Hydrodynamic Modelling of Seine Bay and Estuary in Moderate and Extreme Conditions: With a Focus on Johanna Storm

Seine estuary experiences large tidal ranges (e.g., around 7 m during spring tide) and a long high-tide slack time (roughly three hours). As an embayed estuary located in the English Channel, its dynamics is highly affected by the tidal transformation and mass/momentum exchanges between the English Channel and Seine Bay. However, these mechanisms are not well studied yet. In this investigation, a Telemac2D hydrodynamic model of Seine Bay and Estuary (abbreviated as SBE in this paper) is developed and calibrated for both moderate and extreme conditions. First, the main parameters affecting the tidal dynamics of SBE are identified. Then, the model is enhanced to correct the simulation of tidal propagation in Seine Bay in moderate conditions, thereby reproducing the length and shape of the high-tide slack in the Seine estuary as observed by the in-situ measurements. The enhancement procedure is further confirmed by applying it to another marine-boundary-condition type (i.e., time series of total water level as well as harmonic constituents). Moreover, the results in the bay are validated using remotely-sensed, altimetry data. Then, the model is adapted to simulate the storm-surge events in SBE, focusing on the Johanna storm which caused considerable damage and casualties in March 2008. The calibrated model is used to investigate the tidal dynamics as well as the effects of meteorlogical forces in SBE and can be a useful tool to further study the hydro-eco-morphology of the SBE. This work aims at providing a reliable map of surface water evolution throughout the SBE in stormy conditions. The model will be used in the Cal/Val phase of the SWOT (Surface Water and Ocean Topography) mission to calibrate and validate the SWOT simulator.