Debris flow hazard and mitigation works in Fiames slope (Dolomites, Italy).

Active debris flow scree in the territory of Cortina d'Ampezzo (Dolomites, Italy) seriously threatens the urbanized areas and the state road. In September 1997, following a 25.000 m(3) magnitude debris flow that temporarily dammed the river, a retention basin with a storage capacity of about 15000 m(3), was built upstream the state road. In this paper, the effectiveness of the existing debris basin has been evaluated regarding to the magnitude of the design debris flow. Based on geomorphological and historical data, major events up to 30.000 m(3) may be possible. To avoid destruction caused by debris flows, new mitigation measures are planned. Proper zoning is implemented, but for the state road cannot be relocated, both passive and active mitigation measures must be adopted. A warning system has been designed, based on both the early detecting of debris flow-induced ground vibrations and the overcoming of rain thresholds, for stopping the traffic on the state road in the event of a debris flow. As it is not possible to mitigate debris flows along the flow paths, the most suitable action to reduce the risk is to control the debris deposition. Proper design requires an estimate of the magnitude, frequency and delineation of debris flow hazard and risk of the site, together with fundamental information such as possible runout distances, depth, velocities and impact force of a design debris flow. For this purpose, a two-dimensional flow routing model, FLO-2D, has been used, based on a detailed DEM of the site, data collected in a similar neighbour debris flow monitored site and reasonable assumptions regarding sediment concentration and other rheological parameters. A more effective design of a retention basin is proposed.