Th.W.J. van Asch

A view on some hydrological triggering systems in landslides

Abstract In this paper different types of hydrological triggering systems for debris flows, shallow and deeper landslides are described. The generation of surface run-off and high peak discharges in first order alpine catchments is an important triggering mechanism for debris flows. Failure conditions in shallow landslides can occur when at a critical depth, which is determined by the cohesion of the soil and the slope angle, the moisture content in the soil becomes close to saturation, resulting in a considerable reduction of soil strength. Deeper landslides (5–20 m depth) are in most cases triggered by positive pore pressures on the slip plane induced by a rising ground water level. The assessment of meteorological threshold conditions for shallow landslides (1–2 m) needs more detailed meteorological information than for deeper landslides. In the analyses of the hydrological triggering systems of deeper landslides the presence of a permeable top layer and fissures has to be taken into consideration.

Regional Assessment of the Effects of Land-Use Change on Landslide Hazard By Means of Physically Based Modelling

Physically based models are capable of evaluating the effects of environmental changes through adaptations in their parameters. For landslide hazard zonation, this gives them an edge over traditional, statistically based techniques that require large datasets and often lack the objectivity to achieve the same purpose. Therefore, physical models can be valuable tools for hazard assessment and planning purposes.The usefulness of the model prognosis depends largely on the ability of the physical model to mimic the landscape system. This implies that the model should be calibrated and validated and that the imposed changes do not lead to a radical departure from the present situation.Under the recognition of these constraints, a physically based model has been applied to a 1.5 km 2 catchment in the Alcoy region (SE Spain) to evaluate the effects of land use change on landslide activity. The model couples a transient, distributed hydrological model with a probabilistic assessment of the slope stability. Thus, it is able to assess the spatial and temporal activity of slope instability. For the present situation, validation demonstrates that the probability of failure returns a conservative estimate of the spatial frequency of landsliding. The model has subsequently been applied to two hypothetical land use change scenarios that extrapolate present and likely trends. For these scenarios, the model results indicate a marginal decrease in the spatial frequency of landsliding (aerial extent of instability). However, the decrease in the temporal activity (is total duration of instability over a given period) is substantial under the altered land use conditions. The forecasted change in landslide activity not only affects the relative weight of slope processes in the region. It also has implications for the perceived hazard levels and the landslide hazard zonation of the area. Copyright Kluwer Academic Publishers 2004

Geophysical, geotechnical and hydrological investigations of a small landslide in the French Alps

Abstract Geophysical, geotechnical and hydrological surveys were conducted in the French Alps on a small landslide in black marl material to assess the stability. The geophysical survey, consisting of electromagnetic, geo-electric and relative differences in water content. The data on the basal slip surface, together with geotechnical parameters (cohesion and angle of internal friction), were used to make a stability analysis. This analysis revealed that a groundwater level of 4 m below the ground surface is the critical threshold for reactivating the landslide. The hydrological investigation, which entailed measurements of saturated permeability and pressure heads, indicated that instability depended on long-term wet conditions with little evapotranspiration.

Compaction of loamy soils due to tractor traffic in vineyards and orchards and its effect on infiltration in southern France

Viticulture and fruit culture in Mediterranean areas demand frequent tractor traffic in vineyards and orchards for tillage and for the application of herbicides and pesticides, resulting in soil compaction. The aim of this study was to investigate the extent of soil compaction and its effect on infiltration in vineyards and orchards in an area in southern France, known for its wine and fruit production (Vaucluse). Compaction of both the topsoil and the subsoil was demonstrated with measurements of bulk density, penetration resistance and water retention characteristics. Subsoil compaction was attributed to wheel load, not to tillage, and was alleviated within 5 years after termination of tillage operations in vineyards. No effects of topsoil compaction on infiltration were expected on account of the slight differences in the values of infiltration parameters between wheel tracks and inter-rill areas. Effects of subsoil compaction on infiltration were examined with rainfall simulation tests. Under wet initial conditions and high rain intensities, no effect of soil compaction on infiltration was observed. This implies that the frequent tractor traffic associated with viticulture and fruit culture does not enhance run-off on loamy soils in Mediterranean areas.

Hydrological triggering conditions of landslides in varved clays in the French Alps

Abstract In this paper the meteorological and hydrological conditions are analyzed which trigger shallow and deeper landslides in glacio-lacustrine deposits (varved or laminated clays) in the French Alps. The hydrological system of these landslides consists of a colluvial cover which supplies water into the fissures of the underlying varved clays. From these fissures water can infiltrate more or less horizontally into the silt layers of the varved clays. A combined reservoirs model was used to simulate the water fluctuations in the colluvial cover and the fissures. Both the water level in the fissures and the residence time of water in the fissures are mainly controlled by the amount of water storage in the colluvial cover. Simulations over the last 25 years show that almost each year the fissures are completely filled with water for several months a year. Infiltration experiments in the field show that infiltration into the varved clays occurs mainly by horizontal infiltration into the silt laminae. Calculated infiltration rates from these fissures into the silt layers show that the mean yearly residence time of water in the fissures is not sufficient to fully saturate the clay mass each year. It is therefore concluded that the triggering of the landslide movements is mainly controlled by the development of positive pore water pressures in the fissure system and that the rise of pore water pressures induced by the matrix system of the varved clays only plays a minor role. The calculations also show that drainage of the colluvial cover is a very efficient measure to stabilize the deeper landslides.

Temporal patterns of mass movements in the French Alps

Abstract In massmovement hazard assessment it is important to include a temporal dimension in order to give a better insight into the activity of landslides. In the French Alps the temporal pattern of active landslides in varved clays and terres noires (marls) was studied as well as the temporal occurrence of debris flows. The active landslides in varved clays and terres noires show, in detail quite different patterns of movement which is ascribed to the different hydrological conditions in the landslide bodies. The activity of debris flows is controlled by the occurrence of exceptionally high-intensity rainstorms. This means that the temporal pattern of debris-flow activity will reflect the variability of storm rainfall activity. As a first approach an average recurrence interval of some ten years was detected based on dendrochronological and lichenometrical evidence.

The slope movements within the Mondorès graben (Drôme, France); the interaction between geology, hydrology and typology

The Mondores graben in the south-eastern French Alps is an uncommon structural feature, which originates in a complex polyphasic tectonic evolution. In contrast with its immediate surroundings, with hardly any huge landslides, the Mondores graben is characterised by various types of landslides. A huge sagging caused part of the limestone cliff to subside some 50 m within 50 years. Two recent mud flows that occurred were considered a potential threat to some inhabited places downstream. The hydrologic aspects of the Boulc-Mondores landslide resemble the geology: infiltration (and karst input?) in the rock-sliding zone and exfiltration in the marls resulting in slumps and mud flows. The mass movement interactions could be explained by a structural geology analysis with geodetic monitoring using different techniques. It is also shown that hydrochemistry as well as geophysical surveys are of importance in unravelling the hydrologic systems and the geological subsurface structures. The present paper aims at explaining the geological control of the different slides in view of estimating their potential danger. Understanding the geological structure and its evolution therefore is a necessary prerequisite.

An integrated model to assess critical rainfall thresholds for run-out distances of debris flows

AbstractA dramatic increase in debris flows occurred in the years after the 2008 Wenchuan earthquake in SW China due to the deposition of loose co-seismic landslide material. This paper proposes a preliminary integrated model, which describes the relationship between rain input and debris flow run-out in order to establish critical rain thresholds for mobilizing enough debris volume to reach the basin outlet. The model integrates in a simple way rainfall, surface runoff, and concentrated erosion of the loose material deposited in channels, propagation, and deposition of flow material. The model could be calibrated on total volumes of debris flow materials deposited at the outlet of the Shuida catchment during two successive rain events which occurred in August 2011. The calibrated model was used to construct critical rainfall intensity-duration graphs defining thresholds for a run-out distance until the outlet of the catchment. Model simulations show that threshold values increase after successive rain events due to a decrease in erodible material. The constructed rainfall intensity-duration threshold graphs for the Shuida catchment based on the current situation appeared to have basically the same exponential value as a threshold graph for debris flow occurrences, constructed for the Wenjia catchment on the basis of 5 observed triggering rain events. This may indicate that the triggering mechanism by intensive run-off erosion in channels in this catchment is the same. The model did not account for a supply of extra loose material by landslips transforming into debris flow or reaching the channels for transportation by run-off. In August 2012, two severe rain events were measured in the Shuida catchment, which did not produce debris flows. This could be confirmed by the threshold diagram constructed by the model.

Rainfall-triggering response patterns of post-seismic debris flows in the Wenchuan earthquake area

AbstractSeveral giant debris flows occurred in southwestern China after the Wenchuan earthquake, causing serious casualties and economic losses. Debris flows were frequently triggered after the earthquake. A relatively accurate prediction of these post-seismic debris flows can help to reduce the consequent damages. Existing debris flow prediction is almost based on the study of the relationship between post-earthquake debris flows and rainfall. The relationship between the occurrence of post-seismic debris flows and characteristic rainfall patterns was studied in this paper. Fourteen rainfall events related to debris flows that occurred in four watersheds in the Wenchuan earthquake area were collected. By analyzing the rainfall data, characteristics of rainfall events that triggered debris flows after the earthquake were obtained. Both the critical maximum rainfall intensity and average rainfall intensity increased with the time. To describe the critical conditions for debris flow initiation, intensity–duration curves were constructed, which shows how the threshold for triggering debris flows increased each year. The time that the critical rainfall intensities of debris flow occurrences return to the value prior to the earthquake could not be estimated due to the absent rainfall data before the earthquake. Rainfall-triggering response patterns could be distinguished for rainfall-induced debris flows. The critical rainfall patterns related to debris flows could be divided on the basis of antecedent rainfall duration and intensity into three categories: (1) a rapid triggering response pattern, (2) an intermediate triggering response pattern, and (3) a slow triggering response pattern. The triggering response patterns are closely related to the initiation mechanisms of post-earthquake debris flows. The main difference in initiation mechanisms and difference in triggering patterns by rainfall is regulated by the infiltration process and determined by a number of parameters, such as hydro-mechanical soil characteristics, the thickness of the soil, and the slope gradient. In case of a rapid triggering response rainfall pattern, the hydraulic conductivity and initial moisture content are the main impact factors. Runoff erosion and rapid loading of solid material is the dominant process. In case of a rainfall pattern with a slow triggering response, the thickness and strength of the soil, high hydraulic conductivity, and rainfall intensity are the impact factors. Probably slope failure is the most dominant process initiating debris flows. In case of an intermediate triggering response pattern, both debris flow initiation mechanisms (runoff erosion and slope failure) can play a role.

Landslides: The deduction of strength parameters of materials from equilibrium analysis

Summary Rotational landslides (slumps) varying in size can develop at different slope angles in the same type of cohesive regolith material. Therefore it is possible to set up independent equilibrium equations for these rotational slides of different size. In this way, a good estimate of the mean strength parameters of a certain type of material can be obtained on the basis of a number of these independent equilibrium equations. This can be done by means of regression analyses and the correlation coefficients and variances give a good indication of the reliability of the estimate of the strength parameters. In this paper the strength parameters of two regolith materials on schist and claystone are determined in this way and compared with the strength parameters measured in the laboratory.