The quantification of risk has gained importance in many disciplines, including landslide studies. The literature on landslide risk assessment illustrates the developments which have taken place in the last decade and that quantitative risk assessment is feasible for geotechnical engineering on a site investigation scale and the evaluation of linear features (e.g., pipelines, roads). However, the generation of quantitative risk zonation maps for regulatory and development planning by local authorities still seems a step too far, especially at medium scales (1:10,000–1:50,000). This paper reviews the problem of attempting to quantify landslide risk over larger areas, discussing a number of difficulties related to the generation of landslide inventory maps including information on date, type and volume of the landslide, the determination of its spatial and temporal probability, the modelling of runout and the assessment of landslide vulnerability. An overview of recent developments in the different approaches to landslide hazard and risk zonation at medium scales is given. The paper concludes with a number of new advances and challenges for the future, such as the use of very detailed topographic data, the generation of event-based landslide inventory maps, the use of these maps in spatial-temporal probabilistic modelling and the use of land use and climatic change scenarios in deterministic modelling.

https://www.geo.uzh.ch/~chuggel/riskcourse/vanwesten_landsl_hazard_risk_zonation_bege06.pdf

Landslide hazard and risk zonation—why is it still so difficult?

A view on some hydrological triggering systems in landslides

In this paper we review threshold behaviour in environmental systems, which are often associated with the onset of floods, contamination and erosion events, and other degenerative processes. Key objectives of this review are to a) suggest indicators for detecting threshold behavior, b) discuss their implications for predictability, c) distinguish different forms of threshold behavior and their underlying controls, and d) hypothesise on possible reasons for why threshold behaviour might occur. Threshold behaviour involves a fast qualitative change of either a single process or the response of a system. For elementary phenomena this switch occurs when boundary conditions (e.g., energy inputs) or system states as expressed by dimensionless quantities (e.g. the Reynolds number) exceed threshold values. Mixing, water movement or depletion of thermodynamic gradients becomes much more efficient as a result. Intermittency is a very good indicator for detecting event scale threshold behavior in hydrological systems. Predictability of intermittent processes/system responses is inherently low for combinations of systems states and/or boundary conditions that push the system close to a threshold. Post hoc identification of "cause-effect relations" to explain when the system became critical is inherently difficult because of our limited ability to perform observations under controlled identical experimental conditions. In this review, we distinguish three forms of threshold behavior. The first one is threshold behavior at the process level that is controlled by the interplay of local soil characteristics and states, vegetation and the rainfall forcing. Overland flow formation, particle detachment and preferential flow are examples of this. The second form of threshold behaviour is the response of systems of intermediate complexity – e.g., catchment runoff response and sediment yield – governed by the redistribution of water and sediments in space and time. These are controlled by the topological architecture of the catchments that interacts with system states and the boundary conditions. Crossing the response thresholds means to establish connectedness of surface or subsurface flow paths to the catchment outlet. Subsurface stormflow in humid areas, overland flow and erosion in semi-arid and arid areas are examples, and explain that crossing local process thresholds is necessary but not sufficient to trigger a system response threshold. The third form of threshold behaviour involves changes in the "architecture" of human geo-ecosystems, which experience various disturbances. As a result substantial change in hydrological functioning of a system is induced, when the disturbances exceed the resilience of the geo-ecosystem. We present examples from savannah ecosystems, humid agricultural systems, mining activities affecting rainfall runoff in forested areas, badlands formation in Spain, and the restoration of the Upper Rhine river basin as examples of this phenomenon. This functional threshold behaviour is most difficult to predict, since it requires extrapolations far away from our usual experience and the accounting of bidirectional feedbacks. However, it does not require the development of more complicated model, but on the contrary, only models with the right level of simplification, which we illustrate with an instructive example. Following Prigogine, who studied structure formation in open thermodynamic systems, we hypothesise that topological structures which control response thresholds in the landscape might be seen as dissipative structures, and the onset of threshold processes/response as a switch to more efficient ways of depleting strong gradients that develop in the case of extreme boundary conditions.

https://hess.copernicus.org/articles/13/1273/2009/hess-13-1273-2009.pdf

Threshold behaviour in hydrological systems as (human) geo-ecosystems: Manifestations, controls, implications

Once the instability process e.g. erosion or landslides has been identified on a slope, the type of vegetation to best reinforce the soil can then be determined. Plants improve slope stability through changes in mechanical and hydrological properties of the root-soil matrix. The architecture of a plants root system will influence strongly these reinforcing properties. We explain how root morphology and biomechanics changes between species. An overview of vegetation effects on slope hydrology is given, along with an update on the use of models to predict the influence of vegetation on mechanical and hydrological properties of soil on slopes. In conclusion, the optimal root system types for improving slope stability are suggested.

How vegetation reinforces soil on slopes

In the most destructive and catastrophic landslide events, rocks, soil and fluids can travel at speeds approaching several tens of metres per second. However, many landslides, commonly referred to as slow-moving landslides, creep at rates ranging from millimetres to several metres per year and can persist for years to decades. Although slow-moving landslides rarely claim lives, they can cause major damage to infrastructure and sometimes fail catastrophically, transitioning into fast-moving landslides that can result in thousands of casualties. In addition, slow-moving landslides are highly erosive features that control the landscape morphology in many mountainous regions (such as the California Coast Ranges or the Apennines). The persistent and long-term motion of slow-moving landslides provides an exceptional opportunity to investigate landslide processes and mechanisms. In this Review, we examine the environmental conditions (such as geology, climate and tectonics) of slow-moving-landslide-prone regions, analyse the forcings (for example, precipitation and groundwater, earthquakes, river erosion, anthropogenic forcings and external material supply) that drive their motion and investigate the subsequent implications of the different forcings on landslide dynamics. We then discuss circumstances in which slow-moving landslides can accelerate rapidly, move large distances or even fail catastrophically. Finally, we provide new perspectives and challenges for future landslide research. Some landslides creep at low velocities of a few millimetres to metres per year, forced by external factors such as rainfall, snowmelt, earthquakes or anthropogenic forcings. This Review discusses the recent advances in understanding slow-moving landslides and examines circumstances where slow-moving landslides can accelerate and fail catastrophically.

Life and death of slow-moving landslides

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

Many investigations show relationships between topographical factors and the spatial distribution of soil moisture in catchments. However, few quantitative analyses have been carried out to elucidate the role of different hydrological processes in the spatial distribution of topsoil moisture in catchments. A spatially distributed rainfall—runoff model was used to investigate contributions of subsurface matric flow, macropore flow and surface runoff to the spatial distribution of soil moisture in a cultivated catchment. The model results show that lateral subsurface flow in the soil matrix or in macropores has a minor effect on the spatial distribution of soil moisture. Only when a perched groundwater table is maintained long enough, which is only possible if the subsurface is completely impermeable, may a spatial distribution in moisture content occur along the slope. Surface runoff, producing accumulations of soil moisture in flat flow paths of agricultural origin (field boundaries), was demonstrated to cause significant spatial variations in soil moisture within a short period after rainfall (<2 days). When significant amounts of surface runoff are produced, wetter moisture conditions will be generated at locations with larger upstream contributing areas. Copyright © 2001 John Wiley & Sons, Ltd.

M.R. (Martin) Hendriks

Papers

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

The role of overland flow and subsurface flow on the spatial distribution of soil moisture in the topsoil