Geological Hazard and Risk Evaluation Using GIS: Methodology and Model Applied to Medellin, Colombia

Abstract

Geographic Information Systems (GIS) were used to characterize and evaluate geologic hazards, vulnerability and risk in the northwestern urban area of Medellin, Colombia. Seven factors were considered in assessing hazards: bedrock and surficial geology, structural geology, climate, topography, geomorphic units and processes, land use, and hydrology. Hazard was assessed using a two-step procedure: 1. Each factor was subdivided into category ratings (1, lowest hazard to 10, highest hazard) based on the correspondence of components of that factor with hazards such as slope failure, as indicated by Boulean overlay processes applied to maps of the factors. 2. Each factor was then subjectively weighted, and this weighting value was multiplied by the factor's category rating for a specific site. Summing these values for the seven factors produced an overall rating for the site. GIS-based software (GRASS) used the algorithms for these factors to generate hazard zoning maps of areas prone to landslides, floods, and streambank erosion. After geological hazards were defined for the study area, we estimated the vulnerability (Ve) of various elements for an event of ‘i’ intensity. The zoning of risk was evaluated as a function of the magnitude of hazards versus the vulnerability of human and social infrastructure to be affected by any hazards. Results were obtained as risks of landslide, of floods, and of bank erosion, from zero (very low) to five (extremely high). Combining these analyses with planning requirements, we categorized the study area into 14 classes of land-use suitability for planning procedures; 7 classes defined as areas suitable for human settlement, and 7 classes defined as unsuitable for building, and most effectively reserved for parks and forests. The categories and maps produced in this manner are useful to urban planners concerned with geologic hazard management and mitigation.