Flood Extent

Product Description

This product is a component of the Copernicus Emergency Management Service (CEMS). It is suitable for both near real-time and non-time-critical applications. This means it is applicable for both rapid response as well as planning for future events. All Sentinel-1 SAR data is processed and analysed using a suite of flood detection algorithms to increase the level of confidence in the resulting flood maps. The algorithms have been developed by leading research groups (LIST, DLR and TU Wien). The international consortium responsible for the development of this product operates under a Framework Contract with the European Commission’s Joint Research Centre. Further to those developing the algorithms, this consortium includes the Earth Observation Data Centre for Water Resources Monitoring GmbH (EODC), GeoVille Information Systems and Data Processing GmbH and the International Research Centre in Environmental Monitoring.

Input Data Sources

• Sentinel-1 GRDH

• Additional Data:

• • Copernicus 30 m DEM
• • Global Human Settlement Layer (GHSL)
• • World Settlement Footprint (WSF2015)
• • Global forest change dataset

Required Processing

Pre-processing and ancillary data – Sentinel-1 IW GRDH Vertical-Vertical polarization data is ingested and pre-processed using the Sentinel Application Platform (SNAP). The pre-processed analysis-ready data is 20 m sigma nought calibrated backscatter. The use of the Copernicus DEM for geocoding requires a transformation from orthometric height to ellipsoidal height. This incorporates geoid undulations from Earth Gravitational Models. The Sentinel-1 Global Backscatter Model (S1-GBM) is used to determine areas where flood detection is improbable because of surface structures on radar return signal. A near-global Height Above Nearest Drainage (HAND) index is generated to allow for the exclusion of areas where flooding is unlikely due to the hydrologic-topographic conditions. The affected population of a flood is derived from the GHSL-POP layer of the GHSL dataset. The GHSL and the WSF 2015 datasets are used to determine where flooding should be ignored due to the likelihood of artefacts introduced by interactions between SAR signals and built-up areas. Th3 Global forest change dataset is used for gap-filling in areas where S1-GBM is limited by temporal coverage.

Flood detection – Three flood detection algorithms are applied to the Sentinel-1 data to derive the observed flood extent layer of this product. The first is a change detection algorithm which requires as input the current Sentinel-1 scene, a previous overlapping scene from the same orbit, and the previous flood extent map. The second is a fuzzy logic-based approach using region growing. Inputs are the Sentinel-1 scene, a DEM, a dataset of areas where flooding is not prone and a reference water layer. The third is a probabilistic approach taking in the Sentinel-1 scene, a projected local-incidence layer and harmonic model parameters for seasonal backscatter variation simulation. Pixels are assessed for flooding probability. An ensemble flood mapping algorithm uses the output of all three previous algorithms to decide if each pixel is classed as flooded or not. If 2 out of 3 algorithms determine flooding, then a pixel is classed as flooded.

The output layers of this product are listed here. It should be noted that many further processing steps are required to generate some of these layers and details of these can be found on the product site ( Welcome to GFM | EODC Public Wiki).

Output layers:

• • Observed flood extent (GeoTIFF, shapefile)
• • Observed water extent (GeoTIFF, shapefile)
• • Reference water mask (GeoTIFF, shapefile)
• • Exclusion mask (GeoTIFF)
• • Uncertainty values (GeoTIFF)
• • Advisory flags (GeoTIFF)
• • S1 metadata (KML)
• • S1 footprint (KML)
• • S1 schedule (KML)
• • Affected population (GeoTIFF)
• • Affected land cover (GeoTIFF)

Coverage and Frequency

• Coverage: Global – Sentinel-1 overpasses, 250 km swath

• Frequency: daily, near real-time

• Temporal range: 2014 – present

Resolutions

• Spatial resolution: 20 m

• Temporal resolution: 6 days

Accuracy and Validations

Quality control of this product includes the following activities:

• • Thematic accuracy assessments
• • Product timeliness checks
• • Service availability checks
• • Overall quality assessments

Limitations

• If near real-time applications are desired, the coverage is dependent on the overpass of Sentinel-1, which currently has only a single satellite collecting data.

• Daily coastline dynamics due to tides are not accounted for. This may lead to sand banks being classified as flooded if they are submerged at high tide.

• Narrow “no-data” stripes exist at the start and end of image scenes. This is because adjacent images are not available for processing at the same time. Waiting for all adjacent images to be available would allow this to be solved but would compromise the timeliness.

Delivery Format

Products can be downloaded as layers in raster or vector format. Area of interest selection is possible via a map viewer after user registration.

Accessibility and Cost

Data can be accessed and downloaded via the portal:  Global Flood Monitoring. Users are required to create an account. Data is free to download.