Graph theoretical analysis of braided rivers

Abstract

Braided rivers are one of the most complex and unpredictable natural systems on Earth. Found worldwide, they are easily identified from above by their distinctive morphology (a large network of interlinked channels divided by interspersed sedimentary deposits). Their propensity to dramatically adjust their form during flood events impacts upon millions of people who interact with them. However, their size and the intensity of processes that are active during these episodes of morphological change have presented considerable barriers to the measurement, understanding, prediction and management of braided river behaviour. This research has begun to resolve these issues through the successful development of a new theoretical framework for the study of braided river evolution based on graph theory (a branch of mathematics concerned with network structure and function). Leveraging a recent upsurge in open-access Earth observation data provision, it is now possible to extract network representations of braided rivers globally from satellite imagery. A workflow for the extraction of braided river networks from multi-spectral remotely sensed imagery is described herein, with these networks providing the basis for graph analysis. A key question in network research is how to define functional units, which in the context of braided rivers are reaches. Defining reaches has previously been done arbitrarily, which likely negates key controls that determine the spatial scales over which braided river morphologies evolve. This research proposes a new, physically-based approach to defining reaches in braided rivers that accounts for spatial scaling and network structure. Evolution of braided river morphologies at the reach-scale or greater occurs over timescales that range from event-based change to long-term trajectories of change in the whole braided channel network. Analysis of the topological evolution of a braided channel network is presented to show spatio-temporal variation in connectivity and how this relates to braided river morphodynamics.