Valley Formation by Debris-Flow Incision in the Central Apennines, Italy

Debris flows and related mass movements play a central role in carving mountainous ridge-valley topography where steep channels, typically with slopes greater than 6%, constitute a significant fraction (30-75%) of valley relief by length. At present, however, we have only a rudimentary understanding of the physics of valley formation by repeated debris-flow incision over geomorphically relevant time scales (104-106 yrs). This is unfortunate because in such steep landscapes it is mass flow activity, rather than incision by mountain rivers that determines hillslope gradients and hence limits upland relief. A natural experiment in the central and southern Apennines, Italy provides new constraints on the physics of valley incision by granular mass flows. The region contains numerous examples of small, steep mountain valleys, typically with drainage area less than 5km2 and with almost linear longitudinal profiles. Most are ephemeral and lack classic fluvial features, but many bear signs of scour by granular mass flows, including direct evidence for grain-impact wear. Because many of the Apennine valleys are bounded by active normal faults, whose rates and timing are constrained independently, the mass flows are incising terrain for which we have excellent controls on the boundary conditions that drive downcutting in debris-flow dominated channels. We compare small catchments draining faults with a range of slip rates, including examples of mid-Pleistocene acceleration in rate of throw, to provide information on the morphodynamic response of the channels to temporally and spatially varying rates of base level fall. The apparently nonlinear relationship between valley slope and local throw rate implies that gradient controls event frequency, magnitude, or both. We compare observed morphology with the predictions of a 1D model of debris-flow incision. The model computes changes in valley floor elevation due to scour of weathered material and/or bedrock by repeated debris-flow events, in which the motion of each flow is calculated from a simplified momentum balance. Results suggest that dynamic frictional debris entrainment and/or rock wear is required to explain the longitudinal profiles of Apennines mountain valleys.