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

On the influence of fissure flow on the landslide activity

Water ManagementCivil Engineering and Geoscience

A model of hydrological and mechanical feedbacks of preferential fissure flow in a slow-moving landslide

The importance of hydrological processes for landslide activity is generally accepted. However, the relationship between precipitation, hydrological responses and movement is not straightforward. Groundwater recharge is mostly controlled by the hydrological material properties and the structure (e.g. layering, preferential flow paths such as fissures) of the unsaturated zone. In slow-moving landslides, differential displacements caused by the bedrock structure complicate the hydrological regime due to continuous opening and closing of the fissures, creating temporary preferential flow paths systems for infiltration and groundwater drainage. The consecutive opening and closing pf fissure aperture control the formation of a critical pore water pressure by creating dynamic preferential flow paths for infiltration and groundwater drainage. This interaction may explain the seasonal nature of the slow-moving landslide activity, including the often observed shifts and delays in hydrological responses when compared to timing, intensity and duration of precipitation. The main objective of this study is to model the influence of fissures on the hydrological dynamics of slow-moving landslide and the dynamic feedbacks between fissures, hydrology and slope stability. For this we adapt the spatially distributed hydrological and slope stability model (STARWARS) to account for geotechnical and hydrological feedbacks, linking between hydrological response of the landside and the dynamics of 20 the fissure network and applied the model to the hydrologically controlled Super-Sauze landslide (South French Alps).Water ManagementCivil Engineering and Geoscience

A conceptual model of the hydrological influence of fissures on landslide activity

Hydrological processes control the behaviour of many unstable slopes, and their importance for landslide activity is generally accepted. The presence of fissures influences the storage capacity of a soil and affects the infiltration processes of rainfall. The effectiveness of the fissure network depends upon fissure size, their spatial distribution, and connectivity. Moreover, fissure connectivity is a dynamic characteristic, depending on the degree of saturation of the medium. This research aims to investigate the influence of the fissure network on hydrological responses of a landslide. Special attention is given to spatial and temporal variations in fissure connectivity, which makes fissures act both as preferential flow paths for deep infiltration (disconnected fissures) and as lateral groundwater drains (connected fissures). To this end, the hydrological processes that control the exchange of water between the fissure network and the matrix have been included in a spatially distributed hydrological and slope stability model. The ensuing feedbacks in landslide hydrology were explored by running the model with one year of meteorological forcing. The effect of dynamic fissure connectivity was evaluated by comparing simulations with static fissure patterns to simulations in which these patterns change as a function of soil saturation. The results highlight that fissure connectivity and fissure permeability control the water distribution within landslides. Making the fissure connectivity function of soil moisture results in composite behaviour spanning the above end members and introduces stronger seasonality of the hydrological responses.Water ManagementCivil Engineering and Geoscience

Field investigation of preferential fissure flow paths with hydrochemical analysis of small-scale sprinkling experiments

The unsaturated zone largely controls groundwater recharge by buffering precipitation while at the same time providing preferential flow paths for infiltration. The importance of preferential flow on landslide hydrology is recognised in the literature; however, its monitoring and quantification remain difficult. This paper presents a combined hydrological and hydrochemical analysis of small-scale sprinkling experiments. It aims at showing the potential of such experiments for studying the spatial differences in dominant hydrological processes within a landslide. This methodology was tested in the highly heterogeneous black marls of the Super-Sauze landslide. The tests were performed in three areas characterised by different displacement rates, surface morphology and local hydrological conditions. Special attention was paid to testing the potential of small-scale sprinkling experiments for identifying and characterising preferential flow patterns and dominant hydrological processes.Water ManagementCivil Engineering and Geoscience

Feasibility of soil moisture estimation using passive distributed temperature sensing

Through its role in the energy and water balances at the land surface, soil moisture is a key state variable in surface hydrology and land?atmosphere interactions. Point observations of soil moisture are easy to make using established methods such as time domain reflectometry and gravimetric sampling. However, monitoring large?scale variability with these techniques is logistically and economically infeasible. Here passive soil distributed temperature sensing (DTS) will be introduced as an experimental method of measuring soil moisture on the basis of DTS. Several fiber?optic cables in a vertical profile are used as thermal sensors, measuring propagation of temperature changes due to the diurnal cycle. Current technology allows these cables to be in excess of 10 km in length, and DTS equipment allows measurement of temperatures every 1 m. The passive soil DTS concept is based on the fact that soil moisture influences soil thermal properties. Therefore, observing temperature dynamics can yield information on changes in soil moisture content. Results from this preliminary study demonstrate that passive soil DTS can detect changes in thermal properties. Deriving soil moisture is complicated by the uncertainty and nonuniqueness in the relationship between thermal conductivity and soil moisture. A numerical simulation indicates that the accuracy could be improved if the depth of the cables was known with greater certainty.Water ManagementCivil Engineering and Geoscience