3D high-resolution Romanian Black Sea – Danube Delta coastal geomorphic surveys for change analysis

3D high-resolution Romanian Black Sea – Danube Delta coastal geomorphic surveys for change analysis Andrei Gabriel Dragos1,2, Monica Palaseanu-Lovejoy3, Gabriel Iordache1,4, Irina Stanciu1, Florin Pitea1,2, Maria Ionescu1,2, Adrian Gherghe1, Adrian Stanica1 1 National Institute for Research and Development on Marine Geology and Geo-Ecology (GeoEcoMar), Bucharest, Romania 2 Faculty of Geology and Geophysics, University of Bucharest, Romania 3 U.S. Geological Survey, Geology, Minerals, Energy and Geophysics Science Center, Reston, Virginia, USA 4 Faculty of Geography, University of Bucharest, Romania The security and wellbeing of a community is partially dependent on a critical understanding of the natural environment, landscape evolution, available resources, vital information communication in response to an event or increasing rates of change. The ability to map flooding, erosion, and habitat loss is a key tool in a country's resilience and strategies for mitigation and adaptation against costly natural hazards, and this cannot be done without high-resolution, accurate 3D data. Structure-from-Motion (SfM) photogrammetry is a powerful technique for creating high-resolution 3D digital terrain models (DTMs) from overlapping 2D images at a relatively low cost. We conducted several SfM-photogrammetric surveys in the Romanian Black Sea coastal zone, on the wild protected beaches in front of the Danube Delta - Edighiol barrier beach (July and November 2023) on the southern extent of the Danube Delta, and the Sf. Gheorghe beach immediately North of the southernmost arm of the Danube in the Black Sea (August 2022 and August 2023). Sf. Gheorghe is the only asymmetric active lobe in the Romanian part of the Danube Delta associated with a river-sea confluence barrier island (South of the mouth), which is a very dynamic spit with a large cyclic development. We generated DEMs and orthomosaics at 4 – 5 cm pixel resolution with a vertical mean square error between 5 to 8 cm and a mean error bias of 2 cm or less. In the case of Sf. Gheorghe beach, between 2012 LiDAR survey and 2022 SfM survey, shoreline erosion up to 100 m was observed immediately adjacent of the northern side of the Sf. Gheorghe branch, at the confluence with the Black Sea. The erosional trend increases closer to the confluence on both Sf. Gheorghe bank and Black Sea shore sides. About 2 km North of the Sf. Gheorghe mouth the 2012 and 2022 shorelines coincide, while on the Sf. Gheorghe branch shore, the two left bank positions coincide after 400 m only. The uneven erosion near the confluence point suggests the impact of Black Sea longshore currents due to insufficient sediment from the Danube. Edighiol SfM surveys analyzed coastal dynamics, emphasizing winter storms, inundation, vegetation changes, sand dune shifts, and beach erosion. The Danube Delta Black Sea coast erosion is primarily caused by human activities, including reduced sediment supply, altered sediment pathways (resulting from damming, embankments, and canal cutting), and accelerated climate change. Natural factors like subsidence, sea-level rise, and occasional extreme storms also contribute. SfM surveys provide quantitative analysis for assessing short- and long-term changes influenced by episodic and seasonal events in this dynamic environment.