• Journal of Korea Water Resources Association
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• 제40권11호
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• pp.899-908
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• 2007

In this study, wave reflection due to depression of seabed is calculated by using eigenfunction expansion method. The proper numbers of steps and evanescent modes needed for analysis are suggested by applying the eigenfunction expansion method to bottom topography of which slope or curvature varies. While satisfying shallow or intermediate water depth condition, the optimal figure of depression of seabed is obtained by calculating reflection coefficient for various depressions of seabed. The reflection coefficient with distance between the depression of seabeds is then calculated after arraying the optimal geometry in two and three rows.

• Journal of the Korean Society of Marine Environment & Safety
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• 제22권7호
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• pp.836-845
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• 2016

The variation and structure of the cold water mass around Ganjeol Point during the summer of 2011 were studied using data from CTD observations and temperature monitoring buoys deployed at 20 stations off the southeast coast of Korea. There was a $-12^{\circ}C$ surface temperature difference between the cold water mass and normal water during the monitoring period. Variations in the isothermal lines for surface temperature along the coast showed that the seabed topography at Ganjeol Point played an important part in the distribution of water temperature. Cold water appeared when the wind components running parallel to the coast had positive values. The upwelling -response for temperature fluctuations was very sensitive to changes in wind direction. Vertical turbulent mixing due to the seabed topography at Ganjeol Point can reinforce the upwelling of cold bottom water. From wavelet analysis, coherent periods found to be 2-8 days during frequent upwelling events and phase differences for a decrease in water temperature with a SSW wind were 12-36 hours.

• Journal of Korean Society of Coastal and Ocean Engineers
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• 제33권4호
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• pp.168-178
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• 2021

Numerical simulations were implemented to look into the modified seabed topography due to the presence of breakwaters of varying reflection characteristics. The numerical model was composed of OlaFlow, an OpenFoam-based tool box, and a physics-based morphology model [Seoul Foam]. In doing so, the interaction between the seabed, which undergoes deformation due to siltation and scouring, and the incoming waves was described using Dynamic Mesh. The rubble-mound, vertical, and curved slit caisson breakwaters with varying reflection characteristics resulted in standing waves that differ from each other, shown to have a significant influence on the seabed topography. These results are in line with Nielsen's study (1993) that sands saltated under the surface nodes of standing waves, where the near-bed velocities are most substantial, convected toward the surface antinodes by boundary-layer drift. Moreover, the crest of sand waves was formed under the surface antinodes of standing waves, and the trough of sand waves was formed under the surface antinodes. In addition, sand wave amplitude reaches its peak in the curved slit caisson with a significant reflection coefficient, and the saltation of many grains of sand would cause this phenomenon due to the increased near-bed velocity under the nodes when the reflection coefficient is getting large.

• Journal of the Korean Society of Surveying, Geodesy, Photogrammetry and Cartography
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• 제38권1호
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• pp.1-9
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• 2020

ABL (Airborne Bathymetric LiDAR) is an advanced survey technology that uses green lasers to simultaneously measure the water depths and oceanic topography in coastal and river areas. Seabed point cloud extraction is an essential prerequisite to further utilizing the ABL data for various geographic data processing and applications. Conventional seabed detection approaches often use return waveforms. However, their limited accessibility often limits the broad use of the bathymetric LiDAR (Light Detection And Ranging) data. Further, it is often questioned if the waveform-based seabed extraction is reliable enough to extract seabed. Therefore, there is a high demand to extract seabed from the point cloud using other sources of information, such as geometric information. This study aimed to assess the feasibility of a ground filtering method to seabed extraction from geo-referenced point cloud data by using CSF (Cloth Simulation Filtering) method. We conducted a preliminary experiment with the RIGEL VQ 880 bathymetric data, and the results show that the CSF algorithm can be effectively applied to the seabed point segmentation.

• Ocean Systems Engineering
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• 제4권4호
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• pp.279-291
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• 2014

Smart flanges are used for pipeline and riser repair in subsea. In a typical case in the gas export pipeline project, the end cap bolts of a 4inch smart flange were broken during operation, and in turn leakage occurred. This work presents the detail of three dimensional finite element analysis of the smart flange to support the observed end cap bolts failure. From finite element analysis it turns out that in the presence of external bending moment, an uneven contact distribution is present between seal and end cap, which in turn changes the uniform load distribution on bolts and threaten the integrity of bolts. On the other hand, 3D finite element analysis of interaction between pipeline and seabed is presented by means of Abaqus to explore the distribution of bending moment along the pipeline route. It is found that lateral buckling occurs in the pipeline which introduces large bending moment.

• Journal of Korea Water Resources Association
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• 제35권6호
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• pp.677-684
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• 2002

In this study, a finite element model is employed to simulate the diffraction of waves caused by a change of water depths. The model is firstly applied to the estimation of reflection coefficients of monochromatic waves over a sinusoidally varying topography. Predicted coefficients are compared with those of the eigenfunction expansion method and laboratory measurements. A good agreement is observed. The model is then used to investigate effects of heights of bottom topography and number of ripples on variation of reflection coefficients of monocromatic water waves.

• Journal of Korean Port Research
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• 제14권1호
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• pp.97-105
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• 2000

This paper presents an automatic mesh generation considering water depth, which is based on the depth interpolation. The key feature of this method is that the position of a mesh on any depth in the shallow water area can be generated. The Examples are carried out, and the results are shown to be good. This method is shown to be a useful and powerful tool for the flow calculation for the seabed topography.

• Journal of the Korean Society of Marine Environment & Safety
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• 제22권7호
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• pp.863-868
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• 2016

In this study, the precise shapes of sunken ships and information on seafloor topography were analyzed using data obtained from a multi-beam echo sounder. The state of each sunken ship was analyzed by processing diverse imagery data which was compared with data obtained from past investigations to determine changes in the state and circumjacent seafloor topography. Apparent changes in the seafloor topography around one sunken ship, the "Pacific Friend", were found from stern to bow as a result of continued submarine erosion and sedimentation. In the case of sunken ship "No. 7 Haeseong", the partial collapse of the bow was revealed in the seabed images captured in 2015, though it had still been intact in images captured during the Korea Hydrographic and Oceanographic Agency's investigation in 2011. This partial collapse was presumed to have resulted from the effects of continued tidal currents, the cargo load of the ship and continued corrosion of the ship over a long time on the seabed. Continuous monitoring of residual fuel inside the ship is necessary to avoid leakage and potential marine pollution. By conducting image analysis on these sunken ships, it has been determined that the structural safety of the ships is seriously influenced by tidal currents and seafloor topography, while the hulls will be continuously changed by corrosion. As a result, it can be concluded that the development of prediction and response techniques that take into consideration residual fuel leakage and environmental changes according to the geological characteristics of sunken ships is necessary.

• Journal of Korean Society of Coastal and Ocean Engineers
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• 제31권6호
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• pp.423-433
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• 2019

In order to predict the seabed topography change due to the construction of offshore wind power structures in the west-southern sea of Korea, field observations for tides, tidal currents, suspended sediment concentrations and seabed sediments were carried out at the same time. These data could be used for numerical simulation. In numerical experiments, the empirical constants for the suspended sediment flux were determined by the trial and error method. When a concentration distribution factor was 0.1 and a proportional constant was 0.05 in the suspended sediment equilibrium concentration formulae, the calculated suspended sediment concentrations were reasonably similar with the observed ones. Also, it was appropriate for the open boundary conditions of the suspended sediment when the south-east boundary corner was 11.0 times, the south-west was 0.5 times, the westnorth 1.0 times, the north-west was 1.0 times and the north-east was 1.0 times, respectively, using the time series of the observed suspended sediment concentrations. In this case, the depth change was smooth and not intermittent around the open boundaries. From these calibrations, the annual water depth change before and after construction of the offshore wind power structures was shown under 1 cm. The reason was that the used numerical model for the large scale grid could not reproduce a local scour phenomenon and they showed almost no significant velocity change over ± 2 cm/s because the jacket structures with small size diameter, about 1 m, were a water-permeable. Therefore, it was natural that there was a slight change on seabed topography in the study area.

• Proceedings of the KSRS Conference
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• 대한원격탐사학회 2002년도 Proceedings of International Symposium on Remote Sensing
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• pp.663-663
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• 2002

The National Oceanographic Research Institute of KOREA started to survey for the basic data necessary to territorial sea and EEZ identification and marine development with Multi-Beam Echo Sounder(L3 SeaBeam 2112) since 1996. The Multi-Beam surveys has provided a very new and precise way of describing the morphology and nature of the underwater seabed. Multi-Beam Echo Sounder systems employ sound waves propagating at angles which vary from vertical to nearly horizontal. The locations on the bottom where echoes are generated cover a swath whose port to starboard width may be equal to many times the water depth. Newer Multi-beam bathymetric sonars provide both a beam by beam depth and backscatter amplitude of the bottom. But The backscattered amplitude didn't use for identification of bottom properties because backscatter amplitude effects by the many environmental variables of underwater and seabed. We investigates the utilization of geo-referenced backscatter amplitude and analysis of relationship between The Backscattered Amplitude and Sidescan Sonar imagery from Sea Beam 2112. For the backscattered amplitude imagery mainly represents the properties of sediment, we computed the beam geometry, time-varied amplifier gain, and mainly incidence angle to the topography using bathymetric model at each ping. In this paper, those issues are illustrated, and the angular independent imagery based on swath topographic model is described.