Monti Martani (Umbria, Italy) Alluvial Fans: Hazards Sites and Occurrence

In this paper we proposed modeling and simulation approaches for testing the debris flows occurrence hypothesis. The approach is an empirically and process based, and use multiple physically-based simulations to evaluate hazard down-slope from initiation sites in alluvial fans of the Terni basin-northern area (Umbria, Italy). The northern part of the area is bounded by the M. Martani normal fault that controls the drainage network where produces a large debris piedimont deposition. The main fault scarp is cut by narrow streams that represents the dominant constructional process by three alluvial fans generations. Field-based and remote sensing observations from the area will be used to provide a sound empirical evaluation of the new landslide occurrence hypothesis. In humid, soil-mantled environments in particular, debris flow always originate in fine-scale valleys in steep, rhythmically dissected terrain. Concave planform contours define topographic swales, referred to as "hollows" in the nomenclature, that typically contain colluvial soils significantly thicker than those found on adjacent slope. Areas underlain by massive, resistant bedrock, show that the majority of debris flows originated in colluvium-filled hollows. Hollows consequently define a mappable debris flow hazard. The association of debris flow with hollows is governed by relations between sediment transport, hillslope hydrology and slope stability. Consequently, colluvial deposits in hollows are particularly susceptible to landsliding. Furthermore topographic converge also focuses subsurface runoff into hollows, so high intensity rainfall cells indicate that a lack of historic landsliding from specific hollow is by no means an indicator of future stability. The strong likelihood that the 25m resolution DEM can be used in this project means that we will have the topographic control to do more detailed modeling of hill slope hydrology to account for spatial and temporal variability in groundwater saturation on hillslopes, and the consequences for slope failure. We will use a software tools for multiflow routing of runoff given a precipitation model, a DEM, and a probabilistic model of finding pockets of colluvium on hillslopes which is demonstrated for the synthetic hillslope.