Some considerations on geomorphological hazard assessment for territorial planning

The researches on the geomorphological hazard carried out in the last years in mountain areas of the Modena and Bologna Provinces (Northern Apennines) have shown, in several situations, that instability processes do not completely correspond with those mapped by the Instability Inventory Maps used by the Public Administrations for their territorial planning within the framework of the Province-Coordinated Territorial Plans (named as PTCP). The differences consist in the location, state of activity and/or extent of the instability phenomena which, from a practical point of view, are the characterizing elements of the instability processes.The differences highlighted are due to different causes. One of them is the fact that the Instability Inventory Maps are derived from the Geological maps created for the Regional Geological Map. In the latter, according to its construction criteria, the superficial deposits have been given a secondary importance and just those with a wide extent have been mapped, giving more relevance to the substratum and to the structural-tectonic aspects. Another cause is represented by the very active and intense geomorphological dynamics of the Northern Apennines: geomorphological processes may produce or reactivate instability conditions as it happened during the autumn/winter period of 2008/2009. In that period, after unfavourable meteorological events (coincidence of high precipitations, both solid and liquid, and snow pack melting), many new and reactivated landslide occurred.Following this consideration, the Instability Inventory Maps, that have a great applied significance for territorial planning, should be updated not only on the base of a traditional geological map, but also following an accurate geomorphological cartography (implementation of geomorphological maps at the 1:5,000/1:10,000 scale) which can precisely define the genesis, dynamics and morphometry of instability phenomena. This study should be based on detailed mining of historical and bibliographical data, on consultation of geological documentation (geomorphological and geological maps, professional geological reports etc.), on the interpretation of multi-scale aerial and satellite images of different time periods and on the detailed field survey accompanied by interviews with local inhabitants about past instability events. The great bunch of collected data should be gathered within a Geodatabase through which it is possible to perform a critical cross-validation and synthesis of the knowledge acquired.With the final aim of produce a document suitable for territorial planning, in a following step the geomorphological cartography and the Geodatabase should be used to asses the geomorphological hazard. In this study, geomorphological hazard maps make reference to the Swiss method which complies with the Italian law (DM180/98, L267/98, DPCM 29/9/98). This method is based on a classification of the intensity and frequency of the events for each category of instability processes. This is achieved by means of univocal matrix combinations which allow the definition of various levels of geomorphological hazard (possibly cross-verified by applied-geology investigations).Cartography of hydrogeological instability should include, as a first step, the updating of the Instability Inventory Maps, at least for the areas of future development, by means of geomorphological maps; this should lead to produce, as second step, geomorphological hazard maps should, also suggesting remedial measures in order to solve or at least mitigate processes resulting from geomorphological hazards.