Morphological model for the River Meuse : Grave-Lith v1.0

The current report describes the setup and calibration of the morphological model of the Meuse between Grave and Lith. The river stretch Grave-Lith is located between river-kilometre (rkm) 175 and rkm 202. The river bed is important for many different river functions, as it influences water levels, determines the dimensions of the navigation channel, determines the stability of man-made structures (in and beside the river), and is important for habitats. Knowledge how the river bed develops is important considering: • Design of measures along the main channel (river training works and sediment management); • Evaluation of the effect of measures (Rivierkundig Beoordelingskader); • Aiding in the monitoring of pilot locations (river widening and deepening, eroding banks, ...); • Long term system development considering different maintenance strategies. On the Meuse, there are a few options how to assess these developments, namely expert judgement, computation of equilibrium development for small scale measures in the winterbed of up to 5 km (D-FAST MI, formerly WAQMORF) and since 2020 a preliminary Delft3D FM 1D model of the Meuse. The development of a two-dimensional Delft3D FM model will extend and uniformise the available options to assess the development of the river bed of the Meuse with that of the Rhine and allow for an easier implementation of measures, more detailed information (i.e., two-dimensional data) and at a finer resolution than in the Delft3D FM 1D model. The model developments take place over the course of several years and started in 2023. The work carried out in 2023 is reported in Ottevanger et al. (2024a) and led to model version v0.5 (uncalibrated). The current report is a report that describes the development work of 2025 which resulted in model version v1.0. Once the model of the entire Meuse is ready, a final report will be made that contains a full description of all data, methods and results used for the final version v1 of the model. In the current report a method was refined to set up a submodel for the reach based on the 20m Meuse grid and boundary conditions, initial conditions and weir settings from the 40m hydrodynamic Meuse model. The approach was refined to align as much as possible with the current hydrodynamic models of the Meuse. A data analysis of the bed level changes showed the importance of knowing the locations of dredging as well as the bed level changes caused by maintenance dredging. Using a schematised hydrograph between 2011 and 2021 a net reduction in bed material transport of roughly 10 000m3 (including pores) was found between rkm 176 and rkm 181 at a location where the main channel was lowered. Using the above estimate, it was possible to make a first estimate of suitable parameters for the sediment transport formula. This choice was imposed in the simulation, which lead to a very low first estimate of the bed level changes for the reach. The intial estimate failed to reproduce the propagation of waves in the bed level which migrated at a speed of 50-70 m/year. By increasing the transport rate by a factor 8 the propagation speed of bed level waves was well reproduced. Subsequently, transverse bed slope and secondary flow effects were considered for further two dimensional calibration. A sensitivity analysis of the initial sediment composition and the influence of bed level filtering was also investigated. These showed that imposing the measured sediment composition and bed level measurements without filtering proved to be the best choice. The model Grave-Lith v1.0 is fully calibrated. This means it can be used for application in long-term assessment of measures on the reach. A first test application for the measure Demen-Dieden is currently in progress and is expected to be finalised at the end of 2025. Another important remaining activity is the derivation of discharge hydrographs which can be used for future scenario’s, and the finalisation of the user manual for use of the model.