Influence of the widening of a tributary on confluence morphology: preliminary results

The aim of this work is to obtain a better understanding of the morphological development in alpine river confluences, where special attention is given to the widening of the tributary channel and its influence on bed morphology evolution. Hydraulic tests have been performed in an experimental set-up of asymmetric confluent channels connected in an angle of 90°. The main and post-confluence channels are 3.6 m respectively 4.9 m long while the tributary channel is 4.8 m long. Two different configurations have been considered: one with all channels 0.50 m wide and other where the tributary has been widened (1.0 m wide) over a length of 1.0 m just upstream the confluence. Experiments have been carried out under mobile bed conditions where sediments (poorly sorted grain distribution) are feed on the tributary channel. Preliminary results considering a discharge ratio (Qmain channel/Qtributary) equal to 1 have shown that the influence of the widening of the tributary is quite important on the morphological development in a confluence zone. The enlarged zone is characterized by high variability on water depths, flow velocities and particle size distribution, showing that widening the tributary channel can be adequate to improve ecological values on confluence zones in the framework of river rehabilitation projects.