In the context of river hydraulics, we develop the idea of a richer local zoom model superposed to a given global model. The local zoom model (2D shallow water equations in flooded parts) describes additional physical phenomena which are not represented by the basic global model (1D shallow-water equations with storage area). The weak coupling method presented preserves the integrity of the global 1D model, while it allows to calibrate its input variables (inflow boundary conditions for example) by assimilating local data measured inside the 2D zoom area. The resulting Joint Assimilation Coupling algorithm (JAC) is based on the optimal control method of PDEs systems. Some numerical tests, where we consider a toy flooding event that involves overflowing of the main channel and a moving front travelling over previously dry areas, show the efficiency of the algorithm.