Using High-Resolution Drone-Derived DEMs and Orthomosaics to Analyse Temporal Dynamics of Salt Marshes and Mudflats in the Dutch Wadden Sea

The Wadden Sea, spanning the coasts of the Netherlands, Germany, and Denmark, constitutes a large tidal flats system, shaped by intricate biophysical interactions. Within this system, salt marsh ecosystems situated in the upper intertidal areas hold significant ecological value and provide flood protection. In the Dutch Wadden Sea, salt marshes are bordered by extensive mudflats on the seaward side and dikes on the landward side, restricting their upland migration in response to sea level rise. Often the marshes are sheltered from hydrodynamic forces using groynes or permeable dams made from brushwood. Salt marsh dynamics are influenced not only by complex biogeomorphological interactions within the marsh but also by interactions with the adjacent mudflats. To improve our understanding of variability in salt marsh vegetation extent, cliff formation at the salt marsh edge, mudflat elevation dynamics, and channel formation within the marsh, field data capturing spatiotemporal variations is crucial. However, current data lacks sufficient high-resolution spatial coverage, information on seasonal variations, and mudflat morphological developments.
To address this, we employ a fixed-wing (vertical take-off and landing) drone with RTK to acquire imagery and develop cm-level accurate point clouds, orthomosaics, and high-resolution digital elevation models (DEMs) using structure-from-motion techniques. With the drone flying at an altitude of 100 m, the achieved ground resolution of the DEMs is approximately 2.4 cm with a mapping speed of about 300 - 400 ha in 2.5 h, allowing for the mapping of one entire marsh and mudflat during one low tide. This methodology allows us to collect data seasonally (spring and autumn) at various salt marshes in the Dutch Wadden Sea. In this study, we present unique field data from five salt marshes in the Dutch Wadden Sea, showing the seasonal changes in both mudflat morphology and salt marsh vegetation cover. We examine the bed level differences on the mudflat and the development of the adjacent vegetation cover and establish a relation to local hydrodynamic conditions. The method's spatial and temporal flexibility bridges the gap between in-situ field measurements and coarser resolution airborne measurements or satellite data, improving our understanding of salt marsh dynamics.