# Multitemporal SAR Data and 2D Hydrodynamic Model Flood Scenario Dynamics Assessment
## 1 Introduction
This situation is fast improving, however, thanks to advances in remote sensing, especially Synthetic Aperture Radar (SAR) [17], (e.g., ERS and ASAR with a spatial resolution not smaller than 25 m, or TerraSAR-X, RADARSAT-2 and COSMO-SkyMed characterized by a resolution as refined as 3 m or better [18]), that may be the only source of information available during emergencies, especially in data-scarce environments. In the context of this study, the disadvantage of traditional optical sensors, such as MODIS; is their low spatial resolution (250-500 m) and, moreover, the fact that the number of available flooding-related images could effectively be small because of their sensitivity to cloud cover. Thus, the main merits of SAR satellites, working in the microwave range of the electromagnetic spectrum (1-10 GHz), is their sensitive to water and it is also able to collect data independent of time of day and to observe the Earth's surface even in cloudy and rainy conditions (except for very intense precipitations, which can affect higher frequency sensors [19; 20]). Moreover, SAR offers a high spatial resolution, which can reach up to 1 m [21] with the new generation of X-band instruments, such as COSMO-SkyMed (CSK). Even the problem of the time needed to revisit SAR data (critical for their operational use for flood monitoring) is now overcome thanks to recent satellite constellations (e.g., CSK, Sentinel) [22]. The use of Synthetic Aperture Radar (SAR) data was rather limited in past applications because of the high costs of data and the complexity of their interpretation by non-expert users. Although, mapping flooded vegetation and urban areas can be still considered a critical issue for operational inundation mapping using exclusively SAR data [23], the combination of SAR with modeled flood-related data, continuous in space and time, offers great potential for implementation for near real-time flood forecasting and monitoring. The future implementation and operationalization of a such forecasting system may become a fundamental product for the delineation of risk scenarios to support international or national agencies involved in DRR, and, in general, emergency managers in taking their decisions [12]. In many studies, hydrodynamic models were calibrated using spaceborne SAR imagery (e.g., [10; 15; 18; 22; 23; 24; 25]). However, the hydraulic models calibration is often limited to the use of SAR derived flood extent data, or, even the spatially distributed water depth were also employed, the calibration is performed only in the river channel, and, as a consequence, it cannot work for an extremely flat area with complex topography (levees, banks, bridges, etc.). Furthermore, hydraulic model calibration should consider the important impact of SAR image acquisition timing. In this paper, we focus on the use of temporal series of SAR data, such as those available from the Cosmo-SkyMed constellation of four satellites for the proposed case study located at the Bradano River mouth, in Basilicata (Italy) (Section 2), in combination with a Digital Elevation Model for extraction of flood extent limits and spatially distributed water depth. These SAR- and DEM-based hazard maps are used for calibrating a two-dimensional (2D) hydraulic model in order to show the potential of these satellite images in the spatio-temporal characterization of flood events. When considering that we expect roughness to have most impact on the rate of flooding and receding in the 2D hydrodynamic model, we restricted model calibration to this parameter ([10; 26]). In Section 3, we first describe the key features of the methodologies used for the extraction of flood extent limits and for the estimation of spatially distributed water depth in gentle slope areas with complex topography; later, we introduce the main features of the hydrodynamic model that are utilized and we detail the model calibration methodology framework.