•   Monitoring
•   Forecasting
•   Action & Mitigation
•   ICT & Organisation

RESEARCH DETAILS
RAPID FLOOD MAPPING USING SATELLITE DATA

Operational systems that are running in areas with regular flooding need more data to ensure a good flood forecast and thus a safer living environment. Research has pointed out that in case of a possible flood, there is always a shortage on hydrological gauge data. Also, in remote areas in the world, there is in most cases no real time information on flood extent, which hampers efficient mitigation actions.

In the innovation track Operational Satellite-Based Flood Mapping, Fugro, Deltares, HKV and Haskoning connect near real-time flood information (as measured by satellites) to maps that can be used by the water manager for the two main purposes of improving existing flood forecasting systems and models, and by flood extent mapping for mitigation actions.

A combination of satellite images was used to create flood maps for the Mekong area, where a focus is put on the Tonle Sap lake (Cambodia). Images can be seen in figures 1 - 3. As a lot of the Mekong area is situated in remote areas, flood extent maps are a welcome addition to existing hydrological information. They can be delivered on an operational basis by combining information and displaying them in the Deltares FEWS system. Furthermore, we have created "virtual sensors" that measure water level. These sensors are all made from satellite data (ESA's River and Lake service, SAR and optical data). In this way, more gauge data is added to the flood management system.

Figure 1: The probability of water as derived from low resolution satellite images. The data is processed from ENVISAT-ASAR Global Monitoring data (courtesy of ESA).

Conclusions and recommendations of this project are:

• Flood extent maps with low (1 x 1 km) or medium (0.1 x 0.1 km) spatial resolution are available on a real time basis, i.e. within 2 - 6 hours after caption time;
• A combination of flood maps can generate flood maps on daily basis;
• Probability maps of flooded areas and water level are recommended when working with different satellite images;
• Flood maps can be used to create more ‘virtual sensors' on water level, accurate topography model are needed for this;
• A high resolution topography model will significantly improve flood interpretation;
• Urban areas create in general a more expensive and complex flood map system (i.e. expensive high resolution imagery, heavy reflection of urban architecture, removable flood walls or dykes and connection to flood plain model);
• In each area where the flood maps are implemented a detailed user requirement study is necessary.

In this research, use was made of the following satellite products:

• Radar altimetry (ESA River and Lake, coutersy of ESA and De Montfort University
• Envisat ASAR images (courtesy of ESA)
• MODIS flood map from Dartmouth Flood ObservatoryShuttle Radar Tomography Mission Digital Elevation Model (SRTM)
• Shuttle Radar Tomography Mission Digital Elevation Model (SRTM)

Figure 2: Flood extent maps can be combined with other, existing flood inititatives in the FEWS system. Left is a flood map as derived in this project, right is the validation with an underlying MODIS derived flood maps from Dartmouth Flood Observatory.

Figure 3: Difference between now (top right, global mode, 1 x 1 km) and the potential of higher resolution pictures (150 x 150 m).