The influence of fissures on landslide hydrology

Preferential flow occurs in many soils and it is recognized to influence soil moisture distribution and hydrological fluxes at different scales. Preferential flow paths are formed for example by soil fauna, by plant roots or soil erosion. Water plays an important role in mass movement processes: rainwater or snow melt infiltrates into the soil and recharges the groundwater system. The unsaturated zone controls groundwater recharge allowing for the loss of soil moisture by evaporation and attenuation of percolation towards the groundwater system. An increase in pore water pressure results in a decrease in an effective stress and internal strength of slopes. The preferential fluxes may change the spatial and temporal hydrological response of a landslide and influence intensity, duration and differentiation of mass movement. The quantification of groundwater recharge, especially by means of preferential flow, is a research challenge for an advanced understanding of hydrological systems in hillslopes and landslides. The main difficulties stem from heterogeneity of landslide lithology and spatial and temporal variations of hydraulic properties. The complexity of preferential fissure flow processes, and their high spatial and temporal variability, makes it very difficult to measure the processes in the field and to include them in hydrological modeling. This thesis focuses on preferential fissure flow, where fissure is defined as geo-mechanically induced cracks commonly present in slow-moving landslides, and their influence on landslide hydrological behaviour. Research work included both extended field measurements and hydrological modelling. All experiments described in this thesis were done at the Super-Sauze landslide: a persistently active clay shale landslide that covers 0.17 km2 of surface with the average slope of 25°. The landslide kinematics of the Super-Sauze is controlled by hydrology. The mass movement occurs as a consequence of the rise of groundwater table and hence the development of positive pore pressure. In order to monitor and quantify preferential flow processes on site two methodologies were proposed: Distributed Temperature Sensing (DTS) and combined hydrological and hydrochemical analysis of small-scale sprinkling tests. Both methodologies allowed for qualitative analysis of preferential flow patterns and showed the potential for quantification of dominant hydrological processes observed across the landslide: - qualitative analysis of measured soil temperature variation allowed observing spatial differences in soil moisture state and estimating the location of surface and subsurface water flow paths; - quantitative analysis of measured soil temperature made it possible to detect the spatial and temporal variations in apparent soil thermal conductivity and correlated them with measured soil moisture content; promising empirical relationships were obtained when accounting for local heterogeneities in soil characteristics; - analyses of small scale sprinkling experiments, combining the hydrological and hydrochemical analysis of two consecutive days of sprinkling, were able to capture the dominant hydrological process occurring in the area and show the potential for their quantification; based on the analysis of all available field data, conceptual models of the hydrological responses were proposed. The literature review and the analyses of the extensive field data sets consisting of day-to-day monitoring as well as sprinkling experiments, resulted in the formulation of a conceptual model of the hydrological influence of fissures on landslide activity. Special attention was given to spatial and temporal variation in fissures connectivity, which makes fissures act both as preferential flow paths for deep vertical infiltration and as lateral groundwater drains. These dynamics were included in a spatially distributed hydrological and slope stability model and applied to a ’simple’ landslide. The results highlight that fissure connectivity and fissure permeability play an important role in distributing water within a landslide. Making the fissures connectivity a function of soil moisture content resulted in a strong seasonality of the hydrological response on infiltrating rainwater or snowmelt: increased soil moisture content leads to more lateral water drainage through the fissures towards the lower part of the landslide, while decreased soil moisture content increases the water storage in the fissures. Furthermore, an analysis was made of all available field monitoring data of the Super-Sauze landslide. Hereafter, the distributed hydrological and slope stability model was applied to the Super-Sauze case study. The main objective was to model the influence of fissures on the hydrological behaviour of slow moving landslide and the dynamic feedbacks between fissures, hydrology and slope stability. In addition to hydrological feedback (fissure connectivity being the function of soil moisture content), the mechanical feedback was implemented as a relationship between fissure volume and level of landslide activity. Overall, from this research it can be concluded that preferential fissure flow may significantly influence the timing and duration of the periods of elevated pore pressure conditions in landslides depending on fissure network characteristics, especially fissure volume and connectivity between them. The field measurements outline the spatial heterogeneity of soil hydraulic properties and dominant hydrological processes existing in slow-moving clay shale landslides. The analyses of field data together with presented modelling results confirms the importance of distributed approaches when modelling differential hydrological response of complex heterogeneous landslides and stresses the need for including spatio-temporal changes in soil hydraulic properties of both fast (i.e. fissures) and slow (i.e. matrix) responding domain.Water ManagementCivil Engineering and Geoscience