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SIXTH FRAMEWORK PROGRAMME, SCIENTIFIC SUPPORT TO POLICIES

Project Report 2.2

Deliverable 2.3.2.4

RISK ASSESSMENT METHODS OF LANDSLIDES

J.P. Malet and O. Maquaire

Risk Assessment Methodologies for Soil ThreatsRisk Assessment Methodologies for Soil ThreatsRisk Assessment Methodologies for Soil Threats

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Abstract Landslides in Europe mainly occur in Alpine regions and are related to the erosion-prone areas. Landslides in topsoils, also referred to as ‘Blaiken’ or shallow landslides (Maquaire & Malet, 2006), are caused by a set of preparatory and triggering factors which determine their location, their frequency and their magnitude. Apart from relief, geology, hydrology and climate, the geomorphological changes in connection with land use, as well as vegetation and soil changes may influence landslides in cultural landscapes. Effect of forest vegetation (and particularly the role of the architecture, morphology and properties of roots) on soil fixation, erosion, and water-flow has been measured, modelled and integrated in DSS in EU-funded projects recently (Tasser, 2003). Our limited understanding of the variability of the causes and consequences of landslides and their transient characteristics limits our ability to forecast shallow landslides with mathematical, process-based, models. Consequently, available landslide RAMs are diverse and combine expert judgment, empirical approaches and to a lesser extent mathematical simulations. A trend in harmonizing procedures and proposing standards is nevertheless gaining ground since the execution of various EU-funded projects, though differences in terminology often hamper exchanges of information. The following study initiated within the framework of the RAMSOIL project concerns the landslide threat which is one of the degradation processes of the soils.

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Policy Summary A first analysis of Risk Assessment Methodologies (RAM) for landslides over the EU25 and some other European countries reveals that some countries do not still possess a RAM officially recognized, either because it is in the course of completion, or because the phenomena, when observed in the country, do not engender many damage to conduct to the establishment of risk analysis studies. For example, Denmark has been exposed to some mudslides and Netherlands experienced phenomena of shrinking/swelling following mining activities and collapses in polders, but these two countries do not have any official RAM for landslides. Legislation relating to hazard and risk prevention has often developed after the occurrence of large disasters, like in France after the Plateau d’Assy catastrophe in the 1970s or in Italy after the Sarno-Quindici disaster in 1998. In European countries, the legislative output corresponds to an increasing involvement of the governmental authorities in the face of the growing social demand for security. The state is more or less directly involved in the management of the risks. For instance, In France which is a a centrally-run country, the government agencies decide where the hazardous zones are and what must be done in terms of risk zoning; at the opposite, in federal countries, like Italy, Switzerland and Spain, the national government defines the general framework of risk assessment, but the regional and municipal levels are the main actors (Bonnard et al., 2005). Figure 1: Stage of development of landslide Risk Assessment Methodologies (RAMs) in the European

Union

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Contents Glossary of terms used in landslide Risk Assessment Methodologies ....Error! Bookmark not defined. Abstract.................................................................................................... 1 Policy Summary........................................................................................... 3 Contents ................................................................................................... 4 Introduction: Landslide causes and landslide impacts .......................................... 5 1. Landslide Risk Assessment Methodologies ................................................... 7 1.1. General framework of a landslide rsk assessment:...................................... 7 1.1.1. Hazard analysis ..................................................................................... 8 1.1.2. Consequence analysis.............................................................................. 8 1.1.3. Risk analysis......................................................................................... 9 1.1.4. Risk assessment..................................................................................... 9

1.2. Overview of landslide RAMs in EU25 member state .................................... 10 1.2.1. Status of RAMs over the EU25 ...................................................................11 1.2.2. Historical development of RAMs ................................................................19

1.3. Detailled description of Official RAMs.................................................... 20 1.3.1. Description of the French RAM..................................................................20 1.3.2. Description of the Swiss RAM....................................................................21 1.3.3. Description of the Swedish RAM ................................................................22 1.3.4. Description of the Italian RAM ..................................................................23

2. Comparison of the official RAMs ..............................................................24 2.1. Methodology: indicators and spider graphs .............................................. 24 2.2. Comparison ................................................................................... 25 2.3. Conclusions ................................................................................... 26

3. Options of harmonization ......................................................................26 3.1. Regional differentiation .................................................................... 26 3.2. Current differentiation within RAMs ...................................................... 27 3.3. Options for harmonization.................................................................. 27

Conclusion ............................................................................................... 27 References ............................................................................................... 27 Figures and tables.................................................... Error! Bookmark not defined.

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Introduction: landslide causes and landslide impacts Landslides are classified according to their mechanisms (movement types) and the nature of the displaced material (material type), as well as information on their activity (state, distribution, style), ie. the rate of development over a period of time (Varnes, 1978; Dikau et al. 1996; Cruden and Varnes 1996). Five principal types of movements are distinguished according to the geomorphological classification proposed by Cruden and Varnes (1996) and Dikau et al. (1996).

� Fall A slope of movement for which the mass in motion travels most of the distance through the air, and includes free fall movement by leaps and bounds and rolling of fragments of material. A fall starts with the detachment of material from a steep slope along a surface in which little or no shear displacement takes place.

� Topple A slope movement that occurs due to forces that cause an over-turning moment about a pivot point below the centre of gravity of the slope. A topple is very similar to a fall in many aspects, but do not involve a complete separation at the base of the failure

� Lateral spreading A slope movement characterized by the lateral extension of a more rigid mass over a deforming one of softer underlying material in which the controlling basal shear surface is often not well-defined.

� Slide A slope movement by which the material is displaced more or less coherently along a recognisable or less well-defined shear surface or band. Slide could be rotational (the sliding surface is curved) or translational (the sliding surface is more or less straight). In some cases a slide can change into a mudslide or slump-earthflow, especially on steep slopes, in highly tectonized clays or silty formations (Picarelli, 2001).

A: Rotational slide: more or less rotational movement, about an axis that is parallel to the slope contours, involving shear displacement (sliding) along a concavely upward-curving failure surface, which is visible or may reasonably be inferred’ (Varnes, 1978). B: Translational slide: The material displaces along a planar or undulating surface of rupture, sliding out over the original ground surface.

� Flows A slope of movement characterized by internal differential movements that are distributed throughout the mass and in wich the individual practicles travel separately whithin the mass. Debris flow and debris avalanche: Debris flow is a very rapid to extremely rapid flow (> 1 m.s-1) of saturated non-plastic debris in a steep channel. Characteristic of a debris flow of a debris flow is the presence of an established channel or regular confined path, unlike debris avalanches which are thin, partly or totally saturated and which occur on hillslopes (Hungr et al. 2001). These five types may sometimes be combined or may succeed each other, forming a sixth type: a composite and complex movement, which consists of more than one type (e.g. a rotational-translational slide) or those where one type of failure develops into a second type (e.g. slump-earthflow).

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The shape and size of slope movements vary because of the combination of several preparatory and triggering factors (dissolution, deformation and rupture by a static or dynamic load). They may be controlled by the topography (inclination and shape of the slope), the lithology (physical and geomechanical characteristics), the geological structure (dip, fault, discontinuity), the hillslope hydrology (pore pressures, water contents) or a combination of all these factors. Water intervenes in several ways. First, it acts by changing the state or consistency of materials (transition from a solid to a plastic and finally to a liquid state) (Coussot and Meunier, 1996); secondly, it lightens the terrain due to Archimedes’ thrust (Terzaghi’s law) in relation to water tables increases. The consequence for the slopes is a reduction in the inter-particle forces and the associated friction throughout the length of the rupture surfaces. Finally water acts as an agent in the transport of materials. A failure occurs when the disturbing forces that create movement exceed the resisting forces of the material. Triggering factors may either increase the shear stress, decrease the shearing resistance of the material or both. Table 1 summarizes the most common triggering factors of slope movements. Table 1: Common triggering factors of slope movements (in Van Asch et al., 2007)

Increase in shear stress

� Erosion and excavation at the toe of the slope � Subterranean erosion (piping) � Surcharging and loading at the crest (by deposition or sedimentation � Rapid drawdown (man-made reservoir, flood high tide, breaching of natural

dams) � Earthquake � Volcanic eruption � Modification of slope geometry � Fall of material (rock and debris)

Decrease in shearing resistance

• Water infiltration (rainfall, snowmelt, irrigation, leakage of drainage systems) • Weathering (freeze and thaw weathering, shrink and swell weathering of

expansive soils) • Physico-chemical changes • Fatigue due to static/cyclic loading and creep • Vegetation removal(by erosion, forest fire, drought or deforestation) • Thawing of frozen soils

Possible increase in shear stress and decrease in shearing resistance

• Earthquake shaking • Artificial vibration (including traffic, pile driving, heavy machinery) • Mining and quarrying (open pits, underground galleries) • Swinging of trees

Landslides are characterized by their spatial and temporal occurrence and by their intensity. Intensity can be defined by the volume of the displaced material (in link with landslide depth) and by the velocity of the movement. The intensity a possible slope movement is difficult to foresee as it depends on the magnitude of the triggering event and the environmental conditions (e.g. height of water table) at the onset of the event. The impacts of a landslide increase significantly with the velocity and the travel (or run-out) distance. The slow and progressive movements do not generally present risk for the human lives, except in the case of crises and potential fluidization of the landslide mass, but have large impacts on the infrastructures (buildings, roads, etc...). Populations are more vulnerable to sudden, rapid and intermittent landslides (such as mudflows, debris flows and debris avalanches) but the victims still remain rare. Impacts of landslide events are diverse according to the type of movement. A

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building can face several types of solicitations of different magnitudes; estimating its potential damage and its vulnerability with engineering vulnerability functions is therefore a complex task difficult to apply in practice and necessitating detailed engineering databases (Léone et al., 1996). These damages have a direct cost due to the repairs or to the maintenance of the works, generally supported by the local authorities and the State, and an indirect cost linked to the disturbance of the socio-economic activities (Ministère de l’Ecologie et du Développement Durable, 2004).

Figure 2: Types of slope movements -fall, slide, flow- and associated major type of impact and solicitation on an element at risk (modified from Léone et al., 1996).

1. Landslide Risk Assessment Methodologies

1.1. General framework of a landslide risk assessment Landslide risk evaluation aims to determine the « expected degree of loss due to a landslide (specific risk) and the expected number of live lost, people injured, damage to property and disruption of economic activity (total risk) » (Varnes et al., 1984). So this work begins with the identification and the description of the threat, to succeed towards an evaluation of the claimed exposure and a characterization of the risk. The hazard analysis is based on four fundamental assumptions: a/ Landslides will always occur in the same geological, geomorphological, hydrogeological and climatic conditions as in the past; b/ the main conditions that cause landsliding are controlled by identifiable physical factors; c/ the degree of hazard can be evaluated; d/ all types of slope failures can be identified and classified (Varnes and IAEG 1984; Hutchinson 1995). Generally, landslide risk assessments are carried out using the risk based framework described in Figure 3. The risk assessment begins with analysis of hazard and consequences. Then the process of establishing a measure of risk is referred to risk estimation. Risk assessment ends by risk evaluation where levels of risk will determine prevention measures.

Figure 3: Framework for landslide quantitative risk assessment (Fell et al., 2005)

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1.1.1. Hazard analysis Hazard analysis consists in identifying the landslide mechanisms and in quantifying their corresponding spatial and temporal occurrence (in terms of probability) and their intensity (Fell et al., 2005). To assess landslide hazard several approaches have been developed worldwide (Figure 4), from expert analysis to heuristic, statistical or process-based methods (Glade & Crozier, 2004). Each method possesses its advantages and disadvantages. The choice of a method depends on the objective of the study, the scale of the study and the data available. A detailed overview of the exsting methods can be found in Aleotti & Chowdhury (1999) and Glade and Crozier (2004).

Figure 4: Proposed classification of landslide hazard assessment methods (Aleotti & Chowdhury, 1999)

1.1.2. Consequence analysis Consequence analysis includes identifying and quantifying the element at risk (property, persons, environmental assets, etc), and their vulnerability (conditional probability of damage to property,

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or conditional probability of loss of life and injury; (Fell et al., 2005). It may consist in an inventory of the main assets, to a descriptive or numeric rating of the elements, to the quantification of vulnerability curves.

1.1.3. Risk analysis Risk analysis includes the cross-correlation of the hazard analysis and the consequence analysis. It consists in establishing the probability of occurrence of a catastrophic event, e.g., the probability of live losses, or the probability of a landslide causing N or more casualties (Fell and Hartford, 1997). Several qualitative or quantitative approaches may be used according to the scale of the study, the data available, the experience of the specialist, and the scope and purpose of the hazard and risk assessment (Chowdhury, Flentje, Ko Ko, 2001, Fell et al., 2005). Risk analysis has been introduced by Leroi (1996) as a problem at different scales, with each scale having a well defined meaning and aim (Figure 5).

Figure 5: Landslide risk analysis and mapping at different scales For regional studies, approaches and methods may be largely based on remote-sensing including satellite imagery and aerial photographs. For more detailed studies, use should be made of local knowledge and databases concerning relevant parameters and elements at risk. The approach adopted for urban areas may be qualitative or semi-quantitative. A fully quantitative approach can be attempted but there are limitations due to lack of extensive and accurate data and also due to spatial and temporal variability of factors and parameters and due to other significant uncertainties (Chowdhury et al., 2001).

1.1.4. Risk assessment Landslide risk assessment is the process of making a decision recommendation on whether existing risks are tolerable and present risk control measures are adequate, and if not, whether alternative risk control measures are justified or will be implemented. Risk assessment incorporates the risk analysis and risk evaluation phases. Risk evaluation is the stage at which values and judgement enter the decision process, explicitly, by including consideration of the importance of the estimated risks and the associated social, environmental, and economic consequences, in order to identify a range of alternatives for managing the risks. Risk assessment takes the output from risk analysis and assesses these against values judgements, and risk acceptance criteria (Fell et al., 2005).

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Since the 1970’s risk assessment and management principles, in a qualitative manner, have been practised for landslide hazard zoning for urban planning and highway slope management. In the 1980’s and particularly in 1990’s these have been extended to quantitative methods, and to management of individual slopes, pipeline routes, submarine slopes and more global slope risk management. These developments are described by Varnes (1984), Whitman (1984), Einstein (1988, 1997), Fell (1994), Leroi (1996), Wu et al. (1996), Fell and Hartford (1997), Nadim and Lacasse (1999), Ho et al. (2000), Kvalstad et al. (2001), Nadim et al. (2003), Nadim and Lacasse (2003, 2004), Hartford and Baecher (2004), and Lee and Jones (2004).

1.2. Overview of landslide RAMs in the EU25 member states

A questionnaire has been send to several research institutes and private engineering companies within the EU25 member states in order to proceed to an inventory of methods and data used for assessing landslide risks. The aim of the questionnaire is to examine the current situation of methodologies for landslide risk assessments and to assess its pros and cons, in order to unravel for what reasons an EU member state uses a specific kind of risk assessment methodology. The questionnaire consisted in 27 questions addressing the 4 following items:

- General information on risk assessment: the interest was to obtain general information on methodologies to identify the state of development of RAMs, particularly through the legislative texts, and the bodies linked to the establishment of the methodology.

- Data on hazard and risk: this item allowed to estimate level of accuracy and reproducibility of the RAM according to the scale of the input and output documents and the variety of data used in the analysis.

- Description of the RAM: this item allowed to describe the pros and cons of each RAM and the approach used in the analysis (heuristic, statistical, deterministic, etc).

- Outputs documents of the RAM: this item allowed to characterize the information contained in the output documents and identify the priorities along the process of risk assessment.

Most of the contacts were members or affiliated members of CERG (European Centre on Geomorphological Hazards), one of the 22 research centres of the EUR-OPA Major Hazard Agreement of the Council of Europe. As contacts were available in other countries than the EU25 member states, the questionnaire has been sent o them. Switzerland has also been added in the study because of its pioneering work in the domain of landslide risk assessment. Over 72 questionnaires sent, 30 questionnaires were fulfilled. Finland, Luxembourg, Latvia and Estonia did not answer the questionnaire, but documentary researches revealed that Latvia and Estonia do not possess a landslide RAM (Jelinek et al., 2006). The answers to the questionnaire have been coded in an Excel spreadsheet.

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Countries Official RAM

Official RAM in development

RAM used by an institute

No RAM No answer

Austria × Belgium × Cyprus ×

Czech Republic × Denmark × Estonia × Finland × France × Germany × Greece × Hungaria × Ireland × Italy × Latvia × Lituania ×

Luxembourg × Malta ×

The Netherlands × Poland × Portugal × Slovakia × Slovenia × Spain × Sweden ×

Switzerland × United-Kingdom ×

Table 2: Summary of stage of development of landslide RAM in EU25

1.2.1. Status of RAMs over the EU25 member state

The analysis of the questionnaires and of reference documents (research papers, reports, etc) provide information on landslide RAMs for 23 countries of the EU25 + Switzerland:

- 4 countries have official landslide RAMs: France, Italy, Sweden, and Switzerland. - 9 countries have official RAMs in development: Belgium, Cyprus, Czech Republic, Hungaria,

Ireland, Slovakia, Slovenia, Spain, and United-Kingdom. - 4 countries have RAMs used by research institutes or private engineering offices: Germany,

Greece, Poland, and Portugal. - 7 countries have no specific landslide RAM: Austria, Denmark, Germany, Latvia, Lituania,

Malta and The Netherlands. Some countries have already established laws about natural risks detailing the general framework of risk policy, but the instruments of risk assessments (methodologies, maps, etc) is not yet defined or is in development. The case in some countries is also complex, such as for instance in Spain, where a RAM is being developed but not for all the regions. In the Basque Region or in Andorra, Risk Plans exist but the extension of the procedure at the whole country is still in development. The figures below summarize the main characteristics of the RAMs (official RAM, official RAM in development, RAM used by research institutes or private engineering offices) existing in the EU25 member state.

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* Countries with an official RAM used in practice

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* Countries with a RAM in development

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* Countries with RAMs used by research institutes or private engineering offices

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1.2.2. Historical development of RAMs

The development of landslide RAMs followed always the occurrence of large landslide catastrophes:

- In France, in 1970, the mudslide of Plateau d’Assy caused the death of 40 persons in Haute-Savoie (the most dramatic case of the century), strengthening in France the preoccupations in this domain and bringing the establishment of the first ZERMOS maps (Zones Exposées à des Risques liés aux MOuvements du Sol et du sous-sol; Besson, 2005).

- In Italy, the catastrophic event of May 1998, which caused very large damage and deaths in the municipalities of Sarno and Quindici (Campania) urged the government to provide answers for development regulation. According to a decree named “the Sarno Decree” the government detailed legislative measures at the national level, including the procedure to define landslide risk areas (Bonnard et al., 2005).

- In Switzerland, the flooding of 1987 obliged the federal authorities to update the criteria governing natural hazard protection. The “Federal Flood Protection Law” and the “Federal Forest Law” came into force in 1991. Their purpose is to protect the environment, human lives and property from the damage caused by water, mass movements, snow avalanches and forest fires (Raetzo et al., 2002).

- Sweden also knew some large catastrophic landslides n the 1970s which have influenced the development of land use planning regulations. The Tuve landslide of 30 November 1977 engendered the death of 9 persons and 65 houses were ruined, 500 persons became homeless and fatalities occurred. Subsequent to this landslide the Ministry of Housing and Physical Planning commissioned the Swedish Geological Institute to carry out a survey of the unstable slopes. The government decided also that municipalities should be mapped generally regarding the presence of unstable slopes in built-up areas (Edwards, 2004).

- In Spain, the disasters of Biescas (August, 1996) and of Alicante (September, 1997) conducted the Spanish Senate to exhort the constitutional court to recognize the necessity of including risk prevention measures to reduce the vulnerability of the slopes to natural risks.

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1.3. Detailed description of official landslide RAMs

1.3.1. Description of French RAM Risk prevention plans (PPR: Plan de Prévention des Risques), established by the law of February 2nd, 1995 imply a location of the vulnerable zones exposed to the hazard. The PPR collects informative documents (a note of presentation, a localization map of the phenomena, a hazard map and some tatutory documents (risk zoning map at a scale of 1:10,000 or at 1:5,000 for the urban zones, and a regulation). Inventory of processes The RAM consists first in the elaboration of an informative map of the natural phenomena. It represents on a topographic map at 1:25,000, the observed and known phenomena inventoried from archives, aerial photographs and field work. Hazard map The hazard map is established by a forward-looking approach where areas where any phenomena has been observed can be classified in hazard zone. The map is constructed through the combination of predisposing factors. The susceptibility of the site to landslide is estimated by a qualitative approach and is considered maximal where all the unfavourable factors (slope, lithology, …) are present. Figure 6: Schematic representation of the GIS workflow for the PPR methodology

Map of major asset The inventory of the stakes consists in analyzing the landuse characteristics considering both the existent and the future developments. This analysis allows to identify the major assets such as establishments receiving public (hospital, schools, campsites, etc), strategic buildings (fireman's barracks, water drinkable tanks, etc), areas of major economic activities (industrial buildings, etc) as well as the communication capabilities (roads, railways, power roads, etc) which threatening may aggravate the risks during a major event The cross-correlation of the hazard map and the map of major assets allows to identify qualitatively the main risk areas to be protected. The risk zoning consists in three risk classes (red, blue, white) and delineates zones in which prevention measures have to be taken.

Figure 7: Example of Risk map with the PPR RAM. Thus, red zones can concern zones where the measures of prevention are impossible or too costly, so no construction will be authorized. References: MATE/METL (1997, 1999); DRM (1990)

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1.3.2. Description of Swiss RAM In Switzerland, the assessment of landslides and rockfall risk is carried out in a similar manner as that used for the evaluation of floods, debris flows and snow avalanches. The cantons are required to establish registers and maps denoting areas of hazards and to take them into account in their guidelines for land use planning. The Swiss procedure to estimate the risk declines in three stages. Hazard identification A map of landslide phenomena with a scale at 1:50,000 or 1:5,000 for local plan and associated technical report record evidence and indications of slope stability are created. This is based on the observation and interpretation of landforms, on the structural and geomechanical properties of slope stabilities and on historical traces of previous slope failure (Raetzo-Brülhart 1997). Swiss federal administration has established recommendations for the uniform classification and representation of the hazard and risk (Figure 9). Figure 8: Example of landslide map.

Hazard assessment Some federal recommendations for land use planning in landslide-prone areas and flood-prone areas have been proposed to cantonal authorities and to planners to allow for the development of hazard maps using an intensity/probability diagram (Figure 10).

Figure 9: Matrix used to define the degrees of danger

Indicatives values can be used to define classes of intensity. For example concerning rockfalls the significant criterion is the impact energy in the exposed zone (in Kj). For landslide the long-term mean velocity is used as reference. Probability is defined according to 3 classes limited at 30 and 300 years. Determination of occurrence probability remains very uncertain. Thus the probability of occurrence is generally established for a given duration of landuse. Most slow type landslides are continuous processes, therefore no strict probability of occurrence exists for these landslide types (Lateltin et al. 1997). The maps are created at a scale of 1:25,000 or 1:50,000 for a Master Plan and at 1:10,000 or 1:5,000 for a Local Plan. Risk assessment Less importance has been given to the development of risk maps and only a few examples exist according to the federal guide (BUWAL 1999). Thus, the federal recommendations for the consideration of landslide hazards in land planning (OFAT, OFEE and OFEFP 1997) propose to associate a given hazard level to a general type of action. In red zones, no construction or installation used to shelter people and animals is allowed (prohibition zone). If they exist, buildings cannot be enlarged or reconstructed. References: OFAT-OFEE-OFEFP, 1999; Raetzo et al., 2002; Leroi et al., 2005.

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1.3.3. Description of Swedish RAM Sweden has a long experience of slope stability mapping due to several large landslides that damaged railway tracks and other infrastructures in the 1970s. The mapping and survey of the landslides is carried out by the Commission of the Swedish Rescue Services Agency since 1987. The purpose of the RAM is to develop a slope stability map only for the most developed areas. The maps are intended to support the County Administrative Boards and the municipalities by indicating where there are landslide areas at risk. A prior study defines the list of municipalities to be analysed according to the sum of the products “Relative Relief × Number of Inhabitants”. The Sweden landslide RAM is first based on a general stability mapping derived from aerial photo interpretation, field studies, topographical criteria and a soil map. The potential landslide risk areas are identified by: - the ranking of terrain units by the selected municipalities where the built-up and future development areas are planned, and should be studied in greater detail. - the elaboration of a soil map, including soil type and soil depths. The focus is made on clay and silt soil type, which are very prone to landslides. - the computation of stability calculations based on data from earlier studies, and using relevant process-based models (safety factor calculations). - the ranking of terrain units in terms of stability conditions using contour maps and aerial photo interpretation. The results are mapped on topographical maps at a 1:50,000 scale, according to the classes described in Figure 11.

Figure 10: Hazard map of Eskilshun, SRSA/ Bohusgeo (1996) (Swedish Geotechnical Institute; Fallsvik, 2007). Finally, the risk assessment ends by the design of preventive measures. References: Ahlberg et al., 1988; Fallsvik, 2007.

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1.3.4. Description of Italian RAM In Italy, landslide RAMs are committed to the basin authorities, and are essentially based on a geomorphological approach associating the interpretation of aerial photographs, combined with the analysis of site specific and historical information. Identification of the risk areas to be mapped It consists in the mapping of area bounded by drainage and relief lines (eg. catchment) around the place selected for the landslide risk assessment. Mapping of elementary slopes or watersheds is accomplished at 1:10,000 scale, using large scale topographic maps, and large and medium scale aerial photographs. Multitemporal landslide map Landslide information collected through the interpretation of aerial photographs or mapped in the field is transferred to large scale topographic maps (1:10,000 scale). The different landslide maps are then overlaid and merged to obtain a single, multitemporal landslide inventory map. Landslides are classified according to the type of movement, and their estimated age, activity, depth and velocity. Landslide hazard zones Landslide hazard zones are defined as the areas of possible short-term evolution of an existing landslide, of similar characteristics (type, volume, depth, velocity) identified by aerial photographs and fields observations. They are delineated according to separate landslide scenarios (for the different types of failures observed in the elementary slope) designed using geomorphological inference. Landslide hazard assessment An estimate of landslide hazard is obtained by combining the value of landslide frequency and landslide intensity. The frequency of landslide is obtained through the analysis of historical records of landslide events (Guzzetti et al., 1999). The multitemporal inventory map allows defining 4 classes of landslide frequency in relation to the observation period. Landslide intensity corresponds to the destructiveness of the landslide according to the volume and the expected velocity. Vulnerability of elements at risk A map established at 1:10,000 scale locate the elements at risk (built-up areas, structures and infrastructure). To evaluate their vulnerability, a relationship between the intensity, the type of the expected landslide and the expected damage (according to the known phenomena).

Damage is classified as aesthetic, functional or structural damage. Landslide risk Risk maps are created by correlating the expected damage to the landslide hazard ranked from low to high values.

RS=ƒ (HL, VL) Figure 11: Example of a risk map for the village of Moranno Madonnuccia (Perugia province) (Cardinali, 2002). A: Risk for shallow landslide; B: Risk for deep seated landslides References: Reichenbach et al., 2005; Sorriso-Valvo, 2005; Guzzetti et al. 1999b

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Comparison of the official RAMs The 4 official landslide RAMs have been scientifically compared in terms of documentation, robustness, consistency, ambiguity, applicability and validity. The RAMs in development or the RAMs used by local research institutes or private engineering offices have not been analysed because some procedures were not totally detailed in the questionnaires, or were assumed to be modified.

1.4. Methodology: indicators and spider graphs

The RAMs have been compared according to 5 indicators (RAM Scale, RAM Transparency, RAM Complexity, RAM Cost efficiency and RAM Ambiguousness) which are defined in Table 3.

Table 3: Definition of the scientific indicators to compare the RAMs Indicators Definition Coding value / indicator

Scale This indicator is linked to the availability of documents and the scale of the maps to be produced.

Transparency

It corresponds to the transparency of the human thought and so it depends of the experience of the expert in charge of the assessment. This indicator reveals the applicability of the methodology.

Complexity

The complexity of the methodology is linked to the processing of the input data and the number of output information. The more input data are used, the more complex is the methodology.

Cost efficiency

This indicator presents the profitability of the methodology in terms of means and costs to achieve the objective.

Ambiguousness

This indicator represents the uncertainty in the delineation of hazard and risk zones.

The indicators of the 4 official RAMs are represented with spider graphs. There are as many axes in the spider graph as indicators. Each indicator is coded through an index according to the maximal value observed for each indicator. The value of the index is evaluated according to our interpretation of the questionnaires.

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1.5. Comparison Spider graphs were elaborated only for countries where an official risk assessment methodology is established for landslides. It seemed to us not pertinent to compare the RAMs in development because they may be subjected to some modifications or are not complete. Concerning methodologies employed by research institutes, they are most of the time developed for a specific objective, which sometimes is not for regulatory measures of risk zoning.

Figure 12: Spider graphs of the official landslide RAMs of France, Switzerland, Sweden and

Italy.

The shape of the spider graph are relatively similar, indicating that the RAMs are basically based on the same approach. They indicate however that the French and Italian RAMs are slightly more complex than the Sweden and Swiss RAMs. In European countries, several mapping scales can be used for risk assessment. In the official methodologies, the 1:25,000 scale or 1:10,000 scale are generally used: some local detailed zooms can also be mapped at a 1:5,000 scale. For example in France, a topographic map at 1:25,000 scale enlarged at a 1:10,000 scale and cadastral plans at 1:5,000 scale are used to delineate preventions areas.

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The ambiguousness indicator reveals the precision of the method. All RAMs have mean values of ambiguity. The transparency indicator indicates low value of transparency because all RAMs are based on expert and heuristic analysis, and are thus subject to the thoughts of the experts. All the RAMs present a high value of cost efficiency. For example, iIn France the elaboration of the PPR is very simple and is only based on available and existing data; the objective is to build documents taking into account the known risks rather than to focus on high accuracy.

1.6. Conclusions Official landslide RAMs are similar. They are built on a qualitative approach, at medium scale and where necessary a more detailed local study can be realized with more complex (eg. deterministic) techniques. They conduct all to plans of priority measures and thus lean on local urban plans. Their mai advantages and disadvantages are detailed in Table 4.

Table 4: Pros and cons of current landslide RAMs

Country Methods Advantages Disadvantages

Field geomorphologic analysis

Allow a rapid assessment taking into account a large number of factors

Totally subjective methodology based on the use of implicit rules that hinder the critical analysis of the results

France Italy Sweden

Switzerland Combination of index

maps

• No hidden rules • Automation of the procedure • Standardisation of data management

Subjectivity in attributing weighted values to the single classes of each parameter

The vulnerability component of the risk is often not directly quantified. The vulnerability is mainly determined by the cross correlation of the landuse map and a personal evaluation of the experts. Only the Italian RAM assesses vulnerability by the creation of index according to the nature of the elements at risk. The quantification of landslide risk is often a difficult task, as both the landslide intensity and frequency are difficult to calculate for an entire area even with sophisticated methods. In practice, simplified qualitative procedures are used, such as the one developed in Switzerland, which are also of easier use for a risk mapping over large areas.

2. Options of harmonization

2.1. Regional differentiation There is no regional differentiation for the landslide RAM. The application of a certain methodology depends more on the size of the areas to be mapped, the scale of the output documents and the types of landslide mechanisms observed in the region.

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2.2. Current differentiation within RAMs

All the RAMs use a qualitative approach, at a mapping scale of 1:10,000, and the techniques consist in the investigation of historical archives, field observations, remote sensing techniques and geographical information systems. The main input data are the topography, the geology, the geomorphology and landcover/landuse and an inventory of the elements at risk( eg. Assets or stakes).

2.3. Options for harmonization The landslide threat can cover several expressions in terms of mechanisms (fall, slide, flow, topple, spread), of material being affected (rock, debris, earth, mud) and of thickness (shallow, deep-seated). Options for harmonization of landslide RAMs have been discussed since ca. 10 yers at the international level under the umbrella of the IUGS ‘Joint Technical Committee on Landslides and Engineered Slopes’. Discussion among the landslide scientist community has conducted to a methodological framework for landslide Quantitative Risk Assessment (QRA) independent of the above mentioned landslide expressions. To quantify the landslide risk, this methodology introduces several sub-indicators such as susceptibility (spatial occurrence of a threat), hazard (temporal occurrence and intensity of a threat) and

vulnerability (degree of loss to a given element affected by a hazard), and techniques to quantify and map these indicators at several spatial scales (IUGS, 1997; AGS, 2000). This methodology may result in quantitative maps of landslide hazad and risk expressed in probabilities.

Conclusion At this stage of development of the RAMSOIL project, an inventory and a database of landslide RAMs have been set up. Few countries possess an official landslide RAM, and the ones which exist are already appreciably similar.

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