Hwusub Chun

Analysis of the Wave Spectral Shape Parameters for the Definition of Swell Waves

In the present study, the characteristics of spectral peakedness parameter Qp, bandwidth parameter e, and spectral width parameter ν were analyzed as a first step to define the swell waves quantitatively. For the analysis, the joint probability density function of significant wave heights and peak periods were newly developed. The MCMC(Markov Chain Monte Carlo) simulations have been performed to generate the significant wave heights and peak periods from the developed probability density functions. Applying the simulated significant wave heights and peak periods to the theoretical wave spectrum models, the spectral shapes parameters were obtained and analyzed. Among the spectral shape parameters, only the spectral peakedness parameter Qp, is shown to be independent with the significant wave height and peak wave period. It also best represents the peakedness of the spectral shape, and henceforth Qp should be used to define the swell waves with a wave period. For the field verification of the results, wave data obtained from Hupo port and Ulleungdo were analyzed and results showed the same trend with the MCMC simulation results.

A Study on the Statistical Characteristics and Numerical Hindcasts of Storm Waves in East Sea

In the present study, the statistical analysis on the storm waves in the East Sea have been carried out, and the several storm waves were reproduced by the modified WAM as a first step for the accurate and prompt forecasting and warning against the swell waves in East Sea. According to the present study, the occurrences of the storm waves from the North were the most probable, while the waves from the Northeast were most frequently observed. It was found that the significant wave heights of storm waves from the North and Northern northeast were larger than those of storm waves from the Northeast. But due to long fetch distance, the significant wave periods of storm waves from the Northesast were longer than those of North and Northern northeast. In addition to the wave analysis, the numerical experiments for the storm waves in East Sea were carried out using the modified WAM, and three periods of storm waves in 2013 were calculated. The numerical results were well agreed with wave measurements. However the numerical simulation results in shallow water region showed lower accuracies compared to deep water, which might be due to lower resolution of wind field and bottom topography caused by large grid size, 5 minute, adopted in the present study. Overall computational efficiency of the modified WAM found to be excellent compared to original WAM. It is because the modified WAM adopted the implicit scheme, thereby the present model performed 10 time faster than original WAM in computation time.

Storm Waves on the East Coast of Korea: 20 Years of Wave Hindcasting

ABSTRACT Chun, H. and Ahn, K., 2017. Storm waves on the east coast of Korea: 20 years of wave hindcasting. This paper presents 20 years of wave hindcasting in the East Sea (Sea of Japan) to estimate return periods of storm waves at the east coast of South Korea for coastal risk management. The numerical results were validated against the measured data obtained at eight wave stations in the East Sea. The comparison of correlation coefficients of significant wave heights and wave periods, 0.81–0.95 and 0.70–0.88, respectively, between the numerical results and the measured data shows good agreement. Note that the numerical results agree well with the measurements despite the large computational grid size used. The storm waves on February 2008 were successfully simulated in the present study, and the computed peak periods at Gangneung Port showed good agreement with those measured, even though the coefficients of whitecapping dissipation were not modified. On the basis of 20 years of wave hindcasting results, extreme wave analysis was carried out using the peak-over-threshold method and the generalized Pareto distribution function. According to the extreme wave analysis, significant wave heights occurred at Sokcho and Gangneung on 23 October 2006, and were estimated to have a return period of 100 years. It was found that the waves generated by Typhoon Nabi (T0514) were the most severe in the East Sea during the simulation period. During this typhoon period, a 50-year wave appeared at Pohang, the southern East Sea.

Wave-Induced Reynolds Stress in Three-Dimensional Nearshore Currents Model

ABSTRACT Chun, H. and Suh, K.-D., 2016. Wave-induced Reynolds stress in three-dimensional nearshore currents model. This paper presents the formulation of the wave-induced Reynolds stress (WIRS), i.e. the correlation of horizontal and vertical water particle velocities, based on the Airys wave theory. Even though the correlation of the velocities is zero in a constant-depth water, it has a nonzero value if there are slopes on the bottom and mean water level. Comparison of the vertical gradient of the WIRS term with the horizontal gradient of wave-forcing terms shows that the two terms have the same order of magnitude, which in turn is comparable to that of the radiation stress in the surf zone. For the three-dimensional computation of nearshore currents, the WIRS term is incorporated into a three-dimensional ocean model in the σ coordinates. The numerical model is then applied to two laboratory experiments. To see the effect of the WIRS term on the mean flow, numerical simulations without this term are also carried out. The numerical simulations show that the WIRS term influences the flow properties such as mean water level, longshore current, and undertow.

Wave Hindcasting on the Storm Waves at the Korean Straits of April, 2016

In the present study, the storm waves at the Korean Straits of April, 2016 have been reproduced by the wave hindcasting, and then their characteristics were investigated. Before the wave hindcasting, the wave measurements at the Korean Straits were analyzed. The analysis showed that the waves at the Korean Straits were dominated by the Northeastern waves, same as those in the East Sea. Accordingly, the wave hindcasting was been carried out with the same condition in Ahn et al. (2016). In the numerical results, the maximum significant wave height at the Korean Straits was 5.06 m, and the corresponding significant wave period was 9.2 s. The computed significant wave heights and wave periods were overestimated by 4 cm and 0.8 s, respectively. After the wave hindcasting, the computed significant wave heights and peak periods were compared with the JONSWAP relationship. This comparison showed that the storm waves at the Korean Straits were close to wind waves, not swell.

Analysis of Longshore Currents with an Eulerian Nearshore Currents Model

ABSTRACT Chun, H. and Suh, K.-D., 2017. Analysis of longshore currents with an Eulerian nearshore currents model. In the present study, longshore currents are analyzed with the three-dimensional nearshore currents model in the Eulerian framework. It was found that the model was reduced to the classical one-dimensional longshore currents model for a long, straight, uniform coast, which could also be obtained from the Lagrangian nearshore currents model. It indicates that the longshore current velocities calculated by the Eulerian and Lagrangian models are equivalent to each other for a long, straight, uniform coast. In addition, it was shown that longshore currents with a parabolic vertical profile could be reproduced by the wave–current interaction term in the model. The analysis has been confirmed by the numerical experiments on DUCK94. According to the numerical experiment, the depth-averaged velocities from the present numerical model were consistent with those of one-dimensional longshore currents model at a significance level of 0.05. The present numerical model successfully predicted the parabolic profile of longshore currents outside the surf zone. However, the parabolic profile of longshore currents inside the surf zone was not shown in the present numerical results.