Vegetation root biomechanics and its role in river ecomorphodynamics

Abstract

Successful establishment of riparian vegetation on riverbanks and bedforms depends on river hydrology and related flow and sediment erosion processes. Extreme flow-induced erosion events can uproot vegetation, leading in some cases to failure of bank protection and river restoration schemes. This thesis uses experimental, analytical, and numerical approaches to examine key aspects of the mechanisms of vegetation uprooting by flow. First, the ability of riparian vegetation to respond to different water table regimes is investigated in terms of root growth and resistance. To this purpose, small-scale Salix cuttings were allowed to grow under different water level regimes. At the end of the growing period, extracted samples, obtained through pullout tests, were analysed in terms of root biomass distribution and resistance to external forces. The results demonstrate the driving influence of water and oxygen availability on the vertical configuration of below-ground biomass and thence on uprooting resistance. Second, a free-body model is derived to predict the critical rooting length – a key parameter that determines the probability of flow-induced uprooting of flexible plants at different erosion stages. Model validation is achieved using laboratory and field-scale data. Third, the dynamics of mobilization of stranded living wood logs from alluvial bedforms is investigated experimentally. Pullout test results are used to assess the root resistance of small-scale wood logs at several stages of growth. Trends in below- and above-ground biomass, together with the free-body model, enable detection of ‘biological time windows’ within which re-mobilization becomes possible. The results illustrate that uprooting occurs within two time-lapses, which coincide with particular growth stages of the plant. Finally, a combined analytical and numerical model is derived. This model uses the probability of flow-induced plant uprooting as a proxy to study how perturbations to the natural flow regime may drive riparian ecosystem dynamics towards new and potentially irreversible statistical equilibrium states. The model is applied to an actual case study, in which dam impoundment of a reach of the Maggia River, Switzerland, has led to intense riparian vegetation encroachment with consequent river narrowing. The output of the model sheds light on the type of irreversibility that may arise in riverine ecosystems of severely impounded river basins. The theoretical and experimental results presented in the thesis should be useful to river engineers and managers responsible for river restoration projects, natural flood management schemes, and optimal dam regulation strategies.