EO based assessment of geomorphological impacts caused by heavy rainfall events in high mountain areas

A novel methodology for assessing the catchments most severely affected by extensive heavy rainfall events is presented to support post‑disaster recovery activities. The approach exploits pre‑ and post‑event optical Sentinel‑2 imagery to perform a dual change‑detection analysis. The first component targets land‑cover alterations (Land Cover Change Detection, LCCD), including slope denudation, debris deposition, and alluvial flooding. The second component focuses on variations in the optical properties of surface waters (Water Colour Change Detection, WCCD), such as colour shifts of lake and river waters associated with increased turbidity.

Integrating information on water‑colour change (WCCD) with a more traditional change detection analysis based on variations in vegetation spectral indices (LCCD) is advantageous in high‑altitude environments, where vegetation cover is sparse or absent. The combined change detection compensates for the individual limitations of each method and enhances overall performance by 6–11% and 31–38% compared with the standalone use of LCCD and WCCD, respectively. The final product is a severity map that classifies catchments into increasing levels of impact, derived from the aggregated magnitude of changes detected by both the LCCD and WCCD components.

The methodology relies entirely on freely accessible datasets (Copernicus Sentinel‑2 imagery, the TINITALY 1.1 Digital Elevation Model, and OpenStreetMap layers), and all processing steps are implemented using open‑source software (Google Earth Engine, QGIS, and R), ensuring its potential applicability at the global scale. The approach was tested on two distinct heavy‑rainfall events that affected the northwestern Italian Alps in June and September 2024. Across these case studies, the methodology achieved a correspondence rate of 59–65% between the catchments identified as severely affected and those containing documented natural instability events.