Srebarna is a shallow freshwater lake situated along the right bank of the Lower Danube (391–393 rkm), northeastern Bulgaria. It formed approximately 8000 years ago after the inundation of the riverside terrace, which has shaped its unique hydromorphological and ecological characteristics. In recent times, the natural hydrology of the lake has been modified and managed through a series of embankments and artificial canals which regulate and control the water exchange with the Danube, since their natural connection was interrupted with a dyke built in 1948. Nevertheless, the fundamental importance of the lake’s ecosystem has even increased, considering the degradation and desiccation of the majority of the wetlands on the Lower Danube. Srebarna is a Nature Reserve, designated as a Wetland of International Importance (Ramsar Site) due to its role in biodiversity conservation. The lake is also recognized as a UNESCO World Heritage Site for its ecological significance and function as a key habitat for a wide range of species.

Despite its protected status, Srebarna faces numerous environmental challenges, e.g. eutrophication caused by sediment accumulation, nutrient enrichment and pollution often leading to algal blooms, reduced oxygen levels, and overall poor water quality, especially during the vegetation season. Rapid spread of invasive species also contributes to the degradation of the wetland ecosystem and negatively impact native species diversity. Climate change is a growing threat, as shifts in temperature and precipitation patterns alter the lake's hydrology. Desiccation of periphery pools and overgrowth with common reed and grey willow has been observed in the last years, due to the ever-decreasing water levels of the Danube. In an effort to negate these effects, during 2022 and 2023 three main activities were planned and executed: an excavation of new canal in the western part of the lake, with the main purpose to enhance the flush of accumulated sediments by creating a natural flow between the west and east canals and the main river. In addition, floating mats with reeds, along with willow overgrowth, clogging the canals were removed in order to unblock the canal flow. Significant areas of reed and reedmace were harvested in the peripheral pools in order to increase habitat heterogeneity and to open more water surfaces. About 350000 m³ sapropel sediments were planned for excavation in order to remove nutrients and increase the lake’s water volume.

As part of ongoing conservation efforts for monitoring and assessment of the effectiveness of these restoration activities, advanced techniques to collect up-to-date spatial data were utilised. One key aspect is the use of UAS, which employ airborne LiDAR scanning and photogrammetry for high-resolution data collection. The primary objective is to map the variations in the altitude of the lakebed and to assess the dimensions of the artificial channels used to regulate the water flow between the lake and the main river. Scanning is performed during low-water season when environmental conditions and features can be accurately captured. In the areas where LiDAR scanning is ineffective, e.g within the waterbody, sonar technology, using a boat is employed to gather bathymetric data, providing detailed information about the underwater topography. These tools allow assessment of changes in the lakebed, particularly in the sections that are permanently under water. The lake and its floodplains are divided into 31 non-equal but similar by area sections, covering overall 981.5 ha. To capture comprehensive data for conservation and monitoring purposes, each section is scanned and photographed individually in a single autonomous flight mission using a battery-powered UAV equipped with both LiDAR and RGB camera sensors. The UAV follows flight paths that consist of two to four parallel trajectories, spanning from shore-to-shore across the lake and its adjacent floodplains. This approach is designed to optimize time efficiency and reduce risks, with each flight starting and ending on the same shore, ensuring the UAV’s safe operation. The flight trajectories maintain an average gap of approximately 70 m, and the UAV operates at an altitude of 130 m asl (approx. 120 m above the target surface), with a forward speed of 6.5 m/s and 70% side overlap, allowing dual-mode scanning.

The LiDAR data collected is processed to generate an initial LAS point cloud, which is then converted into a digital elevation model (DEM). Sonar data is integrated into a GIS layer, aligning with the DEM grid cell centres to produce a seamless, improved DEM. This advanced processing technique ensures that the transition between the lakebed and the surrounding dryland surface is smooth and accurate in the final DEM, aiding the evaluation of hydrological changes over time. Photogrammetric data is also processed to generate a complete RGB orthomosaic of the entire lake and its surrounding floodplains. The orthomosaic serves as a visual representation for further analysis, enabling the monitoring of water body changes and the assessment of the wetland vegetation dynamics.

mapping, monitoring, Srebarna, UAS

Tsvetelina Isheva

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