Mesh elevation modifications and upstream boundary adjustment as alternatives to roughness calibration in hydraulic modelling

Hydraulic models are essential for simulating river water levels, but their results often deviate from reality due to systematic errors and modelling uncertainties. These are commonly addressed through roughness calibration, which can obscure underlying model issues and limit reliability under varying flow conditions. This study explores alternative strategies that align simulation results with measurements. We apply: (i) a mesh elevation modification technique to correct spatial representation errors, and (ii) an upstream boundary condition adjustment to refine flow inputs. The approach is tested on the Waal River in the Netherlands, with both adjustments implemented without roughness calibration. The model successfully reproduces observed water levels across the full discharge range, achieving a mean absolute error below 10 cm over two hydrologically distinct years. The findings demonstrate that targeted structural corrections and informed boundary adjustments can improve model performance, providing a complementary approach to traditional calibration-based methods.