Bifurcations in mountain rivers: insights on their hydraulics from field measurements.

Although the importance of studying channel bifurcations is widely recognised, their hydraulic behaviour in shallow, rough mountain rivers has so far received little attention from researchers. Understanding the specific hydraulics of such units is essential for predicting and interpreting their morphodynamic evolution. Water discharge measurements in the incoming channel and distributaries are often difficult to perform in steep streams characterised by high relative roughness (grain size to depth ratio d/h > 0.1), aerated flow, and marked free-surface waves. Nonetheless, recent advances in Acoustic Doppler Current Profiler (ADCP) technologies open new possibilities for studying the flow configuration at stream bifurcations. We monitored the flow repartition in a bifurcation of a mountain gravel-bed river by deploying an ADCP specifically designed for shallow flow conditions. This field campaign was combined with photogrammetric surveys for documenting the geomorphological evolution of the river bed, its surface grain size distribution and structure. Integrating data from these different sources provided useful information on the bifurcation evolution and hydrodynamics. During a period in which the river bed did not undergo noticeable elevation changes, we observed that the water discharge ratio of the distributaries was approximately constant for sensibly different total discharge values. Such result was compared with the outcomes of numerical simulations.