1 except when a significant opal contribution increases the deposition rate during Y . Considered at the scale of the last eustatic changes (lowstand, active transgression and highstand), a clear feedback between climatic and eustatic factors is observed. Whether there was a large fluvial input or not, gravity-induced deposition accelerated during lowstands because of the proximity of the shore and the shelf edge. The active transgression (16,000-6000 yrs B.P.) controlled an increasing deposition in the large submarine valleys cutting the shelf, coincident with the intensification of the monsoon regime (12,000-11,000 yrs B.P.). The formation of unstable deposits was favoured by the increase in depositional rhythm. During the last period of highstand a slowdown of deposition rate was logically related to the positive change in sea level and also to the decreasing runoff and sedimentary fluxes during the last 3000 years.

Net offshore bar migration variability at a regional scale: Inter-site comparison (Languedoc-Roussillon, France)

ABSTRACT Aleman, N., Robin, N., Certain, R., Barusseau, J.-P. and Gervais, M., 2013. Net offshore bar migration variability at a regional scale: Inter-site comparison (Languedoc-Roussillon, France). The Languedoc-Roussillon coastline is a large unit stretching out over 200 km of sandy low coast in a wave dominated environment. The nearshore is characterized by a quasi continuous system of double sandbar that displays a wide range of typology. The interannual sandbar dynamic was investigated using 2D bathymetric profiles and 3D LiDAR imagery. This study has allowed determining the sandbar systems affected by the Net Offshore Migration (NOM). At a regional scale, conditions necessary to NOM development depend on the sandbar morphologies (crescentic or straight), the wave energy and the associated coastal orientation (low or high energy, sheltered area), the coastal structures (harbour and coastal defences) and the nearshore sedimentary budget. The areas where the NOM occurs show cycle dynamic differences. This regional inter-site comparison highlights that nearshore morphology and bar parameters seem to influence the nearshore bar behaviour. Interaction between the nearshore slope, width of the bar zone and the migration rate control the NOM duration. The sandbar volume and the regional wave climate influence also the migration rate of the system. On the Languedoc-Roussillon coast, the sediment grain size does not appear to influence the seaward bar migration.

Lithological, mineralogical and geochemical observations of Senegalo-Mauritanian quaternary shoreline deposits: Possible chronological revisions

Abstract Petrographical, mineralogical and geochemical observations of cemented deposits of Quaternary shorelines in Mauritania and Senegal reveal several lithogenetic phases. Holocene sandstones cemented by micrites of marine origin show aragonitic bioclasts at different stages of calcitization. Pleistocene sandstones show several stages of cementation in phreatic marine waters and in continental vadose waters and contain fully recrystallized aragonitic bioclasts. Radiometric datings are discussed in the light of these observations. A new stratigraphical aragonitic marine Quaternary is proposed in terms of relative tectonic stability: the emerged Pleistocene levels (Aioujian, Inchirian) belong to the high levels of the last interglacial period. The Inchirian, interpreted as a mid-Wurmian high level, is limited to levels at depths of −10 to −30 m. The older Quaternary rocks (Tafaritian) seem to have been deposited within various continental and unstable environments.

Méthodologie pour une simulation des transformations granulométriques de sables de la zone d'avant-côte

Abstract The grain size of mature coastal sands changes rapidly in response to the variation of the energy regime which provokes either erosion or deposition of fractions of the textural assemblage. Whatever the dynamic conditions, questions of paramount importance such as the size of grain populations transported simultaneously, the most common size of the grains in movement and the grain-size range of the sediment fraction involved are still poorly understood. The present paper deals with an attempt to describe this fraction without any consideration of the processes responsible for the observed changes. A major problem results from the fact that the distribution of the mobilized fraction cannot be known and described by direct comparison between the initial and the final grain-size curves. The only way to characterize the mobile fraction between two subsequent grain-size states is to simulate various kinds of removal or deposition. The first question is to express the shape of the actual distributions of the sediments mathematically. The simple Gaussian approximation being irrelevant, distributions can be expressed using advanced computer programmes which can provide mathematical expressions for all distributions. The cubic-spline approximation was chosen. The mobile fraction was assumed to have a Gaussian distribution. Various tests were made in order to simulate qualitative phenomena observed in nature (bimodality, grain-size parameter changes…) and to assess the three parameters which define the characteristics of the fraction deposited or removed: modal value, dispersion of the mobile population and relative amplitude of the change. An application was carried out on the Senegalese coast offshore from the Senegal delta and on the Mediteranean coast in the Gulf of Lions. It suggests that the dynamic agents have a great selectivity. Further developments are considered.

Apport de la géophysique terrestre (géo-radar, sismique) pour l’aide à la gestion du trait de côte

1. CEFREM-UMR 5110, Université de Perpignan Via Domitia, 52 Avenue Paul Alduy, 66000 Perpignan, France. nicolas.robin@univ-perp.fr 2. BRGM, Université de Montpellier, 1039 rue de Pinville, 34000 Montpellier, France. 3. Marseille Université, CNRS, IRD, Collège de France, CEREGE, Aix-en-P., France. 4. BRGM, 3 avenue Claude Guillemin, BP 36009, 45060 Orléans Cedex 2, France. 5. BRGM, 117 avenue de Luminy, BP 168, 13276 Marseille Cedex 9, France.

The Importance of Scientific Knowledge as Support to Protection, Conservation and Management of West and Central African Estuaries

The concluding chapter of this volume of the series “Estuaries of the World”, focused on Western and Central African coastal zone, highlights the complexity of such coastal environments and ecosystems. This chapter insists as well on the need for an increased knowledge of the structure and functions of these coastal ecosystems, while highlighting the life-supporting ecosystem goods and services they provide to humanity, including both the scientific and management implications. The challenge ahead of us will be to enhance the networking possibilities and to increase the capacities of young scientists working on African coastal zones.