Characterization of a landslide‐triggered debris fow at a rainforest‐covered mountain region in Brazil

Abstract

Debris fows represent great hazard to humans due to their high destructive power. Under- standing their hydrogeomorphic dynamics is fundamental in hazard assessment studies, especially in subtropical and tropical regions where debris fows have scarcely been studied when compared to other mass-wasting processes. Thus, this study aims at systematically analyzing the meteorological and geomorphological factors that characterize a landslide- triggered debris fow at the Pedra Branca catchment (Serra do Mar, Brazil), to quantify the debris fow’s magnitude, peak discharge and velocity. A magnitude comparison with empirical equations (Italian Alps, Taiwan, Serra do Mar) is also conducted. The meteor- ological analysis is based on satellite data and rain gauge measurements, while the geo- morphological characterization is based on terrestrial and aerial investigations, with high spatial resolution. The results indicate that it was a large-sized stony debris fow, with a total magnitude of 120,195 m3, a peak discharge of 2146.7 m3 s −1 and a peak velocity of 26.5 m s −1. The debris fow was triggered by a 188-mm rainfall in 3 h (maximum intensity of 128 mm h−1), with an estimated return period of 15 to 20 years, which, combined with the intense accumulation of on-channel debris (ca. 37,000 m3), indicates that new high- magnitude debris fows in the catchment and the region are likely to occur within the next two decades. The knowledge of the potential frequency and magnitude (F–M) can support the creation of F–M relationships for Serra do Mar, a prerequisite for reliable hazard man- agement and monitoring programs.