Dispersion induced by braided river morphology

Braided rivers were deeply investigated as regards their morphodynamical evolution, conversely less interest was shown for the role that their peculiar topology can play in other phenomena, such as transport processes.

Our work deals with the link between the river network structure and the downstream transport of scalars, for example chemical substances (nutrient or pollutant) or suspended sediments. The river network acts as a mixer on the injected substance, thus causing a dispersion effect on the transport process. This phenomenon - known as geomorphological dispersion in the river networks at basin scale - becomes particularly relevant in braided rivers due to the complexity of their networks and their possible crucial impact on fluvial water quality.

Adopting the approach of GIUH theory, we develop a mathematical model for the dispersion in braided rivers. In particular, we assume to inject a given initial distributions of the scalar in the network inlets and aim to compute the outlet discharge by only considering the network properties, i.e. discharges at bifurcations, branches travel times, and network topology.

What we observe from the results is a strong dominance of the network topology over branch-specific hydraulic properties with regard to the outlet distributions, meaning that the properties at the network scale seem to have more influence than those relative to the scale of the individual branch. Moreover, we show how the outlet distributions depend on network properties. We find that, as the network grows, temporal distributions become closer and closer to Gaussians.  This behaviour - expected in light of the central limit theorem - is caused by the succession of bifurcations and reconnections interspersed with branches having different travel times.