Centennial landslide risk evolution in Medellin, Colombia

The metropolitan area of Medellin, Colombia, gradually expanded toward steep hillslopes due to the constraining rugged topography surrounding the city. Steeper hillslopes are more prone to landslides by default. When combined with inadequate planning due to high population pressure, the landslide risk in the city increases disproportionately. 

We analyzed the city's expansion in 18 time steps since 1770. In particular, we explored the empirical relation between urban growth and landslide occurrences. From 1976 to 2023, the city nearly duplicated its area, going from 56 km² to 110 km². We showed that gradually, steeper hillslopes were urbanized, especially since 1941, accompanied by a growing proportion of land in high landslide hazard zones. We also found that landslide activity predominantly occurred on the outskirts of the urban area at any given time, resulting in harm primarily to newly emerged neighbourhoods.

Within the study period, in 2005, when areal images were available, we also categorized the metropolitan area into consolidated urbanization, i.e., compact regular mesh, compact irregular mesh, condominiums, and equipment; and unbound urbanization, i.e., level I: scarce urbanization, level II: road deficit/scattered urbanization, and level III: urbanization scattered around an axis. 

We assigned each category to variables such as hillslope angles, landslide hazards, and socioeconomic strata. Unbound urbanization was identified on steeper hillslopes, coinciding with poorer communities. Hence, we conclude that the persistent expansion of Medellin into landslide-prone areas poses a considerable threat to the population, especially those with limited opportunities in deprived neighbourhoods. By documenting the city's evolution in relation to landslide occurrences, we want to emphasize the pressing landslide risk to local and international policymakers.