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Abstract. Airborne LiDAR bathymetry is an efficient technique for surveying the bottom of shallow waters. In addition, the measurement data contain valuable information about the local turbidity conditions in the water body. The extraction of this information requires appropriate evaluation methods examining the decay of the recorded waveform signal. Existing approaches are based on several assumptions concerning the influence of the ALB system on the waveform signal, the extraction of the volume backscatter, and the directional independence of turbidity. The paper presents a novel approach that overcomes the existing limitations using two alternative turbidity estimation methods as well as different variants of further processed full-waveform data. For validation purposes, the approach was applied to a data set of a shallow inland water. The results of the quantitative evaluation show, which method and which data basis is best suited for the derivation of area wide water turbidity information.

Abstract. In Morocco, the phenomenon of silting affects all the dams, where more than 75 Mm3 of sediments are deposited every year at the bottom of the reservoirs. The aim of our study is to assess the contribution of the bathymetric multi-beam sonar as well as static and mobile laser scanners in the estimation of siltation of the basin of the Tanger-Med dam in the north of Morocco. The proposed methodology consists of performing and fusion of a bathymetric survey by multi-beam echo-sounder with terrestrial laser scanner surveys in static mode and mobile mode. The result of these surveys is used, for the calculation of the siltation volume and the inspection of the upstream facing of the main structure. Siltation calculation was carried out with comparison to a reference survey obtained by single beam sonar. The comparison between these two instruments showed that the multi-beam presents many advantages: high density of the cloud of points acquired and precision. The density allows providing a better description of the dam's bottom. The comparative study between the two types of scanners showed that static scanner offers a better accuracy. However, mobile scanner gives more accessibility of all parts of the dam. As a conclusion, the multi-beam echo-sounder with additional laser scanner data have many advantages in this study: volume calculation precision, reduction of the acquisition time and the enhancement of the acquired point cloud density. We also obtained a global 3d modelling useful for the monitoring of the infrastructure.

Abstract. Artificial reefs (ARs) have become popular technological interventions in shallow water environments characterized by soft seabed for a wide number of purposes, from fisheries/environmental protection and enhancement to research and tourism. AR deployment has the potential for causing significant hydrographical and biological changes in the receiving environments and, in turn, ARs are strongly affected by the surrounding area in terms of spatial arrangement and structural integrity as well as colonization by benthic communities and finfish. In this context, ARs require a systematic monitoring program that a multibeam echosounder (MBES) can provide better than other sampling methods such as visual dives and ROV inspections that are not quantitative and often influenced by water visibility and diver experience/skills. In this paper, some subsequent MBES surveys of the Senigallia scientifically-planned AR (Northern Adriatic Sea) are presented and state-of-the art data processing and visualization techniques are used to draw post-reef deployment comparisons and quantify the evolution of the reef in terms of spatial arrangement and bulk volume. These multibeam surveys play a leading part in a general multi-year program, started simultaneously with the AR design and deployment and aimed to map how the reef structure quantitatively changes over time, as well as it affects the sea-bottom morphology and the fishery resource. All the data, surveyed over years making use of different sampling methods such as visual and instrumental echosounding observations and catch rate surveys, gain a mechanistic and predictive understanding of how the Senigallia AR functions ecologically and physically across spatial and temporal scales during its design life

Abstract. Traditional ship-based bathymetric surveys based on echo sounding are expensive and time consuming. This paper presents a project with the aim of identifying the opportunities of airborne laser bathymetry for the monitoring of the sea bed at the German Baltic Sea coast. Such devices operate with laser signal in the green part of the visible spectrum which is capable to penetrate the water. The depth is determined from the two-way runtime between the water surface and reflections from the ground underneath. Several flight campaigns in representative test areas will be carried out in order to analyze the reachable depths, the accuracies of the acquired points, and the detection of obstacles depending on different water conditions (e.g. turbidity). We discuss some preliminary results of a pilot project and the first campaign of a study area close to the island of Poel, Germany.

Abstract. Knowledge of underwater topography is essential to the understanding of the organisation and distribution of archaeological sites along and in water bodies. Special attention has to be paid to intertidal and inshore zones where, due to sea-level rise, coastlines have changed and many former coastal sites are now submerged in shallow water. Mapping the detailed inshore topography is therefore important to reconstruct former coastlines, identify sunken archaeological structures and locate potential former harbour sites. However, until recently archaeology has lacked suitable methods to provide the required topographical data of shallow underwater bodies. Our research shows that airborne topo-bathymetric laser scanner systems are able to measure surfaces above and below the water table over large areas in high detail using very short and narrow green laser pulses, even revealing sunken archaeological structures in shallow water. Using an airborne laser scanner operating at a wavelength in the green visible spectrum (532 nm) two case study areas in different environmental settings (Kolone, Croatia, with clear sea water; Lake Keutschach, Austria, with turbid water) were scanned. In both cases, a digital model of the underwater topography with a planimetric resolution of a few decimeters was measured. While in the clear waters of Kolone penetration depth was up to 11 meters, turbid Lake Keutschach allowed only to document the upper 1.6 meters of its underwater topography. Our results demonstrate the potential of this technique to map submerged archaeological structures over large areas in high detail providing the possibility for systematic, large scale archaeological investigation of this environment.

Abstract. Satellite-Derived Bathymetry (SDB) has been used in many applications related to coastal management. SDB can efficiently fill data gaps obtained from traditional measurements with echo sounding. However, it still requires numerous training data, which is not available in many areas. Furthermore, the accuracy problem still arises considering the linear model could not address the non-relationship between reflectance and depth due to bottom variations and noise. Convolutional Neural Networks (CNN) offers the ability to capture the connection between neighbouring pixels and the non-linear relationship. These CNN characteristics make it compelling to be used for shallow water depth extraction. We investigate the accuracy of different architectures using different window sizes and band combinations. We use Sentinel-2 Level 2A images to provide reflectance values, and Lidar and Multi Beam Echo Sounder (MBES) datasets are used as depth references to train and test the model. A set of Sentinel-2 and in-situ depth subimage pairs are extracted to perform CNN training. The model is compared to the linear transform and applied to two other study areas. Resulting accuracy ranges from 1.3 m to 1.94 m, and the coefficient of determination reaches 0.94. The SDB model generated using a window size of 9x9 indicates compatibility with the reference depths, especially at areas deeper than 15 m. The addition of both short wave infrared bands to the four visible bands in training improves the overall accuracy of SDB. The implementation of the pre-trained model to other study areas provides similar results depending on the water conditions.

Abstract. High resolution TerraSAR-X/TanDEM-X as well as Sentinel-1 remote sensing Synthetic Aperture Radar (SAR) data are used to determine and monitor the waterline in the Wadden Sea. In this very unique and dynamic coastal region in the southeastern North Sea, tidal flats extend several kilometers away from the coast during low tide with features like tidal inlets and sand banks. Under the influence of tidal water currents transporting large amounts of eroded material, inlets and sand banks move over time; heavy storms can even cause large variations in their extensions in merely a few hours. Observation of these obstacles is crucial for maritime security as high ship traffic is caused by the ports of Hamburg, Bremerhaven, Wilhelmshaven and others. Conventional monitoring campaigns with ships or airplanes are economically expensive and can only provide limited coverage. We present an automatic algorithm with Near Real-Time capability for extracting the waterline at the time of recording from SAR images, which allows for a fast and large scale determination of changes in coastal outlines. The comparison of recent acquisitions of TerraSAR-X and Sentinel-1 to bathymetry data of the Elbe estuary obtained in 2010 reveals significant changes in tidal flat structures.