Uncertainty Quantification for the Gironde Estuary Hydrodynamics with TELEMAC2D

Abstract

In a context of development and implementation of data assimilation techniques in Gironde estuary for flood forecasting, a TELEMAC2D model is used to compute water depth and velocity fields at each node of an unstructured mesh. Upstream, the model boundaries are, respectively, La Réole and Pessac on the Garonne and Dordogne rivers. The maritime boundary is 32 km off the mouth of Gironde estuary, located in Le Verdon. This model, which contains 7351 nodes and 12,838 finite elements, does not take into account overflows. This paper presents a global SA study in the context of flood forecasting in the Gironde estuary. It aims at identifying which input variables should be better described for water levels to be better simulated and forecasted in the estuary. On the one hand, a propagation and quantification of uncertainties by a unidirectional analysis method was carried out. On the other hand, a variance sensitivity study (ANOVA) was carried out, by calculating the total and partial sensitivity Sobol’ indices for all numerical parameters (wind influence coefficient, Strickler friction coefficients for four zones) and time-dependent forcings of the model (rivers discharges and maritime boundary conditions). It led to the identification of parameters and forcings to which the model is most sensitive for each area of the estuary, as well as the identification of very low interdependencies, in order to choose the variables to assimilate later. The standardized variation coefficients for 1981 event as well as Sobol’ indices for 2003 event show a predominance of the influence of the maritime boundary conditions all along the estuary and of the Strickler coefficient corresponding to the zone considered for the estuarine part and the confluence, to which must be added the Garonne discharge as a predominant parameter for the latter. Unsurprisingly, the upstream part of river zones is influenced primarily by the friction coefficient and the respective river flows of Garonne and Dordogne rivers.