Linking Inca Terraces with Landslide Occurrence in the Ticsani Valley, Peru

Since the times of the Incas, farmers in the remote Andes of Peru have constructed terraces to grow crops in a landscape characterized by steep slopes, semiarid climate, and landslide geohazards. Recent investigations have concluded that terracing and irrigation techniques could enhance landslide risk due to the increase in water percolation and interception of surface flow in unstable slopes, leading to failure. In this study, we generated an inventory of 170 landslides and terraced areas to assess the spatial coherence, causative relations, and geomechanical processes linking landslide presence and Inca terraces in a 250 km2 area located in the Ticsani valley, southern Peru. To assess spatial coherence, a tool was developed based on the confusion matrix approach. Performance parameters were quantified for areas close to the main rivers and communities yielding precision and recall values between 64% and 81%. On a larger scale, poor performance was obtained pointing to the existence of additional processes linked to landslide presence. To investigate the role of other natural variables in landslide prediction, a logistic regression analysis was performed. The results showed that terrace presence is a statistically relevant factor that bolsters landslide presence predictions, apart from first-order natural variables like distance to rivers, curvature, and geology. To explore potential geomechanical processes linking terraces and slope failures, FEM numerical modeling was conducted. Results suggested that both decreased permeability and increased surface irrigation, at 70% of the average annual rainfall, are capable of inducing slope failure. Overall, irrigated terraces appear to further promote slope instability due to infiltration of irrigation water in an area characterized by fluvial erosion, high relief, and poor geologic materials, exposing local communities to increased landslide risk.