• Title/Summary/Keyword: freak wave

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Extreme and Freak Wave Characteristics in the Coastal Writers of Korean Peninsula (한국 연안의 극히 파랑환경과 Freak Wave의 특성에 관한 연구)

  • 류청로;윤홍주
    • Journal of Environmental Science International
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    • v.2 no.3
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    • pp.235-243
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    • 1993
  • Extreme environments and freak wave characteristics in the coastal waters of Korean Peninsula are analyzed using the observed wave data. Freak wave has been intensely emphasized as an important environmental force parameter in several recent research works. However, the mechanism and occurrence probability of freak wave are not clarified. The aims of this study we: to summarize the distribution of extreme environment for wind waves, and to find occurrence probability of freak wave in the coastal waters of Korean Peninsula. These extreme sea conditions are discussed by applying extreme value analysis method, and the statistic characteristics are summarized which can be used to the design and analysis of coastal structures. The mechanism and the occurrence probability of freak wave are also discussed in detail using wave parameters in considered with wave deformation in the coastal waters. Key Words : extreme wave, freak wave, extreme analysis, design wave, probability density.

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A Study on the Characteristics of Large Amplitude Ocean Waves (대진폭 해양파의 특성에 대한 연구)

  • Kim, Do-Young
    • Journal of the Korean Society for Marine Environment & Energy
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    • v.12 no.2
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    • pp.61-67
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    • 2009
  • In this paper time series wave data which contain a freak wave is investigated. Various wave characteristics are compared between wave data with a freak wave and without. Among 24 hour wave data measured in the Yura Sea, two adjacent 30 min wave data with and without a freak wave are examined intensively. It is seen that the highest waves do not have the longest wave period. The wave period of the longest period waves is a little longer than the average wave period and much shorter than the significant wave period. Although the sea state is quite high, the Rayleigh distribution fits well to the probability of wave height. The characteristics of the wave spectra do not change much, but the nonlinearity increases for the wave data with a freak wave. The significant wave height without a freak wave is larger than that with a freak wave. Hence, the higher significant wave height does not always increase the probability of the occurrence of the freak waves.

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On the Statistical Characteristics of Freak Wave Occurrence (Freak Wave 발생의 통계적 특성에 대하여)

  • Kim, Do-Young
    • Journal of the Korean Society for Marine Environment & Energy
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    • v.14 no.2
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    • pp.138-145
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    • 2011
  • In this paper time series wave data are simulated by the Monte Calo method using random numbers to generate random phases of the wave signal. The simulated wave signasl are used to study the characteristics of freak waves. Various sea states are represented by combinations of the significant wave height $H_s$ defined in the spectrum method and the significant wave steepness $S_s$. For a fixed value of $S_s$, the probability of the occurrence of the freak wave is decreased as $H_s$ increases. For a fixed value of $H_s$ the probability of the occurrence of the freak wave increases as $S_s$ increases. The average value of the maximum wave height increase as $S_s$ increases, but the average height of freak wave remains the same and the value approaches two times of $H_s$. For the fixed value of $S_s$, average kurtosis of wave elevation increases as $H_s$ increases, but for a fixed $H_s$, the average kurtosis decreases as $S_s$ increases. The average of abnormality index(AI) is around 2.11 irregardless of $H_s$ and $S_s$. The maximum value of AI lies between 2.5 - 3.0. Therefore it is conjectured that AI maximum due to linear focusing is 3.0.

Recent Progress of Freak Wave Prediction

  • Mori, Nobuhito;Janssen, Peter A.E.M.
    • Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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    • pp.127-134
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    • 2006
  • Based on a weakly non-Gaussian theory the occurrence probability of freak waves is formulated in terms of the number of waves in a time series and the surface elevation kurtosis. Finite kurtosis gives rise to a significant enhancement of freak wave generation in comparison with the linear narrow banded wave theory. For fixed number of waves, the estimated amplification ratio of freak wave occurrence due to the deviation from the Gaussian theory is 50% - 300%. The results of the theory are compared with laboratory and field data.

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An Analysis of Statistical Characteristics of Nonlinear Ocean Waves (비선형 해양파의 통계적 특성에 대한 해석)

  • Kim, Do-Young
    • Journal of the Korean Society for Marine Environment & Energy
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    • v.13 no.2
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    • pp.112-120
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    • 2010
  • In this paper time series wave data measured continuously for 24 hours during a storm in Yura Sea Area are used to investigate statistical characteristics of nonlinear waves. The exceedance probability of wave height is compared using the Rayleigh distribution and the Edgeworth-Rayleigh (ER) distribution. Wave data which show stationary state for 10 hours contain 4600 waves approximately. The Gram-Chalier distribution fits the probability of wave elevation better than the Gaussian distribution. The Rayleigh ($H_{rms}$) distribution follows the exceedance probability of wave height in general and predicts the probability of freak waves well. The ER distribution overpredicts the exceedance probability of wave heights and the occurrence of freak waves. If wave data measured for 30 minute period which contains 250 waves are used, the ER distribution can predict the occurrence probability of freak waves well. But it overpredicts the probability of overall wave height If no freak wave occurs, the Rayleigh ($H_{rms}$) distribution agrees well with wave height distribution for the most of wave height ranges. The wave height distribution of freak waves of which height are less than 10 m shows similar tendency compared with freak waves greater than 10 m. The value of $H_{max}/H_{1/3}$ is related to the kurtosis of wave elevation. It seems that there exists threshold value of the kurtosis for the occurrence of freak waves.

Study on Freak Wave Characteristics and Approximation of Wave Spectrum in Uljin Sea Area (울진해역의 Freak wave 특성과 스펙트럼 근사에 대한 연구)

  • Ryu, Hwang-Jin;Hong, Sa-Young
    • Journal of Ocean Engineering and Technology
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    • v.26 no.2
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    • pp.8-13
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    • 2012
  • This paper investigates the statistical properties of waves in the sea area of Uljin, which is located in the East Sea area of Korea. The wave data were measured using AWAC (Acoustic Wave and Current Meter), which was installed at a 16-m water depth from November 2010 to March 2011. The wave data acquisition rate, Hmax, monthly mean Hs, Tz, Tp, and wave direction are summarized. The distributions of Hs and Tz were analyzed using the Hs-Tz scatter diagrams. The measurement wave data were analyzed to investigate freak wave characteristics. By comparing the wave spectrum using the measurement wave data with the wave spectrum obtained by varying the JONSWAP wave spectrum, it was possible to approximate the wave spectrum shape at the Uljin Sea area.

Field Wave Data Analysis for Investigation of Freak wave Characteristics (Freak wave 특성 파악을 위한 파랑관측 자료의 분석)

  • Shin, Seung-Ho;Hong, Key-Yong;Moon, Jae-Seung
    • Proceedings of the Korean Institute of Navigation and Port Research Conference
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    • v.1
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    • pp.175-180
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    • 2006
  • This study is carried out the investigation of nonlinear characteristics of the ocean based on the field wave observation data acquired the western sea area in Jeju island during one year. It is aimed to offer the fundamental data for Freak wave forecasting in real sea. For this, the nonlinearity parameters of ocean waves, which are Skewness, Atiltness, Kurtosis and Spectrum band width parameter, are introduced, and the parameters are compared and discussed with some characteristic wave components, ie, significant wave height, maximum wave height, and so on.

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Occurrence Probability of Freak Waves at Nearshore of Donghae Harbor in the East Sea (동해항 전면 해역에서의 Freak Waves 발생확률)

  • Ahn, Kyungmo;Oh, Chan Young;Jeong, Weon Mu
    • Journal of Korean Society of Coastal and Ocean Engineers
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    • v.27 no.4
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    • pp.258-265
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    • 2015
  • Over the last 20 years, freak waves have attracted many researchers because of their unexpected behaviors and damages on offshore structures and vessels in the ocean and coastal waters. Despite many researches on the causes, mechanisms and occurrence of freak waves, we have not reached consensus on the results of the researches. This paper presents the occurrence probability of freak waves based on the analysis of wave records measured at coastal waters of Donghae harbor in the East Sea. Three freak waves were found which satisfied conditions of m and $H_S{\geq}2.5m$ and $H_m/H_S{\geq}2$. The occurrence probabilities of freak waves were estimated from extreme distributions by Mori, Rayleigh and Ahn, and found to be on the orders of O($10^{-1}$), O($10^{-2}$), and O($10^{-3}$), respectively. The occurrence probabilities of freak waves measured from waves records were estimated between O($10^{-2}$) and O($10^{-3}$), which were located between predictions by Rayleigh and Ahn's extreme probability distributions. However, we need more analysis of wave records obtained from diverse field conditions in order to verify the accuracy of the estimation of occurrence probability of freak waves.

Wave Data Analysis for Investigation of Freak wave Characteristics (Freak Wave 특성 파악을 위한 파랑관측 자료의 분석)

  • Shin, Seung-Ho;Hong, Key-Yong;Moon, Jae-Seung
    • Journal of Navigation and Port Research
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    • v.31 no.6
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    • pp.471-478
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    • 2007
  • This study is carried out the investigation of nonlinear characteristics of the field wave observation data acquired in the western sea area in Jeju island during one year. It is aimed to offer the fundamental data for Freak wave forecasting in real sea. For this, the nonlinear parameters of ocean waves, which are Skewness, Atiltness, Kurtosis and Spectrum band width parameter et al., are introduced, and the parameters are compared and discussed with some characteristics wave components, ie, significant wave height, maximum wave height, and so on. As a results, we know that the parameters describe nonlinear characteristics of observed wave spectrum broadly, are feebly related with occurrence of abnormal maximum wave height, namely freak event, however the Kurtosis, $K_t$ which is a degree of peakness of mode of surface elevation distribution, has better relationship than others.

On the possibility of freak wave forecasting

  • Janssen, Peter A.E.M.;Mori, Nobuhito;Onorato, Miguel
    • Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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    • pp.121-126
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    • 2006
  • Modern Ocean wave forecasting systems predict the mean sea state, as characterized by the wave spectrum, in a box of size ${\Delta}x{\Delta}y$ surrounding a grid point at location x. It is shown that this approach also allows the determination of deviations from the mean sea state, i.e. the probability distribution function of the surface elevation. Hence, ocean wave forecasting may provide valuable information on extreme sea states.

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