Discharge and location dependency of calibrated main channel roughness: Case study on the River Waal

To accurately predict water levels, river models require an appropriate description of the hydraulic roughness. The bed roughness increases as river dunes grow with increasing discharge and the roughness depends on differences in channel width, bed level and bed sediment. Therefore, we hypothesize that the calibrated main channel roughness coefficient is most sensitive to the discharge and location in longitudinal direction of the river. The roughness is determined by calibrating the Manning coefficient of the main channel in a 1D hydrodynamic model. The River Waal in the Netherlands is used as a case study. Results show that the calibrated roughness is mainly sensitive to discharge. Especially the transition from bankfull to flood stage and effects of floodplain compartmentation are important features to consider in the calibration as these produce more accurate water level predictions. Moreover, the downstream boundary condition also has a large effect on the calibrated roughness values near the boundary