BABA (BArre de flot et Bathymétrie Augmentée in French) is a research project that aims to understand the hydro-sedimentary dynamics related to the propagation of a tidal bore in internal estuaries (definition given by Fairbridge in 1980) by addressing unresolved issues in previous ANR MASCARET project. It is part of scientific theme land-coastal-open ocean continuum including continental and marine processes in the interface domains of axis A.1 of CES01. The estuary represents this continuum in macrotidal estuaries, where a tidal wave can travel up to 250km upstream of the river mouth.

A tidal bore is a phenomenon occurring in macrotidal estuaries when the tidal wave front propagates against a river current. It has different geometries, ranging from a very long wavelength wave perceptible only with pressure sensors, to a 2m high breaking wave propagating on the whole river width and at velocities reaching up to 4m/s during spring tides. The following names are distinguished: tidal waves, tidal jumps (TJ) (invisible by eyes but not to sensors tidal wave), tidal bores (visible by eyes with tidal wave crossing the river) and tidal bores with lateral turbid jets (due to the inclined banks geometry, hereafter referred to as TB). The corresponding french terms are: onde de marée, ressaut de marée, barre de flot and mascaret.

The hydro-sedimentary processes associated with TB have been studied in situ during the last decade. The front of TB is characterized by a turbulent flow causing a significant resuspension of bottom sediments and their diffusion in water column. As Tidal Jumps (TJ) are perceptible only with pressure sensors, there is an underestimation of their occurrence in the literature. In the case of the Garonne River, TJ may appear for 90% of tides during low flow period. Even if TJ are low intensity, they are associated with significant acceleration at the tidal front and could have important consequences for sediment dynamics. TJ and TB are an integral part of estuarine processes and can have impacts on estuarine navigation, diffusion of contaminants stored in sediment deposits, banks erosion or biological development. It is a tourist attraction in some regions, where festivals are organized during spring tide such as “Mascarock festival” (Dordogne River, France) or “Bono festival” (Kampar River, Indonesia). A community of surfers provides many videos, articles or books on spots of TB.

The macrotidal conditions alone do not explain the development of this front. Its formation and dynamics are also sensitive to the general morphology of funnel-shaped estuaries. Its development and characteristics would also be sensitive to the spatial evolution of tidal range in estuaries (hyper- or hypo-synchronous), the shape of the tidal wave (tidal asymmetry), the intensity of river flow, anthropization and particularly to local bathymetric variations. Observation of different shapes depending on depth, bank slope, sand banks development or meander sinuosity. Short term, bathymetric variations are mainly induced by erosion/deposition cycles partly controlled by the tidal bore itself, or periods of high or low water.

The estuaries are morphological systems in constant evolution adapting to variations of controlling factors such as the location of base level, long term tidal cycles, river flow, sediment supply and anthropization. Although the factors of TJ and TB (TJ&B) formation and dynamics are identified, they are not yet fully understood due to their spatial and temporal variability. In a context of global change, modifications of estuarine systems are expected in the future. Sensitive to modifications, TJ&B could disappear or conversely, strongly intensify in areas where it has currently no impacts. For example, intense modifications of the morphology of the Seine estuary related to developments for navigation purposes caused a significant decrease of TB intensity or the TB disappearance in some channel sections, observed during the 20th century.

Investigations are needed to characterize the factors controlling TJ&B development in internal estuaries whose bathymetry changes due to the TJ&B-induced erosion. BABA will study the dynamic interactions between bathymetry, sediment transport and TJ&B. The study will not investigate the influence of TJ&B on water column stratification and the muddy plug. The future morphological evolution of estuaries on TJ&B development will not be addressed either, as current projections are not reliable enough. The physical processes will be studied at the scales of laboratory channels (cm) and estuaries (km). The time scale will vary from a few seconds to several years. In situ, the study areas will be the Sée (Mont Saint Michel (MSM) Bay) and the Gironde estuary, located at the confluence of the Dordogne and Garonne rivers. The olds maps data extraction will allow to perform this study on several years. The field measurements will be performed for the duration of BABA. Old and actual bathymetric data will be provided for the fabrication of realistic channel morphologies, the first condition for physical and numerical modelling. All equally important, the data, relating to sedimentary, tidal and meteorology, will be given to support physical and numerical modelling. In laboratory, it will use the open channels of the Hydrodynamics and Environmental Platform (PHE) of the Pprime Institute. Hydrodynamic and sedimentary data, acquired in laboratory under controlled conditions, will enable physical processes to be studied both physically and numerically. The observation time is a few seconds, and the spatial scale varies from cm and m. It is sufficient to obtain the experimental data needed to study the hydrodynamics of TJ&B and their erosive effects.

Anthony BEAUDOIN  
Assistant Professor, ENSI Poitiers (University of Poitiers)

Pprime Institute (CNRS, University of Poitiers, Isae Ensma,  UPR 3346)
CURIOSITY, research team of Department FTC (Fluids, Thermal and Combustion) 

Postal adress: Département FTC, Bât. H2, SP2MI, 11 Bd Marie and Pierre Curie, Site du Futuroscope, TSA 51124, 86073 Poitiers Cedex 9
Email: anthony.beaudoin@univ-poitiers.fr