Map of sites affected by mass-movement and floods
2nd edition

Paola Reichenbach, Fausto Guzzetti e Mauro Cardinali
CNR - IRPI Perugia
U.O. 3.1. GNDCI

The second edition of the "Map of Sites Affected by Mass-movements and Floods" is the latest result of a long term research effort aimed at collecting and analysing historical data on hydrological and geological catastrophes in Italy. With respect to the previous map that showed the location of about 5000 landslide sites and 4000 inundation sites the new map shows the location of more than 15,000 sites (9086 landslides and 6456 floods) affected by catastrophic events in this century. There are 2608 landslide sites and 2317 inundation sites affected recursively, i.e., more than once.

The map was prepared by mapping, as points at 1:100,000 scale, all sites inventoried by the AVI project. Sites for witch the location was uncertain were mapped in the municipality chief town. If compared to the old map the density of affected sites is higher. Dishomogeneities are still present and mostly due to the methodology used to complete the inventory.

The map, far from being a map of hydrological and geological hazard or risk, provides the most accurate and updated synoptic view of the distribution and frequency of geological and hydrological catastrophic events in Italy. Careful analysis of the map shows that most of the Italian territory was historically subject to mass-movements or inundation. All the Italian provinces experienced at least once a landslide or an inundation in the period 1918-1994. An inset shows a preliminary ranking of the Italian territory based on the density of sites. Site density was computed by counting the number of landslide sites, adding the number of inundated sites and dividing it by the extent of each municipality in square kilometres. There are 5497 municipalities (67.8%) that were affected either by landslides (47.6%) or by floods (42.8%) at least once and 22.6% of the municipalities that were affected by both landslides and floods.

The map shows also an update of regional histograms showing the frequency of inundated sites. With respect to the previous edition, the number of inundated sites in each region has largely increased. The number of times each site was inundated is also increased. Some of the peak values correspond to region or province chief towns, reflecting a known bias in the historical database toward hinabited or densely populated areas. For few regions the peak value is the result of the presence in the database of events related to sea storms, squalls, or the "acqua alta".

Lastly the map shows an example of possible use of the historical information inventoried by the AVI project, namely: the assessment of the hydrological and geological hazard and the definition of regional warning or alarm thresholds for the occurrence of hydrological and geological catastrophes.