Flood Analysis using SAR imagery : Water level estimation by fusion between RADAR image and high resolution DEM
Analyse d'inondation par imagerie satellite RADAR : estimation de niveaux d'eau par fusion entre image RADAR et MNT fin
Résumé
Spatial characterization is a recurrent problem in flood risk management. However, spatial characteristics of a flood event are not easily gaugeable directly in the field with current tools. Techniques based on remote sensing imagery, especially on satellite imagery, could be very useful to complete gauging, calculate flood extends and volumes. RADAR sensors, with their exclusive cloud penetration capacity, appear to be of great interest in flood events monitoring. The general aim of our work is to develop way for spatial characterisation of flood events, using RADAR images, in order to help hydraulic modelling. Therefore, we have chosen a three-stage approach. The first stage aims to extract flooded area from RADAR images, the second one to enhance quality of inundated area calculation and to determine waterline elevation, and the last one to use spatial information calculated for flood modelling. Here, we just present the result for the two first stages. RADAR data, which are imperfect and sometimes incomplete, allow us to product only uncertain flood maps. As a consequence, to relevantly extract flooded area using RADAR images, we have to focus on accuracies and uncertainties due to RADAR data and treatments used for mapping, we have to examine uncertainties linked to RADAR data and. These uncertainties are especially linked with RADAR characteristics (e.g.: speckle, spatial and radiometric resolutions, etc.), meteorological conditions (e.g.: wind, rain) and land cover (e.g.: forest and cities). This stage especially enables us to share calculated flood extent into relevant and irrelevant areas. To enhance the calculated flood extent and determine water levels, we match relevant flooded areas with very high resolution DEM thanks to a Geographic Information System (GIS). Moreover, to reduce water level uncertainties and to make water level estimations hydraulically coherent we introduce rules of hydraulic coherence on successive water levels along flood plain. The perspective of these to first steps is to use spatial characteristics calculated with SAR images and DEM to help hydraulic modelling. For example, we aim to use water level estimations as initial condition, or for calibration of a hydraulic model. This kind of improvement of SAR imagery can be very useful to develop automatic methods, which could allow us to calculate future evolution of a beginning flood. The area of interest is the Moselle river between Thionville and Berg-sur-Moselle (France). The accuracy of water level estimations is about ± 50 cm using RADARSAT 1 image of flood (acquired 02-28-1997).