Global Sensitivity Analysis applied to the Telemac2D numerical forecast model of high water levels in the Gironde estuary

Overview & Synopsis

In the context of the development and the implementation of data assimilation techniques in Gironde estuary for flood forecasting, a Telemac 2D model is used to calculate water depths 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 river. The maritime boundary is 32 km off the mouth of Gironde estuary, located in Verdon. This model, which contains 7351 nodes and 12838 finite elements, does not take into account overflows. It has been calibrated on 4 non-overflowing events and then validated on 6 overflowing events. In a first step, a propagation and quantification of uncertainties by an unidirectional analysis method (creation of a set of 2000 members perturbed for each parameter and input forcings and analysis of output water depths) was carried out on the numerical parameters (wind influence coefficient, Strickler friction coefficients for 4 zones) and forcings of the model (rivers discharges and maritime boundary conditions, meteorological forcings). The objective is to determine the variation coefficient of water depths for 13 major events between 1981 and 2016. The exploitation of 1981 event results shows a predominance of the influence of the maritime boundary conditions and the Strickler coefficient for the estuarine part and the confluence, to which must be added the Garonne discharge as a predominant parameter for the latter. Unsurprisingly, river zones are influenced primarily by the coefficient of friction and the respective river flows of Garonne and Dordogne rivers. On the second hand, a Global Sensitivity Analysis (GSA) by variance analysis (ANOVA) was carried out, by calculating the total and partial Sobol’ indices, integrating the time- and/or space dependent forcing variables. It has led to the identification of parameters and forcings to which the model is most sensitive, as well as their inter-dependencies, in order to choose the variables to assimilate. The GSA shows that the tidal signal imposed at the maritime boundary condition and provided by a more extended surge levels model is the key input variable. Moving from the mouth to the upstream part of the Garonne and Dordogne rivers, the influence of the friction coefficient increases and hydrological forcings have a very local influence upstream the rivers.