P. Le Hir

Characterization of intertidal flat hydrodynamics

Abstract The paper reviews the different physical forcings that control tidal flat hydrodynamics. Tidal propagation and cross-shore or long-shore currents, tidal asymmetry, wind-induced circulation, wave propagation and drainage processes are successively considered. Some simple methods are described for estimating cross-shore currents and wave-induced bottom shear stresses, and the results obtained are compared to field measurements on three contrasted sites in Europe. In particular the cross-shore current is shown uniform in the lower part of the flat, and decreasing towards the shore. Bottom friction-induced wave attenuation is simply formulated on gently sloping beds, leading to a maximum wave height that a flat can experience; it is proportional to the water height according to the ratio between the slope and the wave friction factor. The maximum related shear stress occurs at high water and is also proportional to the water depth. Maximum tidal velocities are very similar in the three sites where bottom sediment is muddy, suggesting a relationship between physical stresses and sediment characteristics. The consequences of physical forcings on sediment transport are listed. The bottom shear stress is suggested as the relevant parameter for comparing tidal and wave effects. In general, tide induces onshore sediment transport, whereas waves and drainage favour offshore transport. The processes leading to a possible tidal equilibrium profile are analysed: they involve the intrinsic asymmetry that favours net deposition at high water, and an ebb dominance generated by the resulting bottom profile convexity. Eroding waves are likely to upset such a balance; this equilibrium then reduces to a trend for the system.

Investigation using simple mathematical models of the effect of tidal currents and waves on the profile shape of intertidal mudflats

Abstract Schematic mathematical models have been used to investigate the relationship between hydrodynamic forcing, sediment transport and the equilibrium profile shape of intertidal mudflats. The approach involves representation in the model of the main physical processes. The model is then applied to iterative calculations of sediment transport and morphological change which proceed until a stable situation is achieved. Relationships are established between the shape of the equilibrium profile and external factors such as tidal range, sediment supply and wave conditions. Large tidal ranges are shown to correspond to steeper mudflats, a high sediment supply leads to a less steep mudflat and the action of waves is shown to lead to a mudflat profile which is concave upwards.

Sediment transport over an intertidal mudflat: field investigations and estimation of fluxes within the “Baie de Marenngres-Oleron” (France)

Abstract This contribution analyses the various hydrodynamic forcings responsible for sediment dynamics over the intertidal mudflat of Brouage (Baie de Marennes-Oleron). This mudflat is characterized by a gentle slope and an extensive ridge and runnel network. The sediment dynamics have been thoroughly investigated by continuous measurements of turbidity, waves and tidal currents, core measurements and bed level monitoring. Turbidity measurements highlight the importance of waves to the sediment resuspension over the mudflat. Conversely, higher turbidities during spring tides in the channel, further offshore, indicate the major contribution of the tide to sediment transport within the embayment. Overall, a large amount of mobile sediment is consistently present within the bay, either as suspensions during spring tides, or as fluid mud deposits in the runnels of the mudflat during neap tides. Residual sediment fluxes, computed from the measurements, proved to be onshore during spring tides, but offshore during periods of wave domination. Finally, a sediment dynamics overview for the area is proposed on the basis of the flux estimations.

Circulation and suspended sediment transport in a coral reef lagoon: The south-west lagoon of New Caledonia

The south-west lagoon of New Caledonia is a wide semi-open coral reef lagoon bounded by an intertidal barrier reef and bisected by numerous deep inlets. This paper synthesizes findings from the 2000-2008 French National Program EC2CO-PNEC relative to the circulation and the transport of suspended particles in this lagoon. Numerical model development (hydrodynamic, fine suspended sediment transport, wind-wave, small-scale atmospheric circulation) allowed the determination of circulation patterns in the lagoon and the charting of residence time, the later of which has been recently used in a series of ecological studies. Topical studies based on field measurements permitted the parameterisation of wave set-up induced by the swell breaking on the reef barrier and the validation of a wind-wave model in a fetch-limited environment. The analysis of spatial and temporal variability of suspended matter concentration over short and long time-scales, the measurement of grain size distribution and the density of suspended matter (1.27 kg l(-1)), and the estimation of erodibility of heterogeneous (sand/mud, terrigenous/biogenic) soft bottoms was also conducted. Aggregates were shown to be more abundant near or around reefs and a possible biological influence on this aggregation is discussed. Optical measurements enabled the quantification of suspended matter either in situ (monochromatic measurements) or remotely (surface spectral measurements and satellite observations) and provided indirect calibration and validation of a suspended sediment transport model. The processes that warrant further investigation in order to improve our knowledge of circulation and suspended sediment transport in the New Caledonia lagoon as well as in other coral reef areas are discussed, as are the relevance and reliability of the numerical models for this endeavour.

Cohesive sediment transport processes

Abstract The general aim of the Project “Cohesive Sediment” of the MAST programme “G6 Coastal Morphodynamics” is to advance the knowledge and modelling of physical processes related to cohesive sediment in coastal environments. Better insights are given into deposition modelling, consolidation theories and associated constitutive relationships, laboratory experiments of liquefaction by waves only or combined waves and currents, modelling of flow-sediment interaction and turbulence in the water column, and the conduct of laboratory erosion experiments. Short term recordings of deposition events, wave effects, consolidation and bottom turbulence in muddy environments emphasise the distance which remains to be covered in order to understand and predict the processes in the field.

Development of ALTUS, a high frequency acoustic submersible recording altimeter to accurately monitor bed elevation and quantify deposition or erosion of sediments

In order to accurately measure the level variations of the bed in muddy dominant environments, a selfcontained device has been specifically designed to be used in shallow waters. The submersible developed system is of small size and can be easily deployed by hand on intertidal areas, and eventually be placed on a submerged frame in water. It acts as an echo sounder located at a desired place while emitting 2 MHz acoustic waves towards the bottom. Altitude data corresponding to various detection thresholds are recorded. This device has been designed to precisely survey and quantify short or long-term changes of bed elevation at locations where deposition of (fine) sediments or erosion measurements are of greatest interest for general hydrosedimentological studies and investigation of physical processes, in order to validate relevant numerical models and particularly sediment transport ones. This system has been deployed in association with a multiparameter probe on intertidal mudflats in bay and estuary.

Modelling sand/mud transport and morphodynamics in the Seine river mouth (France): an attempt using a process-based approach

The mouth of the Seine River estuary (France) has undergone marked morphological evolution over several decades mainly due to engineering works aimed at improving access to Rouen and Le Havre harbours. The intertidal areas are decreasing in size and the lower estuary is accumulating sediment and prograding. In order to understand and better describe the major morphological behaviours of the estuary, a morphodynamic numerical model was developed within the Seine-Aval program. At the end of the 1st part of the research program, a validated fine sediment transport model (3D) was available (Le Hir et al., 2001b). As the present morphological study addresses medium-term issues (a few decades), and because of the need to investigate impacts of local structures or events, we chose to use the so-called “process-based approach” starting from the existing model. First, the existing model was upgraded to account for (suspended) sand transport, and to achieve coupling between morphological changes and sediment transport. Erodability of the sediment accounts for the respective proportions of mud and sand. Simulations starting from an arbitrary surficial sediment cover show that the model is able to reproduce realistic sediment patterns. For example, it is able to change the sediment nature on the intertidal flat near Le Havre from sand to mud. Observed structures of suspended sediment are also reproduced: fine particles mainly follow the turbidity maximum whereas significant concentrations of sand grains in suspension are found where the hydrodynamic stresses are intense. Concerning morphodynamics, simulations with real forcing over one year are discussed. The effect of waves on the bathymetric evolution of the mouth is shown and the sensitivity of morphodynamics to the coupling procedure is tested.

An integrated modelling approach to forecast the impact of human pressure in the Seine estuary

Within the framework of the European Water Framework Directive, the Seine-Normandie Water Agency has defined prospective scenarios describing possible trends of evolution of the pressures on water resources. In order to evaluate the resulting water quality improvement or degradation of water bodies in the Seine river basin, an integrated modelling was proposed. The approach consisted in coupling three models, the seneque model for upstream sub-basins, the ProSe model for the Seine river and main tributaries and finally the siam1d model for the downstream estuary. After consistency verification, the integrated model was applied to scenarios proposed by the Seine-Normandie Water Agency. As a result of improvement in the nitrogen treatment by waste water treatment plants, the annual load of ammonia at the basin scale will be reduced by 65%. The oxygen and ammonia criteria in the estuary will improve from “bad” to “good”. However the nitrate criteria will remain “poor”, given the strong influence of non-point sources. Despite a 70–75% drop of the point orthophosphate loads, the criteria for this variable will also remain “poor”. The nutrient levels will be high enough to maintain eutrophication in the system; a general trend to a shift from N-limitation to P-limitation will be accentuated.

Modelling Pb and Cd dynamics in the Seine estuary

The Seine estuary (France) is currently one of the world’s most contaminated estuaries, due in particular to its high cadmium and lead content. Proper understanding of contaminant transfer, transformation and retention mechanisms throughout the estuary and up to the adjacent marine zone require a range of studies involving data collections, experiments, and modeling tool. A multivariable transport model (SiAM-3D) was used to simulate dissolved and particulate transport and it is applied to several calculation grids; a speciation model (MOCO) was used to select key species and obtain a schematic system representation. The coupled model for Cd and Pb was compared with field measurements. The complementary character of various tools (model applied to annual time scale, in situ measurements and experiments) allowed to explore and quantify various hypotheses on the high dissolved cadmium concentrations observed during low river flow. The target is to achieve a compromise between acceptable computing times and adequate result accuracy. Although particle and contaminant behaviour is globally well-reproduced by the coarse grid, calculation errors relating to bayward fluxes and stocks deposited inside the estuary were highlighted after comparison with the fine grid.

3D application of the continuous modelling concept to mud slides in open seas

Most turbidity current models are vertically integrated, and use a parametric entrainment rate to simulate the growth and dilution of the dense layer. The continuous modelling concept integrates all physical processes related to high-concentrated suspensions (stratification-induced turbulence damping, hindered settling and molecular viscosity increase or even viscoplastic behaviour). It is applied in a three-dimensional frame to simulate turbidity currents on a slope. A sensitivity analysis has been carried out in a 2DV configuration. It shows the front celerity is dependent on the initial mass, not so much on the slope, whereas the velocity within the body of the turbidity current is controlled by the slope and vertical mixing processes. The resulting entrainment rate is in agreement with flume experiments. A vertical recirculation is observed at the head of the turbidity current. The contribution of bed erosion is pointed out: such an erosion is generated by the density current and then enhances the density gradient and thus the turbidity current. The application of the 3D model to the 1979 Nice slide and turbidity current is under process. Preliminary results are discussed.