## 2 Data and Methods  
### Earth Observation Data  
In the past decades, satellite remote sensing has been proven to be a very important and cost-effective tool for environmental monitoring [35]. The continuous provision of satellite imagery, with a high frequency of acquisition and a high spatial and spectral accuracy, facilitates the realization of innovative and scalable solutions at various scales (local, regional, and national scales) [36]. In the framework of satellite-based coastal dynamics monitoring and shoreline/coastline extraction, optical and synthetic-aperture radar (SAR) imagery represent complementary and independent sources of information [37]. When the former is available, the different reflection and absorption properties that the land and sea have in the optical spectral range can be extracted [38], while when the latter is available, the different backscattering properties (in terms of signal intensity, dominant scattering mechanism, and/or statistical distribution) that characterize land and sea can be exploited [39]. In this way, the land and sea boundary can be identified even by simply exploiting the inherent capabilities of those imaging sensors. For instance, optical images facilitate easy interpretation but suffer from limited imaging capabilities, being able to operate only during cloud free days, while SAR images can overcome such drawbacks of the optical systems, yet at the expense of a more complex interpretation (speckle noise, geometric distortion, etc.). As a result, the joint use of optical and SAR images has a number of advantages, since a specific object or class that is not observable via passive sensor images might be monitored using active sensor images and vice versa due to the nature of the electromagnetic radiation used [40]. Currently, Copernicus constitutes one of the most comprehensive Earth Observation programmes, helping with its missions the European Commission member states to develop environmental policies and to monitor the results. In this context, the European Space Agency (ESA) is developing the Sentinel missions for the operational needs of the Copernicus programme, where they address issues related to the availability of coarse- and medium-resolution imagery [41]. Sentinel-2 is such an earth observation mission from the Copernicus Programme, which systematically acquires optical imagery at high spatial resolution (10 m to 60 m) over land and coastal waters [42]. The mission is currently a constellation with two satellites (Sentinel-2A and Sentinel-2B) supporting a broad range of services and applications such as agricultural monitoring, emergencies management, land cover classification, or water quality [43]. On the other hand, Sentinel-1 is a Synthetic Aperture Radar instrument. It operates C-Band in four exclusive imaging modes with different resolutions (down to 5 m) and coverages (up to 400 km). It provides dual polarization capability, a very short revisit time, and rapid product delivery. The mission is composed of a constellation of two satellites, Sentinel-1A and Sentinel-1B, sharing the same orbital plane. Sentinel-1 has the ability to transmit and to receive the backscatter signal, either with single polarization or with cross-polarization. Sentinel-1 data are sensitive to physical properties such as surface roughness, local topography, and dielectric constant [44]. Apart from Copernicus, other satellite missions also provide valuable datasets for environmental and coastal monitoring applications. Landsat missions make available a long-term archive of optical imagery with Landsat 8 to provide eight 30 m multi-spectral bands, one 15 m panchromatic band, and two 100 m spectral bands in the thermal region and with a revisit time of 16 days [45]. Moreover, the availability of very-high-resolution data can support the realization of more precise applications. For instance, the Pleiades constellation covers the Earth with a repeat cycle of 26 days, and delivers panchromatic and multispectral images at spatial resolutions of 0.5 m and 2 m, respectively [46].