Computational Aspects of River Hydraulics

Abstract

Research of river systems is expanding enormously. Therefore, the amount of needed measurements is also increasing rapidly. To limit this effort, computer modelling is an interesting tool. The environment ’Femme’ (’a flexible environment for mathematically modelling the environment’) is used to model ecological processes as the transport of nutrients and polluents. Here, the hydraulic part, which is incorporated and based on the Saint-Venant equations, is used to check the influence of river roughness (expressed by the Manning coefficient) on discharges and water levels.
It is shown that the Manning coefficient of a specific river is influenced by the discharge and the amount of vegetation in the river. The variation of biomass over the year includes also a seasonal variation of the Manning coefficient. Next to good numerical approximation of the physical processes in rivers by good calibration, three sensitivity analysis are carried out.
First, the influence of the discharge on the water level is checked and in general, discharges are more sensitive to variations then water levels and are preferred as upstream boundary condition. Second, the influence of discharge and Manning coefficient on celerity and dispersion of waves is studied. Celerity and dispersion are larger when Manning coefficient or discharge is increasing. Third, it is shown that higher Manning coefficients can cause larger back watereffects and can consequently cause flood problems.
As a conclusion, the calibration of the Manning coefficient in any numerical model is indicated at very important to come to accurate results and realistic simulation of the hydraulic characteristics of the river.