Modelling the time variance of the river bed roughness coefficient for improved simulation of water levels

Aquatic macrophytes create seasonally variable changes in river bed roughness. These changes in roughness influence the hydrodynamic behaviour of the river, for example, elevated water levels for the same discharge. In most hydrodynamic river models, the roughness is assumed constant in time, which may lead to high errors in the river water level results. When the river water level results are applied to water quality studies, biased estimates may be obtained of river flow velocities, dilution and other water quality processes. This article investigates how models implemented in two common hydro- dynamic modelling software packages (MIKE11 and InfoWorks RS) can be adopted to take into account the time-varying river bed roughness due to plant growth. Albeit the use of a different approach by the two models, it is shown that both can account for that influence. After application to the Grote Nete river catchment in Belgium, far more accurate river water level results are obtained. Overall, the increase in accuracy of the water level simulations outweighs the increase in model complexity required to enable simulation of the vegetation changes. The model results can also be used to correct discharge estimates based on the measured water levels as shown for this case study.status: publishe