Groundwater module SOBEK : A groundwater module for the hydraulic model of the Rhine between Andernach and Lobith in GRADE

A groundwater module SOBEK has been developed as module for the SOBEK 3 model of the Rhine to represent the water exchange between river and aquifer. Several model runs have been carried out and the simulation results were analysed to assess the usage of the groundwater module SOBEK with respect to three aspects: 1) Added value of modelling river-aquifer interaction in hydraulic models 2) Impact of river-aquifer modelling on extreme flood events and flood statistics 3) Practical aspects of modelling river-aquifer interaction The SOBEK 3 model with groundwater module SOBEK produces good results. This is supported by a comparison with data based on observations, the HyMoG data, for three flood scenarios (1995, 1993, 1988). Comparison runs with the SOBEK-RE model and the GRADE data set show a good match of the SOBEK 3 simulation result, too. Different to earlier approaches for modelling the river-aquifer interaction in SOBEK models, the modelling approach of the Groundwater Module SOBEK uses the leakage-approach, an approach which is well-documented in literature and a widely accepted modelling approach for river-aquifer interaction in computer models. The SOBEK 3 model has already a good level of calibration without the groundwater module SOBEK. Adding the groundwater module SOBEK in its current state does not necessarily improve the model results at all locations. At the downstream locations however, which are considered as the most important ones, the Groundwater Module SOBEK shows already an improvement in terms of match between simulated and observed data. A new integrated calibration of the SOBEK 3 model that involves both the hydraulic model component (roughness parameters) and parameters of the groundwater module SOBEK will provide an even better match between simulated data and observed data. The calibration parameter of the hydraulic model, the roughness, mainly allows to tune the water level. The shape of the discharge curve can only be addressed to some extent with the roughness parameter (wave damping). With the Groundwater Module SOBEK it is now possible to model bank storage, which is technically and physically a temporary withdrawal of water. This process has been reported as significant in literature; now it is incorporated in the model this opens a new dimension for the calibration. Simulation runs of extreme scenarios from GRADE have been carried out with the Groundwater Module SOBEK. The effect of the river-aquifer interaction is also clearly visible for extreme scenarios. The more extreme a scenario, the smaller is the effect of river-aquifer interaction on the model results. This can be explained with the bank storage processes and the effect of inundations that take place under extreme flood conditions on the water level. Modelling river-aquifer interaction with the Groundwater Module SOBEK will thus have an impact on the flood statistics that are produced with the GRADE instrument. However, for an assessment of good quality, more simulations must be carried out. Correction functions that are currently used to achieve model consistency between SOBEK 3 and SOBEK-RE should be updated after the integrated calibration of the SOBEK 3 model of the Rhine. Ideally, the models are sufficiently consistent such that no correction is needed. As next steps we recommend an integrated calibration of the SOBEK 3 model, where the parameters of the hydraulic model (roughness parameters) and the parameters of the Groundwater Module SOBEK are adjusted coherently.