Rapid mapping of potential ground effects of the May 2023 Emilia-Romagna rainstorms.

Rapid mapping of the potential ground effects related to the flood events of May 2023 that affected part of the Emilia-Romagna Region, Central Italy. The study aims to a rapid detection of the areas with highest density events on large-scale and low-resolution. 

The map has been obtained with the methodology described by Notti et al., 2023. This procedure is based on the difference of the Normalized Difference Vegetation Index (NDVI) between the pre-event acquired images and post-event ones. The variation in the index is mostly linked to the processes that occur during the flood event, this allows the functional detection of the phenomena of slope instability. By using the download service of the Copernicus Open Access Hub platform (https://scihub.copernicus.eu/), the following Sentinel-2 images were utilized:

Pre-event Sentinel-2 image: 2022-05-13 (A);

Post-event Sentinel-2 image: 2023-05-23 (B). 

For each image, the NDVI has been computed; then the difference NDVI_post-NDVI_pre was performed. All the areas characterized by a variation of NDVI (DNVI) <=0.3 and slope >=15° have been identified (the slope was obtained from 10 m DTM TIN Italy, Tarquini et al.,2017). Lastly, the obtained results were converted in polygons. Ancillary information, such as average slope (derived where available also the from 5 m DTM issued by Emilia-Romagna Region) and the intersections with roads, buildings, rivers (available as shapefile from “Geoportale of Emilia-Romagna” region, https://geoportale.regione.emilia-romagna.it /), were added to the obtained polygons.

Since the presented work is a rapid, post event, mapping, a very simplified classification was carried out:

1. Slope instability processes (mainly shallow landslides) and secondary hydrographic network;
2. Instability processes mainly related to the bank erosion processes affecting the hydrographic network of the valley floor.

Given the limited availability of the high-resolution images, the mapping is solely based on the NDVI analysis performed on Sentinel-2 images. Consequently, the proposed procedure may generate potential false-positive related to: i) land use changes that occurred during the period; ii) residual effects of cloud cover of the May 23 Sentinel-2 image. For this purpose, a preliminary filtering procedure has been performed. However, local residual effects related to cloud coverage remain at this early stage.

It is important to note that, given the spatial resolution of the Sentinel 2 data of 10 m, the presented product represent an optimal tool to identify phenomena with dimensions greater than 100-200 m².

Further updates and refinements of the map will be made available as the new satellite images and data are progressively analyzed, as well as with the integration of field surveys still in progress.

References

Notti, D., Cignetti, M., Godone, D., and Giordan, D.: Semi-automatic mapping of shallow landslides using free Sentinel-2 and Google Earth Engine, Nat. Hazards Earth Syst. Sci., https://doi.org/10.5194/nhess-23-2625-2023, 2023.

Tarquini S., L. Nannipieri (2017). The 10 m-resolution TINITALY DEM as a trans-disciplinary basis for the analysis of the Italian territory: Current trends and new perspectives. Geomorphology, 281, 108-115.

Weier, J. and Herring, D. (2000). Measuring Vegetation (NDVI & EVI). NASA Earth Observatory, Washington DC.