Pascal Pirlot, Ph.D. Candidate 36th cycle, University of Trento, DICAM

Fluvial bifurcations are natural processes that recurrently appear in all types of reaches in alluvial water streams; in steep, braiding, gravel-bed rivers and flat, lowland meanders and multiply at the ocean where they form deltas. A majority of these bifurcations distribute the water and sediment discharges unevenly along the bifurcates. This discharge asymmetry interacts with the platform and the boundaries of the bifurcation node. The core goal of my work relies on understanding the mechanisms underlying this complex interaction. 

Taking foot on data that furnishes hydraulic parameters at about 200 sandy and gravel-bed bifurcations around the globe, I gather satellite imagery from which the bifurcation planform is parametrized. To do so, I developed a semi-automatic extraction algorithm in which the river banks are collected manually. After this, the algorithm computes the channel width, the curvilinear middle axis in each branch constituting the bifurcation, allowing the computation of variations and averages of the channel width, angle and curvature for each bifurcation branch, the bifurcation and bar head angle and the length of the bifurcates. These parameters represent planimetric forcings that influence the morphodynamic equilibrium at the node. 

The focus is led onto the differences in channel width upstream and downstream of the bifurcation denoted the “Bifurcation Enlargement” and between the bifurcates themselves as the “Channel Width Asymmetry”. The various planimetric parameters show little correlation with the morphodynamic parameters and between themselves. Nevertheless, the distribution of values for the channel width asymmetry shows that a third of the bifurcations is roughly symmetrical and the rest is conspicuously asymmetrical. Concerning the bifurcation enlargement, about a fifth of the observed cases are approximately uniform, while the majority display an effective enlargement.

These typical differences in channel width in the branches influence the equilibrium configurations of the bifurcations greatly. To understand the underlying processes, I study the problem from the mechanical perspective and apply these channel width variations to a physical nodal point relationship. Doing so, I discovered that the used planimetric forcing modifies the topology of the solution of the bifurcation problem and sets the need for a new definition of “balance” and “reference flow”, that are crucial to interpreting equilibrium configurations.

Formally, the channel width asymmetry promotes stability in the bifurcation, offering higher transport capacity, longitudinal uniformity and resistance throughout flow regime variations. On the other hand, effective values of the bifurcation enlargement destabilise the node since the downstream reference flow is shallower and closer to the incipient sediment motion condition.

My work exposes precise parameterized features of observed bifurcations and possible correlations between these features. I also update my understanding of the stability of the bifurcations from the mechanical perspective. My contribution sets the ground for further research, investigating the possibility of retrieving the bifurcation bathymetry from remote sensing in order to test the main outcomes. Also developing a two-dimensional model will confirm the presented results.