Tae-Min Ha

DISTANT TSUNAMI SIMULATION WITH CORRECTED DISPERSION EFFECTS

The physical dispersion of linear Boussinesq equations can be replaced by the numerical dispersion obtained from the dispersion-correction scheme in linear shallow-water equations. A simple dispersion-correction scheme is first proposed and verified by applying it to the problems that has analytical solutions and simple topography. The proposed numerical model is then employed to simulate a distant tsunami. The target tsunami event is the 1983 Nihonkai Chubu Tsunami. The predicted run-up heights of the tsunami accord well with the available field data.

Observations of Run-up and Inundation Levels from the Teletsunami in the Andaman and Nicobar Islands: A Field Report

Abstract In this study, we aimed to document the run-up and inundation of recent tsunamis and loss of life in the December 26, 2004, tsunami in the Andaman and Nicobar Islands. An estimated 1925 people lost their lives and 5555 were reported missing in the Andaman and Nicobar Islands because of the tsunami. The Nicobar group of Islands—namely Great Nicobar, Katchall, Teressa, Nancowry, Trinkat, Car Nicobar, and others—were severely affected by tsunami because they were close to the epicentre. The effect on the Andaman Islands was less severe because of the islands geometry and topography. Because the settlements in the South Andaman Islands are largely confined to sheltered areas like bays that are far from the coast, and more importantly, in elevated areas (except for certain low-elevation, far inland locations like the Sippighat area) there was almost no loss of life. However, damage to property, especially fishing vessels, was high in most of the area, where seawater inundated the land completely. This field survey was conducted at 26 sites; the Little Andaman passenger jetty area recorded the highest run-up of 17.26 m, and the Chidiyatopu area had the highest inundation at 500 m. Furthermore, this survey also supports the assertion that the low-lying areas of Little Andaman were severely affected by the tsunami waves because of their gradual elevation increase.

Numerical Simulation of Oil Spill in Ocean

The spreading of oil in an open ocean may cause serious damage to a marine environmental system. Thus, an accurate prediction of oil spill is very important to minimize coastal damage due to unexpected oil spill accident. The movement of oil may be represented with a numerical model that solves an advection-diffusion-reaction equation with a proper numerical scheme. In this study, the spilled oil dispersion model has been established in consideration of tide and tidal currents simultaneously. The velocity components in the advection-diffusion-reaction equation are obtained from the shallow-water equations. The accuracy of the model is verified by applying it to a simple but significant problem. The results produced by the model agree with corresponding analytical solutions and field-observed data. The model is then applied to predict the spreading of an oil spill in a real coastal environment.

Numerical Study on Reduced Runup Heights of Solitary Wave by Submerged Structures

Most of coastal structures have been built in the surf zone to protect coastal areas. The transformation of waves in the surf zone, in general, is very complicated and contains lots of hazards to coastal communities. Thus, behaviors of waves in the surf zone should be very carefully analyzed and predicted. Furthermore, an accurate analysis of deformed waves around coastal structures is directly related to construction of economic and safe coastal structures because a wave height plays an important role in determination of weight and shape of a levee body or an armoring material. In this study, a numerical model using a large eddy simulation is employed to predict runup heights of nonlinear waves passed a submerged structure in the surf zone. Reduced runup heights are predicted and the characteristics of runup heights in terms of wave reflection, transmission and dissipation coefficients are investigated.

Numerical simulation of solitary wave run-up with an internal wave-maker of Navier-Stokes equations model.

급경사에서의 고립파의 처오름을 예측하기 위해 3차원 수치모형에 내부조파기법을 도입하여 수치모형실험을 수행하였다. 수치모형은 Navier-Stokes 방정식을 유한차분법을 이용하여 계산하는 동수압 모형으로서, 난류의 해석을 위해서 상대적으로 큰 에디(eddy)만을 고려하는 SANS(spatially averaged Navier-Stokes) 방정식을 푸는 LES(large-eddy-simulation) 기반의 수치모형을 사용한다. 엇갈림 격자체계에서 유한차분법을 사용하여 지배방정식을 해석하는 모형으로서 수치기법으로 Two-step projection 기법을 사용하여 SANS 방정식을 풀었으며, Poisson 방정식을Bi-CGSTAB 기법을 이용하여 풀고 압력장을 계산하였다. 또한, 자유수면의 추적을 위하여 2차 정확도의 VOF(volume-of-fluid) 기법을 사용하였다. 먼저 고립파를 3차원 공간의 일정 수심상에서 내부조파하여 해석해와 비교한 후 분산오차에 대해 분석하였다. 그리고 고립파를 내부조파하여 급경사에서의 고립파의 처오름 및 처내림 현상을 예측하고 수리모형 실험결과와 비교 및 분석하였다. 【A three-dimensional numerical model called NEWTANK is employed to investigate solitary wave run-up with an internal wave-maker on a steep slope. The numerical model solves the spatially averaged Navier-Stokes equations for two-phase flows. The LES (large-eddy-simulation) approach is adopted to model the turbulence effect by using the Smagorinsky SGS (sub-grid scale) closure model. A two-step projection method is adopted in numerical solutions, aided by the Bi-CGSTAB (Bi-Conjugate Gradient Stabilized) method to solve the pressure Poisson equation for the filtered pressure field. The second-order accurate VOF (volume-of-fluid) method is used to track the distorted and broken free surface. A solitary wave is first internally generated and propagated over a constant water depth in the three-dimensional domain. Numerically predicted results are compared with analytical solutions and numerical errors are analyzed in detail. The model is then applied to study solitary wave run-up on a steep slope and the obtained results are compared with available laboratory measurements.】

Numerical Study on Tsunami Hazard Mitigation Using a Submerged Breakwater

Most coastal structures have been built in surf zones to protect coastal areas. In general, the transformation of waves in the surf zone is quite complicated and numerous hazards to coastal communities may be associated with such phenomena. Therefore, the behavior of waves in the surf zone should be carefully analyzed and predicted. Furthermore, an accurate analysis of deformed waves around coastal structures is directly related to the construction of economically sound and safe coastal structures because wave height plays an important role in determining the weight and shape of a levee body or armoring material. In this study, a numerical model using a large eddy simulation is employed to predict the runup heights of nonlinear waves that passed a submerged structure in the surf zone. Reduced runup heights are also predicted, and their characteristics in terms of wave reflection, transmission, and dissipation coefficients are investigated.

Numerical Study of Rip Current Generation Mechanism at Haeundae Beach, Korea

ABSTRACT Ha, T.; Jun, K.; Yoo, J., and Park, K.S., 2014. Numerical study of rip current generation mechanism at Haeundae Beach, Korea. Haeundae Beach is one of the most famous tourist attractions in Busan, Korea. Every year, millions of people visit the beach. Recently, however, numerous rip current accidents have occurred along the middle of the beach. For example, in 2013, more than 500 people were swept away by a strong channel of water flowing seaward from the shore and were rescued after being dragged into deeper water. Fortunately, no casualties were reported, but the authorities said that, given the right weather conditions, rip currents could occur again at any time, not only at Haeundae Beach but also at other beaches. Some studies have been conducted to determine the rip current generation mechanism at Haeundae Beach. Previous research suggests that a known mechanism of rip current generation associated with the nodal line area of honeycomb-patterned wave crests was one of the significant factors in rip current occurrences at Haeundae Beach. In this study, we performed numerical experiments to identify the rip current generation mechanism based on the hypothesis proposed in the study mentioned. A well-known Boussinesq equation solver, FUNWAVE-TVD, was employed to simulate nearshore circulation at Haeundae Beach. The model was verified by generating a rip current for a simple beach for honeycomb-patterned incident wave conditions. The model was then used in numerical simulations of wave transformation to identify the rip current generation mechanism for Haeundae Beach, and wave transformation characteristics were examined in detail to identify the possible origins of rip currents. The original bathymetry was modified and applied to simulation of nearshore circulation to understand how reefs located off the shore of Haeundae Beach contribute to rip current generation. The numerical results corresponded to a slightly different rip current generation mechanism. It is thought that honeycomb-patterned incident waves may induce rip current event at Haeundae Beach but the other factors may contribute as well.

Directional Wave Generation in the Navier-Stokes Equations Using the Internal Wave Maker

A numerical modeling has become increasingly popular and more important to the study of water waves with a rapid advancement of computer technology. However, different types of problems are induced during simulating wave motion. One of the key problems is re-reflection to a computation domain at the incident boundary. The internal wave generating-absorbing boundary conditions have been commonly used in numerical wave models to prevent re-reflection. For the Navier-Stokes equations model, the internal wave maker using a mass source function of the continuity equation has been used to generate various types of waves. Nonetheless, almost every numerical experiment is performed in two dimensions and only a few tests have been expanded to three dimensions. More recently, a momentum source function of the Boussinesq equations is applied to generate essentially directional waves in the three dimensional Navier-Stokes equations model. In this study, the internal wave maker using a momentum source function is employed to generate targeted linear waves in the three-dimensional LES model.

Laboratory Investigations on Effects of Water Level Change on Surf-zone Processes

ABSTRACT Yoo, J.; Shin, S.; Do, K.D.; Shim, J.S.; Ha, T., and Jun, K.C., 2014. Laboratory investigations on effects of water level change on surf-zone processes. Tidal level and current often influence the rip current and the morphological changes in the macro-tidal beaches. Effects of water level change on surf zone processes were investigated through laboratory experiments conducted in a two dimensional wave flume. A movable beach was installed using a find sand in the flume starting with 1/50 slope from the offshore and ending with 1/20 slope near the shore. Irregular waves were generated by a piston type wave maker with active absorption. In the meantime, water level was changed gradually by using a pump and drain system to consider waves and tide simultaneously. Herein, three different tidal phases (i.e. flood tide, full tide and ebb tide) were conditioned with a gradual depth change of about 10 ~ 15 cm in the offshore during a period of about 30 minutes per phase except the full tide, while only one wave condition was kept targeting a significant wave height of 12 cm and significant wave period of 1.7 s. Measurements of wave attenuations in the wave breaking zone influenced by the tidal change were made using in-situ sensors and an image-based analysis method. The rates of wave attenuation (i.e. the ratio of wave height to water depth) measured during the both of flood and ebb tides were found to be larger, ranging from about 0.8 to 1.4, compared to those recorded during the full tide with little depth change. In addition, the attenuation coefficient of the flood tide tends to be larger than that of the ebb tide.

Transformation of small-scale meteorological tsunami due to terrain complexity on the western coast of Korea

ABSTRACT Ha, T., Choi, J.-Y., Yoo, J., Chun, I., Shim, J., 2014. Transformation of small-scale meteorological tsunami due to terrain complexity on western coast of Korea. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 284–289, ISSN 0749-0208. On the 4th of May, 2008, a meteorological tsunami occurred in the Yellow Sea and abnormal waves were observed at Jukdo Island, located on the western coast of Boryeong, Korea. The event attracted the attention of many engineers and scientists because it caused extreme waves only at Jukdo Island, unlike the meteorological tsunami event in 2007, which caused abnormal extreme waves along the western coast of Korea. In general, a meteorological tsunami can be forecasted using large-scale meteorological observation data and climate modeling systems. However, a small-scale meteorological tsunami like the event that occurred in 2008, which can cause abnormal extreme waves only in specific coastal areas, should be analyzed using a high-resolution modeling system because water motions can be affected by local terrain. In this study, numerical experiments were conducted to identify the generation and amplification mechanisms of the meteorological tsunami that hit Jukdo Island in 2008. To achieve this objective, we generated virtual meteorological tsunamis using atmospheric pressure disturbances observed in 2008 and simulated the propagation and run-up of the meteorological tsunamis over real topographies by varying specific characteristics of the local terrain. A three-dimensional hydrodynamic model, MOHID, and a Boussinesq-type wave model, FUNWAVE-TVD Version 2.0, were employed to simulate the generation and transformation of the meteorological tsunamis, respectively. The numerical models were first validated by comparing the numerical results with the available tidal records observed during the event, and then used to identify the generation and amplification mechanism of the meteorological tsunami that hit Jukdo Island. Finally, the transformation of a small-scale meteorological tsunami due to terrain complexity is discussed in detail.