# A Combined Approach of Field Data and Earth Observation for Coastal Risk Assessment
## 1 Introduction  
Sea and the coastal areas constitute a driving force for the economic activity based on maritime resources, the so-called \"blue economy\" [1]. Actually, maritime and coastal tourism are the leading sectors of the European maritime economy. The priority objective of the coastal policy should hence be to enhance ecological and landscape resources in order to further increase their attractiveness and economic and social growth. Any enhancing measure strongly requires proper governance of coastal areas through integrated management plans and actions targeted to mitigate and counteract coastal risks [2]. Therefore, a detailed outline of the dynamics of coastal erosion phenomena over different time-scales is necessary for an effective coastal risk assessment and for land-use planning purposes [2; 3]. Growing urbanization, human activities and climate change are exacerbating the impact of shoreline erosion phenomena [4]. Hence, shoreline position and its variability over time are an essential element for coastal management [5; 6], coastal protection structure evaluation [7], validationand calibration of numerical models [8; 9; 10] and climate change studies [11; 12]. The coastline can ideally be defined as the land-sea interface [13], and its position is constantly changing over time, mainly due to the dynamic nature of the sea level that is influenced by astronomical tides, waves, storm surge, set-up, run-up and sediment transport (cross-shore and long-shore currents) [14; 15]. Therefore, the shoreline position must be defined in the most appropriate temporal scale; in the case of a long-term study, a yearly coastline can be adequate [12], whereas in the case of run-up analysis, it may be necessary to sample positions with much greater frequency [9; 16]. In situ measurement techniques have been successfully adopted for shoreline observation obtaining accurate results, such as the ones that involve Global Navigation Satellite System (GNSS) campaigns [5; 17; 18], but a continuous GPS monitoring turns out to be impractical in terms of costs and logistics over a wide scale. Innovative monitoring approaches, based on remote sensing techniques, could be an economically sustainable alternative providing information on shoreline evolution trend and promptly indicating also the occurrence of risk situations [19]. Different remote sensing measurement techniques can be successfully adopted for shoreline observation: optical sensors on both aerial and space platforms [19], LiDAR campaigns [20; 21], Unmanned Aerial Vehicle (UAV) [22; 23; 24; 25], videomonitoring systems [26; 27; 28; 29]. The exploitation of very-high resolution aerial optical images and LIDAR surveys allows very accurate positioning of the shoreline over a wide scale, but their high acquisition costs strongly limit the frequency of monitoring, whereas small UAVs and videomonitoring systems offer a low cost solution but they are able to provide information only about limited areas. Over the years, Synthetic Aperture Radar (SAR) has proved to have a positive cost-benefit ratio for territorial analysis, from small/medium to wide scale and the all-weather day/night imaging capability has made this approach very popular, especially for mapping remote areas or under extreme climatic conditions [19]. The use of satellite SAR sensors has been limited in the past by the low spatial resolution of the images [30; 31]. Since 2007, the new constellations of satellites equipped with high resolution X-band SAR sensors, such as the COSMO-SkyMed (CSK) and TerraSAR-X (TSX) missions, potentially allowed to identify and monitor shoreline evolution with meter or sub-meter level accuracy (according to the selected acquisition mode), thus fostering the exploitation of SAR data for coastline extraction. Moreover, the nationwide acquisition plan named MapItaly has been devised by the Italian Space Agency (ASI), to provide CSK high-resolution satellite SAR images covering the whole Italian territory with a 16-day revisit period [32].