Optimized Predictive Two-Dimensional Hydrodynamic Model of the Gironde Estuary in France.

Abstract : A two-dimensional hydrodynamic model of the macrotidal Gironde estuary (France) is developed and shown to predict the tide propagation with an accuracy of < 10 cm on water level. The 170-km-long computational domain represents, in detail, the central part of the estuary, characterized by a complex multichannel geometry. This model provides an efficient and accurate operational tool by using an optimized finite-element numerical scheme. Particular attention is paid to the offshore boundary conditions and to the calibration procedure. On the offshore boundary, the tidal signal is decomposed into harmonics whose main amplitudes and phases are obtained from a regional tidal model covering the North East Atlantic. Sensitivity studies are conducted to determine the influence of the harmonics number and harmonic constants on tidal prediction at the mouth. Differences up to 20 ∼ 30 cm in water level at the estuary mouth are observed between the scenarios for the tidal forcing. The calibration of the friction coefficient is thus linked to the tidal forcing scenario. A calibration procedure for the friction coefficient is also presented that can be applied to estuarine conditions. The van Rijn method is applied to predict a first set of values for the friction coefficients as a function of grain size and flow parameters. These predicted mean (time-averaged) coefficients are then adjusted with a maximum of 15% to reproduce in a best way water level and velocity measurements.