Lidar images create model to predict landslide risk in coastal Brazil

18 Nov 2024

New approach has identified some 1,000 landslide points in São Sebastião state.

São Sebastião, a municipality on the coast of São Paulo state in Brazil that was partially cut off from the rest of the country in February 2023 after a period of torrential rain, had more than 1,000 landslide points.

That is according to an inventory produced by researchers at the University of São Paulo’s Institute of Astronomy, Geophysics and Atmospheric Sciences and Institute of Geosciences using aerial images collected shortly after the disaster.

The scientists are now analyzing higher-resolution lidar images to find out how the inventory correlates with other variables and to develop a method for mapping the risk of landslides more precisely.

“In this type of analysis, known as a landslide susceptibility assessment, knowledge of the areas in which landslides occurred, including topographical, geological and other data, is extrapolated for other areas.

“We currently have digital elevation models with a spatial resolution [pixel size] of 30 m. The lidar data lets us go a step farther and apply elevation models with higher resolution – as high as one meter – and more precision,” said Carlos Henrique Grohmann, a professor at IAG-USP.

Grohmann is the leader of the project, which is supported by FAPESP, and involves a partnership with the São Paulo State Institute of Geography and Cartography (IGC-SP, an arm of the Department of Budget and Administration). The institute is creating a repository of lidar images for the entire state.

The expectation is that the new methodology will be ready by the end of 2025, so that policymakers and planners can use it to map landslide-prone areas in the municipality more comprehensively.

In February 2023, São Sebastião declared a state of emergency due to the damage caused by a record amount of rain totaling 683 mm in less than 15 hours – the monthly average is 300 mm. At least 60 people died and hundreds were made homeless, and there was significant infrastructure destruction.

New tools

Landslides are common geological processes in mountainous areas, especially those with a tropical climate like Serra do Mar, where São Sebastião is located. In light of their destructive effects, especially in irregularly occupied areas such as squatter settlements, landslides are a focus for scientific studies aiming at identification, characterization, monitoring and prevention.

Grohmann’s group uses drone images and 3D modeling, as well as lidar. The 3D results display the shape of the terrain and specific surface features, characterizing the structure of the vegetation and the topography of the bare ground without the influence of forests or buildings.

In addition to predicting landslides in São Sebastião, the group expects to obtain a morphometric characterization and assessment of surface changes in certain local areas. The inventory that mapped landslide risks in the municipality is published in the Brazilian Journal of Geology. The data is available from Zenodo, a repository of open-access publications and information created to facilitate data and software sharing.