Spatial distribution of trace elements in the surface sediments of a major European estuary (Loire Estuary, France): Source identification and evaluation of anthropogenic contribution

Spatial distribution of living (Rose Bengal stained) benthic foraminifera in the Loire estuary (western France)

Considering the sedimentation behaviour of suspensions, four states of different sediment-water mixtures are usually distinguished, these being (in increasing concentration) dilute suspensions, concentrated suspensions, fluid muds and mud deposits. As the concentration values delimiting these states vary considerably from one cohesive sediment to another, a procedure for their quantitative determination is proposed here. This paper deals mainly with the effects of hindered settling in interacting concentrated suspensions and of sedimentation in fluid muds. In contrast to the settling velocity of individual particles in a dilute suspension, which must be studied statistically as a stochastic variable denoted W, hindered settling and sedimentation velocities can be described by a scalar denoted V, as solids at any particular level of concentrated suspensions and fluid muds settle at the same velocity. On the basis of settlement tests carried out in this study and published data on organic-rich cohesive sediments, a concentration-dependent empirical law for a permeability coefficient (k) has been generated for cohesive sediments, using data from ten estuarine and nine marine environments, based on the one-dimensional Kynch theory of sedimentation. Based on the median diameters of the dispersed mineral fraction, the main provenances of the sediments are: fine to very fine clay from tropical marine/estuarine environments, medium clay to very fine silt from estuaries in western France, and fine to coarse silt from marine (harbour) environments in the Normandy region of France. A general trend for the influence of the grain size of the mineral fraction on the permeability coefficient has been established. It is demonstrated that the concentrations delimiting the different states of sediment-water mixtures can also be related to the grain size of the mineral fraction. Thus, hindered settling and sedimentation processes of muds, similar to the marine and estuarine cohesive sediments considered in this paper, can be studied as generic problems parameterized through a defined median diameter of the dispersed mineral fraction. Results for Loire estuary sediments are presented separately, based on specific tests to analyse the influence of experimental conditions on settlement. Moreover, the concentration values delimiting the different sediment-water mixture states have been largely established for this estuary.

The influence of particle size of the dispersed mineral fraction on the settlement of marine and estuarine muds

Coastal erosion results from a combination of various factors, both natural and humaninduced, which have different time and space patterns. In addition, uncertainties still remain about the interactions of the forcing agents, as well as on the significance of non-local causes of erosion. We focused about the surface sediments in the Marennes Oleron bay, after a general description of the site that has many various activities. The superficial sediments show a mechanical behavior, mainly depends on the fine fraction for a composition that contains up to 60% of sandy material. Fine sediments fraction has a typical yield stress depending naturally of concentration or water content. This yield could be modified slightly or significantly by adding silt or sand. As a result, the rheological measurement sensitivity allows us to characterize five typical sediments that correlate with solid fraction and fine fraction.

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Effect of Coarse Particle Volume Fraction on the Yield Stress of Muddy Sediments from Marennes Ol´ eron Bay

Settling velocity of the suspended sediment in three high-energy environments

Steady-state vertical distribution of cohesive sediments in a flow

The distributions of mud deposits and turbidities are studied in the estuary of the river Loire and in the coastal area of Loire-Atlantique (France). A description of the sediment dynamics is given in regard to tidal streams and water agitation due to waves. The silting up of the harbour sites is investigated in relation with the environmental conditions and local hydrodynamics associated with the harbour infrastructures. Some configurations are suggested for the implantation of fishing ports and marinas, in Loire-Atlantique, taking into account economical and environmental conditions.

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L'envasement dans différents ports de Loire-Atlantique

Des mesures des niveaux de basses mers realisees par les autorites portuaires dans l'estuaire de la Loire montrent qu'entre 1969 et 1993 ce niveau est descendu d'une facon significative. Ceci s'explique par les dragages recents dans le chenal externe de l'estuaire qui ont modifie la propagation de la maree dans ce site, ce qui a conduit a une correction officielle de –0,40 m pour le zero des cotes marines.

Modification de la marée par le creusement du chenal d'un estuaire

Une methodologie pour l'etude de la dynamique sedimentaire des vases dans les estuaires a maree est developpee en prenant comme reference la Loire. Une description de la localisation des zones a forte turbidite et de la dynamique des vases dans les estuaires est presentee sur la base des mesures in situ. La modelisation des principaux processus sedimentaires, tenant compte des proprietes mecaniques et rheologiques des vases, est abordee. Les valeurs des parametres des differents modeles ont ete evaluees pour les vases de cinq estuaires francais, grâce a des mesures en laboratoire. Ces modeles, couples a des observations sur le terrain, sont utiles pour apprehender correctement les problemes d'envasement et d'erosion lies aux travaux d'amenagement en site estuarien.

Dynamique sédimentaire des estuaires

Cet article concerne une approche globale de la vitesse de la phase solide des sediments cohesifs de l'estuaire de la Loire couvrant la tres large gamme de concentrations observee in situ. La vitesse de chute W des matieres en suspension est etudiee comme une variable stochastique. La vitesse de tassement V de la creme de vase est caracterisee par un scalaire puisque les sediments a l'interieur d'une couche de creme de vase descendent sans vitesse differentielle. Enfin, il est montre que la vitesse de chute entravee caracterise une transition entre la vitesse de chute des matieres a l'etat dilue et la vitesse de tassement de la creme de vase.

Yann Delanoë

Papers

Steady-state vertical distribution of cohesive sediments in a flow

L'envasement dans différents ports de Loire-Atlantique

Modification de la marée par le creusement du chenal d'un estuaire

Dynamique sédimentaire des estuaires

Dynamique des sédiments fins de l'estuaire de la Loire sous l'action de la pesanteur