The temporal stability and activity of landslides in Europe with ž ...

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4 change. The basic research questions of objective 2 are: – Does the dated landslide event carry a climate signal? – Are there other signals carried by the event? – Are there any regional coincidences in dated landslide events in Europe? – Are there palaeoclimatic reconstructions in the region under investigation? – Are we able to relate the dated event with general circulation models Ž GCM. predictions? – Is there any information in terms of the lifetime of landslides? ( ) R. Dikau, L. SchrottrGeomorphology 30 1999 1–12 – Are we able to draw first conclusions to continue the European programme on climate and natural hazards Ž EPOCH. synthesis Ž see Table 1 . ? For the second objective, the methodology included geomorphological mapping, a variety of suitable dating techniques, especially lichenometrical and radiometrical methods and standard chronostratigraphic techniques in order to establish time series of landslide incidents. A further, special technique was the use of trees as a tool to study the history of slope activity Ž dendrochronology . . During movement of a slide or parts of the slide, the tree is bent. The redressing of the trees after the incident can be Table 1 The importance of different objectives of the TESLEC project in relation to the previous Ž EPOCH. and subsequent project Ž NEWTECH.
detected by asymmetric ring growth. Using carefully applied statistical filtering techniques, the rate of strain can be dated. A motion activity time series related to different parts of the landslide complex can be obtained ŽCorominas and Moya, 1999; Fantucci and Sorriso-Valvo, 1999 . . 2.3. ObjectiÕe 3: the deÕelopment of a hydrological and slope stability modelling framework for the prediction of landslide actiÕity in a changing climate Landslides are often considered as rigid blocks with a constant safety factor along the slope. However, this approach gives only a general view of the stability of the slope. For a better understanding of complex landslides, the spatial distribution of stability within the landslide complex must be known. Objective 3 linked climatic change model outputs to hillslope hydrology and groundwater models used within slope stability analysis ŽBuma and Dehn, 1998; Bonomi and Cavallin, 1999; Dehn and Buma, 1999; Angeli et al., 1999 . . The location of the ‘‘weakest parts of the chain’’ gives an idea of future distribution on unstable and effectively stable parts of the landslide. In some cases, the hydrological part in the stability model forms, in fact, the trigger determining the activity of the slope. The water balance must be analysed by both simple hydrological grey box models or tank models and the new models of climatic change. Calibration of the hydrological part of the stability model was done with existing piezometric data or back analysis. An important tool can be the use of probability stability models. These models calculate the transition probabilities of the different sections within the landslide. Other models, showing the internal stability distribution of landslides, have been developed. These probability models show the distribution of the safety factor Ž stability coefficient along the slip surface. and may give a direct indication of internal stability. Once the stability model procedures have been made by means of operation and calibration, they will be manipulated to simulate the effect of climatic change. The most important task was to recognise the relationship between changing effective precipitation and groundwater level or piezometric conditions. A search was be made for groundwater stations where this relationship could be developed. This modelling is important because it proves or ( ) R. Dikau, L. SchrottrGeomorphology 30 1999 1–12 5 refuses the use of our existing knowledge of global climatic change in order to predict the spatial and temporal occurrence of landsliding. A further research task was to evaluate various climatic change models with respect to the changing probability of landsliding. Today, many global models of climatic change exist. With regard to questions about environmental changes, one of the most important research tasks was to find the key to how these models can be regionalised to smaller areas or how the generalities can be applied to the specific problems in landslide areas. Because the confidence in the regional changes simulated by GCMs is low and simple interpolation of the coarse grid output is inadequate, downscaling techniques are applied. These objectives are in close connection with two other EC-funded research projects related with landsliderclimate relations Ž Table 1 . . The TESLEC project shows a shift in research focus. While the previously project running from 1991–1993 within the EPOCH Ž Soldati, 1996. was mainly concerned with the development and definition of time classification standards and the reconstruction of past landslide events, the TESLEC project was additionally focused on modelling the current and future climatic impact on single landslides. The continuation of the research tasks in the NEWTECH project was related to a stronger emphasis on single landslide monitoring and modelling and future climate impact scenarios Ž . Corominas et al., 1998 . 3. Study areas The study areas of the TESLEC project were selected in terms of having a variety of landslide types showing different sensitivities to climatic impacts. The areas are located in five European countries Ž Fig. 2. including four different landslide types Ž fall, slide, flow, complex . . The different time scales of investigation are shown in Fig. 3. Most of the studies are related to landslide events during the last 50 years. The study in southern Italy goes back to 1850 using dendrogeomorphological analysis ŽFantucci and Sorriso-Valvo, 1999 . , and for two areas in northern Spain ŽCantabria and Asturias. a first attempt has been made towards chronologies of landslide activities during the last 120,000 years Ž Gonzales-Dıez ´ ´ et al., 1999. and mid
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