Faculty Profiles - CHO Yonghwan

Papers - CHO Yonghwan

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MORPHOLOGICAL EVOLUTION OF CLAY/SAND MIXTURE FOCUSING ON CLAY LOSS Reviewed International journal

Cho Y. H., Watanabe M., Nakamura T., Mizutani N.

page： 593 - 599 2020

Sediment transport calculation considering cohesive effects and its application to wave-induced topographic change Reviewed

Cho Y.H., Nakamura T., Mizutani N., Lee K.H.

Proceedings of the 7th International Conference on Asian and Pacific Coasts, APAC 2013 page： 338 - 345 2020

Language：English Publishing type：Research paper (international conference proceedings) Publisher：Proceedings of the 7th International Conference on Asian and Pacific Coasts, APAC 2013

A sediment transport calculation considering cohesive force is proposed to deal with the transport phenomena of cohesive sediment. In the proposed calculation, each sand particle is assumed to be surrounded by a thin layer of clay. The critical Shields parameter and bed-load sediment transport rate are modified to include the cohesive force acting on the sand particle. The proposed calculation is incorporated into a two-way coupled fluid-structure-sediment interaction model, and applied to wave-induced topographic change of an artificial shallow. Numerical results show that an increase in the content ratio of the clay, cohesive resistance force per unit surface area and water content cause increases in the critical Shields parameter and decreases in the bed-load sediment transport rate, reducing the topographic change of a shallow without changing its trend. This suggests that mixing clay in the pores of the sand particles can reduce the topographic change of shallows.

Scopus

INTRODUCTION OF CLAY LOSS TO MIXED SOIL CALCULATION

CHO Yonghwan, NAKAMURA Tomoaki, MIZUTANI Norimi

Journal of Japan Society of Civil Engineers, Ser. B2 (Coastal Engineering) Vol. 75 ( 2 ) page： I_475 - I_480 2019

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Japan Society of Civil Engineers

This paper describes the method to calculate sediment transport in the mixed soil condition composed of sand and clay considering the change of clay contents due to clay loss. The concepts of the four-phase soil model and the maximum clay contents classifying mixed soil status were introduced into the method. The model incorporating the proposed method was tested under the experimental conditions. From the numerical results, when all the clay in the clay activation layer where clay loss can occur was flowed out, the suspended concentration of clay decreased with time similar to the experimental results. The suspended concentration of clay was remarkable at the offshore and onshore of the movable bed consisting of sand and clay, and the decrease of the clay content on the surface of the bed was confirmed in the same locations due to clay loss. Therefore, the proposed model suggests the possibility to improve the accuracy of sediment calculation in mixed soil condition and to clarify the mechanism of clay loss from the surface of mixed soil.

DOI： 10.2208/kaigan.75.i_475

CiNii Research

REAL-SCALE NUMERICAL ASSESSMENT OF ARTIFICIAL REEF IN IDA BEACH

KIM Shinwoong, NEHASHI Soraka, NAKAMURA Tomoaki, CHO Yonghwan, MIZUTANI Norimi, TAKEUCHI Masayuki

Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering) Vol. 75 ( 2 ) page： I_971 - I_976 2019

Language：English Publishing type：Research paper (scientific journal) Publisher：Japan Society of Civil Engineers

In this study, to evaluate the effectiveness of the FS3M for assessing the countermeasures of beach erosion, the model was applied to the real-scale domain. We used the FS3M, which is a computational fluid dynamic model using the Navier-Stokes equation, and the survey data provided by Mie Prefecture. To address the problem of calculation cost, the numerical method called long-term estimation was used to calculate the evolution of the beach profile on the gravel beach. The validity of FS3M was investigated by applying the parameters used in the reflective type beach to the step type beach. The model reproduced well the position of the berm but shows a problem in the profile change under the water. However, in the process of wave propagation to the rear of the artificial reef, the reduction of the wave energy decreases the wave steepness, and the beach profile change for reflective type occurs predominantly. This fact shows that FS3M can be used to reproduce the beach profile change and evaluate the effects of artificial reef.

DOI： 10.2208/jscejoe.75.i_971

CiNii Research

EXPERIMENTAL STUDY ON THE INFLUENCE OF SPATIAL VARIATION OF CLAY ON THE TOPOGRAPHY CONSISTING OF MIXED SOIL

AKAHOSHI Rei, CHO Yong-hwan, WATANABE Mikiya, NAKAMURA Tomoaki, MIZUTANI Norimi

Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering) Vol. 75 ( 2 ) page： I_193 - I_198 2019

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Japan Society of Civil Engineers

As a part of elucidation of the dynamic characteristics of fine particles considered to be an important factor in maintenance for tidal flat, we carried out a hydraulic model experiment focusing on the change of topography consisting of sand-clay mixed sediments by wave conditions and clay content. The characteristics of the topography and the influence of the spatial distribution of clay on the sediment characteristics of the mixed sediment were investigated. As a result, in the case of sediment consisted of clay and sand, the effect of the cohesion of clay was effective in delaying the topographic change. Mixed sediment was eroded by the movement of sand after clay particle is washed out. Moved sand covered mixed sediment and reduced the erosion.

DOI： 10.2208/jscejoe.75.i_193

CiNii Research

DEVELOPMENT OF VOF-BASED NUMERICAL MODEL APPLICABLE TO UNSATURATED SOIL AND APPLICATION TO SEEPAGE AND EROSION PHENOMENA

NAKAMURA Tomoaki, CHO Yonghwan, MIZUTANI Norimi

Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering) Vol. 75 ( 2 ) page： I_229 - I_234 2019

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Japan Society of Civil Engineers

A simple method to deal with unsaturated soil for the volume-of-fluid (VOF) method was proposed and incorporated into a VOF-based numerical model, FS3M. Furthermore, the FS3M was improved to compute unsaturated seepage flow more accurately. From comparison with experimental data, the predictive capability of the improved FS3M was demonstrated for unsaturated seepage in a dike and overflow-induced erosion of an unsaturated dike. This suggested the usefulness of the improved FS3M in assessing such phenomena related to unsaturated soil.

DOI： 10.2208/jscejoe.75.i_229

CiNii Research

STUDY ON EFFECTIVENESS OF CONFIGURATION CHANGE IN CAISSONS AGAINST OVERFLOW TSUNAMIS

NAKAMURA Tomoaki, KANG Jiyoung, CHO Yonghwan, MIZUTANI Norimi

Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering) Vol. 75 ( 2 ) page： I_773 - I_778 2019

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Japan Society of Civil Engineers

To prevent the overturning of caissons due to overflow tsunamis, the effectiveness of configuration change in the caissons was investigated from hydraulic experiments and numerical analyses. From experimental and numerical results, it was found that the resilience of a trapezoidal caisson was higher than that of a rectangular caisson and an L-shaped caisson because the configuration of the trapezoidal caisson prevented the separation of the overflow and the formation of a vortex reducing an increase in wave pressure acting on the landward surface of the caisson.

DOI： 10.2208/jscejoe.75.i_773

CiNii Research

EXPERIMENTAL STUDY ON EFFECTIVENESS OF GABIONS AGAINST TSUNAMI SCOUR AT LANDWARD TOE OF COASTAL DIKE

NAKAMURA Tomoaki, TANIGUCHI Junya, CHO Yonghwan, MIZUTANI Norimi

Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering) Vol. 75 ( 2 ) page： I_749 - I_754 2019

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Japan Society of Civil Engineers

The effectiveness of gabions against tsunami-induced scour at the landward toe of a coastal dike was investigated from hydraulic experiments. Experimental results showed that the installation of gabions moved a scour hole landward and reduced its maximum depth. This effect was reconfirmed from the hydraulic experiments in addition to existing numerical analyses not considering the leakage of sand and the displacement of gabions. Furthermore, it was found that a scour hole grew rapidly due to sand leakage after the gabions were displaced largely. This suggested that connected gabions could be more effective to reduce the displacement of the gabions and the gaps between the gabions.

DOI： 10.2208/jscejoe.75.i_749

CiNii Research

STUDY ON PERFORMANCE OF ANTI-OSCILLATION TANK AGAINST MOTION OF FLOATING CAISSON AND ITS SIMPLE ESTIMATION METHOD

NAKAMURA Tomoaki, ONUMA Fumito, CHO Yonghwan, MIZUTANI Norimi, EGUCHI Mikiko, KURAHARA Yoshinosuke, TAKEDA Masahide

Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering) Vol. 75 ( 2 ) page： I_403 - I_408 2019

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Japan Society of Civil Engineers

The effectiveness of flume-type anti-oscillation tanks installed on a floating caisson in the pitch and roll directions under regular waves and multi-directional irregular waves was investigated from hydraulic experiments, and a simple numerical method for estimating the effect of the anti-oscillation tanks was developed and applied to the hydraulic experiments. From experimental results, it was revealed that the anti-oscillation tanks can reduce the pitch of the caisson increased at around the natural frequency and its double frequency under the regular waves. Furthermore, the anti-oscillation tanks were found to be effective against not only the pitch but also the roll under the multi-directional irregular waves. From numerical results, it was found that the effect of the anti-oscillation tanks can be estimated reasonably well in terms of the amplitude of the pitch. This suggested the usefulness of the developed method in estimating the most effective water volume in anti-oscillation tanks.

DOI： 10.2208/jscejoe.75.i_403

CiNii Research

EXPERIMENTAL STUDY ON WAVE HEIGHT REDUCTION EFFECT AND DYNAMIC BEHAVIOR OF TEMPORARY FLOATING BREAKWATER

NAKAMURA Tomoaki, YOSHIMURA Kazuki, CHO Yonghwan, MIZUTANI Norimi, KOTAKE Yasuo, SHIBUTANI Yoko

Journal of Japan Society of Civil Engineers, Ser. B2 (Coastal Engineering) Vol. 75 ( 2 ) page： I_781 - I_786 2019

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Japan Society of Civil Engineers

A temporary floating breakwater has been proposed to keep calm wave conditions in an offshore construction site. In this study, its transmission coefficient and dynamic behavior were investigated from hydraulic experiments. Experimental results showed that the transmission coefficient decreased with an increase in the wave steepness and relative breakwater width. Furthermore, the time from the peak of the heave to that of the sway increased with the relative breakwater width and the temporal changes in the heave and sway had a nearly opposite phase for large relative breakwater widths, resulting in the abovementioned decrease in the transmission coefficient.

DOI： 10.2208/kaigan.75.i_781

CiNii Research

NUMERICAL SIMULATION OF TSUNAMI SCOUR AT LANDWARD TOE OF COASTAL DIKE AND EVALUATION OF EFFECTIVENESS OF GABIONS

NAKAMURA Tomoaki, TANIGUCHI Junya, CHO Yonghwan, MIZUTANI Norimi

Journal of Japan Society of Civil Engineers, Ser. B2 (Coastal Engineering) Vol. 75 ( 2 ) page： I_703 - I_708 2019

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Japan Society of Civil Engineers

A three-dimensional coupled fluid-structure-sediment-seabed interaction model (FS3M) was applied to hydraulic experiments on the effectiveness of gabions against tsunami scour at the landward toe of a coastal dike. Numerical results showed that the predictive capability of the FS3M was demonstrated in terms of temporal change in the maximum scour depth for all conditions without the gabions and a small discharge condition with the gabions. From numerical results for a large discharge condition with the gabions, which subsided due to sand leakage in the hydraulic experiments, it was suggested that it was essential to take into account the subsidence of the gabions in accurately assessing the effectiveness of gabions.

DOI： 10.2208/kaigan.75.i_703

CiNii Research

命山基部に使用したPCマクラギの耐津波安定性に関する研究 Reviewed

趙容桓, 山田悠貴, 中村友昭, 水谷法美

土木学会論文集Ｂ２（海岸工学） Vol. 74 ( 2 ) page： I_301-I_306 2018.11

消波ブロック被覆堤マウンド下部の砂地盤の侵食と石かごが与える影響に関する実験的研究 Reviewed

村岡宏紀，中村友昭，趙　容桓，水谷法美

土木学会論文集B1（水工学） Vol. 74 ( 2 ) page： I_595-I_600 2018.11

陸上構造物に作用する津波力に与える浮遊砂の影響に関する研究 Reviewed

中村友昭, 中井祐斗, 趙容桓, 水谷法美

土木学会論文集B3（海洋開発） Vol. 74 ( 2 ) page： I_210-I_215 2018.9

Experimental study on gravel beach deformations under irregular waves Reviewed

Shinwoong KIM, Ryoya NAKAYAMA, Tomoaki NAKAMURA, Soraka NEHASHI, Yonghwan CHO, Norimi MIZUTANI

Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering) Vol. 74 ( 2 ) page： I_731-I_736 2018.9

体積変化量に着目した七里御浜井田海岸の海浜変形特性に関する研究 Reviewed

趙容桓, 小林泰輔, 菊雅美, 中村友昭, 水谷法美

土木学会論文集B3（海洋開発） Vol. 74 ( 2 ) page： I_647-I_652 2018.9

消波ブロック被覆堤下部からの砂地盤の吸い出しに関する実験的研究 Reviewed

中村友昭, 村岡宏紀, 趙容桓, 水谷法美

土木学会論文集B3（海洋開発） Vol. 74 ( 2 ) page： I_258-I_263 2018.9

Failure Mechanism of Armor Blocks of Submerged Breakwater Reviewed International journal

Vol. 85 page： 1166 - 1170 2018.5

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Coastal Research

This paper describes the failure mechanism of armor blocks of a submerged breakwater under regular and long period waves. Laboratory experiments with the scale of 1/50 were conducted in a wave flume. The measured normal and tangential wave forces on the damaged armor blocks were compared with 31 tests for a submerged breakwater with 3 submerged depths and 3 respective wave periods of regular and long period waves. The experimental results described the failure of armor blocks in the threshold conditions occurred mainly at the seaward edge of the crown by normal wave forces under regular waves, and the landward edge of the crown by normal and tangential wave forces under long period waves. Two-dimensional numerical analysis was carried out to investigate the effect of temporal and spatial variation of wave fields around the submerged breakwater on wave force acting on armor blocks. The results demonstrated that the dominant wave forces on armor blocks were induced by the decrease of the pressure near the damaged blocks resulting from the wave breaking for regular waves and the increase in the flow velocity for long period waves. It was confirmed the other blocks on the crown tends to be stable under long period waves.

Scopus

EXPERIMENTAL STUDY ON EROSION OF SAND SEABED UNDER RUBBLE MOUND OF COMPOSITE BREAKWATER COVERED WITH WAVE DISSIPATING BLOCKS AND EFFECTS OF GABIONS

MURAOKA Hiroki, NAKAMURA Tomoaki, CHO Yonghwan, MIZUTANI Norimi

Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering) Vol. 74 ( 5 ) page： I_595 - I_600 2018

Language：Japanese Publisher：Japan Society of Civil Engineers

In this study, 1/55.8-scale hydraulic experiments on a composite breakwater with gabions like large-scale hydraulic experiments were carried out to organize what should be considered in determining the sizes of mound gravel and seabed sand. Experimental results showed that same flow fields around the breakwater were formed as hydraulic experiments without gabions. Similar with the large-scale hydraulic experiments, sand seabed benearth the seaward part of the mound was eroded. However, the time at which this eroded depth reached to that for the large-scale hydraulic experiments needed to be longer than the time determined according to the Froude similarity law. This suggested that further studies are required for the time scale of the erosion of the sand seabed benearth the mound.

DOI： 10.2208/jscejhe.74.5_i_595

CiNii Research

Suspended clay load from mixed soil under regular waves

Akahoshi R., Cho Y.H., Nakamura T., Mizutani N.

Proceedings of the International Offshore and Polar Engineering Conference Vol. 2018-June page： 1387 - 1391 2018

Publisher：Proceedings of the International Offshore and Polar Engineering Conference

In this study, we conducted hydraulic model experiments on mixed soil containing clay and investigate the phenomenon of interParticle clay escaping from sand-clay mixed sediment with different wave conditions and clay content. Furthermore, we examine the effect of the clay escape phenomenon on the topographic change characteristics of mixed sediment. On mixed sediment having a high clay fraction, suspended clay behavior depended on bottom shear stress. On the other hands, in lower clay fraction condition, suspended interParticle clay behavior depended on wave steepness. It was also found that the corrugation gradient and the erosion rate of the clay escaping from the sand Particles have a linear relationship. Through the above investigation, it was suggested that difficult to evaluate the washout of clay from sand Particles only by evaluating critical shear stress.

Scopus

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Papers - CHO Yonghwan