The dynamics and impacts of the December 2017 catastrophic mass flow Villa Santa Lucia, Chile

Landslides and mass flows are dynamic processes that involve the movement of rock, debris and earth down a slope. As a result of the 2017 catastrophic mass flow, these processes have been further established as a significant risk to the population of Chile, and further afield. Through field site investigations, it is possible to develop a greater insight into the mechanisms and conditions that influence the dynamics of these phenomena.

On Saturday 16 December 2017, a catastrophic debris flow (aluvión) partially destroyed the village of Villa Santa Lucía and a 5 km long reach of the Panamerican Highway resulting in 22 fatalities. The apparent trigger was an intense rainfall event of 124 mm in 24h associated with an elevated 0˚C isotherm (1600 m.a.s.l.) that led to the failure of 5.5 - 6.8x10⁶m³  mountainside in the uppermost catchment of Rio Burritos near the SE end of the Cordón Yelcho Glacier. The landslide transformed rapidly into a highly mobile debris flow as it entrained water from the Rio Burritos river and glacier ice from the Cordón Yelcho.

This study characterises the geomorphological impacts and dynamics of the 2017 mass flow. Post-event DEMs, aerial photos and satellite imagery provided the basis for geomorphological mapping and terrain analysis. Fieldwork in January 2019 allowed sampling of mass flow deposits, logging of sedimentary sections and dGPS surveys.

Both erosion and deposition occurred over the Villa Santa Lucía flow path. Erosion occurred more frequently in the first 7.9km of the flow path due to high slope angles and presence of the Rio Burritos that channelised flow. A high proportion of coarse particles in the flow enhanced basal scouring and erosion of the valley sides, resulting in significant flow bulking. A total of 7.6x10⁶m³ – 7.7x10⁶m³  of material was deposited across the latter 6.3km of the flow path.

Sediment sample analysis showed that the flow began as cohesive and viscous in nature in spite of a lack of clay particles and high proportions of sands and gravels. The addition of water from the Rio Burritos reduced the viscosity of the flow as the flow propagated downstream. This resulted in enhanced lobe spreading and particle interactions in the depositional zone. In spite of this water entrainment, the flow remained both sediment and debris rich over its duration.

Catastrophic mass flows like the event at Villa Santa Lucía are likely to become more common around the world in the future as intense rainfall events become more frequent due to the dominance of El Nino Southern Oscillation (ENSO) events. By studying recent catastrophic mass flow events, an insight into the relationship between mass flow triggers and flow composition will be developed. This will allow for greater understanding of how these influence mass flow behaviours. As a result, it may then be possible to predict the rheology and routes of future flows. These predictions have the ability to be used to protect communities from such events in the future.