Riparian vegetation in fluvial environments: linking timescales through flow uprooting

Abstract

In the last decades, the presence of riparian vegetation on riverbanks and floodplains along rivers was acknowledged not only to improve water quality and heal biological diversity but also to contribute to river evolution processes. When water flow runs over vegetated areas, averaged velocity profile is affected by the presence of stem, branches and leaves, sediment transport changes according to modified turbulence and bed shear stresses and soil shear strength is altered by root binding. As a result, bed scour, bank erosion and accretion, bar migration and width adjustment processes lead to different river morphology evolution. Conversely, flow and sedimentary patterns influence vegetation dynamics, by shaping barebed deposits available for colonisation and by affecting mortality rate, through burying and uprooting processes. This PhD work examines the uprooting process of both pioneer seedlings and established vegetation driven by flow and bed erosion, whose role is to reduce root anchorage, at various spatial scales ranging from a single plant to a river reach. The main purpose of this research is to illustrate the links between temporal scales regarding the hydro-morphological evolution of fluvial systems, such as bed scour development, flood duration and return period, and those proper of biological components with regards to both growth and decay rates of riparian vegetation. The results of this PhD research show the existence of cross-related temporal scales between riparian vegetation and river morphodynamics and demonstrate their relationships with flood return period and event duration. As a final result, this research hints the capability for river to select species and cover according to hydrological regime and biological properties. This is crucial in fluvial environments altered by climate change, where alien species may replace native ones. It also underlines the importance of taking into account riparian vegetation dynamics, effects and interactions to guarantee the reliability of long-term river morphodynamics modelling and the success of river maintenance and restoration strategies.