Spatial distribution of sand dunes along the Bulgarian Black Sea Coast: inventory, UAS mapping and new discoveries

Coastal sand dunes are amongst the world's most sensitive and dynamic landforms. Unfortunately, during the last thirty years, heavy anthropogenic alterations have been observed, encompassing the greater part of the Bulgarian Black Sea coast (BBSC), which has changed the land-sea interactions significantly. As a consequence, the depositional coast has shrunk to 131 km or 25% of the aggregate Bulgarian Black Sea shoreline length. Although our research reveals that 86% of BBSC dunes are included in the Natura 2000 network of protected sites established under the Habitats Directive (Council Directive 92/43/EEC 1992), they are often heavily modified, subjected to environmental vandalism and destroyed due to mismanagement or lack of accurate information and prevention. These facts were the main reason for carrying out an inventory of the Bulgarian Black Sea coastal dune systems in 2021-2022. Our research aimed to identify all dune systems/ sand dunes, update their spatial distribution and classify the observed coastal sand dunes landforms along the BBSC. The article demonstrates a successful methodology for combining unmanned aerial systems (UAS), Structure-from-Motion (SfM) photogrammetry, in situ sediment sampling, video imaging and verification and GNSS-RTK ground control points for coastal mapping. As of June 2022, over 97% of the Bulgarian shoreline has been surveyed with this technique, excluding military areas and national security sites. Based on the acquired data, as of 2021, the shoreline length was estimated to be 518.7 km at a scale 1:5000. The integrated UAS approach includes using Digital surface models (DSM), raster orthophotomosaics (OM) and 3D models, based on SfM photogrammetry to analyse the coastal topography, detect dune forms and update their spatial distribution. Throughout the inventory, 46 beach-dune systems were identified along the BBSC, which were divided into 62 dune sectors. The area of coastal dune systems was estimated at 988.21 ha (0.0089% of Bulgaria) and a total length of 73 km (14% of the shoreline). A comprehensive geomorphological analysis of the relationships between landforms morphology, aeolian and morphodynamic processes, vegetation density and type was the basis for the coastal dune landforms (CDLs) or dune systems to be classified into primary (312 ha; 32%) and secondary (676 ha; 68%). Additionally, the CDLs were classified according to Natura 2000 habitats: fixed (grey) dunes (546.27 ha; 55.28%), wooded dunes (222.61 ha; 22.53%), shifting (white) dunes (150.30 ha; 15.21%), embryonic dunes (68.3 ha; 6.91%) and humid dune slacks (0.94 ha; 0.09%). The highest positioned CDLs on the Balkan Peninsula were registered at perched Sozopol Sand Dunes (61 m a.s.l.) and cliff-top dunes at Arkutino (50.2 m a.s.l.). The multi-temporal analysis of photogrammetric DSMs and raster OMs showed the permanent loss of five dune systems in the Pomorie-Burgas-Rosenets coastal sector. The accrued UAS data approach allowed us to identify and map eight dune systems for the first time: Zlatni Pyasatsi (Panorama), Asparuhovo (Varna), Byala, Atanasovska Kosa, Central Beach (Burgas), Chernomorets, Kavatsite (partly) and Rezovo-Kastrich. A high anthropogenic footprint was registered on 50.7 ha (5.1%) of the entire dune surface. In the final stage of the study, human interventions that caused degradation and permanent loss of dunes (12 ha) over the last 15 years along the BBSC were shown. The main causes for dune degradation along BBSC have been documented, such as massive tourism development after the socialist period, road construction, recreational pressure exerted on the dunes, human trampling, lack of designated footpaths in areas with fixed and mobile dunes, off-road vehicles and parking lots (especially at camping sites), dumping of garbage and anthropogenic marine litter on the sand dunes etc.