• Journal of the Korean Society of Marine Environment & Safety
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• v.25 no.5
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• pp.589-600
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• 2019

In order to investigate the flow structures around Geumo archipelago on Southern Waters of Korea, water movements were measured for 25 hours during spring tide in May and neap tide in September 2002 using ADCP (Acoustic Doppler Current Profiler) attached to a running boat. Dominant directions of ebb and flood current at spring tide are SE-NW, representing the average flow rate of approximately 40cm/s in the surface layer. However because of the topographical reason, the direction and speed of the flow in the narrow waterway sea area around the northwest of Gae Island were different. There was no notable baroclinic component of tidal flow at spring tide. This indicates that the sea area has been actively engaged in vertical mixing due to island wake or eddy due to narrow waterways, shallow water depth and rapid flow rate around archipelago. At neap tide, dominant directions of tidal flows are SSE-NNW and the average flow rate in the surface layer is about 85 percent of the spring tide. The duration and intensity of the flow direction are shorter and less dominant than the spring tide. It is expected that asymmetrical tidal mixing will occur due to vertical velocity shear and horizontal eddies. From daily mean tidal flows obtained from the ADCP observation, it was found that the northwest of Gae Island have flows in NW~NE, the west of Geumo Island have the average currents of up to 21 cm/s WSW~SSW and counterclockwise circulation or eddy currents are formed in the west of Sori Island.

• Journal of the Korean Society of Marine Environment & Safety
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• v.29 no.5
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• pp.520-524
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• 2023

Regalecus russellii, which spends most of its life in the deep sea, occasionally appears in coastal waters. However, the reasons for its appearance remain unclear. In Korea, R. russellii mainly appears along the eastern coastal waters, and most of them are caught in fishing gear, such as gill nets, or are stranded on the shore; nevertheless, the frequency of appearance is extremely low. Even if found, this species is often identified to be morphologically similar to Trachipterus ishikawae, and comprehensive analysis to identify the species through sample collection is limited. Consequently, information on the biological characteristics of R. russellii appearing in the coastal waters of Korea is scarce. Herein, the anatomical characteristics of R. russellii caught in a gill net off the Gangwon-do coast on March 14, 2023, were analyzed, and coastal water temperature was measured using an ocean buoy. Our results showed that the individual was male, its total length was 320 cm, body weight was 27.52 kg, body length was 26.62 cm, gonad weight was 619.45 g, and liver weight was 218.71 g. The stomach was full of euphausiids. The water temperature changed drastically at 15-30 m roughly a week before the R. russellii individual was caught, and the subsurface water temperature was lower than 10 ℃. Our findings provide baseline data to understand the ecological characteristics of R. russellii appearing along the eastern coast of the Korea.

• Journal of the Korean Society of Marine Environment & Safety
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• v.29 no.6
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• pp.562-575
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• 2023

In an effort to improve water quality, the South Korean government has implemented measures to increase seawater circulation in Saemangeum Lake. We analyzed the effect of increasing the frequency of seawater circulation based on salinity levels and bottom water exchange in the lake, using an environmental fluid dynamics code model. When the sluice gate opening and shutting frequency increased from once to twice per day, the internal water level of Saemangeum Lake increased by up to ~0.7 m. The salinity increased by 2.12 psu near the western breakwater and decreased by 1.18 psu near the freshwater inlet. We analyzed the extent of bottom water exchange using a particle tracing method and observed that the residual rate of particles shallower than 5 m in water depth decreased by 2.52% in Case 2 (opening and shutting twice per day) compared to Case 1 (opening and shutting once per day). This indicates that increasing the frequency of sluice gate opening and shutting would promote enhanced bottom water exchange. Consequently, the increased salinity and bottom water exchange associated with increased seawater circulation are expected to improve water quality in Saemangeum Lake.

• Korean Journal of Ichthyology
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• v.35 no.4
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• pp.270-277
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• 2023

Since its introduction in 1973, the bass, Micropterus salmoides, has spread to dams and rivers in Korea, adversely affecting native fish communities. In this study, after complete removal of bass from Bukchosan Reservoir, an farm reservoir that has lost its function, an attempt was made to indirectly confirm the adverse effects of bass on fish by comparing fish communities in reservoirs of a similar size where bass inhabit. The study was conducted three times a year from 2019 to 2022. As a result of the study, in Bukchosan Reservoir, where the species composition of fish was judged to be simplified due to the habitat of bass, only four species such as Carassius auratus and Rhinogobius brunneus that could be used as a food source for bass appeared before the removal of bass, but after the removal of bass. The appearance and increase of the population of the introduced small freshwater fish (Oryzias sinensis, Rhinogobius giurinus) were confirmed. When comparing Gangjeong Reservoir, Susim Reservoir, which are other reservoirs inhabited by bass investigated in this study, and Bukchosan Reservoir, this result is judged to show a trend of stabilization and recovery of fish species after removal of bass in small reservoirs with high predation pressure of bass.

• Journal of the Korean Society of Marine Environment & Safety
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• v.30 no.1
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• pp.20-31
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• 2024

This study was conducted to investigate the changes in the structure of benthic communities resulting from aquaculture activities and to assess the benthic health status of surface sediment in Gangjin Bay, a region known for concentrated shellfish aquaculture on the southern coast of Korea. Survey stations were divided into longline culture, bottom culture, and non-cultivation areas. The spatiotemporal distribution of physiochemical factors such as the grain size, water temperature, salinity, and total organic carbon in Gangjin Bay showed no significant differences between sampling stations. However, the species number, density, and diversity were relatively lower at the sampling stations in the bottom culture areas than at the other stations throughout the entire survey period. Cluster analysis and principal coordinates analysis also clearly distinguished the benthic communities in the bottom culture areas from those in the other sampling areas. At the sampling stations in the longline culture and non-cultivation areas, Scolectoma longifolia and Sigambra tentaculata, which are indicator species of organically enriched areas, appeared as dominant species. However, excluding some stations influenced by physical factors such as the water depth and current speed, the occupancy rate was not high. The health assessment results, conducted using the fisheries environment assessment method, revealed good conditions with Grades 1 and 2 across the entire area. However, an examination of the spatiotemporal changes in benthic communities and the benthic health index indicated that the benthic environment in the bottom culture areas was affected by physical disturbances.

• Journal of Korea Water Resources Association
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• v.51 no.9
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• pp.827-837
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• 2018

The stabilities of sliding and overturning of caisson and bearing capacity of mound against eccentric and inclined loads, which possibly happen to a composite caisson breakwaters, have been analyzed by using the technique of multiple failure modes. In deterministic approach, mathematical functions have been first derived from the ultimate limit state equations. Using those functions, the minimum cross section of caisson can straightforwardly be evaluated. By taking a look into some various deterministic analyses, it has been found that the conflict between failure modes can be occurred, such that the stability of bearing capacity of mound decreased as the stability of sliding increased. Therefore, the multiple failure modes for the composite caisson breakwaters should be taken into account simultaneously even in the process of deterministically evaluating the design cross section of caisson. Meanwhile, the reliability analyses on multiple failure modes have been implemented to the cross section determined by the sliding failure mode. It has been shown that the system failure probabilities of the composite breakwater are very behaved differently according to the variation of incident waves. The failure probabilities of system tend also to increase as the crest freeboards of caisson are heightening. The similar behaviors are taken place in cases that the water depths above mound are deepening. Finally, the results of the first-order modal are quite coincided with those of the second-order modal in all conditions of numerical tests performed in this paper. However, the second-order modal have had higher accuracy than the first-order modal. This is mainly due to that some correlations between failure modes can be properly incorporated in the second-order modal. Nevertheless, the first-order modal can also be easily used only when one of failure probabilities among multiple failure modes is extremely larger than others.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.26 no.5
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• pp.267-277
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• 2014

The conventional performance-based design method for the solid-wall caisson breakwater has been extended and applied to the perforated-wall caisson. The mathematical model to calculate the sliding distance of a perforated-wall caisson is verified against hydraulic experimental data. The developed performance-based design method is then compared with the conventional deterministic method in different water depths. Both the expected sliding distance and the exceedance percentage of total sliding distance during the structure lifetime decrease with decreasing water depth outside the surf zone, but they increase with decreasing water depth inside the surf zone. The performance-based design method is either more economical or less economical than the deterministic method depending on which design criterion is used. If the criterion for the ultimate limit state is used, the former method is less economical than the latter outside the surf zone, whereas the two methods are equally economical inside the surf zone. However, if the breakwater is designed to satisfy the criterion for the repairable limit state, the former method is more economical than the latter in all water depths.

• Proceedings of the Korean Institute of Navigation and Port Research Conference
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• 2004.11a
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• pp.169-174
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• 2004

To design ship channel is one of important factors for planning and developing a port. In most case, the core factors for designing ship channel are the layout and width of dvnnel provided the net underkeel clearance is assessed as safety. In this study, Variable Bumper Area(VBA) model is applied to design and assess ship channel. This model reflects ship's principle dimension, ship domain theory, ship speed, conning officer's ship handling skill and experience and all external forces which cause leeway, set and drift and the change of ship maneuvering characteristics. Full Mission Ship Handling Simulator is used to analyze ship dynamic data according to conning officer's ship control, external forces, etc. This model uses Domain-index for assessing the efficiency and safety of the channel. The proposed model is applied to Ulsan new port plan which has a channel width of 1.5 times the length if the largest vessel, a radius if 5 times the length of the largest vessel in a curve of 57 degree centerline angle and SBM facility adjacent to the lateral edge if channel. The result of this study shows tint the width and radius of channel curve are suitable for the target ship but the difficulty of ship handling is caused by the large course change and SBM located in the vicinity if channel.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.25 no.5
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• pp.317-326
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• 2013

Probabilistic design methods can consider uncertainties of design variables and are widely used in the design of vertical breakwaters. The probabilistic design methods include a partial safety factor method, reliabilitybased design method, and performance-based design method. Especially the performance-based design method calculates the accumulated sliding distance during the lifetime of the breakwater or during a design storm. Recently a time-dependent performance-based design method has been developed based on the first-passage probability of individual sliding distance during a design storm. However, because the allowable criteria in the first-passage probability method are not established, the stability of structures cannot be quantitatively evaluated. In this study, the allowable first-passage probabilities for two limit states are proposed by calculating the first-passage probabilities for the cross-sections designed with various water depths and characteristics of extreme wave height distributions. The allowable first-passage probabilities are proposed as 5% and 1%, respectively, for the repairable limit state (allowable individual sliding distance of 0.03 m) and ultimate limit state (allowable individual sliding distance of 0.1 m). The proposed criteria are applied to the evaluation of the effect of wave-height increase due to climate change on the stability of the breakwater.

• Journal of the Korean Society for Marine Environment & Energy
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• v.19 no.1
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• pp.7-17
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• 2016

The present study experimentally considers dynamic performance of large floating wave-offshore hybrid power generation platform in extreme conditions. In order to evaluate the motion performance of the large floating hybrid power generation platform, 1/50 scaled model was manufactured. A mooring line was also manufactured, and free-decay and static pull-out tests were carried out to check the mooring model. A mooring line table was introduced to satisfy the water depth, and environmental conditions were checked. Motion responses in regular waves were measured and complicated environmental conditions including wave, wind, and current were applied to see the dynamic performance in extreme/survival conditions. Maximum motion and acceleration were judged following the design criteria, and maximum offset and mooring tension were also checked based on the rule. The characteristics of hybrid power generation platform are discussed based on these data.