Hydrogeological and Climatic Risks: The Emblematic Case of an Exceptional Debris Flow in Central Apennines (Italy)

The present work aims to highlight a particular event where there was a concomitance of particular hydrogeological and climatic conditions, unfortunately in a scenario which could be repeated. In fact, the south-eastern slope of Mount Vermenone (Central Apennines, Italy) was affected by a severe landslide movement of debris flow type. The phenomenon was triggered in the mid-high portion where a vast and thick debris accumulation (coarse, loose, and weakly cemented materials) was present. The aforementioned materials, in addition to being located on a steep slope, were frequently in a condition of water saturation during the winter, given the presence of springs and the characteristic impluvial form in which they were concentrated. In January 1997, a rapid snowmelt increased the water saturations in the thick debris, so that the materials became unstable on the slope and the movement is quickly activated. A fast and dense debris flow was triggered, moving along the steep slope down to the main valley floor: Here, the high energy flow did not stop but continued, spreading its accumulation almost to the built-up area. Only after several days and months the stream was able to resume its course, incising the huge mass of alluvial materials that can still be seen today at the edge of the plain. In addition, the groundwater resource also has been compromised as some springs in the area have changed their regime or even disappeared. The event was of considerable impact, and given its exceptional nature, studies on the dynamics of the landslide phenomenon were carried out. These in particular were conducted on the possibility of reactivations and also toward the monitoring of the adjacent areas. In this regard, a first response from the installed instruments was that no movement was recorded, and therefore, we are facing a very high threshold before triggering, probably due to the high permeability of the materials.