Numerical modelling of the impact of climate change on the morphology of Saint-Lawrence tributaries

This thesis investigates the impacts of climate-induced changes in discharge and base level on the morphology of Saint-Lawrence River tributaries for the period 2010--2099. The selected tributaries (Batiscan, Richelieu, Saint-Maurice, Saint-Francois and Yamachiche rivers) were chosen because of their differences in size, flow regime and morphological setting. Not only will these tributaries experience an altered hydrological regime as a consequence of climate change, but their base level (Saint-Lawrence River water level) will also change. A one-dimensional (1D) morphodynamic model (SEDROUT), originally developed for aggrading gravel-bed rivers, was adapted for the specific context of the Saint-Lawrence lowland tributaries in order to simulate sand-bed rivers with variable daily discharge and downstream-water level fluctuations. A module to deal with sediment routing in channels with islands was also added to the model. The enhanced model (SEDROUT4-M), which was tested with small-scale simulations and present-day conditions in four tributaries of the Saint-Lawrence River, can now simulate a very wide range of river-morphodynamic problems. Changes in bed elevation and bed-material delivery to the Saint-Lawrence River over the 2010--2099 period were simulated with SEDROUT4-M for the Batiscan, Richelieu and Saint-Francois rivers for all combinations of seven tributary hydrological regimes (present-day and those predicted using three global climate models (GCM) and two greenhouse gas emission scenarios) and three scenarios of how the base level provided by the Saint-Lawrence River will alter (no change, gradual decrease, step decrease). The effects on mean annual sediment delivery and bed elevation differ between GCM and seem to be related to whether the river is currently aggrading, degrading or in equilibrium, which highlights the importance of investigating several rivers using several climate models in order to determine trends in climate change impacts. Despite the fact that mean daily discharge and mean annual maximum discharge remain close to their current values in the three GCM scenarios for daily discharge, marked changes occur in the mean annual sediment transport rates in each simulated tributary. This is due to the important effect of more frequent large individual flood events under future climate as well as a shift of peak annual discharge from the spring towards the winter, which results in increased variability of bed-material transport rates. Some complications with the 1D modelling approach to capture the complex geometry of the Saint-Maurice and Saint-Francois rivers suggest that the use of a two-dimensional (2D) approach should be seriously considered to accurately simulate the discharge distribution at bifurcations around islands. The Saint-Francois River is used as a test case for the 2D model H2D2, which performs well from a hydraulics point of view but which needs to be adapted to fully simulate morphological adjustments in the channel.;Keywords. morphodynamic model, Saint-Lawrence River, climate change, bed-material transport, base level, flood risk, effective discharge, recurrence interval, half-load discharge.