HYDROLOGICAL THRESHOLDS FOR THE PREDICTION OF LANDSLIDES AND FLOODS IN THE UPPER TIBER RIVER BASIN

F. Guzzetti, M. Cardinali & P. Reichenbach (CNR-IRPI Perugia, via Madonna Alta, 126 - 06100 Perugia, Italy)

In the upper section of the Tiber River basin (4,147 km2) in Central Italy the occurrence of landslide and flooding was related with hydrological characteristics of triggering meteorological events. Information on the occurrence of landslide and flood was available from a nation-wide inventory. Mean daily discharge values at the Ponte Nuovo gauging station were digitised for the period 1925-1941 and 1951-1980 and charted in five-years intervals. 532 meteorological events, broadly defined as a series of consecutive days having mean daily discharge value exceeding 100 m3/sec, were identified. For each event an estimate of the approximate flood volume was also computed. The highest mean discharge value was chosen as representative of the whole series and was used to assign the event to the appropriate discharge class. Multiple flooding events having more than one peak discharge value were treated as single events.

Information on landslides or floods were available for 56% of the 532 events. Only about 5% of the meteorological events triggered numerous landslides and/or caused extensive flooding in the basin. Of all the events that exceeded 400 and 700 m3/sec, 50% and 90%, respectively, triggered landslides or caused flooding somewhere in the basin. All events with a flood volume greater than 250 million m3 triggered mass movements and caused flooding. These considerations can be used to define the probability that a meteorological event of a given magnitude will trigger a hydrogeological catastrophe in the basin. However it will not be possible to define the exact time or location of the expected landslide or flood. Future developments will extend the analysis to the entire Tiber basin, taking into account other gauging stations, to obtain a better spatial and temporal resolution of the prediction. The possibility of using other indexes representative of the general hydrological behaviour of the basin, such as the weighted cumulative discharge volume, will also be tested.