Tae-Hwa Jung

Finite Element Solution of Linear Waves on a Sloping Bottom Boundary

ABSTRACT Jung, T-H.; Son, S., and Ryu, Y. 2017. Finite element solution of linear waves on a sloping bottom boundary. A new, finite-element solution of linear water waves, which can be applied to a nonvertical bottom boundary, is introduced in this study. The present solution can be applied to regions in which the water depth gradually approaches zero, such as coastlines. To obtain this solution, the entire domain is divided into three subregions. In the up-wave and down-wave subregions, analytical solutions are used. In the middle region, which occupies most computational domain, the standard Galerkin finite-element method is applied. The introduced numerical method is compared with an analytical solution to show its validity.

An Observational and Numerical Study of Storm-Induced Morphologic Changes at Sanpo Beach, Korea

ABSTRACT Son, S.; Kim, J.; Yoon, H.-D.; Jung, T.-H.; Do, K., and Shin, S., 2017. An observational and numerical study of storm-induced morphologic changes at Sanpo Beach, Korea. In: Lee, J.L.; Griffiths, T.; Lotan, A.; Suh, K.-S., and Lee, J. (eds.), The 2nd International Water Safety Symposium. Journal of Coastal Research, Special Issue No. 79, pp. 334–338. Coconut Creek (Florida), ISSN 0749-0208. This study was conducted to investigate morphological changes at Sanpo Beach due to the storm through field observation and numerical simulation. Sanpo Beach located on the southeast coast of Korea had been significantly affected by typhoon Goni while some changes in beach profile was expected due to the strong storm-induced currents. Series of field observations on the beach topography, bottom bathymetries, shorelines, sand samples, and incident waves were carried out in order to investigate the morphological changes at Sanpo Beach during the typhoon season. In particular, Real Time Kinematic-Global Positioning System (RTK-GPS) and single-beam echo-sounder were deployed to collect beach topography and bottom bathymetry data, respectively. Observational results represented spatial and temporal changes of shoreline and sand bars during a storm event. For more close look at the process of morphologic changes derived by the sediment transport mechanism, field-scale numerical simulations were performed in parallel using Xbeach. Xbeach is one of the widely-used, open-source models which calculate sediment transports and morphologic changes by hydrodynamic processes of short (and/or long) waves, wave-induced setup and unsteady currents. Bathymetric evolutions, as well as shoreline changes predicted by the numerical results, revealed good agreements with observational data.

Field Observation and Numerical Modelling of Rip Currents within a Pocket Beach

ABSTRACT Shin, S.; Nam, J.; Son, S.; Kim, I.H., and Jung, T.-H., 2017. Field observation and numerical modelling of rip currents within a pocket beach. In: Lee, J.L.; Griffiths, T.; Lotan, A.; Suh, K.-S., and Lee, J. (eds.), The 2nd International Water Safety Symposium. Journal of Coastal Research, Special Issue No. 79, pp. 229–233. Coconut Creek (Florida), ISSN 0749-0208. Understanding the occurrence mechanism of rip currents are very important in terms of the sediment transport and the water safety issues. In order to investigate the rip current occurrence mechanism, field observations were carried out at Chunjin beach and a numerical model was employed to simulate rip currents by using the field observation data. The collected data set includes the beach topography, bottom bathymetries, shoreline, sand sample, and incident waves for each season. The field observation data showed the spatiotemporal variations of shoreline and sand bars. The results also showed that the rip currents were generated near the crescentic sand bar. For better understanding the mechanism of rip currents, numerical simulation using phase- resolving, three-dimensional 3D Non-Hydrostatic WAVE Model (NHWAVE) was also carried out. The model considered incoming wave conditions collected from the field observation to look at the nearshore current generation patterns. Through this numerical model, this study could have investigated vertical profiles of nearshore wave-induced current while the field observation data represent depth uniform currents. The numerical simulation showed a good agreement with the field observation and provided useful information for nearshore circulation.

Kinematics of Overtopping Flow on Breakwater Top

ABSTRACT Ryu, Y.; Jung, T.-H., and Kim, Y.-T., 2017. Kinematics of overtopping flow on breakwater top. In: Lee, J.L.; Griffiths, T.; Lotan, A.; Suh, K.-S., and Lee, J. (eds.), The 2nd International Water Safety Symposium. Journal of Coastal Research, Special Issue No. 79, pp. 234–238. Coconut Creek (Florida), ISSN 0749-0208. This study carried out experiments to investigate the behavior of overtopping flows in the vicinity of a caisson breakwater with dissipating blocks. Overtopping flows over a breakwater show horizontal momentum that causes significant damage to objects such as buildings, vehicles, or persons on the top. There is a need to understand overtopping flows on the top related to wave conditions to mitigate damage. This study measured front velocity and impinging distance of overtopping flows by monochromatic waves to examine flow behavior on the top and estimate overtopping flows over the rear slope of the breakwater model in varying wave conditions. Relationships among the parameters were also used to induce empirical equations through dimensionless analysis.

Control of Wave Reflection Using Energy Damping Layers in the Mild-Slope Equation

외해에서 내해로 진입하는 파는 다양한 요인들에 의해 회절, 굴절, 천수, 반사 및 쇄파의 과정을 거쳐 변형이 된다. 이러한 파의 변형 과정은 매우 복잡하여 해석적으로는 해를 구할 수 없고 수치 해석을 통해서 계산을 해야 한다. 수치 해는 해석 해와는 달리 이산화 과정을 거치기 때문에 필연적으로 오차 를 수반한다. 이러한 오차를 최소화하여 신뢰할 만한 해를 구 하는 것은 수치 해석을 통한 연구에서 매우 중요한 부분을 차 지한다. 정확한 수치 해는 적절한 지배방정식과 경계조건을 통해서 구할 수 있다. 본 연구에서는 위 두 가지의 조건 중에 서 경계조건과 관련된 연구를 수행하였다. 파랑과 관련된 경계 조건에는 대표적으로 개방경계조건(open boundary condition) 과 반사경계조건이 있다. 개방경계조건은 경계영역에서 파의 재반사없이 계산영역밖으로 파가 진행하는 것을 의미하며, 반 사경계조건은 구조물이나 지형에 의해 진행하던 파의 일부분 이 파 진행의 반대방향으로 되돌아가는 것을 의미한다. 경계조건 기법 중 비교적 근래에 개발된 스폰지 경계층 기 법은 Larsen and Darcy (1983)에 의해 제안된 기법으로, 경계 층 안으로 진입하는 파의 에너지를 인위적으로 감소시켜 반 사파를 없애는 방법으로 N-S 방정식이나 Boussinesq 방정식 에 적용되어 널리 활용되고 있다(이성대 및 김문정, 2014; 최 준우, 2015). 지금까지 스폰지 경계층은 개방경계조건의 대안 으로 활용되었다. 그러나 실무적인 측면에서는 이러한 외해의 개방경계조건못지 않게, 구조물을 만나거나 지형을 만나서 발 생하는 부분반사가 훨씬 빈번하게 발생가능한 내해측의 구조 물경계조건도 적절히 고려해야 한다. 따라서 본 연구에서는 선형파 이론을 이용하여 구조물 등에 의해 부분반사가 발생 Abstract

Vertical Structure of Rip-currents in the Nearshore Circulation

ABSTRACT Son, S.; Jung, T.H., and Shi, F., 2016. Vertical structure of rip-currents in the nearshore circulation. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 1402 - 1406. Coconut Creek (Florida), ISSN 0749-0208. A number of numerical models have been proposed over the past few decades to investigate rip currents in nearshore circulation zones. Rip currents are the offshore-directed flow generated mainly by wave breaking and momentum transfer over varying topography and they are important in relation to the management of coastal preservation, maintenance, and development. To date, a number of rip current studies have been conducted using depth-integrated two-dimensional (2D) or quasi-three-dimensional (3D) models based on wave-averaged formulations. However, compared with 3D hydrodynamic numerical models, these models are not capable of providing the accurate vertical profiles of current velocity. In this study, we examine the vertical variation of velocities induced by rip-currents using a phase-resolving, 3D Non-Hydrostatic WAVE model (NHWAVE) where an idealized rip channel is used to generate nearshore circulation. The vertical variation of velocities for several locations is investigated and compared with a depth-integrated numerical model for better understanding of depth-dependent hydrodynamics.

A Comprehensive Sensitivity Analysis of Tsunami Model System to the Parametric and Input Uncertainties

ABSTRACT Jung, T.-H.; Son, S. and Lynett, P., 2016. A Comprehensive Sensitivity Analysis of Tsunami Model System to the Parametric and Input Uncertainties. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 1117 - 1121. Coconut Creek (Florida), ISSN 0749-0208. In the present study, it is examined the tsunami modelling sensitivity to the various input factors based on the simulation results of 2004 Indian Ocean Tsunami. The numerical tests were implemented using coupled shallow water equation model, COMCOT(Liu et al., 1998) and Boussinesq model(Son et al., 2011) on the multi-grid system. Four different configurations(CASE 1 through 4) where three factors were controlled independently had been set up and simulated. From the numerical results, it was examined that some of defined factors may have control over the level of prediction accuracy in tsunami impacts on the nearshore area. It was shown that hydrodynamic performances calculated by Boussinesq model(Son et al., 2011) were very distinct from those by COMCOT-only. Most importantly, this study also suggests that more thorough and dedicated investigations on the diffusive errors embedded inherently in numerical models are required for the future study.

Treatment of Inclined Boundaries in a Finite Element Model for the Mild-Slope Equation

A numerical skill for effective treatment of inclined boundaries in a finite element method is introduced. A finite element method has been frequently used to simulate hydraulic phenomena in a coastal zone since it can be applied to irregular and complex geometry. In case elliptic partial equations are governing equations for a finite element model, however, there is a difficulty in treating boundary conditions properly for cases in which boundaries are vertically inclined. In this study, a method to treat such inclined boundaries using Bessel functions for a finite element method is introduced and compared with analytical solutions.