**1. Introduction**

The coast is a very dynamic and varied environment, constantly changing in response to complex interrelations between landforms and processes operating across a wide range of spatial and temporal scales. Understanding shoreline dynamics, for integration into process-based modelling, for example, and being able to predict future evolutions in the context of relative sea-level rise, requires an accurate characterization of the geomorphic processes acting at the sediment–water interface [1–5]. Sediment reworking on the beach and nearshore due to waves and currents traduces by the formation of topographic complexity (herewith generally called roughness), such as sedimentary bedforms. These

**Citation:** Bertin, S.; Stéphan, P.; Ammann, J. Assessment of RTK Quadcopter and Structure-from-Motion Photogrammetry for Fine-Scale Monitoring of Coastal Topographic Complexity. *Remote Sens.* **2022**, *14*, 1679. https:// doi.org/10.3390/rs14071679

Academic Editors: Simona Niculescu, Junshi Xia and Dar Roberts

Received: 7 March 2022 Accepted: 29 March 2022 Published: 31 March 2022

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topo-morphological features are the imprint of the processes that shaped them, and, as such, measuring bedforms can provide meaningful information on hydrodynamic forces [6–10]. Besides, bedforms are roughness elements that modify the flow and dissipate energy [11,12]. They are, thus, interesting markers of the sediment bed propensity towards resistance to entrainment and dynamic equilibrium.

Different types of bedforms generally coexist in the coastal environment (e.g., Refs. [13–15]), whose dimensions and shapes depend on sediment availability, sediment type (e.g., size distribution and shape) and driving mechanisms (e.g., waves, currents, wind). Examples of small-scale bedforms are wave and current sand ripples, or gravel-bed clusters, with dimensions or length scales generally not exceeding a few tens of centimeters. Larger-scale bedforms include varied bar and trough morphologies (e.g., ridge and runnels) in the surf and swash zones [16], other rhythmic patterns most often found on the upper beach, such as beach cusps [17,18], as well as aeolian sand dunes. In between those, there are a wide range of topographic variations that take part in the continuum of roughness scales and contribute to the overall structural complexity of the seabed, backshore and dune system.
