Community-Based Flood Risk Assessment Using GIS for the Town of San Sebastián, Guatemala

The municipality of San Sebastián, considered one of the most prone to natural hazards in Guatemala, was selected as one of the pilot areas of the UNESCO program on “Capacity Building for Natural Disaster Reduction”. The town is located within the Samalá basin, near the active channel of the river, which is heavily affected by lahars related to the active SantiaguitoSanta María volcanic complex. The town is threatened both by lahars as well as by flooding. As is often the case in towns such as San Sebastián, very little data (e.g., cadastral, topographical, hydrological and meteorological) was available to perform a flood risk assessment based on traditional hydrological modeling. To perform a basic flood risk assessment, an alternative method for producing several sets of data was implemented by combining aerial photo interpretation, the use of data questionnaires in a community-based field data collection campaign, and subsequent analysis using Geographical Information Systems (GIS). A cadastral base map with an attribute database was generated as key inputs for flood hazard and vulnerability assessment. Information collected by means of a small-scale survey on a home-by-home basis, in cooperation with a local NGO, was used to reconstruct the last flood that affected the town during Hurricane Mitch in 1998. Based on this historic flood scenario vulnerability curves relating water height and speed with expected level of damage were constructed for three of the main elements at risk within the urban area: buildings, building contents, and road network. Vulnerability maps were generated using GIS and combined with cost information to derive estimated losses. The resulting cadastral database containing physical and social information at the parcel level as well as detailed information on flood hazard, vulnerability and expected losses, can be used by municipal authorities as a valuable tool in municipal planning, including a wider range of applications in addition to risk management.