The Causes, Dynamics and Impacts of the December 2017 Catastrophic Mass Flow Aluvión: Villa Santa Lucía, Chile.

Abstract:

Periods of intense rainfall have been identified as a key trigger in catastrophic mass flow events in South America. The December 2017 aluvión at Villa Santa Lucía, Chile, is a prime example of a rainfall-triggered catastrophic mass flow in a paraglacial environment. This event caused 22 deaths and damaged 50% of the buildings in Villa Santa Lucía, submerging agricultural land. 124 mm of rainfall over a 24-hour period caused the failure of 4.1±0.76 x 10⁶m³ mountainside in the uppermost catchment of Rio Burritos. Post-event DEMs and aerial photographs provided the basis for geomorphological mapping and terrain analysis. Mass flow deposits were collected in the field in January 2019 to determine flow composition and its effects on rheology. Velocity and discharge rates were back calculated from superelevation measurements taken in the field and on aerial imagery. The flow travelled 11.9 km at an average velocity of 15 m/s±17% and eroded 33x 10⁶m³ sediment down the flow path, as well as 0.28±0.05 x 10⁶m² vegetation. The total volume of the flow was eight times larger than the initial failure, highlighting the importance of flow bulking. High concentrations of sands (53%) and gravels (43%) resulted in a sediment-rich debris flow which was viscous in spite of a low clay content (<0.22%). The flow deposited 3.9±0.72 x 10⁶m³ of material over the alluvial fan where Villa Santa Lucía is located. Catastrophic mass flows such as Villa Santa Lucía are likely to occur more frequently in South America in the future as intense rainfall events become more common as a consequence of global climate change. By studying recent catastrophic mass flow events, a greater understanding of the factors which control flow rheology, particularly relating to erosion and bulking, which is essential to reduce and mitigate against these hazards.