Local tributary widening for river rehabilitation

The hydro-morpho-sedimentary processes in confluence hydrodynamic zones (CHZ) subject to a local widening of the tributary were investigated in a series of laboratory experiments that were representative of the 20 major confluences of the Upper Rhone River in Switzerland. Three discharge ratios (low, intermediate and high ratios of tributary to main channel discharge) were investigated for each of four tributary configurations (reference configuration with constant tributary width and three geometries of the local tributary widening). In all experiments, the local tributary widening caused substantial morphodynamic changes in the CHZ and moderate sediment redistribution in the post-confluence channel. At the entrance of the local widening, the tributary main flow corridor gradually widened. Towards the confluence mouth, it narrowed and deflected in downstream direction because of a zone of flow stagnation at the confluence upstream junction corner. Dry zones and flow stagnation zones occurred in the widening zone outside the tributary main flow corridor. The results indicate that a local tributary widening considerably enhances the variability of bed substrate, velocity and depth in the CHZ, and offers an appropriate habitat for a wider range of biota than the homogeneous channelized reference configuration. None of the investigated experiments resulted in adverse impacts on water surface elevation and thereby flood safety. On the basis of the laboratory experiments, the appropriate geometry of the local tributary widening is discussed, and quantitative guidelines for practical application are proposed.