Radar remote sensing and elevation models in flood risk mapping
The mapping of flood-prone areas constitutes a primary technical input for spatial planning, the management of critical infrastructure and compliance with regulatory frameworks such as the European Floods Directive. In this context, Earth observation using synthetic aperture radar (SAR) sensors in orbit offers a differential capability: the detection of surface water bodies regardless of cloud cover conditions and solar light availability, which is decisive during flood episodes, precisely when optical platforms present the greatest operational limitations. The physical principle underpinning this capability lies in the behaviour of the radar signal over calm water surfaces, which act as specular reflectors and generate a low backscatter response that is readily discriminable from moist soils, flooded vegetation or built structures. The combination of multi-temporal SAR imagery with high-resolution digital elevation models makes it possible not only to delineate the extent of an active flood, but also to model the spatio-temporal progression of the flood event and to identify preferential accumulation areas linked to local topography. This integration forms the basis of the risk mapping products that feed early warning systems and the emergency plans of the competent authorities. From an operational standpoint, the revisit frequency of the space segment is a critical parameter. Extreme hydrological events evolve on timescales of hours, which demands an acquisition cadence compatible with the dynamics of the phenomenon. Current SAR satellite constellations have significantly reduced revisit intervals over a given area of interest, although effective coverage remains conditioned by orbital parameters and mission scheduling windows. The fusion of data from multiple satellites, including platforms from different operators or institutional programmes such as Copernicus, is common practice for increasing the temporal density of image series during an emergency. The integration of these observation products with hydrological models and cadastral databases introduces an additional level of technical complexity, but also substantially broadens the operational utility of the information. Operators of transport, energy or sanitation networks can incorporate flood risk layers derived from satellite imagery into their asset management systems, identifying exposed sections or installations and prioritising maintenance or structural reinforcement interventions. For public administrations, the availability of methodologically traceable and up-to-date mapping reinforces the technical basis of spatial planning instruments and civil protection plans within the framework of current European legislation.
NASSAT - Network Satellite Systems