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

Hydrodynamic river models are used to predict water levels along the river and support decision making in river management. Therefore, the model predictions need to be sufficiently accurate. To increase the accuracy of the predictions, hydrodynamic river models are calibrated and validated. Often the hydraulic roughness coeffi cient is calibrated because it is the most uncertain parameter in hydrodynamic river models (Pappenberger et al., 2005).
The physical bed roughness can vary along the longitudinal direction of the river due to differences in bed sediment. Moreover, as discharge increases, river dunes grow leading to an increasing bed roughness (Julien et al., 2002). Therefore, it is hypothesized that the calibrated main channel hydraulic roughness is mostly sensitive to the discharge and location in longitudinal direction of the river. The calibration study of Warmink et al. (2007) confirms this hypothesis but does not explain why the calibrated roughness varies.
Our objective is to explain why variations in the calibrated roughness parameter occur and whether its value depends on the location or discharge used for calibration. We use a case study on the River Waal in The Netherlands.