Eco-morphodynamic modelling for gravel bed rivers

Abstract

Rivers provide vital services to society. Most of them are linked to floodplains, and riparian areas, which are key nexus between terrestrial and aquatic ecosystems. However, during the last centuries, the exploitation of river resources has led to a progressive degradation of riparian ecosystems, and loss of their associated services. The hydromorphological condition of rivers, largely impacted by channelization, is recognized as a key indicator for assessing riparian ecosystem integrity, and it is nowadays preventing rivers to achieve a good ecological status. In this context, the development of quantitative, predictive tools in support of river restoration plans urges. River morphology is widely recognized to result from the interactions between vegetation, flow and sediment transport. Riparian plants are known to modify flow patterns and sediment transport during floods, depending on specific above- and below-ground plant traits such as canopy and root biomass and density. In turn, hydromorphological processes influence the establishment, growth, and survival of plants. However, our understanding, and the ability to quantify, the co-evolution of vegetation and river morphology remain limited. In this research, we aim at quantifying the key underlying processes of this coevolution, which have so far been mostly described in a qualitative way, by developing modelling approaches able to identify and disentangle them. We focused on gravel bed rivers, common in Alpine regions and temperate environments. First, we investigated the hydromorphological and biological conditions needed for plants to establish on gravel bars after seed dispersal. By analysing a series of aerial images between 1996 and 2017 of a reach of the Alpine Rhine river, we found that vegetation has grown since 2005 along the more stable (steady) bars, while not on the more active, migrating bars. To interpret the observations, we developed a simple model that searches for the minimum time period needed for plants to resist hydrodynamic disturbances and successfully establish, and explored the influence of bar morphology and seed dispersal. Second, we studied the role of plant roots in mediating river morphodynamics, which has so far been poorly investigated in gravel bed rivers. We developed a model accounting for the main root-related feedbacks, and quantified their importance in a simplified river channel configuration. We found that the uprooting, that is, the mechanism by which plants are removed by flow erosion, is fundamental for describing river eco-morphodynamics, and that the effect of different root characteristics depend on the balance between root anchoring resistance and the erosion potential of the flow. Third, we investigated the role that both above- and below-ground plant traits have on vegetation survival to floods, and ultimately on biogeomorphic patterns. Motivated by observations in the Alpine Rhine river, we studied an alternate bar river morphology and tested different trade-offs between above- and below-ground plant traits. We found that differences among trade-offs emerge only when plant biomass develop enough to interact with river morphology, which may underpin a transition from a geomorphic to a biogeomorphic state. In addition, we were able to explore different mortality mechanisms of plants and their relation with plant traits. To conclude, in this research we were able to quantify eco-morphodynamic processes relevant for, but not limited to, gravel bed rivers and to shed light on their importance.