A Comparative Performance Analysis of Popular Deep Learning Models and Segment Anything Model (SAM) for River Water Segmentation in Close-Range Remote Sensing Imagery

Accurate segmentation of river water in close-range Remote Sensing (RS) images is vital for efficient environmental monitoring and management. However, this task poses significant difficulties due to the dynamic nature of water, which exhibits varying colors and textures reflecting the sky and surrounding structures along the riverbanks. This study addresses these complexities by evaluating and comparing several well-known deep-learning (DL) techniques on four river scene datasets. To achieve this, we fine-tuned the recently introduced 'Segment Anything Model' (SAM) along with popular DL segmentation models such as U-Net, DeepLabV3+, LinkNet, PSPNet, and PAN, all using ResNet50 pre-trained on ImageNet as a backbone. Experimental results highlight the diverse performances of these models in river water segmentation. Notably, fine-tuned SAM demonstrates superior performance, followed by U-Net(ResNet50), despite their higher computational costs. In contrast, PSPNet(ResNet50), while less effective, proves to be the most efficient in terms of execution time. In addition to these findings, we introduce a novel river water segmentation dataset, LuFI-RiverSnap. v1 (Dataset link), characterized by a more diverse range of scenes and accurate masks compared to existing datasets. To facilitate reproducible research in remote sensing and computer vision, we release the implementations of the fine-tuned SAM model (Code link). The findings from this research, coupled with the presented dataset and the accuracy achieved by fine-tuned SAM segmentation, can support tracking river changes, understanding river water level trends, and exploring river ecosystem dynamics. These can also provide valuable insights for practitioners and researchers seeking models tailored to specific image characteristics with practical means in disaster risk reduction, such as rapid assessments of inundations during floods or automatic extractions of gauge data in watersheds

Influence of Structural Elements on the Spatial Sediment Displacement around a Jacket-Type Offshore Foundation

This research advances the understanding of jacket-type platform induced local and global erosion and deposition processes for combined wave–current conditions. To this end, a laboratory study was carried out comparing the equilibrium scour depth for two structural designs that are differentiated in the geometrical distance of the structure’s lowest node to the seabed. Measurements of local scour depths over time have been conducted with echo sounding transducers. An empirical approach is proposed to predict the final scour depths as a function of the node distance to the seabed. Additionally, 3D laser scans have been performed to obtain the digital elevation model of the surrounding sediment bed. Novel methodologies were developed to describe and easily compare the relative volume change of the sediment bed per surface area due to structure–seabed interaction, enabling spatial analyses of highly complex erosion and deposition patterns. The seabed sediment mobility around the structure is found to be highly sensitive to a change in node distance. The decrease of the node distance results in a higher erosion depth of sediment underneath the structure of up to 26%, especially for current-dominated conditions, as well as an increased deposition of sediment downstream of the structure over a distance of up to 6.5 times the footprint length. The results of this study highlight the requirement to consider the interaction of the structure with the surrounding seabed within the design process of offshore structures, to mitigate potential impacts on the marine environment stemming from the extensive sediment displacement and increased sediment mobility

Spatio-temporal analysis of scour around complex offshore foundations under clear water and live bed conditions

This study investigates scour around offshore wind foundations, focusing on two complex structures with varying degrees of flow blockage: a novel hybrid gravity-based jacket (structure “A") and a conventional four-legged jacket (structure “B"). As offshore structures like jackets become more prevalent, mainly due to their structural stability and growth of offshore wind energy in general, understanding scouring phenomena around complex structures is crucial. Laboratory tests under steady flow clear-water and live-bed conditions, with measurements of 3D laser scans for test durations of 15, 90, and 420 min were conducted. In addition the scour development over time was measured and analyzed with eight echo sounders. The findings confirm that scouring around complex structures displays significant variability in dependency of the structure type, making standardization a challenging task. However, some common trends can be derived. Under live-bed conditions, both types of structures exhibit global erosion, regardless of the complexity or flow obstruction of the structure. The spatial erosion depth, relative to the footprint, is markedly higher (2.5 times) for the gravity-based structure as opposed to the jacket structure. In clear-water conditions, no global scour was observed for both structures and a very similar spatial erosion depth was reached after 420 min

Role and Impact of Hydrograph Shape on Tidal Current-Induced Scour in Physical-Modelling Environments

For physical model tests, the time-varying characteristics of tidal currents are often simplified by a hydrograph following a shape of a unidirectional current or by resolving the tidal velocity signal into discrete steps of constant flow velocity. The influence of this generalization of the hydrograph’s shape on the scouring process in tidal currents has not yet been investigated systematically, further increasing the uncertainty in the prediction of scour depth and rate. Therefore, hydraulic model tests were carried out to investigate and quantify the influence of the hydrograph shape on the scouring processes under tidal currents. Several different hydrographs including those with continuously changing velocities, constant unidirectional currents, square-tide velocities and stepped velocity time series were analyzed. Results show that the scouring process in tidal currents is characterized by concurrent sediment backfilling and displacement which can only be reproduced by hydrographs that incorporate a varying flow direction. However, if only a correct representation of final scour depths is of interest, similar scour depths as in tidal currents might be achieved by a constant, unidirectional current, provided that a suitable flow velocity is selected. The effective flow work approach was found capable to identify such suitable hydraulic loads with reasonable practical accuracy

Influence of scour and scour protection on the stiffness of monopile foundations in sand

Offshore structures are subject to cyclic effects from wind and waves, as well as altered seabed. Local and global scour can form around the foundation elements, which in turn can affect the structures’ response in terms of serviceability, eigenfrequency and fatigue limit state. If a scour protection is installed, a stiffening effect is to be expected. The paper investigates the influence of local scour and scour protection on the soil-structure interaction of an exemplary monopile foundation in sandy soil with a numerical finite element model, in which the soil behaviour is modelled by the advanced HSsmall material law. With that, the effect of the depth and the geometry of a scour hole on the load deformation behaviour and in particular on the stiffness of a monopile is quantified. It is shown that the exact geometry of a conical scour hole is of minor importance for the effect on the monopile stiffness. However, scour depths of around 1.5 times the pile diameter, which are often to be considered in design, strongly reduce the stiffnesses with reduction factors of around 2.5. Furthermore, the positive influence of a scour protection layer on the soil-structure interaction is included in the comparison. First order estimates for a typical offshore configuration reveal that an installed scour protection can increase the stiffnesses of the monopile-soil system under operation considerably, mainly dependant on the thickness of the protection layer. The study exemplarily quantifies that the presence of scour protection implies an increase in structural integrity as a so far underrated added value and may actually be an effective mitigation measure for dysfunctional monopile foundations in offshore wind industry

Volume-based assessment of erosion patterns around a hydrodynamic transparent offshore structure

The present article presents results of a laboratory study on the assessment of erosion patterns around a hydrodynamic transparent offshore foundation exposed to combined waves and currents. The model tests were conducted under irregular, long-crested waves in a scale of 1:30 in a wave-current basin. A terrestrial 3D laser scanner was used to acquire data of the sediment surface around the foundation structure. Tests have been conducted systematically varying from wave- to current-dominated conditions. Different volume analyzing methods are introduced, which can be related for any offshore or coastal structure to disclose physical processes in complex erosion patterns. Empirical formulations are proposed for the quantification of spatially eroded sediment volumes and scour depths in the near-field and vicinity of the structure. Findings from the present study agree well with in-situ data stemming from the field. Contrasting spatial erosion development between experimental and in-situ data determines a stable maximum of erosion intensity at a distance of 1.25 A, 1.25 times the structure’s footprint A, as well as a global scour extent of 2.1–2.7 A within the present study and about 2.7–2.8 A from the field. By this means, a structure-induced environmental footprint as a measure for erosion of sediment affecting marine habitat is quantified

Spatiotemporal Scouring Processes around a Square Column on a Sloped Beach Induced by Tsunami Bores

Tsunamis continue to pose an existential threat to life and infrastructure in many coastal areas around the world. One of the risks associated with tsunamis is the formation of deep scour holes around critical infrastructure and other coastal buildings, compromising their structural integrity and stability. Despite its importance, tsunami-induced scour is still given limited and simplified consideration in design guidelines for coastal structures. To further improve the understanding of tsunami-induced scour processes, and thus provide the basis for safer design of coastal structures, novel large-scale laboratory experiments have been conducted. The experiments featured a unique combination of boundary conditions, including a square coastal structure on a sloping and dry sandy beach. Single broken solitary waves were used to simulate tsunami bores. The spatiotemporal scour development directly at the square column was monitored by a high-resolution camera system, allowing a detailed description of the highly dynamic flow and scour process. Differences in the scour process between the wave runup and drawdown phases are described, and maximum and final scour depths are given as a function of inundation depth, wave height, and distance of the column from the shoreline. The scour process is characterized by several distinct phases of varying intensity and scour rate, the sequence of which varies depending on the location on the sides of the column. It is shown that the drawdown phase has a large influence on the overall scour development, adding up to 58% to the scour depth obtained during the wave runup phase. As a result of significant sediment infilling during the drawdown phase, the maximum scour depths achieved during the drawdown phase are up to twice the final scour depths at the end of a test. This discrepancy between final and maximum scour depths is greater than in previous studies using a flat sediment bed. The results of this study therefore help to interpret scour depths measured during field investigations after a tsunami event and provide a basis for extending design guidelines for coastal structures

Low-cost UAV monitoring: insights into seasonal volumetric changes of an oyster reef in the German Wadden Sea

This study aims to quantify the dimensions of an oyster reef over two years via low-cost unoccupied aerial vehicle (UAV) monitoring and to examine the seasonal volumetric changes. No current study investigated via UAV monitoring the seasonal changes of the reef-building Pacific oyster (Magallana gigas) in the German Wadden Sea, considering the uncertainty of measurements and processing. Previous studies have concentrated on classifying and mapping smaller oyster reefs using terrestrial laser scanning (TLS) or hyperspectral remote sensing data recorded by UAVs or satellites. This study employed a consumer-grade UAV with a low spectral resolution to semi-annually record the reef dimensions for generating digital elevation models (DEM) and orthomosaics via structure from motion (SfM), enabling identifying oysters. The machine learning algorithm Random Forest (RF) proved to be an accurate classifier to identify oysters in low-spectral UAV data. Based on the classified data, the reef was spatially analysed, and digital elevation models of difference (DoDs) were used to estimate the volumetric changes. The introduction of propagation errors supported determining the uncertainty of the vertical and volumetric changes with a confidence level of 68% and 95%, highlighting the significant change detection. The results indicate a volume increase of 22 m³ and a loss of 2 m³ in the study period, considering a confidence level of 95%. In particular, the reef lost an area between September 2020 and March 2021, when the reef was exposed to air for more than ten hours. The reef top elevation increased from -15.5 ± 3.6 cm NHN in March 2020 to -14.8 ± 3.9 cm NHN in March 2022, but the study could not determine a consistent annual growth rate. As long as the environmental and hydrodynamic conditions are given, the reef is expected to continue growing on higher elevations of tidal flats, only limited by air exposure. The growth rates suggest a further reef expansion, resulting in an increased roughness surface area that contributes to flow damping and altering sedimentation processes. Further studies are proposed to investigate the volumetric changes and limiting stressors, providing robust evidence regarding the influence of air exposure on reef loss

Oyster Reef Surfaces in the Central Wadden Sea: Intra-Reef Classification and Comprehensive Statistical Description

The Pacific oyster (Magallana gigas) is an invasive species in the Wadden Sea transforming parts of it permanently. M. gigas, as an ecosystem engineer, builds reef structures that are characterized by highly complex and variable surfaces consisting of densely packed, sharp-edged individuals connected with cement-like bonds. To investigate the interactions between reef structure, shape and formation and wave as well as tidal currents, an understanding of the surface roughness is essential. This work reports on observations of oyster reefs for which seven new structural classes (Central Reef, Transitional Zone, Cluster I, Cluster II, Patch I, Patch II, and Garland) are proposed. For each class, high resolution Digital Elevation Models (DEMs) have been elaborated based on Structure-from-Motion (SfM) photogrammetry and analyzed using spatial statistics. By determining probability density functions (PDFs), vertical porosity distributions, abundances, orientations and second-order structure functions (SSFs), topographical parameters that influence the hydraulic bed roughness have been determined. The results suggest, that by applying the structural classification and their distinct topographical roughness parameters, the oyster reef surfaces can be described appropriately accounting for their complexity. The roughness accounts to a total roughness height kt = 103 ± 15 mm and root-mean-square roughness height krms = 23 ± 5 mm. These values were found similar across all structural classes, yet the shape of the PDFs reveal differences. With decreasing abundance, the distributions become more positively skewed and are characterized by more extreme outliers. This is reflected in the higher statistical moments, as the skewness ranges between Sk = 0.4–2.1 and the kurtosis between Ku = 2.2–11.5. The analysis of the orientations and the SSFs confirms anisotropic behavior across all structural classes. Further, the SSFs reveal the oyster shells as significant roughness elements with exception of Cluster I and II, where the clusters are identified as significant roughness elements. The provided set of topographical roughness parameters enhances the knowledge of oyster reef surfaces and gives insights into the interactions between biogenic structure and surrounding hydrodynamics. The new intra-reef classification allows for more accurate determination of the overall roughness as well as the population dynamics of the habitat forming oyster. Combined with hydraulic measurements, the results can be used to estimate the hydraulic bed roughness induced by the oyster reef surfaces. Copyright © 2022 Hitzegrad, Brohmann, Pfennings, Hoffmann, Eilrich, Paul, Welzel, Schlurmann, Aberle, Wehrmann and Goseberg

Oyster Reef Surfaces in the Central Wadden Sea: Intra-Reef Classification and Comprehensive Statistical Description

The Pacific oyster (Magallana gigas) is an invasive species in the Wadden Sea transforming parts of it permanently. M. gigas, as an ecosystem engineer, builds reef structures that are characterized by highly complex and variable surfaces consisting of densely packed, sharp-edged individuals connected with cement-like bonds. To investigate the interactions between reef structure, shape and formation and wave as well as tidal currents, an understanding of the surface roughness is essential. This work reports on observations of oyster reefs for which seven new structural classes (Central Reef, Transitional Zone, Cluster I, Cluster II, Patch I, Patch II, and Garland) are proposed. For each class, high resolution Digital Elevation Models (DEMs) have been elaborated based on Structure-from-Motion (SfM) photogrammetry and analyzed using spatial statistics. By determining probability density functions (PDFs), vertical porosity distributions, abundances, orientations and second-order structure functions (SSFs), topographical parameters that influence the hydraulic bed roughness have been determined. The results suggest, that by applying the structural classification and their distinct topographical roughness parameters, the oyster reef surfaces can be described appropriately accounting for their complexity. The roughness accounts to a total roughness height kt = 103 ± 15 mm and root-mean-square roughness height krms = 23 ± 5 mm. These values were found similar across all structural classes, yet the shape of the PDFs reveal differences. With decreasing abundance, the distributions become more positively skewed and are characterized by more extreme outliers. This is reflected in the higher statistical moments, as the skewness ranges between Sk = 0.4–2.1 and the kurtosis between Ku = 2.2–11.5. The analysis of the orientations and the SSFs confirms anisotropic behavior across all structural classes. Further, the SSFs reveal the oyster shells as significant roughness elements with exception of Cluster I and II, where the clusters are identified as significant roughness elements. The provided set of topographical roughness parameters enhances the knowledge of oyster reef surfaces and gives insights into the interactions between biogenic structure and surrounding hydrodynamics. The new intra-reef classification allows for more accurate determination of the overall roughness as well as the population dynamics of the habitat forming oyster. Combined with hydraulic measurements, the results can be used to estimate the hydraulic bed roughness induced by the oyster reef surfaces. Copyright © 2022 Hitzegrad, Brohmann, Pfennings, Hoffmann, Eilrich, Paul, Welzel, Schlurmann, Aberle, Wehrmann and Goseberg