Numerical Modeling of Plant Root Controls on Gravel Bed River Morphodynamics

The role of vegetation in shaping the geomorphology of rivers and deltas, along with tidal and estuarine environments, is widely recognized. While mutual interactions between flow, plant canopy, and morphodynamics have been extensively investigated, similar studies considering plant roots are limited. Here we present results from a numerical model that quantify the feedbacks of both the aboveground and belowground vegetation on gravel bed river morphodynamics. Plant root biogeomorphic feedbacks, that is, uprooting and root-enhanced riverbed cohesion, are quantified through the description of the vertical root distribution. By investigating the evolution of the riverbed of a straight gravel channel with a vegetated patch, we show that uprooting is the primary plant root biogeomorphic feedback determining the evolution of the riverbed and the competing influence of the potential flow erosion versus uprooting depth mediates the plant root controls on morphodynamics. These findings broaden our understanding on the role played by plant roots on gravel bed river morphodynamics. Plain Language Summary Vegetation living at the interface of water and terrestrial areas represents a key element to understand and predict how rivers change their shape. In fact, plants affect but depend on numerous physical processes linked to water flow and transport of sediments in rivers. Research commonly investigates these processes considering only the aboveground part of vegetation, which consists on a system of branches, foliage, and stems interacting with water flow when submerged. However, what assures plant anchorage to the ground helping plant to resist erosion and strengthens sediments increasing their cohesion is the belowground vegetation, the roots, which is often disregarded. In this study, we include a description of plant roots, by their vertical distribution, into a model simulating river morphodynamics to investigate their role. By means of numerical experiments in a simple river channel configuration, we show that vegetation removal by erosion is the most important process controlling riverbed evolution and that conditions under which plant roots mostly influence such evolution depend on the balance between erosion and root resistance. This can help to broaden our view on the topic and to address questions yet unexplored.