*1.1. High-Resolution Topographic Surveying and Photogrammetric Remote Sensing of Coastal Morphology*

In tidal environments, daily fluctuations in the water level mean that the waterworked seabed can be measured using topographic surveying, with the maximum coverage typically achieved during the lowest low tides. Previous work largely relied on RTK-GNSS, with a rover eventually mounted on a mobile platform, such as a quad, for monitoring the evolution of barred beaches and other coastal landforms [12,19]. The DEMs generated have resolutions ranging from a few meters cross-shore to tens of meters alongshore, in link with the topographic complexity and bedform orientation, while survey coverage can reach up to several kilometers alongshore, allowing to account for the rhythmic nature of the coastal morphology (e.g., rip-channel systems, cusps and megacusps). Common to point-based surveying techniques, enhancing the spatial resolution is generally counter-balanced by increased acquisition time or reduced spatial coverage [20–22], which means that using RTK-GNSS for the fine-scale monitoring of coastal topography remains problematic.

In contrast, remote sensing techniques, such as UAV photogrammetry, have greatly improved, rendering very-high-resolution (e.g., submeter) topographic surveying at a large scale (e.g., beach scale) feasible. Using off-the-shelf drones and cameras, ground sampling distances (GSD) as small as a few centimeters can be achieved during typical beach surveys [23–25]. Photogrammetric processing relies on the SfM method, which is based on the scale-invariant feature transform (SIFT) algorithm [26] for matching corresponding points in images, and uses external information, mostly taking the form of GCPs measured independently, for scaling and optimizing the photogrammetric model. Yet, previous work showed that the resulting DEM quality (e.g., horizontal and vertical precisions) is largely variable (e.g., Refs. [23,27,28]) and depends on a wide range of parameters, among which survey design (e.g., flying height, image overlap, camera orientation, GCP number and distribution) and processing strategies (e.g., direct georeferencing, camera calibration and point-matching methods) play a critical role.

Photogrammetry has been used previously in different domains of research to measure sedimentary bedforms of various dimensions, including particle clusters [29,30], bed undulations, such as sand [31–33] and gravel bars [34–36], and sand dunes [37–42], with measurements of small-scale bedforms generally performed in the laboratory. Hence, it is not yet known if fine-scale water-worked topographies can be efficiently and effectively measured using UAV photogrammetry in the field, such that there is currently a paucity of data for assessing coastal topographic roughness and shaping processes.
