Recent landslides in the Chanchán River basin (Ecuador) – The case study of Chunchi and Alausí.

The Chanchán River basin, located in the Chimborazo province in the Northern Andes of Ecuador, is affected by several factors contributing to slope instabilities. The high frequency of Landslides results in significant social and economic impacts. Hereby, two extraordinary landslides have occurred recently, being firstly in the Chunchi canton in 2021, affecting 704 people and damaging linear infrastructure such as two bridges and road sections, and secondly in the Alausí canton in 2023, causing even 75 casualties, while damaging first- and second-order roads, and collapsing 427 m of railway. In the present research, landslide geomorphology and ground lithology were studied in the field and using geophysical techniques. Potential triggering factors were also evaluated through the analysis of the rainfall data from the last decade, recent seismic events, and changes in land use within the study area. All these conditions allowed to focus on the similarities between the two catastrophic events.

The results yielded that the Chunchi landslide was a combination of rotational, creep, and flow failure types. In this case, the soils involved were superficial a superficial layer of silty sand and a deeper one of clayey sand, while the water table was detected at a depth of approximately 4 m due to recent rainfall. Conversely, the Alausí landslide was of the rotational type, primarily affecting colluvial soils and secondary volcanic lahars up to 4 m depth, volcanic lahars with blocks of dacitic or andesitic lavas from 4 to 13 m and further below altered volcanic tuffs.

After studying the potential triggering factors, the seismic effect was dismissed in both cases, as no significant seismicity occurred in the areas shortly prior to the landslides. In the Chunchi landslide, the rainfall registered before the event is similar to those of previous years, although the infiltration of water due to the agricultural use of the soil and deforestation may have contributed to the event. Alternatively, the most probable triggering factor of the Alausí landslide was the accumulated rainfall, as an increase of up to 600% was recorded in the three months preceding the event, compared to the average values of recent years.

Extreme climate events are expected to become more intense in the coming years due to factors such as climate change. This, on its own or combined with potential changes in land use, may be able to result in an increase of the frequency of landslide events in the Chanchán River basin.

Acknowledgements

This work is supported by the funding scheme of the European Commission, Marie Skłodowska-Curie Actions Staff Exchanges in the frame of the project UPGRADE – GA 101131146.