Evolution of the hydromorphodynamics of mountain river confluences for varying discharge ratios and junction angles

Mountain river confluences are characterized by narrow and steep tributaries that supply abundant sediment load to a main channel that, in turn, provides the dominant flow discharge. In addition, bed sediments consist of poorly sorted mixtures that promote bed armoring. The knowledge of the hydrodynamics and morphodynamics of mountain river confluences is sparse because most of the existent studies on confluence dynamics focus on lowland confluences. This study aims at examining the influence of the junction angle (a) and discharge ratio (Q(r) = Q(t)/Q(m)) on flow dynamics and bed morphology of mountain river confluences. This study presents the results of six laboratory experiments in which three discharge ratios were tested (Q(r) = Q(t)/Q(m) = 0.11, 0.15, 0.23) with two different junction angles (alpha = 90 degrees and 70 degrees). The experiments were conducted under movable bed conditions and with continuous sediment supply to both flumes. Measurements consisted of systematic bed topography and water surface surveys performed at different instants during the experiments and at equilibrium, i.e., when the outgoing sediment rate coincided with the incoming and bed topography reached a steady state. The results show that the discharge ratio and the junction angle parameters are major controls of the dynamics of mountain river confluences. Also, the evolution of bed morphology and flow dynamics for varying junction angles and discharge ratios present some patterns that contrast with those reported for lowland confluences. Among these patterns are the different flow regimes adopted by the tributary for different junction angles and the decrease of the height of the bank -attached bar for increasing discharge ratios. Moreover, results show that the abundant sediment load of the tributary plays a major role on the dynamics of this type of confluence. This load resulted in a marked bed discordance that, in turn, influenced flow dynamics and bed morphology of the confluence. (C) 2015 Elsevier B.V. All rights reserved.