Debris flow hazard mitigation using GIS: an example in the F. Menotre basin (Central Italy) - Conference "Fast slope movements-Prediction and prevention for risk mitigation

Debris flows are important geomorphic events in a wide variety of landscape (Armanini, Michiue, 1997). 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 nonerosive 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. Furthermore quantitative debris flow hazard assessment are complicated by the observation that debris flows typically involve only a portion of the colluvium stored in a hollow and several debris flows can be released from a single hollow over many years. For planning purpose, there is a need for a relatively simple, objective method for describing hollows, estimating the colluvium stored within hollows, and comparing the relative debris flow hazard between hollows. Such a system would help to institutionalize professional recognition of the geomorphic input into land management and planning processes, and potentially could be used to prioritize mitigation for sites exposed to up-slope debris hazards.