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The proposed novel "BathySent" approach for coastal bathymetric mapping, using the Copernicus Sentinel-2 mission, as well as the assessment and specification of the uncertainties of the derived depth results, are the objectives of this research effort. For this reason, Sentinel-2 bathymetry retrieval results for three different pilot sites in Greece (islands of Kos, Kasos, and Crete) were compared with ground-truth data. These data comprised high-resolution swath bathymetry measurements, single-beam echosounder measurements at very shallow waters (1–10 m), and the EMODnet DTM 2018 release. The synthetic tests showed that the "BathySent" approach could restitute bathymetry in the range of 5–14 m depth, showing a standard deviation of 2 m with respect to the sonar-based bathymetry. In addition, a comparison with the "ratio model" multispectral technique was performed. The absolute differences between conventional Earth Observation-based bathymetry retrieval approaches (i.e., linear ratio model) and the suggested innovative solution, using the Sentinel-2 data, were mainly lower than 2 m. According to the outcome evaluation, both models were considered to provide results that are more reliable within the depth zone of 5–25 m. The "ratio model" technique exhibits a saturation at ~25 m depth and demands ground calibration. Though, the "BathySent" method provides bathymetric data at a lower spatial resolution compared to the "ratio model" technique; however, it does not require in situ calibration and can also perform reliably deeper than 25 m. [ABSTRACT FROM AUTHOR]

The modern climate trend and population growth have dramatically increased the need for maximization of the net benefit from the existing storage space in freshwater reservoirs. However, sedimentation in reservoirs through physical deposition and/or slope failures is a major threat to their productivity and life expectancy. In this context, the sedimentation impact on the sustainability of Mornos Lake/Reservoir, which is exceptionally vital for the ~ 3.1 million inhabitants of Athens, had to be evaluated. Therefore, a meticulous geophysical survey of the reservoir bed was conducted in 2015 for the very first time. Bathymetric, sidescan sonar, and seismic profiling datasets, all integrated with real-time kinematic (RTK) positioning, were analyzed for a realistic evaluation of the storage capacity loss. Approximately 18.2 × 106 m3 of lacustrine sediments derived through physical wedge-type deposition process and ~ 800,000 m3 of material produced by slope failures have covered the bottom since reservoir commissioning in 1981. This configures an average storage capacity loss of ~ 0.07% per year, which, however, is one of the lowest rates worldwide. Moreover, the 108-m-deep reservoir basin can presently accommodate a maximum active water volume of ~ 740 × 106 m3. The siltation pattern and sediment transport pathways in the reservoir are principally controlled by vigorous turbidity underflows, which deliver sediment mainly to the dam area (deposition thickness up to ~ 7 m) as well as to the pumping area (deposition thickness up to ~ 4 m) posing there a future risk; nevertheless, according to the predicted lake bathymetry, this risk will be negligible till 2045. [ABSTRACT FROM AUTHOR]