Our environment and society are affected by climate change in many ways. Amongst others, the intensification of the hydrological cycle is resulting in more extreme drought and precipitation events, leading to an increase in the frequency and intensity of flood events. Although heavy monsoons, hurricanes and cyclones long seemed far-away phenomena for Western Europe, this impact also became painfully clear in that region during the summer of 2021.
Synthetic Aperture Radar (SAR) is particularly suited to monitor floods from space, thanks to its ability to penetrate clouds and its independence of an external illumination source. The Copernicus program has boosted the field of SAR remote sensing with the launch of the Sentinel-1 constellation, the first SAR sensors to provide free imagery and global coverage. Throughout the past years, many SAR-based algorithms for flood mapping have been developed, ranging from single scene to time series based and from manual to highly automated approaches. Typically, a high degree of automation and global applicability are pursued, to enable fast mapping independent of the flooded region. However, by doing so, locally available information might not be fully exploited.
The TerraFlood algorithm (Landuyt et al., 2021) was originally developed with and for the Flanders Environment Agency, responsible for operational water management in the Flanders region. The algorithm combines hierarchical thresholding and region growing, both on the pixel and object level. Aiming to fully exploit locally available data, it requires a SAR image pair (containing a flood and pre-flood image) and several ancillary data layers, including elevation, land cover, and flood risk, as input. The output map discriminates permanent water, open flooding, long-term flooding, possible flooding, flooded vegetation, and possibly flooded forests from dry land. Invisible forested areas, forested areas for which the flood state is unknown, are indicated too.
The algorithm’s accuracy and robustness, both for emergency mapping and automated monitoring, were assessed based on maps of 36 flood events that occurred between 2016 and 2020. Besides near-real time flood maps, also reduced products like flood recurrence maps can be provided. End users are provided free and easy access to all products through the Terrascope (1) platform. The algorithm immediately proved its value during and after the summer 2021 floods. As these floods hit hard in the Walloon region, maps were also provided to and used by several Walloon institutions. Feedback from both end users will be used to further improve the algorithm and service. Additionally, potential improvements for vegetated and dense urban areas, the two main pitfalls of the TerraFlood algorithm and Sentinel-1 imagery in general, remain under investigation.
(1) Terrascope (www.terrascope.be) provides analysis-ready satellite data and derived products. Registered users can access Sentinel-1, -2 and -5P and PROBA-V data as well as land cover, vegetation indices and elevation products.
L. Landuyt, F. M. B. Van Coillie, B. Vogels, J. Dewelde and N. E. C. Verhoest, "Towards Operational Flood Monitoring in Flanders Using Sentinel-1," in IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, vol. 14, pp. 11004-11018, 2021.