Coupled Morphodynamics of River Bifurcations and Confluences

Multithread fluvial environments like anastomosing and braided rivers are fundamentally directed by the continuous concatenation of channel bifurcations and confluences, which distribute flow and sediment among different branches that are reconnecting further downstream. A large number of theoretical, experimental, and numerical studies conducted in the last two decades have provided a clear picture of stability conditions for river bifurcations. However, most analyses are focused on the dynamics of bifurcations alone, ignoring the possible mutual interaction with downstream confluences. In this work, we study the morphodynamic equilibrium and stability conditions of a bifurcation-confluence loop, where flow splits in two secondary anabranches that rejoin after a prescribed distance. Through the formulation of a novel theoretical model for mobile bed confluences, we show that the dominating anabranch (i.e., that carrying most discharge) is subject to an increase of the water surface elevation that is proportional to the square of the Froude number. This effect causes a decrease of the slope of the dominating anabranch, which acts as a negative feedback that increases the stability of the bifurcation-confluence system. A linear analysis of the coupled model reveals that the stabilizing effect exerted by the confluence depends on the ratio between the length of the anabranches and the average water depth, independently of channel slope and Froude number. Ultimately, this effect is potentially able to stabilize the loop even when the sediment is mainly transported in suspension, a condition which makes the classic stabilizing mechanism (i.e., the topographic effect at the bifurcation node) practically ineffective.