12. What types of maps on landslides exist?

Geomorphological maps may in many cases be too complicated to be read by non-specialists. This is the reason why planners and engineers are generally provided with derivative maps which appear simpler from a purely graphic standpoint and take into account only those processes and landforms that are connected with slope instability. The product can be a geomorphologically-based landslide hazard map. Many examples, carried out according to different methodologies, are known in literature. The first example should refer to the French ZERMOS (Zones Exposées aux Risques de Mouvement du Sol et du Sous-sol) hazard maps which have been produced for several sample areas at a scale 1:25,000. The hazard (actual and potential) is assessed and rated taking into account distribution of landslides, evidence of instability, as well as geological, geomorphological and hydrological aspects.

Another significant example is provided by Kienholz (1978) who carried out a study in the Grindenwald, an area of the Swiss Alps affected by mass movements and avalanches; first a geomorphological map was produced and from this a natural hazard map was obtained. The degree of hazard was assessed for a high number of previously defined geomorphological units using checklists.

When the casual factors of landslides can be recognised, measured and mapped and the analysis of the distributions of the different factors controlling landsliding can be made, indirect mapping can be carried out.

An original method aiming at the elaboration of hazard susceptibility maps has been defined and tested in a few sites in Italy and has been adopted by the National Geological Survey as the official procedure for the compilation of geological hazard maps (Amanti et al., 1992). This method includes both indirect and direct mapping procedures. In fact, this approach consists, on the one hand, of the analysis of the causes of instability (ascribable to both slope and fluvial processes) which are outlined in a series of thematic maps (geological, hydrogeological, vegetational etc.) and weighted by means of specific procedures, with the aim of producing a first derivative map (integrated analysis map) depicting the distribution of potentially unstable areas. On the other hand, by means of geomorphological investigations, the analysis of the effects of instability is carried out in order to give an outline of landforms and processes, both active and inactive, referable to geomorphological instability. The cartographic documents produced during this phase are a geomorphological map and a second derivative map (map of geomorphological dynamics) showing the areas which are at present unstable. The comparison and critical discussion of these two derivative maps lead to the production of a “geomorphological hazard map”, a concise document depicting present and potential unstable areas subdivided according to the causes of instability.
A partly similar approach was used for research on landslide hazard in Tuscany (Italy) which led to the elaboration of several landslide maps, accompanied by geological information permitting the assessment, albeit qualitatively, of the ratio between the degree of stability attributed to an area and its structural characteristics. In these hazard maps the following categories are distinguished: highly hazardous areas, including active and dormant landslides, areas characterised by high potential instability linked to the morphological aspects, areas prone to landsliding owing to lithological aspects, areas of medium stability and finally stable areas (Nardi et al., 1985).

Jones (1992) refers to a further approach, Land Systems Mapping, which is placed among indirect mapping techniques and is based on the subdivision of the study territory into limited areas (land systems, land units, land elements, passing from large scale to small scale subdivisions) in which predictable combinations of landforms, soils, vegetation and superficial processes take place. This approach, created with the goal of producing basic maps for rural planning, has been extremely useful in landslide hazard assessment since it allows large portions of territory to be quickly classified providing a useful regional picture for data collection and storage.
In the last decade many other methods has been proposed for the assessment of landslide hazard and risk all over the world (e.g. Heinmann et al.. 1998, Raetzo et al., 2002; Santacana et al., 2003; Zezerè et al. 2004; Guzzetti et al.. 2005) because the quantification of risk has gained importance in many disciplines. Anyhow the problem of attempting to quantify landslide hazard and risk is still difficult to solve (Cascini et al. 2005, van Westen et al., 2005).

In Italy, the zoning of areas subject to hydro-geological risks has been regulated since 1998 by a specific law and its associated application decree, which were released after the “Sarno” mudflow disaster (go to 13.2. Sarno study case). However, these juridical documents contain only some general guidelines for a common working methodology to be adopted for hydro-geological risk assessment in all of the Italian territory, and leave Basin Authorities free to define their own specific guidelines and, upon will, involve in this process research institutions and private consultants. This led to a large differentiation of procedures among Basin Authorities, and in practice the only standards adopted nation wide regard the work steps to be adopted, the goal of the investigation (that is the subdivision of risk in 4 classes) and the constrains to be adopted on high risk zones in terms of land use limitation and land management policy.

In particular, the risk zoning was calibrated according to the 4 risk classes defined by the Central Goverment as follows:
i) Very high (R4): human life loss and destruction of buildings, infrastructure and environmentall as well as interruption of economic activities are expected;
ii) High (R3): victims, functional damage to buildings and infrastructure, as well as partial interruption of economic activities are possible;
iii) Medium (R2): limited damage to buildings, infrastructure and environmentalmay occur;
iv) Low (R1): social, economic and environmental damageare of marginal relevance;

The results obtained for the territory of the National Authority of Liri – Garigliano and Volturno river basin (a territory of about 12,000 km2 located in Central-Southern Italy) are summarized in the following (Cascini, 2005). Inside the study area, undeveloped areas affected by dormant, active or potential landslides were mapped and classified according to the risk classes defined and the Cruden and Varnes (1996) suggestions; these areas were considered worthy of different attention levels.

To zone the risk and attention areas, geological, geomorphological and soil cover maps were preliminarily compiled. Subsequently, using these maps as well as aerphoto interpretation and available informations, 30,000 landslides together with their surroinding areas, and zones potentially affected by fast slope movements were mapped using Varnes (1978) classification, creep evidence, state of activity and other criteria described by Cascini (2005)(Fig.8).