Abstract
In rivers, temporary dams formed by landslides, rock falls or debris flows pose a severe threat to human life, public infrastructure and private assets. Besides the on-site impacts from the mass movement itself, these phenomena can cause disastrous off-site impacts by backwater inundation or catastrophic outburst floods. These secondary fluvial hazards can affect areas at substantial distances both up-and downstream of the site where the blockage occurred. In the present case study, the formation of rockslide dams and their subsequent impacts are viewed as a cascade of linked geomorphological and hydrological phenomena and assessed accordingly. The case study area extends over the two municipalities of Stilfs/Stelvio and Prad am Stilsferjoch/Prato allo Stelvio in the Autonomous Province of Bozen/Bolzano (South Tyrol) in Northern Italy, where the upper part of the slope above the village of Trafoi is at risk of a sudden failure. The present work builds on previous modelling attempts that predicted deposition heights up to 75 m in the channel of the Trafoi River. In this paper, we qualitatively identify all involved processes and create scenarios of various dam heights and dam breach processes. Modelling results show that the river downstream of the temporary dam features a channel conveyance sufficient for the peak discharge associated to a dam height of 4 m whereas dam heights of 16 m and 75 m would lead to severe flood events within the downstream municipality of Prad am Stilfser Joch. The modelled peak discharge values range from 35 m³s-1 to 2554 m³s-1 , which fits well with values reported from other dam breach occurrences.