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Numerical simulation of inundation processes and subsequent drying of floodplains is a challenging issue for hydrodynamic models. In particular, classical approaches are designed for explicit schemes, hence they are time- consuming and prevent fully implicit time integration. In this work, a new wetting – drying algorithm with implicit time-stepping is developed based on piecewise linear Runge - Kutta discontinuous Galerkin approximations for the shallow water equations. The principle of this algorithm is to set a limitation for the fluid depth and a blending parameter in order to ensure local mass conservation and rapid transition of a wet - dry interface. This scheme is integrated into the Second-generation Louvain-la-Neuve Ice-ocean Model (SLIM). The method is first validated by means of the analytical Thacker test case, showing its well-balancing property. Then, it is applied to the Tonle Sap Lake in the Cambodia floodplain, an integral component of the Mekong River Basin. This lake plays an important role in livelihoods of the Southeast Asian riparian residents and has a unique flow feature: it constitutes a storage area for flows coming from the Mekong River during the flooding seasons and then, during the dry seasons, it provides water to the Mekong delta. As a consequence, the Tonle Sap Lake experiences significant water level fluctuations during a year, with large variations of the total flooded area. The simulation results are investigated for two distinct flow seasons, pointing to the well-foundedness of the new wetting-drying scheme.

Numerical simulation of inundation processes and subsequent drying of floodplains is a challenging issue for hydrodynamic models. In particular, classical approaches are designed for explicit schemes, hence they are time- consuming and prevent fully implicit time integration. In this work, a new wetting – drying algorithm with implicit time-stepping is developed based on piecewise linear Runge - Kutta discontinuous Galerkin approximations for the shallow water equations. The principle of this algorithm is to set a limitation for the fluid depth and a blending parameter in order to ensure local mass conservation and rapid transition of a wet - dry interface. This scheme is integrated into the Second-generation Louvain-la-Neuve Ice-ocean Model (SLIM). The method is first validated by means of the analytical Thacker test case, showing its well-balancing property. Then, it is applied to the Tonle Sap Lake in the Cambodia floodplain, an integral component of the Mekong River Basin. This lake plays an important role in livelihoods of the Southeast Asian riparian residents and has a unique flow feature: it constitutes a storage area for flows coming from the Mekong River during the flooding seasons and then, during the dry seasons, it provides water to the Mekong delta. As a consequence, the Tonle Sap Lake experiences significant water level fluctuations during a year, with large variations of the total flooded area. The simulation results are investigated for two distinct flow seasons, pointing to the well-foundedness of the new wetting-drying scheme.