Introduction

Located in the Alps, Switzerland is a small, hazard-prone country (covering 41,300 km2 with 7.4 million inhabitants) exposed to debris flows, earthquakes, floods, forest fires, hail storms, slides, rock falls, snow and ice avalanches and wind storms. Landslides can be caused by ground conditions inherited from geological or glacial history, influenced by preparatory causal factors linked to climate or human activities and by triggering causal factors related to weather, public works or earthquakes. The geological structure of Switzerland is essentially the result of a collision of the African and European plates over millions of years. Fifty-seven percent of its surface lies in the Alps, 30% in the Molasse Basin (Swiss Plateau) and 13% in the Jura. Rainfall is abundant (500 mm in Central Valais, 2,500 mm on Säntis at 2,503 m asl) and evenly distributed throughout the year. Towards the interior of the Alps, the timberline rises from 1,700–2,400 m and the permanent snowline from 2,500–3,200 m asl.

The shaping of the present landscape has taken place over the past two million years. In the Swiss Alps, numerous slopes are affected by small movements related mostly to ancient landslide mechanisms of post-glacial age and to the progressive failure of rock slopes induced by weathering and water pressure in fault systems. We estimate that more than 6% of the Swiss territory (2,500 km2) has been affected by landslides. During the last 30 years, landslides have caused an average annual direct financial damage of about US $20 million (Fig. 1) and a death toll of 1 person per year (Schmid et al. 2004).

Fig. 1
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Distribution of landslides that caused financial damage between 1972–2002 in Switzerland, modified from Schmid et al. 2004

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The new federal laws on forest and flood protection (LACE 1991; LFo 1991), which were established after the flooding events of 1987, are based on an integrated approach to protect people and assets from natural hazards (PLANAT 2005). The purpose of these new laws is the protection of the environment, in particular human life and high value property, from natural hazards with a minimum of structural countermeasures. Therefore, of particular importance are non-structural preventive measures. The four main elements of the natural risk management strategy in Switzerland are hazard assessment, definition of protection requirements, planning of measures and emergency planning (Fig. 2).

Fig. 2
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Strategy for landslide risk management in Switzerland

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Landslide hazard assessment

The new Federal Ordinances on Flood and Forest Protection (OACE 1994; Ofo 1992), require the cantons to establish hazard maps which have to be incorporated in regional master plans and local development plans. The cantons are responsible initiating hazard mapping. The federal authorities subsidize this activity up to 70% of its cost. The techniques for developing landslide hazard maps are outlined in the federal recommendation “Code of practices for landslide hazard and land use planning” (OFAT, OFEE and OFEFP, 1997), issued in 1997.

Register of events

An indispensable prerequisite for hazard identification is information about past events. Recommendations for the definition of a uniform register of landslides have been developed, including special sheets for each phenomenon (slides, rock falls, debris flows) and each canton is currently compiling the data for its own register. These databases, called “StorMe”, are transferred to the Swiss Agency for the Environment, Forests and Landscape to allow an overview of the different natural disasters and potential associated damage in Switzerland.

Classification of landslides

Slides can be classified according to the estimated depth of the slip surface (<2 m: shallow; 2–10 m: intermediate; >10 m: deep) and the long-term mean velocity of the movements (<2 cm/year: sub-stabilized; 2–10 cm/year: slow; >10 cm/year: active). These depth and velocity parameters are not always sufficient to estimate the potential danger of a slide. Differential movements must also be taken into account as they can initiate the toppling of buildings or opening of cracks. Rock falls are characterized by their speed (<40 m/s), the size of their elements (stone diameter <0.5 m, block diameter >0.5 m) and the volumes involved. Rock avalanches with huge volumes (>1 million m3) and rapid velocity (>40 m/s) can also occur, although these are rare. Due to heavy precipitation, debris flows and very shallow slides are frequent in Switzerland. Most of these are moderate in volume (<20,000 m3) and of rapid velocity (1–10 m/s). These phenomena are dangerous and annually cause fatalities and traffic disruptions.

Maps of landslide phenomena

A map of landslide phenomena and an associated technical report record evidence and indications of slope instability as observed in the field. The map presents phenomena related to dangerous processes and delineates vulnerable areas. Field interpretations of these phenomena allow landslide-prone areas to be mapped, based on the observation and interpretation of landforms, on the structural and geotechnical properties of slope instabilities and on historical traces of previous slope failures. Some recommendations for the uniform classification, representation and documentation of natural processes have been established by the federal administration (OFEE and OFEFP 1995). The different phenomena are represented by different colors and symbols. An additional distinction is made between potential, inferred or proved events. According to the scale of mapping (e.g. 1:50,000 for the Master Plan, 1:5,000 for the Local Management Plan), this legend may contain a large number of symbols. This approach allows maps from different parts of the country to be easily compared.

Landslide hazard map

The hazard is defined as the probability of a potentially damaging natural phenomenon within a specific period of time in a given area (IDNDR 1993). Hazard assessment implies the estimation of the intensity of an event over time. Mass movements often correspond to gradual phenomena (slides) or unique events (falls, debris flows). It is indeed difficult to make an assessment of the return period of a large rock avalanche, or to predict when a dormant landslide may reactivate.

For simplification, three levels of intensity are considered, high, medium and low (Table 1). Regarding probability, the same three levels, high, medium and low, are used with the corresponding return periods 1–30, 30–100 and 100–300 years. The work to be done for a potential hazard is therefore to determine its intensity for the chosen levels of probability at selected points in a specified area. This is achieved by various means, for instance by modelling the corresponding processes (mathematically or by a physical model, calibrated by past events). Indicative values can be used to define classes of high, medium and low intensity (see Table 1).

For rock falls, the significant criterion is the impact energy in the exposed zone (the sum of translation and rotation energy). The 300 kJ limit corresponds to the impact energy that can be resisted by a reinforced concrete wall, as long as the structure is properly built. The 30 kJ limit corresponds to the maximum energy that oak-wood stiff barriers (railway sleepers) can resist. For rock avalanches, the high-intensity class (E>300 kJ) is indeed always reached in the impact zone. The target zones affected by block avalanches of low to medium intensity can only be roughly delineated.

Most slides are characterized by continuous movements, sometimes with associated phases of reactivation. A low-intensity movement has an annual mean velocity of less than 2 cm/year. A medium intensity corresponds to a velocity ranging from 1 to approximately 10 cm/year. The high-intensity class is usually assigned to shear zones or zones with clear differential movements. It may also be assigned if reactivated phenomena have been observed or if horizontal displacements greater than 1 m per event may occur. Finally, the high-intensity class can be assigned to very rapid, shallow slides (velocity >0.1 m/day). In the area affected by slides, field intensity criteria can be directly converted to danger classes. For debris flows, the intensity depends on the thickness of the potentially unstable layer. The boundaries defining the three intensity classes are set at e: 0.5 and 2 m.

Unlike floods and snow avalanches, mass movements are usually non-recurring processes. The return period, therefore, has only a relative meaning, except for events involving rock and block avalanches and debris flows which can be correlated with recurrent meteorological conditions. The probability of a mass movement should generally be established for a given duration of land use. Thus the probability of potential damage during a certain period of time or the degree of safety of a specific area should be taken into account, rather than the actual frequency of an event.

Table 1 Criteria for intensity of different landslide hazards
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The occurrence probability of rock falls should be estimated by taking into account traces of former events that occurred during the last 300 years. This allows zones of low, medium or high probability of mass movement to be established. Rock avalanches are usually unique events, hence it is recommended not to subdivide them into high-, medium- or low-probability zones. Sectors with active movements, widening cracks or isolated rock avalanches originating in a dangerous zone must be considered as “red zones” in the hazard maps. Most slides are continuous processes, therefore no strict probability of occurrence exists for such mass movements. Periods of landslide activity are often related to precipitation events (Raetzo et al. 2002) and should be related to the probability of specific meteorological conditions (for example, continuous precipitation associated with snow melting).

In principle, the probability scale does not exclude rare events, nor does the intensity scale exclude high-magnitude events. Hazards with a very low probability of occurrence are usually classified as “residual dangers” under the standard classification. In the domain of dangers related to mass movements, the lower limit for a “residual danger” has been set for an event with a 300-year return period.

The next step is to classify the results according to the matrix diagram which combines intensity and probability (Fig. 3).

Based on the results of the modeling processes and with respect to Figure 3, it is then possible to determine the expected hazard level for any point on a map. The end-result is the hazard map, in which “red” means high, “blue”, moderate and “yellow”, low hazard level. For landslides, “yellow–white hatched” would imply very low hazard. The word “danger” (inappropriately used in the Swiss recommendations for hazards, due to the lack of an equivalent German term) or hazard thereby denotes the degree of exposure of persons, buildings and/or infrastructure to a potential hazard of a specified level. As an example, Fig. 4 shows the difference between a landslide map of phenomena (intensity map) and the corresponding landslide hazard map.

Hazard maps for a Master Plan for instance are normally drawn at a scale of 1:25,000–50,000. For a more detailed Local Plan on the other hand, on the basis of which clear restrictions may be formulated, a scale of 1:10,000–1:5,000 is used.

Fig. 3
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Diagram of hazard levels as a function of probability and intensity

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Meaning of landslide hazard map for land use planning

The federal recommendations are proposing codes of practice for the transposition of hazard level in terms of potential damage for the purpose of land use planning, as shown in Table 2.

Definition of protection requirements

The differential safety concept was introduced into the Natural Risk Management Strategy after 1987. The determination of different levels of safety, and thus of the corresponding design event is a decision of major technical and economic consequences. In former days, the design for protection works was generally based on an event with a return period of 100 years (E 100). Today, a differentiation of protection objectives is applied (Fig. 5) for floods, as well as for snow avalanches, for landslides, for debris flows and for rock falls. According to the importance or value of structures and assets to be protected, the respective degree of safety can be chosen. Where property with a very high value has to be protected, it is recommended to increase the degree of safety and to use a design event which is higher than the centennial event. In contrast, for agricultural land, the degree of safety can be much lower.

Fig. 4
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Samples of a landslide phenomena map (top) and landslide hazard map (bottom) (courtesy of B. Loup of the Canton of Fribourg)

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Coping with residual risk

Many disasters in the past demonstrate that a complete safety measure cannot be achieved. A particular risk always remains called residual risk. An assessment of the impact of a landslide or rock avalanche, which exceeds the design event, provides information about the residual risks. If such a disaster occurs, the non-permanent (emergency) measures can provide additional safety for the population at risk and can reduce the number of casualties considerably. Local authorities have then to be involved in the installation and operation of early warning systems, in the preparation of evacuation schemes, and in the training of rescue units.

State of hazard mapping and perspectives

After the publication of the federal recommendation to establish hazard maps for land use planning in 1997, an intensive program of mapping started in Switzerland and regional authorities (cantons) are actively participating to this effort. The present state of landslide hazard mapping (Fig. 6) shows that 50% of the Swiss territory is already analyzed by landslide hazard maps at the scales from 1:25,000 to 1:5,000 (Lateltin et al. 2004).

In Switzerland, less importance has been given to the development of risk maps and only a few examples exist, according to the federal guidelines (BUWAL 1999). The methodology is based on a “three-stage procedure”. Stage 1 provides information on qualitative classification of risk for object categories (protection deficit based on hazard map). Stage 2 provides information on semi-quantitative risks for object types (number of fatalities, property damage). Stage 3 provides information on quantitative risks for individual objects (number of fatalities, extent of damage in Swiss francs). A lot of uncertainties still remain in the calculation of fragility curves (vulnerability of the individual objects) according to different landslide intensities and more research is needed.

Hazard maps and local management plans

Planning measures

The assessment of landslide hazards and risks is not a final goal in land planning; the aims corresponding to the respective laws and ordinances (LAT 1979; LFo 1991; OFo 1992) imply the planning and implementation of selected measures, in order to insure protection against acceptable events of a given magnitude that depends on the set protection objectives (Fig. 5). The first priority in selecting measures implies appropriate land use planning, either at a regional scale (master plan) or at a local scale (local management plan).

Table 2. Transposition of hazard level for land use planning
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Fig. 5
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Differential safety goals against landslides according to different objects

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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, namely:

  • Red zone (high hazard): In principle, no construction or installation used to shelter people and animals is allowed (prohibition zone). If they exist, buildings cannot be enlarged or reconstructed. The planned development zones should be declassified. Transformations in existing buildings are only allowed if the risk is decreased (through appropriate protection measures), but such measures should not be carried out in order to increase the use of the land. A systematic evacuation of the inhabitants of the buildings located in red zones is not foreseen, but provisions must be taken to insure their safety in case of emergency, through the development of evacuation plans.

  • Blue zone (moderate hazard): Buildings are allowed in these zones only under certain conditions depending on the type of hazard (prescription zone). These conditions may include the requirement of additional studies (e.g. geological and/or geotechnical expert advice in order to obtain a building permit, monitoring data), of special construction techniques (e.g. raft foundation for buildings), of appropriate protection measures (e.g. drainage of the ground) or of particular planning measures (e.g. minimum distance to an existing channel in which debris flows may occur). Particularly sensitive objects like hospitals or homes for elderly people should not be authorized in such zones, nor should major development projects. The local authorities can define special additional rules (e.g. limited density of construction).

  • Yellow zone (low hazard): It is possible to build in these zones, but landowners should be informed of the existing hazard (awareness zone). In these cases, adequate prevention measures at the scale of the plot of land or of the whole slope may reduce the potential damage or the size of this low-hazard zone. It is required that special protection measures are taken for the sensitive objects mentioned above. Particular attention should be paid in this case to excavations required for buildings, as they can generate instability problems.

  • Yellow–white hatched zone (very low hazard): This zone highlights the residual risks related, for instance, to rock fall potential events of a very low probability. Standard buildings are allowed without special requirement, but special protection measures must be taken for sensitive objects. Installations with an increased potential for indirect damage (e.g. major oil tanks) are to be avoided.

These zones, that are first determined in the hazard maps according to scientific criteria, have to be adapted according to the existing local management plans that require an updating every ten years according to the land planning law. Indeed, hazard zones have restricted legal value until they are introduced in the local management plans and construction rules with specific requirements, so that they are approved by the local council or the population in small communes. Through this public approval process that needs good communication, the requirements related to landslide hazards are generally well accepted by the population.

Fig. 6
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Present state of landslides hazard mapping in Switzerland

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Any change to the local management plan should also be submitted to the endorsement of the cantonal authorities, so that homogeneity in the management of landslide hazard zones is insured. For this purpose, many cantons have appointed a special natural hazards commission formed of representatives from political authorities, administrative officers, scientists and public insurers, that can analyse each critical situation requiring an urgent action and each change in local management plan when the hazard level is modified by new natural or artificial conditions. This commission may also propose a new policy with respect to endangered zones, as occurred in canton Fribourg after the Chlöwena landslide crisis in 1994, when all the communes with building areas in red zones were designated and required to suspend building activities there (Vulliet and Bonnard 1996; Loup 2003).

These prevention measures are globally accepted and applied in the different cantons of Switzerland, but some modifications of the federal recommendations are presently being studied to increase the systematic applicability of the hazard map criteria, in particular considering specifying zones with high potentials for reactivation processes that might increase hazard levels. It appears, however, necessary that land use planning must be adapted to the political traditions of each canton so as to favor the acceptance of such measures.

Protection measures

In second priority, and only in specific cases where they can be cost-effective, like for debris flow channels crossing inhabited zones, the recommended prevention actions may include protection measures. This need is mainly due to the intense development of tourist infrastructure in some exposed zones, causing an unacceptable risk level.

The difficulty in such cases resides in the selection of design criteria and a detailed risk analysis implying several potential landslide scenarios of different probabilities of occurrence must be carried out. The trend to consider recent events, like the major debris flows recorded in 1987, 1993, 1999 and 2000, as standard cases for the design of protection structures must be avoided, as they sometimes correspond to extremely rare phenomena from which complete protection would be an excessive goal, or, as past events have seriously modified the predisposition factors for a new disaster, the risk of a future new severe event is reduced.

A significant case illustrating the implementation of a new prevention strategy is the tourist resort of Sörenberg in the Swiss Alps, Central Switzerland (Loat and Zimmermann 2004). The village developed during the last 40 years to about 800 housing units on an old debris flow deposit zone. These debris flows originate from a large deep-seated slide above the village (Fig. 7).

Fig. 7
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Sörenberg view from the opposite side after the event of 1999

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In May, 1999, after a winter season during which large quantities of snow were recorded, the slide was reactivated and debris flow of 30,000 m3 occurred, followed by several secondary events. These flows expanded fortunately in an open space between the two main parts of the village, where some public development was planned in the near future. Damage in the building zones was limited, but the communal authorities decided to establish a comprehensive hazard map and propose long-term solutions for the protection of the village.

The hazard map corresponding to the existing situation pointed out a large red zone including numerous houses and no possible extensions or relocations of the village, which was not acceptable. Thus the following protection goals were defined in order to design limited protection works:

  • No casualties for any event are accepted

  • No damage is accepted for a 30-year return period event

  • Limited damage is accepted for a 100-year return period event

  • A limited protection for assets is insured for a 300-year return period event

Thus, after intense discussions with the population and the cantonal authorities, three types of measures were taken:

  • Land-use planning measures: they include the prohibition of new constructions and normal building maintenance works only in the red zone; no new planned residential areas in the blue zone, but possible new buildings in existing housing lots, if local protection works are carried out, whereas limited modifications to existing buildings are allowed, provided that safety measures are taken (e.g. no dormitories in the basement); in the yellow zone, local protection is recommended for new buildings.

  • Structural measures: several protection works such as dams, retention basins and discharge channels, are proposed to reduce the risks and modify the hazard level in the exposed zones. These structures are designed for a 100-year return period event, but substantial damage might be expected in the case of a worse disaster, nevertheless some inhabited areas will stay in red zones after the completion of structural measures, which requires emergency measures.

  • Emergency planning: includes long-term monitoring, but also visual observations and wire sensors in case of major rainfall events, and planning of evacuation routes.

The major difficulty now lies in taking the respective decisions concerning the sharing of the construction costs for the protection works, between the canton, the commune and the private owners. This means that the question of how much the respective partners are ready to pay in order to insure safety and development at the same time is not yet solved, mainly because a correct perception of the risks induced by landslides is not yet achieved. While the consciences of the citizens and of the authorities are open to discussions of risks after a major event or a disaster, the situation is not at all the same for potential major landslides that have not affected the population yet, so that the citizens and the local authorities prefer to deny the existence of risks or pretend the risk level is not increasing (Bonnard et al. 2004). A comprehensive education campaign is then needed.

Communication

In order to allow an efficient management of landslide-prone areas, the thorough communication of the existing hazards and their consequences is of primary importance. The official record of the plots of land is not appropriate for this aim as the instability of the ground is not an objective and permanent characteristic. But the Commune of Belmont s/Lausanne in the canton of Vaud, half of which is formed of landslide-prone areas, has set up an interesting and original initiative to facilitate the transmission of information. A detailed study of all boreholes, maps and expert advice regarding the stability of slopes has allowed the preparation of a public record of slope instability conditions for each plot of land, accompanied by advice on the possible solutions to improve the situation and on the expected hazard level (Noverraz 2002). The potential owners or builders are thus duly informed of the instability conditions of any plot of land and are in a position to take the necessary precautions.

The publication of such information caused some fear of the effect of this negative publicity on the prices of the plots of land, but some recent checks on the market did not confirm this. A continuous effort should therefore be undertaken at all levels to improve knowledge of landslide-prone areas and to communicate such data because the major costs are mainly due to ignorance of the real nature of these hazards.

Conclusions

In Switzerland, important efforts have been made to apply the same strategies and similar approaches for the assessment and management of all kinds of natural hazards (snow avalanches, floods, debris flows, rock falls and slides). The vulnerability of society grows constantly as a result of the growing population density, of an enormous increase of economic assets in risk zones, as well as of the increased mobility of the population in particular in exposed zones. In addition, changing environmental conditions (such as global warming) may aggravate the threat of landslides in the Alps. Land use planning and the resulting zoning laws at the cantonal and communal levels are among the effective tools for landslide risk management in Switzerland. Since 1991, new federal regulations require the 26 Cantons to establish hazard maps and zoning for mass movements so as to restrict development on hazard-prone land. The cantonal authorities are participating actively in this hazard mapping to prevent and reduce the potential losses and the number of victims, in particular by better land use planning. Mapping works are still in progress and 50% of the Swiss territory is currently covered by hazard maps for landslides at different scales. In specific cases, the application of a strict management policy, carried out with the approval of the citizens, allows a significant decrease of landslide risks.