• Journal of The Geomorphological Association of Korea
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• v.19 no.3
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• pp.71-82
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• 2012

This research was carried out to estimate annual erosion and retreat rates by using datum-point and to identify the characteristics and causes of seasonal variations of sea-cliff slope in Pado-ri, Taean-gun. In the result, the erosion and retreat rates of sea-cliff were increased from spring to summer. The rates were increased rapidly between August and October, caused by the effects of extreme weather events such as severe rainstorms and typhoons, etc. Since then, the erosion and retreat rates of sea-cliff were decreased gradually, but the rates were increased again in winter due to the storm surge and mechanical weathering resulting from the repeated freezing and thawing actions of bed rocks. The factors that affect erosion and retreat rates of sea-cliff include the number of days with antecedent participation and daily maximum wave height. In particular, it turned out that the erosion is accelerated by strong wave energy during storm surges and typhoons. The annual erosion and retreat rates of study area for the past two years(from May 2010 to May 2012) were approximately 44~60cm/yr in condition of differences in geomorphological and geological characteristics at each point. These erosion and retreat rates were found to be higher than results of previous researches. This is caused by coastal erosion forces strengthened by extreme weather events. The erosion and retreat process of sea-cliff in the study area is composed by denudation of onshore areas in addition to marine erosion(wave energy).

• Journal of the Korean Society of Combustion
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• v.21 no.2
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• pp.22-28
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• 2016

High efficient and environment friendly combustion technologies are used to be operated an extreme condition, which results in unintended flame instability such as extinction and oscillation. The use of electromagnetic energy is one of methods to enhance the combustion stability and a microwave as electromagnetic wave is receiving increased attention recently because of its high performance and low-cost system. In this study, an experiment was performed with jet diffusion flames induced by microwave. Micro jet was introduced to simulate the high velocity of industrial combustor. The results show that micro jet flames had three different modes with increasing oxidizer velocity; attached yellow flame, lifted flame, and lifted partially premixed flame. As a microwave was induced to flames, the overall flame stability and blowout limit were extended with the higher microwave power. Especially the interaction between a flame and a microwave was shown clearly in the partially premixed flame, in which the lift-off height decreased and NOx emission measured in post flame region increased with increasing microwave power. It might be attributed to increase of reactivity due to the abundance of radical pool and the enhanced absorption to thermal energy.

• Journal of the Korean Institute of Navigation
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• v.13 no.2
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• pp.23-36
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• 1989

Marine casualities in the high sea are mainly classified into the breakage of hull and capsize , of which the latter occurs frequently to a small craft and container vessels by extreme rolling. The aim of this study is to develop shiphandling techniques for the prevention of ship's large rolling by way of evaluating dangerous degree of rolling in heavy weather. In this study, rolling motion is analized by using statistical method as follow : (1) 8 sample ships is presented for calculation. (2) Analized sea state are Beaufort scale 7 and 10 (wind velocity 30kts and 50kts respectively) and significant wave height is put as 5.2m and 11.2m. (3) The formula recommended by International Towing Tank Conference (ITTC) is used to calculated the wave spectrum. The results of this study are as follow : The results of this study are as follow : (1) Most of the vessels with beam of 20 meters or less was found to be capized in the waves abeam under the sea condition of Bearfort scale7(30kts). (2) For the vessels range 20m to 30m was found safe under the sea conditions of Bearfort scale 7(30kts) and imminent danger under the sea condition of Beaufort scale 11(50kts). (3) It is proved that any vessel could be capsized by heavy rolling regardless of vessel's size whenever the motion is synchronized with waves abeam. This study concludes that the navigator, especially at night , must anticipate the exact wave direction, referring to the wether report and coastaline, not to lay the vessel in the serial wave abeam.

• Atmosphere
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• v.30 no.1
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• pp.1-15
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• 2020

This study analyzed and compared development mechanisms leading to heat waves of 2016 and 2018 in Korea. The European Centre for Medium-Range Weather Forecasts Reanalysis Interim (ERA Interim) dataset and Automated Surface Observing System data are used for synoptic scale analysis. The synoptic conditions are investigated using geopotential height, temperature, equivalent potential temperature, thickness, potential vorticity, omega, outgoing longwave radiation, and blocking index, etc. Heat waves in South Korea occur in relation to Western North Pacific Subtropical High (WNPSH) pressure system which moves northwestward to East Asia during summer season. Especially in 2018, WNPSH intensified due to strong large-scale circulation associated with convective activities in the Philippine Sea, and moved farther north to Korea when compared to 2016. In addition, the Tibetan high near the tropopause settled over Northern China on top of WNPSH creating a very strong anticyclonic structure in the upper-level over the Korean Peninsula. Unlike 2018, WNPSH was weaker and centered over the East China Sea in 2016. Analysis of blocking indices show wide blocking phenomena over the North Pacific and the Eurasian continent during heat wave event in both years. The strong upper-level ridge which was positioned zonally near 60°N, made the WNPSH over the South Korea stagnant in both years. Analysis of heat wave intensity (HWI) and duration (HWD) show that HWI and HWD in 2018 was both strong leading to extreme high temperatures. In 2016 however, HWI was relatively weak compared to HWD. The longevity of HWD is attributed to atmosphere blocking in the surrounding Eurasian continent.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.30 no.6
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• pp.286-297
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• 2018

In the present study, wave measurements at KOGA-W01 were analyzed and then the numerical wind waves simulations have been conducted to investigate the characteristics of wind waves in the Yellow sea. According to the present analysis, even though the location of the wave stations are close to the coastal region, the deep water waves are prevailed due to the short fetch length. Chun and Ahn's (2017a, b) numerical model has been extended to the Yellow Sea in this study. The effects of tide and tidal currents should be included in the model to accommodate the distinctive effect of large tidal range and tidal current in the Yellow Sea. The wave hindcasting results were compared with the wave measurements collected KOGA-W01 and Kyeockpo. The comparison shows the reasonable agreements between wave hindcastings and measured data, however the model significantly underestimate the wave period of swell waves from the south due to the narrow computational domain. Despite the poorly prediction in the significant wave period of swell waves which usually have small wave heights, the estimation of the extreme wave height and corresponding wave period shows good agreement with the measurement data.

• Smart Structures and Systems
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• v.26 no.3
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• pp.291-309
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• 2020

To study oceanic and meteorological problems related to climate change, Korea has been operating several ocean research stations (ORSs). In 2011, the Gageocho ORS was attacked by Typhoon Muifa, and its structural members and several observation devices were severely damaged. After this event, the Gageocho ORS was rehabilitated with 5 m height to account for 100-yr extreme wave height, and the vibration measurement system was equipped to monitor the structural vibrational characteristics including natural frequencies and modal damping ratios. In this study, a mass reallocation method is presented for structural model updating of the Gageocho ORS based on the experimentally identified natural frequencies. A preliminary finite element (FE) model was constructed based on design drawings, and several of the candidate baseline FE models were manually built, taking into account the different structural conditions such as corroded thickness. Among these candidate baseline FE models, the most reasonable baseline FE model was selected by comparing the differences between the identified and calculated natural frequencies; the most suitable baseline FE model was updated based on the identified modal properties, and by using the pattern search method, which is one of direct search optimization methods. The mass reallocation method is newly proposed as a means to determine the equivalent mass quantities along the height and in a floor. It was found that the natural frequencies calculated based on the updated FE model was very close to the identified natural frequencies. In conclusion, it is expected that these results, which were obtained by updating a baseline FE model, can be useful for establishing the reference database for jacket-type offshore structures, and assessing the structural integrity of the Gageocho ORS.

• Proceedings of the Korean Institute of Navigation and Port Research Conference
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• 2004.11a
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• pp.159-166
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• 2004

The beach, a margin between the sea and the land, is an extremely dynamic zone, for it is here that the motion of the sea interacts with the sediment, rock of the land or the artificial barriers. In order to prohibit or retard erosions due to the extreme Typhoon or storm induced waves, man has constructed these of temporary or more permanent nature, but they caused problems of other erosions from the secondary effect of them and a bad influence on the seascape. In considering the energy available to accelerate sediment transport and erosion in the surf zone, where the waves are broken, and offshore beyond the breaker line, the wave height and the wave period should be taken account. Hence, we tried to present an applicability of the submerged artificial Bio-reefs analyzing waves by a numerical model such that they could reduce the wave power without the secondary effect and restoration of marine ecologies. A new technique of beach preservation is by artificial reefs with artificial and/or natural kelps or sea plants. By engineering the geometry of the nearshore reef, the wave attenuation ability of the feature can be optimized Higher, wider and longer reefs provide the greatest barrier against wave energy but material volumes, navigation hazards, placement methods and other factors require engineering considerations for the overall design of the nearshore reefs.

• Journal of Korean Society of Disaster and Security
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• v.13 no.2
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• pp.53-63
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• 2020

In this study, the two-dimensional flow analysis model Hydro_AS-2D model was used to simulate the situation of flooding in Seongsangu and Uichang-gu in Changwon in the event of rising sea levels and extreme flooding, and the results were expressed on three-dimensional topography and the optimal evacuation path was derived using BIM technology. Climate change significantly affects two factors in terms of flood damage: rising sea levels and increasing extreme rainfall ideas. The rise in sea level itself can not only have the effect of flooding coastal areas and causing flooding, but it also raises the base flood level of the stream, causing the rise of the flood level throughout the stream. In this study, the rise of sea level by climate change, the rise of sea level by storm tidal wave by typhoon, and the extreme rainfall by typhoon were set as simulated conditions. The three-dimensional spatial information of the entire basin was constructed using the information of topographical space in Changwon and the information of the river crossing in the basic plan for river refurbishment. Using BIM technology, the target area was constructed as a three-dimensional urban information model that had information such as the building's height and location of the shelter on top of the three-dimensional topographical information, and the results of the numerical model were expressed on this model and used for analysis for evacuation planning. In the event of flooding, the escape route is determined by an algorithm that sets the path to the shelter according to changes in the inundation range over time, and the set path is expressed on intuitive three-dimensional spatial information and provided to the user.

• 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.

• Ocean Systems Engineering
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• v.2 no.2
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• pp.99-113
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• 2012

Steel Catenary Risers (SCR) are the simplest and often the most economic solution compared to other riser types such as flexible pipe, riser towers, top tensioned risers, etc. The top of a SCR is connected to the host platform riser porch. The other end of the SCR connects to flowlines from subsea wells. The riser touchdown point (TDP), which is the location along the riser where contact with the sea floor first occurs, exhibits complex behaviors and often results in compression and fatigue related issues. Heave dynamic responses of semisubmersibles in extreme and operating sea states are crucial for feasibility of SCR application. Recent full field measurement results of a deep draft semisubmersible in Hurricane Gustav displayed the considerable discrepancies in heave responses characteristics between the measured and the simulated results. The adequacy and accuracy of the simulated results from recognized commercial software should be examined. This finding raised the awareness of shortcomings of current commercial software and potential risk in mega investment loss and environmental pollutions due to SCR failures. One main objective of this paper is to attempt to assess the importance and necessity of accounting for viscous effects during design and analysis by employing indicator of viscous parameter. Since viscous effects increase with nearly third power of significant wave height, thus newly increased metocean criteria per API in central Gulf of Mexico (GoM) and even more severe environmental conditions in Western Australia (WA) call for fundamental enhancements of the existing analysis tools to ensure reliable and robust design. Furthermore, another aim of this paper is to address the impacts of metocean criteria and design philosophy on semisubmersible hull sizing in WA and GoM.