Rockfalls change the runout and frequency of debris flows at Punta Nera (Eastern Italian Alps)

Abstract

In the Dolomitic region, many debris flow basins have headwaters characterized by extremely steep slopes of bare outcropping rock. Cliffs are commonly incised by multiple chutes which rapidly deliver runoff at their base where the channels incise scree deposits. Debris flows mobilize and transport sediment along such ephemeral channels following intense summer convective rainstorms. In the debris flow basin of Punta Nera, a series of rockfalls, which occurred in the upper part of the headwater between 2013 and 2016, caused dramatic changes in the debris flow dynamics. Starting from the summer of 2014, the runout of debris flow events increased so much that it reached the national road, which runs at the toe of the debris flow fan, attracting media attention and prompting the adoption of protective measures. Here, we use newspaper reports, direct observations, aerial and terrestrial photograph, monitoring data and topographic surveys to document the rockfalls and the debris flow activity. The sudden increase in sediment availability changed the magnitude of events, their runout and the critical conditions for debris-flow occurrence.