Discharge estimation in natural channels with significant lateral inflow

The indirect measurement technique for discharge estimation, based on water level data analysis in two different river sections, is extended to the case of existing significant tributary flow between the two sections. The methodology is based on the calibration of a numerical flow routing algorithm, solving the Saint-Venant equations in diffusive form along the main channel between the two measurement sections. The first calibration parameter is the average Manning's roughness coefficient holding in the main channel, the other ones are relative to the rating curve of each tributary channel, estimated per unit roughness coefficient and per unit slope immediately before the junction of the tributary with the main channel. The new methodology is tested against different flood events observed along one gauged river reach: the Alzette river basin, in Luxembourg. The applied MAST hydraulic model solves the diffusive wave equations using the observed stage hydrograph at the upstream section as boundary condition. The observed stage hydrograph at the downstream section is used as benchmark function for the computation of the objective function to be minimized trough calibration. Field tests include comparison of the numerical results with historical discharge and water depth data available in the Alzette river.