12. What types of maps on landslides exist?

Mapping is often a necessary step in landslide hazard studies. Maps may consist either of basic documents such as geomorphological maps and landslide maps or of landslide susceptibility maps and hazard zonation maps.

  • What are they used for?

Mapping can be defined as:

  • direct, when the landslide extent and distribution are recognised on a geomorphological basis with the aim of extrapolating slope stability assessments to the rest of the investigated area;
  • indirect, when the possibility of landslide occurrence is assessed with or without consideration of landslide distribution.

Direct mapping is usually obtained through aerial photo-interpretation and ground surveys, together with bibliographical and archive investigations. Indirect landslide maps are built on the base of the factors (mainly geomorphological and geological) that may favour their occurrence. Therefore these maps represent areas in which there is possibility of landslide triggering.

Landslides inventory mapsare effectively and easily understandable products by both experts, such as geologists and geomorphologists, and by non experts, including decision makers, planners and civil defence managers

  • Where can I get these maps?

The maps can be found in public authorities’ offices, research agencies and scientific reviews, journal and books.

Landslides typology: a1d) active earthflow; a2b) dormant slide; a2d) dormant earthflow; a2g) dormant complex landslide Map of the hydrogeological disturbance of the Regione Emilia Romagna (https://applicazioni.regione.emilia-romagna.it/cartografia_sgss/user/viewer.jsp?service=dissesto, scale 1:5000)

By Alessandro Pasuto1, Mauro Soldati2, & Doriano Castaldini2
1 CNR – IRPI, Padova (Italy)
2 Dipartimento di Scienze della Terra, Università di Modena e Reggio Emilia (Italy)

Mapping is often a necessary step in landslide hazard studies. Maps may consist either of basic documents such as geomorphological maps and landslide maps or of landslide susceptibility maps and hazard zonation maps. Mapping can be defined as direct, when the landslide extent and distribution are recognised on a geomorphological basis with the aim of extrapolating slope stability assessments to the rest of the investigated area, or indirect, when the possibility of landslide occurrence is assessed in relation to the controlling factors, with or without consideration of landslide distribution (Hansen, 1984).

Direct mapping is usually obtained through aerial photo-interpretation and ground surveys, which vary proportionally according to the scale adopted for the study, together with bibliographical and archive investigations. The result can be inventory maps which record landslide distribution over an area (e.g Rybár, 1973; Alger & Brabb, 1985; Guzzetti & Cardinali, 1989). Landslides inventory maps are effectively and easily understandable products fot both experts, such as geologists and geomorphologists, and for not experts, including decision makers, planners and civil defence managers (Galli et al., 2008).

Fig. 1 The Camporaghena landslide map, a large multiple rotational slide located in the Tuscan side of the Northern Apennines. The Red and Orange colours indicate, respectively active and dormant sectors (From Nardi et al., 1990).
Fig.2 The Camporaghena landslide, which was pre-existing, was reactivated by the 1920 Garfagnana-Lunigiana strong earthquake and contributed to the damage caused by this seismic shock in the village. Wide portions of the landslide are still active and frequently involve the main road and several houses. (From Nardi et al., 1990).

The Landslides inventory maps are generally the basis for further detailed studies aiming to assess future failures. The information contained in these maps may also include landslide type, bedrock lithology, superficial deposits, slope angles, engineering geological zonation etc.

In Italy an homogenous and updated Inventory of Italian landslides has been implemented by APAT (Geological Survey of Italy/Land Protection and Georesources Department). The Italian Inventory of landslides (IFFI) holds over 460,000 phenomena occurred in the Italian territory.
APAT published the IFFI Inventory on the Internet as well as the report on the Project (APAT 2007) with the aim of disseminating information about landslides to national and local administrations, research institutes, geologists, engineers and citizens .
Go to IFFI Inventory web site (http://www.apat.gov.it/site/it-IT/Progetti/IFFI_-_Inventario_dei_fenomeni_franosi_in_Italia/)

Fig. 3. Landslide index (%) in Italy (by IFFI Inventory web site)

In the Emilia – Romagna Region, the Instability Inventory Maps, created on the base of traditional geological maps, have a great applied significance for territorial planning. In fact, it is forbidden to built up on “areas affected by active landslides” (landslides which are currently active or that have been reactivated since the last 30 years) meanwhile it is allowed to build up on ” areas affected by dormant landslides” (landslides that have not showed signs of activity since the last 30 years and that could be reactivated by their original causes) or on “Potentially instable areas” (quaternary deposits affected by evident superficial morphogenetic processes) only if prevenction and mitigation measures have been adopted.

Fig.4 .Extract of the Instability Inventory Maps at the scale 1:5,000. Casona Sud area in the Modena Apennines. In red “Areas affected by active landslides”; in yellow ” Areas affected by dormant landslides” , in violet the study area (from Provincia di Modena, 2006, see also http://www.provincia.modena.it/page.asp?IDCategoria=7&IDSezione=863 http://archiviocartografico.regione.emilia-romagna.it/)
Fig. 5. Panoramic vieuw from East to West of the Casona Sud area in the Modena Apennines (Photo D. Castaldini)

Geomorphological maps are another product of direct mapping. These are generally at a larger scale (less than 1:25,000) than those described above and form a very useful basis for landslide hazard studies since they include details on general slope morphology and related processes, and define the type of failure and degree of activity as well as the features associated with landslides. Geomorphological mapping may be easily and usefully applied to the study of mass movements.

There exist many significant examples of landslide-orientated geomorphological maps, such as that of Brundsen et al. (1975), carried out with aim of identifying landslide hazard in an area of Nepal where the construction of a road and a bridge was proposed, or that of Panizza (1990) where the landslides surrounding the town of Cortina d’Ampezzo (Italian Dolomites) are mapped in detail. An interesting attempt to include the temporal aspect in landslide mapping was made by Flageollet (1994) in the above mentioned Dolomite area. The temporal information reported on the computer-based landslide map of the area of Cortina d’Ampezzo is related to periods of activity during the Holocene and to the frequency and incidence (first time failure or removement failure) of the movements. An updating of the Panizza (1990) map has been carried out by Pasuto et al. (2005).

Fig.6. Extract of the Geomorphological map of the Cortina d’Ampezzo area (Dolomites, Italian Alps) at the 1:20,000 scale (from Pasuto et al., 2005).
Fig.7. Panoramica view of the Cortina d’Ampezzo area of Fig. 5 (Dolomites, Italian Alps)

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).

Fig. 8. Example of landslide inventory map (from Cascini, 2005)

Starting from these data, danger maps were then implemented by adopting velocity estimates as well as of the source and propagation areas potentially affected by first stage movements (Fig.9).

Fig. 9. Example of landslide danger map (from Cascini, 2005)

Simplified hazard maps as well as simplified vulnerability maps for all the towns (450) were produced. Referring to the Varnes’ formula, the risk classes (Fig. 10) were obtained by overlapping hazard and vulnerability maps. An example of map containing the attention and risk classes previously defined is shown in Fig. 11.

Fig. 10. Nominal scale for risk level evaluation (from Cascini, 2005)
Fig. 11. Example of map containing attention and risk zones (from Cascini, 2005)


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