HAZARD ASSESSMENT AND LAND-USE PLANNING IN SWITZERLAND FOR SNOW AVALANCHES, FLOODS AND LANDSLIDES Chapter 6 The previous chapters describe the assessment of various natural hazards and mitigation measures that may reduce the consequences of these natural hazards. Land-use planning and the resulting zoning laws are among the most effective tools for the prevention and mitigation of disasters resulting from natural hazards. Effective planning and zoning require consideration of all the natural hazards that could afkct a location in a commensurate and consistent manner. In Switzerland. new Federal legislation requires the Cantons to establish hazard maps and zoning for floods, snow avalanches and mass movements to restrict development on hazard-prone land. Areas highly exposed to natural hazards have to be mapped at regional or local scales. A coherent code of practice for landslides, snow avalanches and floods is now available, taking into account the possible threat to human life and important facilities in settled areas. The codes of practice also include some practical criteria to establish hazard maps based on intensity and return period of events. The transposition of these maps for the purposes of land-use planning is proposed for planners and policy makers. This example of combined assessment of natural hazards in Switzerland provides guidance for similar combined or composite assessments of natural hazards in other areas that are prone to multiple natural hazards. Randa from the rest of the valIey for two weeks. In 1994, a prehistoric landslide experienced a strong reactivation with historically unprecedented rates of displacement up to 6 mlday, thus causing the destruction of the village of Falli- Holli (41 houses, loss of US $15 million). Future climatic warming and unfavourable development of forests could lead to increased debris flow hazards in the periglacial belt of the Alps. 6.2 6.1 ally. In SWITZERLAND: A HAZARD-PRONE COUNTRY Located in the Alps, Switzerland is a small “hazard-prone” country (covering 41 300 kmz with 6.7 million inhabitants) natura1 exposed to disasters, such as debris flows, earth- quakes, floods, forest fires, hail storms, landslides, rockfalls, snow avalanches and wind storms. Protection against natural disasters is incomplete and suitable protection does not exist in many places or no longer exists owing to changes in the use of the environ- ment. Catastrophic floods took place in the summer of 1957 (damages of US $ 1 billion), as well as in 1990 and 1993. Ftoods cause damages amounting to US $150 million annu- i\ugust 1995, a debris flow (40 000 m3) cut the highway in Villeneuve, near Montreux, destroying some houses and vineyards (loss: US $15 million). Yearly, snow avalanches kill more than twenty-five people. In 1356, the city of Basel was destroyed by a violent earthquake (Intensity TX on XISK scaIe, 1 5LO victims), and this highly industrial area remains under the threat of future earth- quakes. hlore than eight per cent of the Swiss territory may m3 of fallen be affected by landslides, mainly in the Prealps and the Alps. The Randa rock avalanche of 1991 (30 million rock debris) cut off the villages of Zermatt, Tasch and REGULATIONS Switzerland is a Federal country where 26 Cantons are sovereign in principle: the central authorities only have jurisdiction in those domains determined by the Federal Constitution and all other state powers automatically belong to the Cantons or to the communities. Each Canton has its own government, constituting laws and regulations within the framework defined by the relevant Federal laws. The prevention and management of natural disasters follow the same rules. The legal and technical background conditions for protection from mass movements have undergone considerable changes during the past few years. The flood of 1987 prompted the federal authorities to review h e criteria governing the protection of the habitat against natural hazards. A farmer regulation, the Federal Law for Land-use Planning of 1979, required each Canton to elaborate a Master Plan, including a map of scale 150 000, designating, among others, the hazardous territories. At the communal level, a Local Plan, including a map of scale 1:5 000, was requested for apportionment of land-use (e.g., agriculture, settlements) taking into account natural hazards. Due to the lack of Federal subsidies, the Cantonal and communal authorities didn’t support such investigations, which restricted the use of their own land. Therefore, in many places in Switzerland, these maps are still lacking. Two new regulations, the Federal Law on Flood Protection and the Federal Forest Law, came into force in 1991. Their purpose is to protect the environment, human lives and objects of value from the damaging effects caused by water, mass movements, snow avalanches and forest fires. Following these new regulations, the main emphasis is now placed on preventive measures to an increasing extent. Therefore, hazard assessment, the differentiation of protection objectives, the purposeful planning of measures and the limitation of the remaining risk are of central importance. The Cantons are required to establish registers and maps depicting endangered areas, and to take hazards . into account for their guidelines and for the purposes of land-use planning. For the elaboration of the hazard Chapter 6 - Hazard assesjtnenf and land-use planning in Switzerland for snow, avalanches,joods and landslides registers and hazard maps, the Federal government is providing subsidies to the Cantonal authorities up to 70 per cent of the costs. Land-use planning and the resulting zoning laws are among the most effective instruments to prevent substantial losses and casualties caused by natural hazards in sensitive areas. HAZARD MANAGEMENT 1. WHAT MIGHT HAPPEN AND WHERE? 62 6.3 The identification of natural hazards, the evaluation of their impact and the general risk assessment are decisive steps towards the selection and dimensioning of adequate protec- tive measures. Therefore, a three step procedure has been proposed and is shown in Figure 6.1. 6.3.1 Hazard identification: What might happen and where? Some recommendations for the uniform classification, representation and documentation of natural processes (e.g., snow avalanches, floods, debris flows, landslides and rockfalls) have been established by the Swiss Agency for the Environment, Forests and Landscape, and by the Federal Office of Water Management (Kienholz and Krummenacher, 1995). From these, the elaboration of the map of phenomenon should be based on an uniform legend. According to the scale of mapping (e.g., 150 000 for the Cantonal Master Plan, 1:s 000 for the Communal Local Plan), their legends offer, in a modular manner, a great number of symbols. For the 1:5 000 scale map, more symbols are available within an “extended legend”. However, for a hazard assessment map to he deemed adequate. it must meet certain minimum information requirements. These requirements are contained in the “minimum legend. This “minimum legend is a basic list of information that is included in each map used for hazard assessment. The map of phenomenon is based upon fieldwork and can be supported by other information, if available (e.g., floods records, geological maps, geodetic measurements, aerial photography). The various phenomena (landslide, flood, debris flow and snow avalanche) are represented by different colours and symbols. Additional distinction is made between potential, inferred or observed events. Following the recommendations and the uniform legend, maps can be established exhibiting the different hazardous phenomena within an area of investigation. Owing to nation-wide consistent application of procedures, maps from different parts of the country can easily br compared. Based on the Federal recommendations, harmonized Registers of Events are under development. Each register will include special sheets for every phenomenon (snow avalanches, landslides. rockfalls, debris flows and floods). Each Canton is responsible for its own register. Finally, these registers or databases will be transferred to the Federal Forest Agency,allowing the agency to overview the different natural disasters and damages in Switzerland, according to the standards of classification. (Hazard identification) Documentation: thematic maps (e.g., topography, geology) * aerial photography geodetic measurements * register of events * map of phenomenon 2. HOW AND WHEN CAN IT HAPPEN? I i * (Hazard assessment) Determination: simulation, modelling 3. HOW CAN WE PROTECT OURSELVES? (Planning of courses ofaction) Transposition for: * land-use planning (code of practices) * preventive and protective measures warning systems, emergency planning Figure 6.1 - Procedure for collecting data on natural haz- ards and planning courses of action The hazard assessment: How and when can it happen? .3.2 Hazard means the probability of occurrence of a potentially damaging natural phenomenon within a specific period of time in a given area. Hazard assessment implies the determination of a magnitude or intensity over time. Mass movements often correspond to gradual (landslides) or unique events such as rock avalanches, which are extremely rapid flows of dry debris created by large falls and slides. It is, therefore, difficult to evaluate the return period for a mas- sive rock avalanche or to predict the reactivation of a latent landslide. For processes like rockfalls, snow avalanches, floods or debris flows, it is much easier to evaluate their probability of occurrence. A rockfall is defined as a relatively free-falling newly detached segment of bedrock of any size from a cliff or steep slope. Some Federal recommendations for land-use planning in landslide-prone areas (Lateltin, 1997) and in flood-prone areas (Petrascheck and Loat, 1997) have been proposed to the Cantonal authorities and to planners for the establish- ment of hazard maps using an intensity-probability diagram. Similar recommendations have existed since 1984 for snow (OFF and IFENA, 1984). Hazards maps established for the Master Plan (e.g., scale 150 000) display all hazard-prone zones at the Cantonal level. The classification is made in a simple way: endangered or not endangered areas. Based on a diagram combining inten- sity and probability, hazard mapping for the Local Plan (e.g., scale 1:5 000) represents four classesor grades ofhazard: high danger (darkgrey).moderatedanger (grey),low danger (light 64 snow communities are required. Events involving large numbers of people have to be avoided, in blue zones as much a5 Phenomenon Rockfall kj Chaprer 6 - Hazard iissessmenr and land-use planning in Switzerland for snow, avalanches, floods and landslides The criteria for intensity cqn be summarized in the fol- lowing manner (OFF and IFENA, 1984; Lateltin. 1997; High - People endangered inside the building. - Substantial damage to the building leading to possible Petrascheck and Loat, 1997): inremiry destruction. iwedium intensity - People endangered in the field, but not inside the building. - Slight to medium damage to the building. Low intensity - People not endangered, neither in the field nor in the building. - Slight damage to the building. The detailed criteria for intensity were chosen accord- ing to TabIe 6.1 (OFF and IFENA, 1984, Lateltin, 1997; Petrascheck and Loat, 1997)- 6.4 CODES OF PRACTICE FOR LAND-USE PLANNING Absolute safety is impossible to achieve. In the different Federal recornmendations for natural hazards of landslides, floods and snow avalanches, acceptable risks are given for the different grades of hazard. A generally acceptable risk has been fixed a priori, which considers only events up to a return period of approximately 300 years. That is, the possi- bility of damages resulting from events with return periods greater than 300 years is considered acceptable. The Federal recommendations are principally valid for cities and towns. For other forms of land use, a corresponding scheme can be derived in a similar way. The codes of practice for land-use planning in areas prone to natural hazards (mass movements, flooding and avalanches) can be summarized as follows. In red zones, building is strictly prohibited. In blue zones. building is possible but with restrictions, provided certain safety requirements are met. Buildings exposed to hazards have to be designed corresponding to the hazards possible impacts. To minimize fatalities. the establishment of warning systems and evacuation plans by the Low intensiv Medium Ec30kj < 2 cmiyear possible. In yellow zones, .building is possible without restriction. When implementing risk management, the Cantons are required to proceed in the following manner: (a) Establishment of registers and hazard maps; ( b ) Incorporation of areas prone to various hazards into the Cantonal Master Plan; and ( c ) Controlling the application of the guidelines in the communal construction regulations (Local Plan). CONCLUDING REMARKS 6.5 The Swiss Federal recommendations for land-use planning in areas prone to natural hazards promote disaster preven- tion by restricting development on unstable or endangered zones. Within the alpine realm, rock avalanches, landslides, snow avalanches or floods will occur repeatedly, as long as the processes of erosion and denudation are present. Increasing potential for losses (increasing vulnerability), a growing recognition for the need to increase the protection of individuals, and a decreasing acceptance of periI from natural processes, has led to a situation where action was required. In such cases, land-use regulations were enacted. The Swiss Federal recommendations outlined in ths chap- ter may contribute to the prevention of substantial losses and casualties in the future. However, much information must be gathered and analysed, and the di€ferent Cantonal authorities must participate actively to successfully reduce future losses from natural-hazards. REFERENCES com- i’econornie des eaux et Office f6dCral de I’environ- nement, des for6ts et du paysage, Diffuseur Office rnatkriel, no et du V 30 k] < E < 300 (V Landslide Debris flow W > 2 cm/year) dm/year D<1 rn and V, < 1 m/s Flood T < 0.5 m mZ/s 0.5 m < T < 2 m or 0.5 m2is < V, T < 2 m2/s - V, or T < 0.5 kN, r n 2 Snow avalanche kN/m2 P < 3 < P < 30 kN/mz 3 ~~ ~~ ~~~ . 6.6 Kienholz, H. and Krummenacher, B., 1995: Recomman- dations: Ligende moduluble pour la cartographie des phe‘nomkner. SCrie Dangers naturels, Ofice fidiral de de f&diral central des imprimes mande 310.022/f, 19 pp. Lateltin, O., 1997. Recommandutionspour la prise en compfe du danger de rnent d~ terrrtorre, SCrie Dangers naturels, Office f6dCral High E > 300 kJ m/event V > dm/day or H > 1 D>1 rn and Vw > 7 m/s T > 2 m m2/s or V, T > 2 V,: P > 30 kN/m2 mouvements de terrain dam l’amhage- Table 6.1 - Criteria for intensity €. ktnetrc energy V P mean annual velocity of landslide avalanche pressure exerted dlsplacement H. on an obstacle horizontal flow velocity D thickness of debns front water depth T: Comprehensive risk assessment for natural hazards fidCral de l’epviron- fkderal de 1’Pconomie des eaux et Office nement, des forets et du paysage, Diffuseur Office central des imprimes et du mathel, no de com- mande 3 I O.O23/f, 42 pp. des forCts federal pour l’ktude de la Office fedCral et Institut neige et des avalanches (OFF and IFENA), 1984: Directives pour la prise en compte du danger d’avalnnch- es lors de I’exercice d’activirds touchnnt ii l’orgunisarion du territoire, Diffuseur Office fkddral central des no. 23 PP* l‘amk- Petrascheck, A and prise en compte 65 imprimts et du mattriel, de commande 310.020/f, Loat, R., 1997: Recomrnandationspour la du danger d’inondatwns Office nugernent fCdCral de dans naturels, territoire, Sthe dangers l’tconomie des eaux et Office federal de l’en- vironnement, des forks et du paysage, Diffuseur Office et du rnatkriel, no de com- du fddhal central des imprim6 mande 804.201/f, 32 pp.