## 2 Remote Sensing Technologies for the Monitoring of Coastal and River Delta Regions
### Remote Sensing Technologies for Deriving Land-Use and Land-Cover (LULC) Changes  
Optical remote sensing imaging, including multi-spectral and hyperspectral RS imaging, is beneficial for extracting coastline information [128; 129; 130; 131; 132], deriving long-term land use and land cover change [133], retrieving nearshore bathymetry [134], mapping coastal wetlands [135; 136], and detecting coastal erosion [137]. Numerous archived images with coarse, moderate, and high resolution from optical remote sensing satellites, including Terra/Aqua MODIS, Landsat, Sentinel, SPOT, IKONOS, WorldView, etc., have principally been utilized for long-term monitoring of land-cover and land-use modifications in river deltas [138]. Many studies have presented extended investigations of coastal and river delta zones using optical remote sensing image time series [138; 139; 140; 141; 142; 143; 144]. For instance, [PERSON] et al. [139] monitored the land-use change in the Pearl River Delta using Landsat TM data from 1988 to 1996, revealing that urban areas increased by more than 300%. The work of [145] investigated land cover changes, magnitude, and urbanization of Jing-Jin-Ji, the Yangtze River Delta, and the Pearl River Delta, relying on the classification of Landsat TM and HJ-1A/B images. The results show that urban land areas have increased by about 28,000 km\({}^{2}\), and coastal wetlands and cropland have simultaneously decreased from 1990 to 2010. The authors of [146] used Landsat and HJ-1B images to detect the spatio-temporal dynamics of land-use and land-cover Mekong Delta from 1979 to 2015. During this period, large-scale inland planting land has been transformed into aquaculture ponds. Landsat images taken during the period from 1995 to 2010 show that the Yellow River Delta landscape has dramatically been changed with increases in urban and cultivated aquatic areas displacing naturally vegetated areas [142]. Landsat images from 1999 to 2013 were used to identify land-cover alterations in the Parana River Delta in South America [143]. However, a presentation of accurate and long-term multi-class land-cover and land-use products of the world deltas is rare. Land-cover and land-use change maps of global deltas with 10 to 30 m spatial resolution would be needed using data generated from Landsat, Sentinel-2, and SAR satellites. Acquiring cloud-free images with optical remote sensors for coastal areas located in cloudy and rainy areas is  
Figure 1: Map of the spatial distribution of the 48 most critical river deltas worldwide considered here [126]. The base map is a global map of lands surface elevation [taken from the GEBCO_2020 Grid [127]] with a spatial resolution of 15 arc s.  
particularly challenging. However, other observation techniques, such as SAR, are not typically hindered by cloud cover. Unlike primarily applied to images acquired by optical satellites, a rapidly advancing research field concerns the development and application of change detection methods with SAR images at the microwave band [147; 148]. SAR data are, thus, used for mapping the changes of Earth's surface due to natural phenomena or linked to anthropic activities. In this context, SAR microwave images are exploited because they bring complementary information, being the electromagnetic radiation of microwaves able to penetrate forest canopies. Many features can be extracted from sets of SAR images and the most commonly used estimator for the extraction of changes is the backscattering coefficient [149], a fundamental indicator of the ground conditions which is sensitive to surface roughness and depends on the polarization of the transmitting/receiving waves. Polarimetric decomposition through eigenvalue analysis of the complex data allows the identification and categorization of the type and contribution of the involved scattering mechanisms. These studies provide meaningful and thorough exploitation of SAR polarimetry [150], including the phase information discarded in studies using the backscattering coefficient.