A preliminary study of tide propagation and sediment dynamics in a macrotidal estuary influenced by the presence of a tidal power station

HydrodynamicsAbstrac

Leaf litter degradation in highly turbid transitional waters: preliminary results from litter-bag experiments in the Gironde Estuary

The rates of decomposition of oak (Quercus robur) leaves have been examined using litter bags in a very high turbidity macrotidal estuary, the Gironde Estuary (S.W. France). The first experiments show a marked decrease in the decomposition rate of oak leaves at the water-sediment interface (mud-contact: anoxic conditions, reduced physical fragmentation) in comparison to the water column. The results point out the impact of hydrodynamic conditions on leaf litter degradation in such fluvio-estuarine systems. Regarding the aquatic-terrestrial linkage, our observations suggest direct changes in leaf decomposition kinetics and then, a potential delay on the recycling and transport processes of coarse particulate organic matter, especially in a context of modification of the natural water flow, due to global and land use changes

Residual transport model in correlation with sedimentary dynamics over an elongate tidal sandbar in the Gironde Estuary (Southwestern France)

This study qualitatively describes by a multidisciplinary approach hydrodynamic and residual sediment transport over a sandbar in a macrotidal environment. The sandbar is elongated and attached to a headland that is located in the Gironde estuary (southwestern France). Bathymetric surveys, side-scan-sonar sonograms, grain-size trends of surficial sediment, and 2DH numerical simulations of current patterns were used to define a conceptual model for the dynamics of this sandbar. The sandbar is oriented anticlockwise with respect to the main flow. The channel-oriented bar face is ebb-dominated, whereas the opposite face is flood-dominated, resulting in a clockwise residual transport. The line of sediment flux convergence corresponds to the crest of the bar. The residual flow over the bar appears to originate from the regional estuarine flow. The dominant process responsible for the maintenance of the sandbar seems to be the depth-averaged main tidal flow rather than secondary circulation due to headlands or the interaction between the tidal flow and the seabed. To generalize this regional conceptual model a qualitative comparison is made with the theories of sandbar dynamics due to tidal currents (Pingree and Maddock 1979, Zimmerman 1981, Huthnance 1982, and others). The seabed stability approach developed by Huthnance (1982) seems to be the most reliable theory to explain the Saint-Georges Bank dynamics, even though many natural factors are not taken into account

Structure verticale des courants associés à la propagation de la marée dans la Garonne-impact du mascaret

Les ressauts en translation tidaux (parfois dénommés mascarets) représentent un phénomène naturel non linéaire et non hydrostatique, pouvant avoir une forte intensité et ayant un fort impact sur les écosystèmes fluvio-estuariens. Nous présenterons des résultats expérimentaux issus de la première campagne de mesure de grande ampleur au niveau international, réalisée sur la Garonne. Nous analyserons en particulier les forts cisaillements de vitesse liés au passage du ressaut et des ondes secondaires associées, qui génèrent une forte remise en suspension des sédiments

Utilisation of numerical and statistical techniques to describe sedimentary circulation patterns in the mouth of the Gironde estuary

Sediment distribution is well defined in the mouth of the Gironde estuary but the sediment dynamics is still poorly understood. The aim of this work is to analyse residual circulation of non-cohesive sediment due to the action of the main hydrodynamic factor: tidal currents. A depth-averaged numerical model emphasises the residual circulation of the Gironde estuary, which is characterised by residual tidal gyres. The comparison between sediment transport calculated from the numerical model and from a statistical analysis of the sedimentary distribution leads to the definition of a simple scheme of non-cohesive sediment circulation in the Gironde estuary’s mouth

Approche de l’évolution contemporaine (-50 ans à nos jours) de l’estuaire de la Gironde sous modifications des forçages hydro-climatiques et des contraintes anthropiques : Projet 2009-2011 – Réseau de recherche littoral aquitain - Atelier Gironde

Les propositions présentées ci-après pour la période 2009-2011 s'inscrivent dans le prolongement de celles proposées précédemment (2007-2008) et elles constituent ainsi les "briques" d'un projet plus vaste de moyen terme, visant une meilleure compréhension du système girondin : "Fonctionnement, évolution et trajectoire de l'écosystème estuarien sous contraintes naturelles et anthropiques

Approche de l’évolution contemporaine (-50 ans à nos jours) de l’estuaire de la Gironde sous modifications des forçages hydro-climatiques et des contraintes anthropiques : Projet 2009-2011 – Réseau de recherche littoral aquitain - Atelier Gironde

Les propositions présentées ci-après pour la période 2009-2011 s'inscrivent dans le prolongement de celles proposées précédemment (2007-2008) et elles constituent ainsi les "briques" d'un projet plus vaste de moyen terme, visant une meilleure compréhension du système girondin : "Fonctionnement, évolution et trajectoire de l'écosystème estuarien sous contraintes naturelles et anthropiques

Turbidity in the fluvial Gironde Estuary (southwest France) based on 10-year continuous monitoring: sensitivity to hydrological conditions

Climate change and human activities impact the volume and timing of freshwater input to estuaries. These modifications in fluvial discharges are expected to influence estuarine suspended sediment dynamics, and in particular the turbidity maximum zone (TMZ). Located in southwest France, the Gironde fluvial-estuarine system has an ideal context to address this issue. It is characterized by a very pronounced TMZ, a decrease in mean annual runoff in the last decade, and it is quite unique in having a long-term and high-frequency monitoring of turbidity. The effect of tide and river flow on turbidity in the fluvial estuary is detailed, focusing on dynamics related to changes in hydrological conditions (river floods, periods of low discharge, interannual changes). Turbidity shows hysteresis loops at different timescales: during river floods and over the transitional period between the installation and expulsion of the TMZ. These hysteresis patterns, that reveal the origin of sediment, locally resuspended or transported from the watershed, may be a tool to evaluate the presence of remained mud. Statistics on turbidity data bound the range of river flow that promotes the upstream migration of TMZ in the fluvial stations. Whereas the duration of the low discharge period mainly determines the TMZ persistence, the freshwater volume during high discharge periods explains the TMZ concentration at the following dry period. The evolution of these two hydrological indicators of TMZ persistence and turbidity level since 1960 confirms the effect of discharge decrease on the intensification of the TMZ in tidal rivers; both provide a tool to evaluate future scenarios

Acoustic turbulence measurements of near-bed suspended sediment dynamics in highly turbid waters of a macrotidal estuary

Sediment-turbulence interactions near the bed are still poorly understood in highly turbid estuaries, especially in the presence of fluid mud layers. This results primarily from the difficulty in measuring co-located velocity and suspended sediment concentration (SSC) at sufficiently high rate to resolve small turbulent flow scales. In this paper, we show how a set of commercial acoustic and optical bacicscattering systems known as ADCPs, ADVs and OBSs, can be deployed and used in a complementary way to perform large-scale profilings of tidal current and SSC combined with high-resolution velocity and SSC measurements in the highly turbid near-bed zone. The experiment was done in the Gironde estuary (France) which is well known for its turbidity maximum zone characterized by high SSC values, above 1 g l(-1) near the surface. A first simple inversion method is proposed to convert the backscattered acoustic intensity measured with ADV into SSC data in the highly turbid near-bed zone. Near-bed SSC data from the OBS are used to compensate for the important acoustic sediment attenuation effect at an acoustic frequency of 6 MHz. No a priori knowledge of acoustic backscattering properties of mud suspensions is required with this calibration procedure. We obtain an attenuation coefficient for mud suspensions of 0.28 m(2)/kg at 6 MHz leading to a good agreement between the SSC timeseries from the three ADV receivers and the OBS over the entire tidal cycle. The obtained SSC data are then analyzed with respect to the near-bed velocity, Reynolds shear stress and turbulent kinetic energy (TKE) timeseries in order to identify the relevant sediment transport processes during the tidal cycle. Significant differences in bed shear stress and TICE levels are found between ebb and flood stages with effects on near-bed sediment dynamics. During the ebb, maximum levels of tidal current, bed shear stress and TKE are associated with a reduction of near-bed sediment concentration (from 400 kg m(-3) down to 100 kg m(-3)). Bed liquefaction process is assumed to occur at this moment with the presence of highly concentrated mud layer and a possible lutocline at a distance of less than 20 cm above the bed. During the first 1.5 h of flood, turbulent activity remains moderate. The near-bed flood current is then inhibited very abruptly while a sudden increase in SSC occurred above the bed. Assuming that the ADV is able to estimate relevant turbulent erosion fluxes, the co-located velocity and SSC are multiplied and compared with settling flux measurements made onboard under quiescent water conditions. The mean sediment settling fluxes (averaged over 3 min) increase with SSC and are in relative good coherence with fluxes in quiescent water below the hindered regime, for SSC below 15 g l(-1). Reducing averaging time from 3 min to 30 s allows to increase the range of turbulent fluxes and SSC values, up to 99 g l(-1). At this scale, fluxes keep increasing quasi-linearly at higher SSC, suggesting the inhibition (delay or reduction) of the hindered settling regime as previously shown by Gratiot et al. (2005) from laboratory experiments. However, the 3-min averaged concentration field remained too low to conclude definitively on the effectiveness of such a process. Further analysis conducted at higher SSC regimes and under fully verified equilibrium are necessary

Vers l'évaluation des conséquences écologiques des évolutions hydromorphologiques de l'estuaire de la Gironde : Une approche préliminaire combinant SIG et modélisation hydrodynamique

International audienceEstuaries are composed of a mosaic of habitat providing various ecological functions for many biological communities and many goods and services to society. At the interface between continents and oceans, estuaries in general, and particularly large estuaries, are also environments which concentrate important ecological issues but also many environmental climatic and anthropogenic pressures. The Gironde estuary, one of the largest European estuaries, is thus an emblematic case. Historically, it has undergone major evolutions of its hydromorphosedimentary characteristics due to impacts of climate change and human activities. The aim of this work is to assess the ecological consequences of these changes through the study of the evolution of surfaces of estuarine available habitats and the knowledge of ecological functions associated with these habitats. To do this, we propose to combine two approaches: ecological and hydromorphosedimentary. First, work was undertaken to determine, from scientific literature, tolerances and preferences of representative species of the ecosystem for each of the parameters determining estuarine aquatic habitats (depth, salinity, sedimentary facies ...). Thus, combining available map data and results from a hydrodynamic model implemented in the Gironde, we can map the spatial distribution of these species in the estuary using a geographic information system (GIS). Then, we characterized the ecological functions associated with each habitat (eg. nursery role for sole, migration for eels ...). On the other hand, old maps of the environment have been digitized and analysis was undertaken to characterize the evolution of morphology, hydrological and sedimentary dynamics of the estuary in recent decades. The potential distribution maps we obtained allowed defining a kind of ecological potential that can be used as a reference for assessing effects of development issues, management measures or climate change scenarios. Results highlighted small changes in functional habitats distributions but high vulnerability of some of them (nursery grounds)