Limited equilibrium and finite element modeling of critical groundwater conditions driving the Achoma landslide, Central Andes REgion, Arequipa, Peru

The Colca Valley has historically been subject to a variety of geological hazards such as landslides, rock falls, and debris flows. On June 18, 2020, a rotational landslide occurred near the community of Achoma in the Colca Valley. The event destroyed agricultural land, impacting the finances of many families, and the displaced material crossed the Colca River and created a dam that increased the risk of flooding for the towns upstream. Because of the potential for future movement at this site or similar landslides nearby in the future, it is important to understand the potential triggers and therefore future management strategies for the area. Field mapping, computer modeling of slope stability, records of rainfall and agricultural activity, and observations of leakage from a water conveyance structure upgradient were used to test various groundwater and infiltration scenarios and identify the critical groundwater conditions that affected the landslide. These scenarios include three possible contributions to the triggering of the landslide: rainfall, local irrigation, or leakage from the water conveyance system. The modeling results indicate that the cause was most likely a combination of factors: weak lacustrine materials within the landslide mass, irrigation activity in the area, high antecedent rainfall, and an incremental increase in the water table that may be related to leakage of a tunnel section of the water conveyance project.