Sébastien Lévy: Analyse de l’évolution morphologique d’une partie du bassin versant de la rivière Chacoura, Québec, Canada.

Sébastien Lévy
Co-supervisors: Prof. Michel Jaboyedoff, Professeur Jacques Locat
Expert: Alexandre Loye

The large plains of Eastern Canada sensitive clays are cut by many rivers, in a way that their slopes have been and are still affected by landslides. They play a major role in the modelling of the landscape of these regions, in addition to induct a risk for human infrastructures. The role of erosion as a trigger of landslides is also important.

On the Chacoura River, north of Louiseville (Quebec), several large scars of landslides, more or less recent, are visible. A first inventory of landslides and areas of erosion was carried out in 1984 on some series of aerial photographs, covering a period from 1948 to 1979. It suggests that the majority of recorded landslides are located in areas that were previously affected by erosion. In this Master Thesis, a detailed analysis of aerial photographs, dating from 1948 to 1997 and a LiDAR-Digital Elevation Model (DEM) dating from 2007, has been done, using four different approaches: (1) a map of the phenomena on a portion of the river, which is considered as very active, was drawn by identifying various elements such as landslides, limits of the slope, position of the river, area covered by the forest and agricultural drainage structures. (2) A quantitative analysis based on the previous map has shown the temporal evolution of landslides in terms of area and volume, using statistical relationships. (3) The study focused on two flows, which happened in 1976 and 1990; it consisted of a detailed analysis of the evolution of their perimeter, area and the initial conditions of the slope. The comparison of DEMs was used to estimate volumes eroded in these flows, and in the entire zone of interest. (4) The last part focused on the general evolution of the river valley, and particularly the identification of areas where erosion could be more active in the future. This study was based on transverse and longitudinal profiles, and using the Sloping Local Base Level (SLBL) method.

The results show that the location of landslides is firmly linked to the presence of typical specific topographic features, such as (1) the shape of the meandering river, (2) the flow of agricultural drainage, or (3) the erosion at the toe of the slope. The study of landslides over a period of 59 years shows also that (4) the major landslide scars in this region are in fact the sum of several events of lesser importance. The analysis of the development of Chacoura River valley indicates that (5) it is still at an early stage of development. Besides (6) the lower part of the river seems to be relatively stable, however the upper part and the tributaries will probably face significant changes in the next 10,000 years. Finally hazard maps, based on the measured distances of flow retrogression have been created for the St-Léon-le-Grand region.