1. INTRODUCTION. EXAMPLE OF SECHILIENNE ROCKFALL (France)

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1 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 1 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 2 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 3 Christophe Bonnard, EPFL-LMS 1. INTRODUCTION 2. IMIRILAND PROJECT 3. VULNERABILITY ASPECTS 4. MEANING OF LANDSLIDE RISK 5. RISK MANAGEMENT 6. CASE STUDIES IN EUROPE 7. CASE STUDIES IN SOUTH AMERICA 8. CONCLUSIONS General view of the Séchilienne landslide (from uphill). In the foreground, the small village of Grand Serre in Séchilienne 1. INTRODUCTION Definition of hazard Probability of occurrence of a given danger within a specific period of time, considering the potentially involved area and the intensity of the damaging phenomenon. Definition of vulnerability Degree of loss for a given element at risk, or set of such elements, resulting from the occurrence of a natural phenomenon of a given magnitude. Elements at risk Objects including any land, resources, environmental values, buildings, infrastructures, economic activities and/or persons in the areas, that are exposed to natural and technological hazards. Risk Consequences of a landslide hazard, expressed by the product of the occurrence probability and of the consequences. DETERMINATION OF DIRECT AND INDIRECT POTENTIAL IMPACTS Direct impacts Damaging consequences resulting from the direct effect of a landslide mass on the elements at risk. These direct impacts are assessed at a local scale. Indirect impacts Damaging consequences resulting from the secondary physical effects of a landslide mass on the environment (for example a failing landslide dam) or from the secondary economic effect of the hindrances caused by the landslide (for example interruption of tourism traffic due to a rockfall on a road). These indirect effects have to be reassessed at a regional scale. FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 4 PROBABILITY OF OCCURRENCE OF A LANDSLIDE Landslides are not repetitive phenomena like avalanches or floods, so that a strict definition of their probability of occurrence cannot be given. For sudden exceptional phenomena, there may be a link between their probability of occurrence and the probability of the triggering mechanism (conditional probability). The probability of occurrence is often assessed with respect to the number of similar events having occurred in a region over a long period of time (analysis of historic data). However a same landslide,especially if it is large, may develop into different scenarios each of one may be qualified by a certain probability. FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 5 EXAMPLE OF SECHILIENNE ROCKFALL (France) Present local zone of rockfall Potential huge rockfall Zone of slope deformation Road exposed to initial rockfall River exposed to landslide dam hazard Towns and large industries downstream Dwellings and farms in the impact area Springs for water consumption in the valley immediately downstream of the exposed zone Actions undertaken: Displacement of road Creation of new river channel Building of protection dam Construction of diversion tunnel FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 6 IMIRILAND PROJECT : Investigation and mitigation of large lanslide risks Danger identification : determination of the lansdlide characteristics Hazard analysis : quantitative assessment of potential landslide development according to different scenarios Consequence analysis : determination of potential impact on exposed elements, considering their value and the different hazard intensities Risk assessment : computation of risks for each scenario of a given probability and for each type of impact Analysis of indirect effects of landslide scenarios and of possible interactions between different types of hazard >Need for multidisciplinary investigations 1

2 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 7 VULNERABILITY ASPECTS Physical vulnerability Social vulnerability Environmental vulnerability Economic vulnerability Representation of vulnerability values Value of exposed objects - Computation of real value - Use of index values FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 8 Physical vulnerability Vp Rate of damage or destruction to buildings, infrastructures, service lines The value of Vp depends on : - the nature of the exposed element - the type of landslide mechanism and the hazard level - the structure of the building or work - the building materials - the foundation system - the size and shape of the element -its age For slides with differential movements, Vp depends mainly on the location of the building with respect to secondary scarps. FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 9 Risk components: - Physical risk - Social risk - Environmental risk - Economic risk 4. MEANING OF LANDSLIDE RISK Combination of risks : The risks for each category corresponding to different scenarios can be added, but total risks have no real meaning FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 10 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 11 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 12 Eastern part of Switzerland, canton of Graubünden Bern Sedru n Area : 1.5 km 2 River basin of Vorderrhein Volume : 100 million m 3 Case of Sedrun landslide (Graubünden, Switzerland) 1650 inhabitants Bern Rhein Vorderrhein P. Oberalp Sedrun P. Gotthard Strem river Average velocity : 5-40 cm/year (locally 80 cm/year) Drun river Sedrun Railwa y Cantonal road Rhein river 2

3 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 13 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 14 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk B Profile 26 F B F F B 24 Approximative limit of the unstable zone A D 516 A F Displacements in cm/year Displacements Point 514 Point 516 Point 26 Point 27 Point 23 Point 24 Point 2 Point 3 Point 5 Point 7 Point Time in years 3 m FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 16 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 17 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 18 Scenarios 1 st scenario Scenario 1: Mass landslide Scenario 2: Very high volume landslide with debris flow Scenario 3: Rockfall and formation of a dam and a lake Scenario 1: Mass landslide on the right side of Drun Tobel at Plauncas Tgamos. Dam in Drun Tobel and debris flow. Plauncas Tgamos Mass landslide Formation of a lake Dam Debris flow 3

4 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 19 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 20 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 21 2 nd scenario Heavy rainfall 3 rd scenario Scenario 2: Very high volume landslide on the right side of Drun Tobel with debris flow. High volume landslide Debris flow Scenario 3: Rockfall from summit 2458 in Val Strem and formation of a dam and a lake. Brutal rupture of the dam due to percolation possible, or overflowing. FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 22 Formation of a lake FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 23 Hypotheses for the selection of scenarios FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 24 Dam Rockfall Among the various possible scenarios which can be assumed with respect to the observed geological conditions, mechanisms and movements, three cases of potential behaviour with different probabilities of occurrence have been selected as being the most reliable ones. Debris flow These investigation hypotheses should be confirmed at a later stage, for instance by numerical modeling, when appropriate data are obtained by boreholes. Within the IMIRILAND Project the modeling carried out simulating the effect of the glacier retreat did not give the expected results, as the present observed movements are the largest at the top of the slope, whereas in the modeling they were quite reduced. 4

5 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 25 Station Children s holiday camp Railway FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 26 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 27 Sport center Data about Sedrun Strem river School Cantonal road Drun river 1650 inhabitants 9 hotels 15 restaurants 5250 beds Alptransit building site FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 28 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 29 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 30 5

6 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 31 Consequences of 1st scenario FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 32 Risk analysis FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 33 Consequences of 2nd scenario Scenario 1: ~ 1.3 mio Physical cost : 2 million SFr. Economical consequences : Railway to Oberalp pass closed for 1 day Environmental consequences : Limited damage on fields and Drun river bed FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 34 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 35 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 36 Risk analysis Consequences of 3rd scenario Risk analysis Scenario 2: ~ 7 mio Scenario 3: ~ 8 mio Physical cost : 11 million SFr. Physical cost : 13 million SFr. Economical consequences : Railway closed for 3 days, cantonal road closed for 1 day, impact on reputation of resort Environmental consequences : Important damage on fields, Drun river bed and Strem river bed Economical consequences : Railway closed for 2 days, cantonal road closed for 2 day, impact on reputation of resort Environmental consequences : Important damage on fields, Drun river bed and Strem river bed 6

7 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 37 Comparison between two methods of consequences quantification Value of exposed objects Sedrun : estimated value by object Ex: value of a house : SFr. Ceppo Morelli : relative value by object Ex: house in residential area : Assets rel. value FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 38 Comparison between two methods of consequences quantification Vulnerability of exposed objects Sedrun : estimated degree of damage by object (in relation with danger intensity and specificities of the construction) 0-100% Ceppo Morelli : estimated degree of damage by object (in relation with danger intensity) 0-1 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 39 Comparison between two methods of consequences quantification Consequences for exposed objects Sedrun : estimated value vulnerability Ex: damaged house : = SFr. Ceppo Morelli : relative value vulnerability Ex: damaged house in residential area : = consequence rate of FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 40 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 41 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 42 Management strategy and practical measures Appropriate land planning limitation of construction in danger zone Protection works to prevent overflowing of the Drun river or the Strem river. Evacuation of deposit materials Survey, monitoring of landslide activity and warning system Although no appropriate data allow the computation of the probability of occurrence of the different scenarios considered, they can be qualitatively assessed as : Case 1 : high probability (approx. 1/10) Case 2 : medium probability (approx. 1/100) Case 3 : low probability (approx. 1/1 000) and then quantified with reference to the Swiss recommendations. The corresponding potential damage computed shows that the highest risk is related to case 1 for physical damage; but when considering indirect impacts, case 2 might be the most severe. The communal authorities are elaborating similar landslide scenarios and installed a continuous monitoring of the displacements of some points, but for the moment they consider only snow avalanches in their local plan; as these phenomena have induced several disasters in the past. Case of Ceppo Morelli Landslide (Italy) Landslide zone in Anzasca Valley in the north of Piemonte Affected by frequent rock block fall, as well as by a rockslide in the upper part of the slope, which may induce a large rockfall Frequent critical phases (in particular 1977, 2000) Some houses are exposed, but the major risk affects the access road to Macugnaga resort station, as well as the river (risk of damming) 7

8 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 43 Quantification of hazard FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 44 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 45 COMBINATION OF RISKS Movements measured after October 2000 Modelling of rock block fall Modelling of rockfall propagation of different size (3 scenarios) Probabilities of the scenarios : 1/15 1/115 1/600 Assessment of values of exposed elements by index values When risks of different kinds for several scenarios have been identified, the question rises of how they can be combined. It is more meaningful to add the respective risks for any scenario, which correspond to a given aspect (physical, social, environmental, economic). Adding the risks concerning the physical and social aspects means to give a value to the human life, which is difficult. The different risks related to physical, social, environmental and economic aspects are presented for the case of Ceppo Morelli, in italy, which is threatened by rock blocks and rockfall of different intensities. The major impact consists in the potential blockage of the road which leads to the resort station of MACUGNAGA. FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 46 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 47 FORM OSE POST-GRADUATE COURSE Landslide vulnerability and risk 48 INDIRECT RISKS RISK MANAGEMENT CONCLUSIONS A potential major rockfall may cause several indirect risks : - impeding the transit traffic in the valley - affecting the tourist activity in Macugnaga - Interrupting the electricity supply in this village - affecting the environment (forest, fauna) - Above all, causing a landslide dam in the valley which might induce major damage once it would fail. Indeed a major flood release in the vallley would cause major destructions in all the villages downstream, down to Torino. A series of infrastructures would be affected. One of the main duties of land planning implies the consideration of natural constraints, that include the natural hazards. The potential conflicts between the consequences of natural hazards and the foreseen land use has to be avoided. Plannig include short and long term prospective action, at the regional and local scale. It also includes actions aiming at saving lives when facing disasters, through mitigation actions. At a regional scale. The master plan considers the presence or absence of dangers through an indicative map. At a local scale, the local management plan needs a more detailed hazard map especially in the built areas or in the zones in which it is planned to build. The the hazard level must be specified and corresponding preventive measures have to be taken (prohibition, instructions, information). The existence of possible risks has to be put forward at an earlier stage so that new constructions or infrastructures are not exposed to hazard. In order to determine such hazard levels, it is necessary to build scenarios and assess their zone of impact. After a disaster it is first required to establish the safety zones in which the natural phenomena will develop (new river beds), then determine the appropriate areas where buildings will be safe, and finally plan and carry out the required access roads. The construction of new roads as a first objective is an error. 8

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