TB appear in special configurations of sea and estuary environment and by specific tide and meteorological conditions. How can the data contained in ancient maps (bathymetry, sediment) be extracted, analyzed and crossed with meteorological and tidal data to explore the mutations of the TB physical manifestation through history (since 19th century)? The study of natural past configurations and conditions is an application of recent methods, used to study past submersions. It consists in old maps data extraction and past tidal and meteorological material collection to provide numerical models with past datasets. From a qualitative point of view, the study of the impact of TB on the navigation (shipwrecks) and the riverbanks inhabitants (flooding) will provide historical markers of TB intensity variations and the impact of TB on society.

TBs have been observed in many estuaries worldwide. At the beginning of the 21st century, tidal bore characterization in natural environments was based on qualitative observations. In the last decade several quantitative field studies on TB have been devoted to the analysis of propagation and turbulence properties, associated sediment transport and sedimentary sequence. Most of field studies focused on (i) spring tide periods (with the most intense TB) without interest for the transition periods between neap to spring tide (from no tidal wave impact, up to TJs, undular and breaking TBs), (ii) small scale processes without interest on spatial evolutions of these waves in relation with river morphology and (iii) water column excluding interface processes (water/sediment and water/air). Some studies show data from spring to neap tide or relationship between TB and morphology but analysis stay concentrated on breaking bore events, on a single tide and at a specific site. Studies covering successive tides from neap to spring tides, are concentrated mainly on evolutions of water depth and bore shape excluding the spatially measurements of bore celerity, current velocity and sediment processes. Several questions remain unanswered: what is the impact of bathymetric variations along the river transverse profile on the shape of the tidal front (TJ&B)? What are the hydrodynamic forces acting on the bottom (erosion processes) for different tidal front shapes (from neap to spring tide)? What is the real impact of TJ&B on bathymetry evolution on the scale of a river channel (a few hundred m longitudinally)? Spatialized in situ studies cover an entire spring tide cycle including interface processes are currently lacking, which is explained by the difficulty to instrument large estuaries, during the tidal bore passage.

TB is the object of many investigations in laboratory. During the last years, several works have studied the air–water properties of unsteady breaking bores with Eulerian and Lagrangian velocity measurements using intrusive and non-intrusive techniques and also sediment transport in the water column.  It was demonstrated very complicated fluid-particle-bed interactions in the rapidly varied flows during bore events and showed the effect of the adverse longitudinal pressure gradient force and the transient flow reversal on particle acceleration and upstream advection of bed particles. What are the main hydrodynamic forces generated by the TB on the bottom and on the sedimentary beds? Moreover, it was shown that an instability of the TB front generates some coherent structures that invades the whole flow. A new method of TB creation was discovered recently and which is based on a generation mechanism similar to that observed in nature with the inversion of the ebb by a very short flood thanks to the connection of our medium open water channel to a tranquilization chamber with a waterfall controlled by a weir that disappears by turning off the pump flow rate. It differs from the historical method which consists in cutting the flow with a guillotine inducing an intumescence counter-propagating on a constant river flow. With respect to the experimental state of the art, preliminary works have been done who looked to the effect of slope on the deceleration of TB albeit using simple local measurements of the water depth as a function of time. Very recently, report preliminary results on the effect of channel curvature on the TB propagation with the same experimental limitations namely no spatial-temporal measurements and no particles image velocimetry of the entire flow fields. What is the impact of the real/simplified trapezoidal shape of real rivers compared to the simple rectangular shape used in laboratory experiments on the propagation and type of TBs? How is the theoretical prediction on the classification of TB supported by time-resolved particles velocimetry and free surface measurements in the laboratory and what is its robustness for changing geometries? What are the hydro-mechanical solicitations during the passage of the TB which start the erosion process?

By using Notus for 3D intensive numerical simulations and by developing Lagrangian methods, 4 classes of TB relative to the flow under the free surface have been identified during the ANR project MASCARET: TB dominated by the stubbles with trajectories of coiled ribbon fluid particles and TB dominated by the current with undulating ribbon trajectories with or without reversal of river current. A classification of TB types and particle trajectories was predicted through the implementation of a non-linear wave-current interaction model in an ideal flat bathymetry without side effects using the Froude number based on the river depth. Estuary and river channels are non-rectangular and present most of the time a variable cross-section with a nearly trapezoidal shape.  It was showed that the presence of sloping side banks has a strong impact on TB dynamics. From field observations in the 2 main French tidal bore estuaries, the Seine and Garonne estuaries, they identified 2 undular bore regimes around a transition Froude number FT of approximately 1.1. An abrupt decrease of the secondary wave steepness occurs when Fr goes below FT. This transition is in accordance with that observed for undular bores propagating in a trapezoidal channel and was theoretically explained. The difference of the TB wave steepness for the two regimes around FT, is associated with a strong difference of the fluid acceleration at wave fronts. What is the impact of the transition around Fr = FT, on the TB-induced sediment transport and the morphodynamic evolution of rivers? What is the dependence of FT with the slope of side banks?

There have been many works addressing the development of numerical models for the study of water flow and erodible in the rivers. Most of existing numerical models are based on the solving of Shallow-Water-Exner equations. When dispersive wave propagation is predominant, the hydrostatic shallow water equations are not anymore, a relevant model, and non-hydrostatic variants must be considered. The propagation of tidal bores is rich in flow regimes with different characteristics. A transition appears between undular tidal bores dominated by non-hydrostatic effects, and dispersive-like undular tidal bores associated to a hydrostatic wave refraction process due to the variations of the bathymetry within each section. A numerical and theoretical study of this transition has been performed. In the paper, which provides a prediction of the observation, the authors underline the dynamic process of development and stabilization of finite wavelength undular bores, in which the equilibrium between nonlinearity, dispersion and dissipation is established. A modified version of section-averaged Boussinesq equations has recently been proposed in 1D. The main features of the wave train’s free surface have been reproduced using channel Favre waves cases with a fixed and linear geometry, different in the natural environment. The absence of explicit dissipation terms in propagation, such as the nonlinear shallow water, Boussinesq and Serre-Green-Naghdi equations usually assuming irrotational ideal flow, requires some implicit or explicit closure for these effects. The interaction between modelling and numerics becomes relevant. Numerical dissipation can be a surrogate for the lack of physical dissipation, and indeed it does allow to obtain a qualitatively correct description of the underlying processes. However, the dependence on mesh size makes this approach less reliable when a robust quantitative prediction of bore dynamics is necessary. Is it possible to simulate the propagation of different types of tidal bores in real rivers by means of actual models based on the nonlinear shallow water, Boussinesq and Serre-Green-Naghdi equations? Does solving the system of shallow water equations, the Exner equation, and the advection-dispersion equation allow to study the interaction between river bathymetry and tidal bores?