Debris flows hazard mitigation: an example in the F. Menotre basin (central Italy)

Debris flows are important geomorphic events in a wide variety of landscape. Although repeated landsliding causes bedrock to underlie many hill slopes of mountain belts, some landslide debris remains and is stored as a thin colluvial cover, particularly in "hollows". These colluvial pockets act as slope failure "hot spots" by focusing infiltration of storm runoff, leading to local groundwater concentration above perched water tables and therefore enhanced failure potential. Consequently, colluvial deposits in hollows are particularly susceptible to landsliding. Once a failure occurs, hillslope sediment transport processes refill the scar, resulting in a cycle of gradual colluvium accumulation and periodic instability. Hillslope debris moves down-slope as a result of hillslope processes and where overland flow is either non-erosive or infrequent colluvium accumulates along the line of descendent. So the association of debris flow with hollows is governed by relations between sediment transport, hillslope hydrology and slope stability. Hollows consequently define a mappable debris flow hazard source areas. With these goals in mind we propose a GIS model where we have provided a record of landslide activity (debris flow events) in response to specific storm over areas of diverse geology, geomorpholgy, microclimates and vegetation. When combined with information on the physical properties of hillslope form and gradient, rainfall characteristics, and travel distance, these inventories could provide a foundation for the development of accurate and meaningful susceptibility maps. In particular susceptibility index identify those geologic units that produced the most debris flow in each study area in response to specific rainfall condition. To examine relations between geologic units and debris flow susceptibility, we calculated an index of debris flow susceptibility for each geologic unit in each representative elementary area. This is done by dividing the number of landslide initiation locations within a particular unit by the areal extent of that unit in the study area. The final aim of our research has been the identification of the terrain attributes related to the occurrence of debris flow and to quantify their relative contribution to the hazard assessment. The modeling work has focused on slope failure in the Menotre basin because the connection between severe storms and debris flows is clear and we had access to appropriate data to constrain the modeling.