## 1 Introduction  
Estuarine and coastal ecosystems belong to the most heavily used and threatened natural systems worldwide [1]. Currently, more than 40% of the world's population lives in coastal regions, a quarter of which is situated less than 10 m above sea level [2, 3]. Floods, saltwater intrusion, tsunamis, subsidence, and erosion are among the natural risks to which coasts are exposed [4]. In this context, global change increases the pressure on coastal ecosystems through a rising global mean sea level, changing rainfall and related freshwater discharge patterns, and the likelihood of more frequent or more severeroughs and storms [3, 5]. The IPCC 2019 Special Report on Oceans and Cryosphere [5] further states the important role of non-climatic anthropogenic drivers for the increasing vulnerability of low-lying coastal communities. In contrast to the long-term impact of sea level rise, human-induced changes can rapidly modify coasts over short periods [5, 6]. Profound understanding and monitoring of coastal processes are therefore crucial for protecting a large number of people in the face of a changing world. Coastline change is a powerful indicator for characterizing and comparing dynamics within the coastal zone [7]. The observation of coastline dynamics allows for continuous and quantifiable coastal monitoring. The definition of the 'coast' goes beyond the binary, linear separation of land and water and can rather be described as the zone that is generally affected by ocean water. According to [PERSON] and [PERSON] (2005) [8], a comparable definition of the 'coastline' is required when studying coastline dynamics over a period of time. However, the terms 'coastline' and'shoreline' have been used interchangeably in coastal research [9; 10; 11]. [PERSON] et al. (2005) [12] suggests a differentiation of the terms 'coastline' and'shoreline' by scale, where 'coastline' describes the land-water boundary at regional to global scales, while'shoreline' marks the precise boundary between the shore and the water. In this paper, 'coastline' will refer to the annual mean high-water boundaries on large scale, while'shoreline' will be utilized for single acquisition waterlines on smaller scales. Drivers of long-term coastline changes are diverse and often interrelated [13]. Natural coastal erosion may be caused by near-shore currents, wind, waves, or weathering [14]. Coasts that are usually supplied by river sediment likely erode as a consequence of a reduction in river discharge through decreasing rainfall or snowmelt [15; 16]. Human-induced coastal erosion is often associated with the construction of river dams [17; 18], sand mining [19; 20; 21], cutting of coastal mangrove forests [22], and the construction of coastal infrastructure, such as groynes, ports, inlet jetties, breakwaters, and sea walls [23; 24; 25]. Moreover, anthropogenic climate change is already contributing to global mean sea level rise, which is projected to be a major cause of coastal erosion in the future [5; 26]. Natural coastal accretion on the other hand most commonly occurs at deltaic coasts, where large quantities of fluvial sediment get distributed along the coast through littoral drift [12]. Energetic breaking waves, coastal landslides, and rockfall, as well as tidal flat formation, may also temporally prograde coastlines [12]. Anthropogenic coastal accretion encompasses artificial (beach) nourishment, artificial coastlines, land reclamation, and mangrove planting [12; 27; 28]. In the past, coastline changes have been monitored using historical photographs, coastal maps, land-based surveys, seabed mapping, and temporally sparse aerial datasets [10]. These techniques are rather time-consuming and expensive. The emergence of publicly available, global satellite archives has revolutionized coastal research, allowing for continuous monitoring with large spatial and temporal coverage [8; 22; 29; 30; 31].