Journal of Japan Society of Civil Engineers, Ser. B2 (Coastal Engineering)
Online ISSN : 1883-8944
Print ISSN : 1884-2399
ISSN-L : 1883-8944
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Showing 1-50 articles out of 234 articles from the selected issue
Paper
  • Takehito KAMIKAWA, Takaaki SHIGEMATSU
    2019 Volume 75 Issue 2 Pages I_1-I_6
    Published: 2019
    Released: October 17, 2019
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     Most of pile-supported piers that have been used in port and harbor in japan faced to the sever deterioration of slat damage. The purpose of this study is to investigate the conditions of splash occurrence by wave under mild weather in front of the pile-supported pier by carrying out laboratory experiment and numerical calculation. According to the experiment, it is found that there are two patterns of splash occurrence. One of them is that splash occurs on the vertical wall and the other is that at the convergence point of surface waves surrounding the circular cylinder. The calculation results show the potential which OpenFOAM can simulate the wave refraction and convergence and can predict the variation of wave convergence location due to the conditions of waves and pile-supported pier.

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  • Natsuki MIZUTANI
    2019 Volume 75 Issue 2 Pages I_7-I_12
    Published: 2019
    Released: October 17, 2019
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     In this study, the internal velocity field of dam-break flows on a dry bed was investigated, to estimate bed shear stress using the space-time average. The wave number spectrum of velocity fields at the same height was calculated from the PIV results. The width for spatial average was decided based on the peak wave number to calculate the vertical profiles of mean horizontal velocity and Reynolds stress. The profiles indicated that layers under y = 2 mm strongly affected by the bottom friction. The bed shear stress calculated from the friction velocity was of the same order as that obtained from the Manning formula, but indicated larger fluctuations with time variation. Further, it was indicated that bed shear stress caused by friction velocity was related to the local water surface slope.

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  • Nguyen Xuan TINH, Hitoshi TANAKA, Wenzheng SONG
    2019 Volume 75 Issue 2 Pages I_13-I_18
    Published: 2019
    Released: October 17, 2019
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     This study presents a numerical analysis of bottom boundary layer and shear stress under tsunami wave using a high-precision k-ω numerical model. The simulations have been made for hypothetical shoaling tsunami from the tsunami source area to nearshore region to investigate characteristics of the bottom shear stress acting on the sea bed. It is found that a very thin boundary layer is formed beneath tsunami at the tsunami source area and further developed as it propagates to the shallow region. On the other words, the tsunami boundary layer does not span entire the water column as assumed in the steady flow theory even though the long wave condition is fully satisfied. Consequently, the steady friction law is not valid in most entire computational domain although the shallow water equation is applicable. As a result, there are remarkably underestimated the bottom shear stress by the steady friction law due to the steep velocity gradient in the boundary layer. Therefore, it is necessary to take into account such unsteady boundary layer characteristics in the numerical simulation of tsunami wave and estimation of tsunami-induced bed shear stress in the nearshore region.

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  • Nagendram VEERAPAGA, Gubash AZHIKODAN, Tetsuya SHINTANI, Katsuhide YOK ...
    2019 Volume 75 Issue 2 Pages I_19-I_24
    Published: 2019
    Released: October 17, 2019
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     In order to discuss the variation of saltwater intrusion and mixing types in terms of estuarine bed topography, planform shape, channel cross-section and multi-branch, a total of 25 numerical experiments were carried out with a conceptual estuary using a three-dimensional hydrodynamic simulator. The wavy bottom of the channel had less salinity intrusion length (SIL) compared with the flat bottom (reference case) under the constant tidal range and freshwater discharge. This is because of the decrease in velocity of the gravitational flow due to the bottom drag force as well as the trapping of saltwater in the bottom hollows. The SIL was increased in the case of funnel shaped estuary compared with the straight (constant width) channel. For the channel cross-section, the temporal variation of SIL was highest in the case of the triangular cross-section and the mixing condition was changed from partially mixed to stratified with the change in cross-sectional shape from triangular to parabolic and then to the rectangular cross-section. The results from the multi-branch indicated that the sub-channels with different length would affect the saltwater intrusion and mixing condition in the estuary. It was shown from the present numerical experiments that the saltwater distribution in the estuary was significantly affected by the planform shape, bed topography and channel cross-section.

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  • Kazufumi TADA, Keisuke NAKAYAMA, Yutaro NAKANISHI, Daisuke SASAKI, Kat ...
    2019 Volume 75 Issue 2 Pages I_25-I_30
    Published: 2019
    Released: October 17, 2019
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     “Submerged Aquatic Vegetation model (SAV model)” using object-oriented programming that enables analysis of the interaction between wave-current and aquatic plants has been developed in the previous studies. In this study, we investigated mass transport under oscillational conditions with eelgrass meadow by using SAV model. Also, we made an attempt to clarify how eelgrass capture dissolved inorganic carbon (DIC) that determines CO2 partial pressure in water. As a result, it was found that the shorter the length of eelgrass is, the more the stokes drift transport DIC. In addition, it was demonstrated that the net absorption of DIC largely depends on the leaf length of eelgrass, and the effect of waves is secondary compared to uniform flow.

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  • Akio NAGAYAMA, Yuki MORIMOTO, Takatomo MIYAKE
    2019 Volume 75 Issue 2 Pages I_31-I_36
    Published: 2019
    Released: October 17, 2019
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     Sand steam hot spring using underground hot spring water has been used for a long time on the sandy beach of Ibusuki coast of Ibusuki city, Kagoshima prefecture. In recent years, a coastal maintenance project has been implemented in this coastal area. Therefore, detailed on-site observation has carried out about the effusion of underground hot spring water. In this paper, we developed a multiphase flow solver that adds a calculation term to reproduce flow and temperature fields in porous media. Then, we examined the influence of the tide level on the temperature in the sandy beach and the effusion of the hot spring water. As a result, it was possible to reproduce the phenomenon that the temperature field in the sandy beach fluctuate greatly with the tide level fluctuation. And we clarified the phenomenon that the vertical upward flow was always generated at the shoreline position at the time of the low tide.

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  • Takatomo MIYAKE, Nobuyuki ONO, Kenki KASAMO, Shohei NAKAMURA, Takaya F ...
    2019 Volume 75 Issue 2 Pages I_37-I_42
    Published: 2019
    Released: October 17, 2019
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     In Ibusuki port coast, a coastal improvement project is in progress applying the idea of integrated shore protection system combining several coastal works and beach nourishment. For the construction of these works, the local groundwater environment system named as “natural sand-steaming hot spring” should be carefully conserved, because that is an important tourism resource in the region. This study intends to evaluate the impact on the hot-spring groundwater system by the beach nourishment work. In this purpose, we conducted in-situ field measurements to monitor the variations of the groundwater levels and sand temperature in the sandy beach by placing test nourishment sands.

     The results show that any significant effects have not been obtained in the areas behind the seawall and neighboring region of the nourishment, whereas, the marked risings in groundwater level and sand temperature have been observed in the beach nourishment area.

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  • Kenki KASAMO, Nobuyuki ONO, Takatomo MIYAKE, Toshiaki YUKIMARU, Nobuhi ...
    2019 Volume 75 Issue 2 Pages I_43-I_48
    Published: 2019
    Released: October 17, 2019
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     In Ibusuki port coast, a coastal improvement project is in progress applying the idea of integrated shore protection system combining jetties, offshore breakwaters, seawalls and beach nourishment. In the project, the local groundwater environment named “natural sand-steaming hot spring” should be carefully conserved, because that is an important tourism resource in the region. With the aim of evaluating the impact of the beach nourishment on the hot spring groundwater, this study develops a basic model to analyze the flow and temperature variations of the hot spring groundwater in this site. The developed model is found to reproduce well both the steady groundwater levels in the land region and the time varying ones with tidal level fluctuations in the coastal region. The developed model also describes the coastal temperature fields well in the following aspects; in the upper layer, the temperature drops rapidly by cooling of the seawater during flood tides and it grows hotter as the tidal level decreases; in the lower layer, the temperature variations with tide becomes less with decrease of the influence of in-filtering seawater flows.

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  • Yasunori WATANABE, Kazunao SUGIMURA, Kento YAMASHITA, Ayumi SARUWATARI
    2019 Volume 75 Issue 2 Pages I_49-I_54
    Published: 2019
    Released: October 17, 2019
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     This paper experimentally identified mechanisms of air-water heat transfer during growth of wind waves, on the basis of simultaneous measurements on water temperature, wind-driven current and waves. When capillary surface disturbances occur in an early stage of wave generation, a wind boundary layer evolve into turbulent one containing streamwise counter-rotating vortices, which induces convergent and divergent surface flows to vary thickness of heat boundary layer over the water surface, and thus modify heat flux coming into ocean. In an equilibrium state of wind waves, as capillary waves break on the gravity wave face, the heated capillary surfaces are entrained into bulk water, resulting in local variations of surface temperature. The subsurface momentum boundary layer contributes to downward heat transport from the renewed surfaces.

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  • Toshinori OGASAWARA, Kenta MASUDA, Keigo SUGAWARA
    2019 Volume 75 Issue 2 Pages I_55-I_60
    Published: 2019
    Released: October 17, 2019
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     The experiments were carried out using a wind tunnel water tank capable of generating thermal stratification with strong vertical temperature gradient like the polar region. The influence of thermal stratification from neutral stratification to unstable stratification on the generation and development mechanism of wind waves was examined based on wave heights and visualization data obtained from experiments. It is clarified that the development process of wind waves is greatly affected in the state of thermal stratification where there is a vertical temperature gradient. In particular, in unstable stratification compared with neutral stratification, wave height and flow velocity both increased by about 3 times, and it was found that kinetic energy of wind current and wave component extends to about twice the depth of significant wave height.

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  • Masashi TAKAGI, Nobuhito MORI, Junichi NINOMIYA, Tomoya SHIMURA, Yusuk ...
    2019 Volume 75 Issue 2 Pages I_61-I_66
    Published: 2019
    Released: October 17, 2019
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     Wave-induced turbulent kinetic energy (TKE) plays an important role on the numerical simulation of sea surface mixing. This study has been conducted to improve the parameterization of sea surface turbulent energy fluxes induced by white-capping based on the observation data. The data indicates that TKE fluxes coefficient α proposed by Feddersen and Trowbridge (2005) has dependence on the difference between wave and wind direction. In addition, when the waves follow wind direction, α is weakly dependent on wave steepness. Based on the optimized bulk equation, the coupled atmospheric and ocean simulation of typhoon, Haiyan(2013), has been conducted. The results show the difference on the spatial TKE distribution near sea surface and macroscopic characteristics of typhoon. It is clear that wave-dependence on upper ocean mixing in atmosphere-ocean coupled model is important.

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  • Ayumi SARUWATARI, Junichi OTSUKA, Yasuyuki BABA, Teruhiro KUBO, Hideak ...
    2019 Volume 75 Issue 2 Pages I_67-I_72
    Published: 2019
    Released: October 17, 2019
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     The contribution of bubbles to the air-ocean heat exchange during passages of extreme typhoons was investigated based on a field observation at the ocean. We proposed an ultrasonic backscatter model estimating bubble population and heat flux through the bubble interface. Bubbles produced in an extreme weather were found to be entrained to an ocean surface layer to rapidly exchange heat with the sea water, resulting in vertical transfer of heat in the ocean.

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  • Yasunori WATANABE, Takumi NONAKA
    2019 Volume 75 Issue 2 Pages I_73-I_78
    Published: 2019
    Released: October 17, 2019
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     This paper presents surfactant effects on geometric and kinematic features of experimental bubble plumes. The both bubble size distributions in surfactant water and seawater are well approximated by a lognormal distribution, while bubbles in pure water deviate from the distribution. We also found analogous features in mean diameter, population and terminal velocity of bubbles produced in the surfactant and sea waters. The behaviors of coalescence and collapse of buoyant residual bubbles on as still water surface highly depend on the surfactant concentration.

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  • Eizo NAKAZA, Satoshi TANAKA, Ryo MOTOYASHIKI, Nobutoshi MIYAZATO, Masa ...
    2019 Volume 75 Issue 2 Pages I_79-I_84
    Published: 2019
    Released: October 17, 2019
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     Using CADMAS-SURF numerical simulation models the generation mechanisms of wave breaker types are explained clearly. The strength of the return flow in the very thin layer adjacent to the water surface of the wave front plays a vital role in determining the type of wave breaker that occurs. The particle velocity along the orbital motion of a wave is affected by the return flow in the thin layer at the wave front. In addition to the 4 types of wave breakers shown in previous research, the existence of a resonance mode breaker is newly revealed. A resonant oscillation in the swash zone can cause the maximum amount of wave runup. The mechanism for maintaining position while surfing a breaking wave is also discussed.

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  • Takaaki UDA, Yasuhiro OOKI, Shingo NAGANUMA, Shingo KIKUCHI, Yasuro OH ...
    2019 Volume 75 Issue 2 Pages I_85-I_90
    Published: 2019
    Released: October 17, 2019
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     Locally-formed troughs in the surf zone, which cannot be recognized by ordinary people, and troughs which are related to the formation of longshore and rip currents, were measured using two web cameras. They were installed between artificial headland Nos. 29 and 30 on the Hokota coast, facing the Pacific Ocean and imageries were taken simultaneously. From the imagery data, the characteristics of local topography in the surf zone were studied in terms of the prevention of accidents in the surf zone. It was found that web cameras were useful for the observation of the nearshore topography and nearshore current.

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  • Takashi FUJIKI, Nobuhito MORI, Koji KAWAGUCHI
    2019 Volume 75 Issue 2 Pages I_91-I_96
    Published: 2019
    Released: October 17, 2019
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     We applied the Gaussian Mixture Model (Mixture) as the spectral partitioning method to the directional wave spectrum observed in Japan coast for a year and investigated its multi-modality in each season quantitatively. As a result, we found apparent features on the ratio of unimodal or multi-modal wave systems for windsea and swells in each season and observation stations along the Japanese coast. Unimodal wave system count for more than 50% in every station and especially 90% for high sea state in Akita facing to the Japan sea. The ratio of unimodal windsea increased up to 50% in winter only in Akita whereas the unimodal windsea in the stations facing to the Pacific Ocean accounted for only about 10%, and the unimodal or multi-modal swell was dominant through a year.

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  • Maki CHIWATA, Tomoya SHIMURA, Junichi NINOMIYA, Nobuhito MORI
    2019 Volume 75 Issue 2 Pages I_97-I_102
    Published: 2019
    Released: October 17, 2019
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     The extreme value analysis of wave height has been used to estimate design value of coastal structures. Generally, the extreme wave heights are obtained based on the observations or hindcasts, simply. However, the influence of the weather systems on the extreme value distribution is not taken into consideration. This study analyzes statistical characteristics of extreme wave heights and understands relations between extreme wave height distribution and its dependence on weather systems based on long-term analysis and observed data. Additionally, wave simulation in bay scale is conducted to analyze the impact of the terrain factors into the extreme distribution in local scale.

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  • Toshikazu KITANO, Genta UENO, Satoshi MORI, Ryota MORI
    2019 Volume 75 Issue 2 Pages I_103-I_108
    Published: 2019
    Released: October 17, 2019
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     Confidence intervals have been calculated to evaluate the estimation error for the return levels of significant wave height for our desired return periods. Now that numerous ensembles datasets are avaialavle as output of the numerical sumulation by climate models, even the sample distributions of the estimated return levels be able to result from the enough number of datasets. This study gives the mathematical form for the probability densitiy functions for the sample estimates for the return levels on the theoretical background of the extreme value statistics whose idea will be as simple as that of the nonparametric estimation of the order statistics.

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  • Hiroyuki KATO, Jiro ENDO, Naoki FURUICHI, Masayuki FUDO, Masahito INOU ...
    2019 Volume 75 Issue 2 Pages I_109-I_114
    Published: 2019
    Released: October 17, 2019
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     In recent years, the fishing port area is frequently damaged by the gaint typhoon and the explostive cyclones. However, it is said that the significant wave height has not increased over time.

     In this study, we use the maximum significant wave height to estimate the design deepwater wave, focusing on the maximum significant wave height, we summarized long-term trend of the maximum significant wave height, the influence of trend of maximum wave height on design deepwater wave in fishing port area and the influence on the stability of the structure by the design deepwater wave was examined. As a result, in recent years, the largest significant wave height of the year has often occurred and it is suggested that it is necessary for the rational design of the fishing port structure to review the design offshore wave at an appropriate time.

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  • Sooyoul KIM, Chisato HARA, Yoshinosuke KURAHARA, Yamato NISHIYAMA, Mas ...
    2019 Volume 75 Issue 2 Pages I_115-I_120
    Published: 2019
    Released: October 17, 2019
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     GMDH-based one week wave prediction models are developed using three data sets of global wave forecast models, JMA GWM, NOAA WW3 and ECMWF ERA5, and applied to four stations on the Japanese coast. As a result of the application to four stations, it was found that the one week wave prediction model improves the accuracty of the predicted significant wave height and period. In addition, the best performing set, among the four potential data sets, was found to be a combination consisting of three global wave forecast sets. The study showed that the GMD-based one week wave prediction model is affected by specific global wave forecast data.

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  • Yoshihiro UTSUNOMIYA, Eriko MATSUFUJI, Yoshimitsu SUZUKI, Yasuhiro YOS ...
    2019 Volume 75 Issue 2 Pages I_121-I_126
    Published: 2019
    Released: October 17, 2019
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     The current ensemble forecasts are inadequate as the input for short-term and medium-term forecasts of waves because their tempo-spatial resolutions are coarse. However, the meso-scale ensemble weather forecasts will be available from June 2019. The forecasts add perturbation to the initial condition of the numerical prediction Meso-Scale Model (MSM) with 5 km spatial resolution. The present study examines the availability of wave forecasts for the uncertainty using meso-scale ensemble weather forecasts. The main conclusion of this study are as follows. (1) Using pressures and winds of coarse spatial resolution, it is possible that judgement as to whether it is a typhoon and estimate of the center position and pressure. (2) The accuracy of simple empirical formula is enough for practical, except for significant wave periods. (3) By using simple empirical formula, it is possible to identify the ensemble member that storm surges and waves are highest.

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  • Kazuki MASUDA, Junichi NINOMIYA, Takehisa SAITO
    2019 Volume 75 Issue 2 Pages I_127-I_132
    Published: 2019
    Released: October 17, 2019
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     In this study, we constructed neural network (NN) models to predict Yorimawari waves at Toyama Bay, and conducted accuracy verification and sensitivity analysis for high wave over 5 meters that occurred after February 2008. From the analysis results, the influence of input-output relation was evaluated quantitatively. Using 3 types of sensitivity analysis methods were examined the applicability of the sensitivity analysis to the large networks to predict of extreme phenomena. As a result of analysis, it is shown that sensitivity analysis is suitable method that can consider time series, and the optimal learning condition of NN and influence of input-output relation differ according to the route of the bomb cyclone. It has become clear that it is necessary to learn by changing the composition of the model in the route developed around the Japan Sea or the Japanese archipelago and the route developed in the Pacific Ocean.

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  • Tracey H. A. TOM, Sooyoul KIM, Masahide TAKEDA, Yoshinosuke KURAHARA, ...
    2019 Volume 75 Issue 2 Pages I_133-I_138
    Published: 2019
    Released: October 17, 2019
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     Currently there are three kinds of global wave forecasts for more than one week in the world; however, when targeting a location nearshore on the coast of Japan, the accuracy of their predictions, from the spatial resolution point of view can only be estimated. This study proposed to improve the one week wave prediction accuracy using an artificial neural network with global wave forecasts data, similar to our Group Method of Data Handling (GMDH) model of one week ahead wave prediction. At the target NOWPHAS location of Port Hitachinaka, the predictions of significant wave heights agreed well with the observed ones when using two global wave data by both the NOAA wave model and ECMWF wave model; for significant wave periods, the data by the Japan Meteorological Agency wave model and ECMWF wave model gave the best performance. It was found that it is not necessary to use all three kinds of global wave prediction values to improve prediction values. The results of this study using an artificial neural network are similar to those using GMDH method.

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  • Yuji ARAKI, Nobuhito MORI, Tomohiro YASUDA
    2019 Volume 75 Issue 2 Pages I_139-I_144
    Published: 2019
    Released: October 17, 2019
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     Although researches of impact assessment of climate change are studied energetically and needs of projection of oceanographic phenomenon have been increasing, simulating oceanographic phenomenon using numerical models requires computational cost. This study predicts time series of oceanographic phenomenon such as wind speeds and wave heights from spatial atmospheric phenomenon using Deep Learning (CNN: Convolutional Neural Network). This study also examines difference of the effects of physical factors of atmospheric phenomenon and hyper parameter of CNN. The physical factors mean combination of explanatory variables, input ranges of atmospheric phenomenon, and so on. The hyper parameters of CNN mean mini-batch size and number of epochs. Inputting the instantatneous value of pressure field could predict wind speeds for Ise Bay and a point on the Pacific Ocean. Inputting the time history of wind speeds could predict wave heights for Ise Bay and Tottori with high precision because it enables to consider the effect of swell. Developed CNN can predict high waves by inputting the time series of past atmospheric phenomenon and increasing training periods (number of training data).

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  • Tracey H. A. TOM, Ai IKEMOTO, Hajime MASE, Koji KAWASAKI, Masahide TA ...
    2019 Volume 75 Issue 2 Pages I_145-I_150
    Published: 2019
    Released: October 17, 2019
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     Numerical wave prediction models require a large amount of computational power to timely complete the required calculations. Artificial Neural Networks (NN) have been introduced to perform predictions at a lesser computational cost and increased processing speed. Deep learning and specifically Convolutional Neural Networks (CNNs) have become accepted for various image recognition applications. Motivation for the examination of wave prediction by deep learning came from the success of CNNs in vision applications and the similarity of meteorological weather grid data to visual images. This study investigates a deep learning technique using the Japan Meteorological Agency’s Grid Point Value Mesoscale Model to predict wave height and period along Japanese coasts of the Sea of Japan. In particular, this study uses the Xception deep learning architecture with depthwise separable convolution to obtain improved wave height and period prediction over artificial neural networks, and gets overall success results.

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  • Naoyuki INUKAI, Naoki TAKAHASHI, Hidetoshi SAITO, Jun ABE, Takahiko KI ...
    2019 Volume 75 Issue 2 Pages I_151-I_156
    Published: 2019
    Released: October 17, 2019
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     A water accident occurred in August 2017 at Koga beach. This beach has the groins at the both ends of the beach. When the accident occurred, two children played at the shallow depth area of the edge of the beach, however big waves suddenly incidented to the beach, and the children carried away to the offshore. In this research, we tried to grasp the reason why the accident occurred. Firstly, we conducted the field survey, and we interviewed the rescuers of Fire Department and Japan Coast Gurad of this resion, furthermore, we got the geographic feature of the domain. After the survey, we made the geographical grid data for the numerical simulation. Secondary, we analyzed the wave data of a wave observatory where locates about 20km away from the beach. As the result, the wave from NNE direction was prevalent in this domain. However, Koga beach is located at the base of a peninsula, therefore, when the wave incidents from NNE, the peninsula impedes the wave incident directly. Therefore, we consider the wave height decreases. When the accident occurred, the wave direction fluctuated from NNE to NNW, therefore, wave incidented directly and the wave height increased. We considered that this phenomenon was the one of the reason why the accident occurred. Thirdly, we tyied to grasp when the same phenomenon might occur by the weather chart. As the result, we considered when the low pressure passses the south of the beach and passes the east, the same phenomenon might occur.Finally, we simulated the wave dynamics in the beach when the wave increased. As the result, the wave velocity became over 3 m/s around the accident occurred point.

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  • Tomonori SAITA, Takuro SAWAI, Akira TAI, Akihiro HASHIMOTO
    2019 Volume 75 Issue 2 Pages I_157-I_162
    Published: 2019
    Released: October 17, 2019
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     Tidal flats play an important role in the water environment of the surrounding sea area. In this study, a field observation and numerical analyses were carried out to elucidate characteristics of the resonances in Imazu tidal flat and a relationship between low-frequency water level variations in the Imazu tidal flat and characteristics of secondary undulations in Hakata Bay. As a result, it is clarified that long-period waves with a period roughly from 35 minutes to 60 minutes set up a resonance in Imazu tidal flat. On the other hand, four major types of oscillation in Hakata bay due to the incidence of long-period waves were identified. And it is considered that the water level fluctuation is amplified at the area off Imazu tidal flat by an oscillation of around 60 minutes or 35 minutes. In addition, wind-induced oscillations in Hakata Bay by northeast or southwest winds can be periodic that brings about a resonance in Imazu tidal flat.

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  • Masato YOKOBORI, Yoshimitsu TAJIMA
    2019 Volume 75 Issue 2 Pages I_163-I_168
    Published: 2019
    Released: October 17, 2019
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     This study investigated the influence of complex shape of the fringing reef on inundation characteristics along the coast through the case study at Faleolo in Samoa when Cyclone Val hit the site in 1991. Through the field survey, it was found that the recorded water level fluctuation had clear peak spectrum in low frequency components with period of several minutes. Numerical model was then applied to represent the behavior of wind waves and wave-induced water level fluctuations on and around the fringing reef under the attack of Val. The numerical results reasonably represented the actual inundation confirmed in the field survey and the analysis of the computed results revealed the dominant influence of wave setup and resonance of low frequency fluctuations on locally amplified water level on the reef.

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  • Toru YAMASHIRO, Mako FUKADA, Tomonori SAITA, Toshiyuki ASANO, Kazuyosh ...
    2019 Volume 75 Issue 2 Pages I_169-I_174
    Published: 2019
    Released: October 17, 2019
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     Field observations of ocean long waves propagating eastward were made in winter to early spring of 2012 and 2018 in relation to the occurrence of secondary undulation at the west coast of Kyushu. The period of secondary undulations at Nagasaki, Sakitsu, Kami-koshiki Island and Makurazaki were confirmed from the observational results. Moreover, it was suggested that the field observations of sea-level variations at Fukuejima Island and Meshima Island (Uttchima Island and Nakano-shima Island) are useful to monitor ocean long waves causing secondary undulation at Nagasaki and Kami-koshiki Island (Makurazaki).

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  • Toshinori ISHIKAWA, Yoshiaki SATO, Ryo SHIMADA, Tsutomu KOMINE
    2019 Volume 75 Issue 2 Pages I_175-I_180
    Published: 2019
    Released: October 17, 2019
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     There are from 2,000 to 3,000 rescues every summer season on the beaches of Japan. Also, almost all drowning accidents are caused by the rip current. In order to reduce the rip current accidents, new rip current safety is required. Therefore, we developed a new technology that can detect the rip current and notify beach users and lifesavers using the Internet of Things (IoT) and the Artificial Intelligence (AI). The rip current is automatically detected by an analysis of the AI engine from the camera's image data on the beach. This information is automatically provided to a monitor on the beach. It is effective for the risk awareness of the beach user. Furthermore, if the beach user enters the rip current area, lifesavers can take early action from the warning on their wearable device. In this study, we verified the detected rip current by the AI analysis, using a field measurement, an image analysis and a numerical simulation. As a result, the detected rip current area mostly accorded with actual rip current areas which shift due to tide and wave conditions. Also, the success rate of 78 % for the image analysis and 63 % for the numerical simulation respectively were confirmed during the observation period.

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  • Takaaki UDA, Yasuro OHTANI, Shingo NAGANUMA, Shogo KIKUCHI, Yasuhiro O ...
    2019 Volume 75 Issue 2 Pages I_181-I_186
    Published: 2019
    Released: October 17, 2019
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     On August 20, 2018, field observation was carried out in Kashiwakuma and Katsushita areas along the Kashimanada coast. Floats attached with a small GPS were thrown into the surf zone, and subsequent movements were measured. The diffusion of the dyed tracer was also measured by using a drone. Because artificial headlands have been constructed at both ends of Katsushita area, nearshore current was affected by the headlands, inducing a rip current. Observation results showed that northward longshore current developed well in both areas, and the longshore current was blocked by the headlands, causing strong rip current of a velocity over 1 m/s near the headlands. Then, this rip current passed the tip of the headlands, forming the separation zone of the current immediately north of the headland.

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  • Nguyen Van KHANH, Akio OKAYASU, Tsuyoshi IKEYA, Daisuke INAZU, Van Pha ...
    2019 Volume 75 Issue 2 Pages I_187-I_192
    Published: 2019
    Released: October 17, 2019
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     In recent years, in Vietnamese Mekong Delta (VMD) riverbank erosion and collapse have been excessively occurring in many rivers, especially in small rivers, and threatening people living near the riverbank not only their properties but also their future, even their lives. Erosion and collapse are predicted to increase significantly under the influence of tidal range, sea level rise (SLR), and land subsidence. To confront with erosion and riverbank collapse, small rivers should be intensively studied together with large rivers as most recent studies. However, making a research on small rivers in VMD will be very difficult because of the lack of hydraulic data. The main objective of this study is to demonstrate how to conduct a practical flow modelling for a small tidal river in case of only time-series water level at the mouth available. The results of this study are concentrated in three important points. First, a new searching method for convenient interpolation methods was proposed to reproduce the river bathymetry with sparse depth samples. Second, it was found that Riemann boundary condition is very helpful in case of lack of upstream discharge data but need to be modified to be compatible with 2D flow model. Finally, it was demonstrated that flow model of a river can be easily simulated in long period by applying downstream tidal data and Riemann condition. This research will be helpful for other studies with similar field conditions in the future.

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  • Kazuya WATANABE, Yusuke HORII
    2019 Volume 75 Issue 2 Pages I_193-I_198
    Published: 2019
    Released: October 17, 2019
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     Frequent occurrence of water level rise caused with increase in abnormal weather such as the local torrential rain by the influence of the global warming. Therefore, apprehensive the saline water intrusion was generated at the tidal area due to water level rising and the river mouth topography change in recent years. Saline water intrusion was generated to go up upstream by the ocean waves during winter at the Sea of Japan side river mouth especially. Hence, riverine environment was affected from saline water intrusion. It was in particular thought river mouth shape have a high impact on tidal discharge and saline water intrusion. However, it was not understood in sufficient elucidation of the mechanism of the increase of tidal discharge saline water intrusion. For this reason, it was very important to examine the long-run effect of river shape on tidal discharge and saline water intrusion.

     In this study, it was investigated about the hydraulic characteristics by the field observation and data analysis target at Yoneshiro river where the extension of the sandbar was remarkable. As a result, the amount of change of the river mouth width ΔB grows large more than up to approximately 20m from a previous week. It was revealed that river-mouth closure up to approximately around 10m occurred by the reverse case. Saline water intrusion was not confirmed in Yoneshiro River which was developed of the sandbar. It was thought to cause the river-mouth closure by the development of the sandbar. Furthermore, discharge of wave set-up was about 70m3/s from calculation of the tidal discharge.In addition, it became clear from river mouth width rate of change α that there were many changes of the river mouth width in 500m3/s or less river discharge in Yoneshiro River.

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  • Nobue OKADA, Yusuke UCHIYAMA
    2019 Volume 75 Issue 2 Pages I_199-I_204
    Published: 2019
    Released: October 17, 2019
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     Mesoscale eddies are ubiquitous in the world ocean that frequently collide with currents and other eddies, leading to substantial alteration of the associated dynamics. To investigate fates and dynamical roles of pinch-off eddies from the Kuroshio Extension (KE) Jet, an eddy detection method was applied to ARMOR3D, a 3D global currents and density data set diagnosed from satellite and in-situ observations. Most of the detected pinch-off eddies are cyclonic on the south side of the KE region, while anticyclonic eddies are predominant on the north side. The former occurred 2.64 times as often as the latter, although in the entire North Pacific, cyclonic eddies occurred only 12.1% more frequently than anticyclonic eddies. We succeeded to determine pinch-off eddies recolliding with the Kuroshio by introducing a threshold distance between the Kuroshio path and terminal positions of eddies. Comparisons of the surface temperature and salinity before and after the eddy collisions clearly exhibit that the alteration of the Kuroshio path due to eddy collisions was quite influential in modifying the oceanic structures. It was also suggested that the stability of the Kuroshio path in the KE region is a key influencer on vigorous generation of such colliding pinch-off eddies.

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  • Yu KANEKO, Neriezza OLAP, Yuta UMEDA, Katsuhide YOKOYAMA
    2019 Volume 75 Issue 2 Pages I_205-I_210
    Published: 2019
    Released: October 17, 2019
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     The effect of saltwater intrusion and its characteristics on water distribution at the diversion point of Arakawa and Sumidagawa were examined under low river flow condition, and the following conclusions were obtained. (1) Strong saltwater intrusion was witnessed in Arakawa with the stratified condition during neap tide and well-mixed condition during spring tide. (2) Saltwater intrusion in the Sumidagawa was low compared to the Arakawa due to the differences in topography, and hence the topographical factors such as meandering, channel width, and bed height can influence the saltwater intrusion. (3) There was almost no water flow from Arakawa to Sumidagawa during ebb tide, which can be attributed to the high discharge per unit width for Arakawa than Sumidagawa. These results suggest that discharge and topographical factors are interrelated to the saltwater intrusion and diversion.

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  • Waku KIMURA, Eiji MASUNAGA, Taichi KOSAKO, Yusuke UCHIYAMA
    2019 Volume 75 Issue 2 Pages I_211-I_216
    Published: 2019
    Released: October 17, 2019
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     This study investigated mass transport and dispersion around Izu-chain islands off the Japan mainland using 3-D Lagrangian particle tracking simulations modeled in a high-resolution reginal ocean modeling system (ROMS). The Kuroshio, eddies and tides largely influence physical processes in the study area. Details of transport processes involving the Kuroshio, eddies and tides have not been understood well. However, these processes are important for understanding oceanic environments and ecosystems in the North Pacific Ocean. The vorticity is enhanced by the tidal forcing in the whole study area. The tidal forcing enhances the horizontal diffusion of particles within a two-days scale. By contrast, in the longer time scale than two days, the dispersion of particles is suppressed by the tidal forcing, because large scale motions are suppressed by tides. The frequency spectrum of the particle velocity shows semi-diurnal peaks and high frequency (low frequency) is enhanced (decreased) by the tidal forcing. The tidal forcing promotes high frequency and sub-mesoscale eddies and suppresses low frequency and large scale motions.

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  • Eiji MASUNAGA, Daiki ASAOKA, Shunsuke KOMURO, Shunichi MATSUMOTO, Masa ...
    2019 Volume 75 Issue 2 Pages I_217-I_222
    Published: 2019
    Released: October 17, 2019
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     This study investigates stratification and mixing structures caused by effects of winds and coastal topography by making use of high resolution observed data from the YODA Profiler. Observed data show small scale complicated temperature (density) distributions and baroclinic internal waves generated by wind forcing. The Wedderburn number with a two layer assumption well explains baroclinic and mixing structures along the wind direction. On the other hand, bay and cape shaped topographies cause spatial variations of the wind forcing input, which results in wavy baroclinic strucrures across the wind direction. The observed barolinic and mixing structures directly influence distributions of the phytoplankton and oxygen concentration. This implies that small scale baroclinic structures are key to understand harmuful algae blooms and hypxoa in coastal regions.

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  • Koji KAWASAKI, Masaki NIMURA, Tomokazu MURAKAMI, Shinya SHIMOKAWA, Yor ...
    2019 Volume 75 Issue 2 Pages I_223-I_228
    Published: 2019
    Released: October 17, 2019
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     In this study, we calculated storm surge inundation at the head of Ise Bay by using a coastal circulation model with unstructured grids “FVCOM”. The typhoon conditions were set to be Isewan Typhoon in 1959 and potential typhoon based on the SRES A1B scenario of IPCC. The numerical results for Isewan Typhoon were recognized to be in agreement with the maximum value of storm surge deviation. Coastal structures were also confirmed to be able to protect landside area from storm surge on Isewan Typhoon scale. We examined inundation process due to storm surge induced by potential typhoon and the mitigation effect of coastal structures. The properties of unstructured grid model were also confirmed through the comparison with the numerical results using structured grids including coastal structure treatment.

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  • Masaki NIMURA, Koji KAWASAKI, Tomokazu MURAKAMI, Shinya SHIMOKAWA, Sat ...
    2019 Volume 75 Issue 2 Pages I_229-I_234
    Published: 2019
    Released: October 17, 2019
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     In recent years, sea level rise and intensification of typhoon which are induced by global warming have been feared. It is, therefore, important to estimate the flood characteristics caused by potential maximum typhoon, and evaluate the mitigation effect of tide gate on storm surge. In this study, we simulated storm surge around Nagoya port caused by two potential maximum typhoons, which are the present and SRES A1B scenarios, using improved “T-STOC” for storm surge analysis. The numerical results revealed that maximum storm surge level is about 2m higher than Isewan Typhoon in 1959 and a wider range of below-sea-level area is flooded. It was also found that closed tide gates can delay the start time of inundation, but the evacuation time is very hard to keep once the storm surge flows over the gate.

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  • Yuhi HAYAKAWA, Keisuke MIZUTO, Kazuki YAGISAWA, Shigeo OYA, Sooyoul KI ...
    2019 Volume 75 Issue 2 Pages I_235-I_240
    Published: 2019
    Released: October 17, 2019
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     In this study, we conducted a series of inuduation simulatoins for several storm events that induced the catastrophic strom surges on the Hokkaido coast and investigated critical points of the surge and wave simulation for understanding flood risk. Results of the storm surge simulation showed that the inundation begins from low level grounds and a quay of fishing port. Setting the proper height on the quay of port is important to prevent the surge-induced inundation. It was found that the storm surge height changes with the different wind speed limit in Nemuro. In addition, there are no large difference in storm surge height on other coasts due to the wind speed limit. Therefore, it was found that suitable wind speed limitation is critical to the accurate prediction of the storm surge and wave in a region such as the Nemuro strait.

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  • Jun YOSHINO, Yurie ITAGAKI, Tomonao KOBAYASHI
    2019 Volume 75 Issue 2 Pages I_241-I_246
    Published: 2019
    Released: October 17, 2019
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     In order to estimate the maximum potential storm surge (MPSS) at Nemuro and Hanasaki in eastern Hokkaido, Japan, we conducted track-ensemble experiments of an extratropical cyclone (“bomb cyclone” in December 2014), using an extratropical cyclone bogussing scheme based on the potential vorticity (PV) inversion technique. The extratropical cyclone bogussing scheme developed in this study could create a variety of initial conditions, in which a low-level cyclone with high PV anomaly was slightly shifted from the actual position. The MPSS at Nemuro facing deep water was about 1.5 m due to the effects of wind driven current and pressure drop. The MPSS at Hanasaki facing shallow water was about 1.1 m due to the effects of Ekman transport and pressure drop. It is concluded that the maximum sea-level anomalies in both Nemuro and Hanasaki were strongly influenced by slight differences of paths of the bomb cyclone.

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  • Yoshio HATADA, Koichi INAGAKI
    2019 Volume 75 Issue 2 Pages I_247-I_252
    Published: 2019
    Released: October 17, 2019
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     Characteristics of a difference of peak time between wave heights and storm surge deviations during each storm at Tokyo Bay are discussed based on hourly hindcasts data during 38 typhoon and 56 non-typhoon storms in the past 45 years. Occurrence rates of the differences for the peak time are illustrated by histograms at 10 selected sights. At the sights along the western coast, the wave heights takes earlier peak than the storm surge deviations. In contrast, the storm surge deviations at the eastern coast show earlier peak than the wave heights. And the peak time differences increase toward the outer sights of the bay. The rate at which the peak of wave heights and that of storm surge deviations occur simultaneously at Tokyo Bay is estimated to be approximately once every 4 years and is shorter than that of 6 years at Ise Bay.

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  • Hisamichi NOBUOKA, Naoyuki EBINE
    2019 Volume 75 Issue 2 Pages I_253-I_258
    Published: 2019
    Released: October 17, 2019
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     On the Pacific coast of East Japan, extratropical low pressure system developed significantly and caused storm surge disasters several times. In this study, a probabilistic cyclone model was been constructing that can estimate largest class of storm surges of which return period is around 1000 years. By using all low pressure system track data for this construction, it is generally difficult to calculating powerful low pressure system. On the other hand, the low pressure simulated by the probabilistic low pressure model which used only strong low pressure becomes stronger than the past low pressure system, and become higher storm surge. In addition, the results of extreme value statistical analysis on storm surges showed a good relationship by using of past low pressure and probabilistic low pressure systems.

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  • Takuto HAGA, Kazuhiro FUJIWARA, Katsumi SEKI, Taro ARIKAWA
    2019 Volume 75 Issue 2 Pages I_259-I_264
    Published: 2019
    Released: October 17, 2019
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     A combination of quasi three-dimensional and three-dimensional models is required for reasonable and rapid real-time storm surge forecasting. It is also important to clarify the adaptability conditions of quasi three-dimensional and three-dimensional models. However, there are few studies which examined the adaptability conditions on minor islands because the possibility of storm surge disaster at these islands is quite low compared to the inner bay. In this study, we examined the applicability of quasi three-dimensional and three-dimensional models during the 2017 Hurricane Irma which caused storm surges around Caribbean Sea. We also examined the accuracy of computed three-dimensional velocity induced by its hurricane passage. As a result, it was clarified that three-dimensional model is necessary for modeling storm surge on minor islands. This is because the multi-layered cells contribute to the propagation of storm surge when water depth become small. The computed storm tide become high due to the wind set-up around the island area, but this storm tide caused by wind set-up effect can be estimated properly using three-dimensional model.

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  • Tomoya KATAOKA, Yasuo NIHEI
    2019 Volume 75 Issue 2 Pages I_265-I_270
    Published: 2019
    Released: October 17, 2019
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     The storm surge induced by Typhoon Jebi caused flooding in the city areas around the Takahashi River in Hyogo Prefecture. To investigate the river-related flood disaster, we have conducted in-situ survey for measure the height of high-water marks and a two-dimensional plane flow simulation. The river-related flood occurred due to the increase of river water level induced by the storm surge, and mainly caused by the overflow from bridges whose elevation is lower than that of levees. After that, the flooding water spreaded for thirty minutes. The inundation area is approximately 0.11 km2, and the maximum inundation depth was 0.62 m. On the other hand, the rainfall hardly contributed to this disaster because the peak time of water level induced by the rainfall was before 31 minutes.

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  • Yasuyuki BABA, Teruhiro KUBO, Nobuhito MORI, Yasunori WATANABE, Tomohi ...
    2019 Volume 75 Issue 2 Pages I_271-I_276
    Published: 2019
    Released: October 17, 2019
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     During the summer season in 2018, three typhoons (No. 20, 21, 24) approached around the Kii peninsula, and the severe wave conditions caused by the typhoon Jebi (No. 21) and the typhoon Trami (No. 24) are observed at the observation tower of Shirahama Oceanographic Observatory, DPRI, Kyoto University.

     The observed maximum significant wave height due to the typhoon Jebi and Trami reach 9.60m and 11.09m, respectively. These high waves caused heavy damages to measurement equipments and fixtures on the observation tower.

     High wave conditions due to the typhoons mentioned above consists of two periods. The swell component is dominant during the first half, and the swell and wind-wave components are superposed during the second half.

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  • Sooyoul KIM, Nobuhito MORI, Tetsuya TAKEMI, Yoko SHIBUTANI, Tomohiro Y ...
    2019 Volume 75 Issue 2 Pages I_277-I_282
    Published: 2019
    Released: October 17, 2019
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     The study conducted a series of storm surge and wave simulations caused by Typhoon Jebi using a parametric wind and pressure field model, and mesoscale atmospheric model WRF to hindcast observed storm surges and waves in Osaka Bay. Appropriate typhoon radiuses and the limit of wind speed for wave depending sea surface drag coefficient were examined. As results of the experiment, we could simulate the observed storm surge and wave at Osaka and Kobe with errors of 0.3 m and 0.5 m in maximum, respectively.

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  • Nobuhito MORI, Tetsuya TAKEMI, Sooyoul KIM, Yoko SHIBUTANI, Tomohiro Y ...
    2019 Volume 75 Issue 2 Pages I_283-I_288
    Published: 2019
    Released: October 17, 2019
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     Using high-resolution numerical weather prediction model and coupled storm surges and waves, we conducted pseudo-prediction experiments on the maximum wind speed, maximum storm surge height, and maximum wave height for 2018 Typhoon Jebi. The central pressure and maximum wind speed of the typhoon become more accurate as the forecasting time length becomes shorter. The minimum central pressure and maximum wind speed of typhoon at landfall are reasonably estimated 3 days and 2.5 days before the landfall, respectively. In the 4 days length prediction, the peak appearance time of the event is estimated about 12 hours later, but the accuracy is increased as forecast length becomes shorter. The difference of the storm surge height between the prediction and hindcast is less than 0.5 m at the maximum. The reliable prediction of storm surge can be obtained from 2 days ago of landfall, quantitatively.

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  • Yuji KOZONO, Masaaki SAKURABA, Kazuya NOJIMA
    2019 Volume 75 Issue 2 Pages I_289-I_294
    Published: 2019
    Released: October 17, 2019
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     Typhoon No. 21 occurred in September 2018. This typhoon caused damage of storm surge to the coastal area of Osaka Bay. In addition, the drifted ships collided with the bridge of Kansai International Airport. It is necessary to consider the effect of wind during storm surges. In this study, storm surges by the typhoon were calculated for verification. A drifting prediction model considering the effects of wind was developed. Furthermore, the track of ship drifting was verified. As a result, the calculated results agree well with the observed pressure, wind speed and tide levels in Osaka Bay. As a result of comparing the calculation track of the ship and the record track, the calculated results were in good agreement with the record track. The proposed model was shown to be useful in predicting the behavior of drift during storm surges.

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  • Fumiya INOUE, Wataru ECHIZENYA, Rikito HISAMATSU, Kei HORIE
    2019 Volume 75 Issue 2 Pages I_295-I_300
    Published: 2019
    Released: October 17, 2019
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     We conduct the control experiment (CTL) and the virtual experiment which assumes Kii Peninsula is the ocean (NoBay) of the typhoon 1821 by the WRF model and investigate influences of the geography on the storm surge in Kobe city. The result of CTL shows favorable wind field inducing the storm surge and almost reproduces wind observation of AMeDAS data at Kobe Airport. On the other hand, the gradient wind applied on the Best Track Data is dominantly a westward wind and greatly different from the characteristics of AMeDAS data at Kobe Airport. The maximum 30-minute averaged wind speed in Kobe city of NoBay decreased by 6% compared to CTL. This suggests Kii Peninsula strengthens the wind around Kobe city. These results suggest the wind field by physical typhoon simulation including effects of the land is essential for the accurate assessment of storm surge risks.

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