Akashi Strait Research Articles

Optimal site selection and potential power assessment for tidal power generation in the Seto Inland Sea, Japan, based on high-resolution ocean modelling and multicriteria analysis

In response to Japan's ambitious pursuit of carbon neutrality by 2050, this paper investigates the potential for tidal power generation in the Seto Inland Sea (SIS), Japan, utilising high-resolution ocean modelling and an analytic hierarchy process (AHP) combined with GIS-based spatial analysis. The analysis incorporates a 100 m resolution tidal current and water depth layers from the ocean model. The criteria used for evaluation were divided into main criteria, namely, proximity to existing infrastructure, wave height, and shipping density, and subcriteria, namely, distance from ports, power stations, highly populated areas, and coastlines. The identified optimal locations include the Naruto Strait, Akashi Strait, Matsushima Island, Hanaguri Strait, Funaori Strait, Taizaki Island, Kurushima Strait, Obatake Strait, and Tsuwaji Strait. With a focused analysis of the Obatake Strait in Yamaguchi Prefecture, analytical assessments of potential tidal power were conducted by considering two tidal turbines, Openhydro and Voith. The results indicate a power supply for approximately 100,000 households and a reduction of approximately 198,000 t-CO2 per year in the most optimistic scenario. In a less optimistic scenario, with the installation of only 10% of the maximum capacity, a power supply for approximately 10,000 households and a reduction of approximately 19,800 t-CO2 per year are observed. As Japan targets carbon neutrality by 2050, this research offers vital and practical insights for policymakers, energy planners, and environmentalists, identifying the Seto Inland Sea as a strategic area for tidal power development, aligning with Japan's commitment to a sustainable and resilient energy landscape.

Temporal variations in the copepod community structure during the sand lance fishery season near the Akashi Strait, eastern Seto Inland Sea

Temporal variations in the copepod community structure during the sand lance fishery season near the Akashi Strait, eastern Seto Inland Sea

Generation Mechanism of Tidally-driven Whirlpools at A Narrow Strait in An Estuary

This study investigates the generation mechanism and influence of the whirlpools in the Naruto Strait on the surrounding marine environment using state-of-the-art high-resolution numerical ocean circulation modeling in a quadruple nested configuration. The Naruto whirlpools is recognized as an extraordinary seascape that the local governments and the citizens seek to register as a world natural heritage site. We found that the pronounced pressure gradient force associated with the meridional surface elevation difference was induced by a phase difference of two bifurcating major tidal waves. These waves originate from the Kitan Strait, and ultimately produce intense tidal currents at the Naruto Strait. One branch of the tidal waves propagates counter-clockwise along Awaji Island through the Akashi Strait, while the other occurs directly from the Kii Channel. As such, the whirlpool emerges as a large number of sub-mesoscale eddies, primarily due to the horizontal shear instability of tidal currents energized at the narrow topography between two headlands that extend into the strait. A dipole of overturning vertical circulations appears underneath the whirlpools with convergent downward transport at the strongest tidal current near the center of the strait; this causes efficient vertical mixing. This three-dimensional non-linear mixing promotes a time-averaged southeastward mass transport that extracts water and materials from the Harima-nada Sea into the Kii Channel.

Influence of marine vessel emissions on the atmospheric PM2.5 in Japan’s around the congested sea areas

In recent years, PM2.5 concentrations in Japan have decreased as China’s measures against the emission of air pollutants were strengthened and the subsequent transport of air pollutants to Japan decreased. On the other hand, along the coast of the Seto inland sea in Japan, the PM2.5 concentration remains high. In this study, in order to evaluate the impact of air pollutants from marine vessels on PM2.5 along the coast of the Seto inland sea, PM2.5 was seasonally collected in the vicinity of a congested sea lane (Akashi Strait) in 2016 and 2017, and a receptor-source analysis was performed to determine the main components of the collected PM2.5. In Japan’s congested sea lane, the vanadium (V) concentration was very high and showed a strong correlation with the nickel (Ni) concentration. Also, the V/Ni ratio rose when the wind blew from the sea lane. Positive Matrix Factorization (PMF) analysis clarified that the contributions from marine vessel emissions to PM2.5 at the current observation sites were 2.5–2.7 μg m−3 (17.3–21.4%), and the marine vessel emissions were the main source of PM2.5 along the coast of the Seto inland sea. Fuel oil regulations for marine vessels to be introduced in January 2020 are expected to improve the air quality of coastal areas.

The age of Yodo River water in the Seto Inland Sea

Abstract The age of Yodo River water, the largest river flowing in the Seto Inland Sea (SIS), was calculated using the constituent-oriented age and residence time theory (CART) in order to understand the long-term material transport and environmental issues in this semi-enclosed sea. The Yodo River water was found to have a mean age of 152 days for the entire SIS. Inside the SIS, the age of Yodo River water shows spatial variation ranging from 97 days in Osaka Bay, 199 days in the Harima-Nada, and 450 days in other areas. The age of Yodo River water shows an apparent seasonal variation in the SIS. Tracing water from the Yodo River reveals two pathways for its movement around Osaka Bay: one brings ~5/8 river water directly into the Kii Channel through the Kitan Strait; the other brings ~3/8 river water into the Harima-Nada through the Akashi Strait that eventually enters the Kii Channel through the Naruto Strait. The mean age of Yodo River water is 128 days at the Kitan Strait, 125 days at the Akashi Strait, 203 days at the Naruto Strait, and 271 days at the Bisan Strait. The above estimation considers the return of old Yodo River water into Osaka Bay that does not change the spatial distribution of Yodo River water age inside Osaka Bay but increases the value of water age by ~80%. Without tidal forcing, the mean ages of Yodo River water in Osaka Bay and the Harima-Nada increase by 6% and 51%, respectively, and nearly all the Yodo River water flows directly into the Kii Channel through the Kitan Strait. Without considering baroclinic forcing, the mean ages of Yodo River water in Osaka Bay and the Harima-Nada increases by 52% and 51%, respectively, while the influence of local winds and the Yodo River discharge on the age of Yodo River water are negligible.

Biological Aspects of Large Whitespotted Conger (Conger myriaster) in the Akashi Strait, Eastern Seto Inland Sea, Japan

Biological Aspects of Large Whitespotted Conger (Conger myriaster) in the Akashi Strait, Eastern Seto Inland Sea, Japan

G1700105 リスク評価に適用する事故シナリオ同定手法に関する研究

Probabilistic risk assessment (PRA) is a technique for evaluating the safety of nuclear power plants. In particular, the level 1PRA has general versatility and has been utilized in various fields. In the level 1PRA, the identification of accident scenarios is very important. It is performed by top-down analysis or bottom-up analysis. In the previous study, the marine accident in Akashi Strait was analyzed by the bottom-up analysis method. However, the identification of accident scenarios required a more effective analysis to extract factors without slipping. So, in this study, we discuss accident scenario identification methods from both top-down and bottom-up analysis method. An example of the analysis of marine accidents was shown by these methods. The bottom-up analysis was referred to the previous study. As a top-down analysis method, the Master Logic Diagrams (MLD) was performed. By comparing the analysis result of MLD to the previous study, we discuss the effectivity of the identified accident scenario by both top-down analysis and bottom-up analysis.

高解像度海洋モデルによる大阪湾・播磨灘における処理水分散シミュレーション

Anthropogenic land-derived materials in wastewater have been recognized to influence on the water quality and the marine ecosystem in Osaka Bay and Harima Nada. Towards a rigorous environmental assessment of these two semi-enclosed estuaries, a triple-nested high-resolution oceanic modeling framework based on ROMS is developed to investigate the dispersal and dilution of urban wastewater effluent released from the subsurface nearshore outfall of Tarumi Sewage Treatment Plant near Akashi Strait. The effluent dispersal in the northern Osaka Bay is two-layered and highly reversal in the upper and lower layers due to stratification influenced by freshwater input mainly from Yodo River. The estimated cumulative tracer flux for the four-month period is found to accumulate by about 75% in Osaka Bay.

Output Characteristics of Power Generation System for the Navigation Buoy using Tidal Currents

Operators of navigation beacons have been searching for self-contained energy sources, as it is difficult to supply buoys in the ocean from the commercial power net. Solar, wind and wave-powered renewable energy mechanisms have been developed for navigation buoys. The advantage of tidal currents over other renewable energy sources is they are independent of the weather, so the available power can be calculated. In this paper, a Darrieus-type water turbine using the energy in the local tidal current was developed as energy conversion device for powering a navigation buoy, and the characteristics of the water turbine were observed in a water channel experiment. The energy converter of the electric power generation system was installed in a buoy in a pilot project in the Akashi Strait in Japan. As the result, the changes in generated electric power with current speed were observed. Sufficient energy was obtained from the tidal currents to operate this navigation buoy.

Several Geotechnical Design and Construction Issues with Akashi Strait Bridge

The Akashi Kaikyo (Strait) Bridge, one of the major bridges of the Honshu-Shikoku bridge network connecting the main island and the Shikoku island, is the world's longest suspension bridge with a center span of about 2,000 m. The bridge was constructed over the Akashi Strait and opened to traffic in 1998 after a construction period of about 10 years. The granite stratum in the Akashi Strait was very deep, while the granite stratum provides the foundations for the other long span bridges which have been constructed in Japan. Three of the four foundations had to be constructed on either a weakly cemented gravelly soil deposit or a sedimentary soft rock layer. The geological structure was carefully investigated and the geotechnical properties, in particular the strength and deformation characteristics of these geomaterial types, were carefully evaluated. To this end, a comprehensive series of advanced triaxial compression (TC) tests were performed on rotary core tube samples of sedimentary soft rock after it was found that the due design was not possible based on unconfined compression tests. Both CD and CU TC tests and cyclic undrained triaxial tests were performed on rotary core tube samples (30 cm in dia.) of the gravelly soil. Based on the data, the long-term and seismic stabilities of the foundations were evaluated. The importance of strain- and pressure-dependent nonlinear stress-strain behaviour at small strains, as well as accurate strain measurements in the laboratory stress-strain tests, was confirmed by analyzing the full-scale behaviour of the foundation during and after construction and comparing the back-calculated stiffness values with those obtained from laboratory stress-strain tests, pressure-meter tests, plate loading tests and field shear wave velocity measurements.

Holocene ostracod palaeobiogeography of the Seto Inland Sea, Japan: impact of opening of the strait

Abstract. In this study, published Holocene ostracod data for Osaka, Hiroshima, and Iyo-nada Bays within the Seto Inland Sea (SIS), Japan, are re-examined. Five major faunal changes are recognized at different stratigraphic horizons (Horizons 1–5) in the Holocene sedimentary record. Horizons 1 and 5 (faunal changes) were most likely triggered by the respective openings of the Bisan-seto (and resulting formation of the SIS) and Akashi Straits. Three other major faunal changes, at Horizons 2–4, may also be linked to the opening of straits and resulting formation of bays, although the exact nature of these events is less certain. This re-examination indicates that the opening of straits extensively affected the benthic inner-bay community during the Holocene transgression in the SIS. Further high-resolution studies are needed for confirmation of faunal changes triggered by opening of straits.

Estimation of far-field horizontal and vertical turbulent diffusion coefficients from the concentration field of a wastewater plume near the Akashi Strait

We carried out a field study of the plume discharged by a near-shore wastewater outfall near the Akashi Strait, Japan. Using an Acoustic Doppler Current Profiler and a tow-body CTD, we measured the near-surface salinity and temperature fields in the region throughout an M2 tidal cycle. We filtered the data in T–S space to remove water masses other than the wastewater, and then used the adiabatic mixing assumption to calculate the concentration of wastewater in the far field of this plume. Averaging the T–S fields of repeated surveys over a time period during which the tidal regime did not change substantially, allowed comparison of the time-averaged plume with the analytical solution for a plume diffusing in both the horizontal and vertical dimensions. The resulting vertical turbulent diffusion coefficients agreed well with those resulting from Thorpe scales determined via a vertically-profiling CTD, as well as with the canonical value for open channel flow of Dz = 0.067hu*. The corresponding horizontal turbulent diffusion coefficients, however, were two orders of magnitude larger than those typically observed in straight channels, and an order of magnitude larger than those observed in meandering rivers. This is likely a result of enhanced horizontal mixing due to barotropic eddies generated by the interaction of strong tidal flow with headlands and levees, as well as due to the time-varying nature of tidal flow, and baroclinic spreading of the buoyant wastewater plume.

ステレオ画像を利用した船舶の検出に関する基礎的検討

Radar is essential to safety navigation for ships. However, it is not perfect equipment and there exist marine accidents. The authors propose using stereo vision that automatically detects ships and/or other objects and measures their 3D locations in order to assist usual navigation equipment. This paper focuses on a novel method to detect ships from stereo images. The proposed method is composed of detection of edges from images, confirmation of corresponding points between left and right images, and final judgment of ships from 3D measurement results. An experiment, which used twelve pairs of stereo images obtained at Akashi Strait, was conducted. The result indicates good possibility of the proposed method, although there is room for improvement in order to ensure the detection.

Behavior of a Small Pulsed River Plume in a Strong Tidal Cross-Flow in the Akashi Strait

We carried out a field study of a small river plume emptying into Osaka Bay near the Akashi Strait in western Japan, to understand the fate of its urban-runoff-laden waters. As the river is weak compared to tidal currents, we find that the behavior of the plume is strongly influenced by phasing between tidal stage and currents (a westward-traveling progressive tidal wave). When the tidal stage rises, sea water floods up the river, and concentrated river effluent cannot be seen in Osaka Bay. During most of the river’s ebb, a topographic eddy transports river effluent towards the energetic Akashi Strait, while strong vertical shear in the tidal flow mixes the effluent with seawater. However, there is a short interval of time during which the direction of tidal flow in the Strait changes direction and the magnitude of the current is weak. At this time, shear is weak enough to allow a stratified plume of concentrated river water to form, and this plume is driven offshore, and into Osaka Bay, by baroclinic circulation. A sewage outfall, which is located very close to the river mouth, is subjected to these same conditions and therefore exhibits similar behavior. Understanding the relation between tidal phase and plume behavior will be useful to Kobe City, as well as other cities in a similar environment, for minimizing the environmental effects of their wastewater and non-point-source runoff.

Middle Pleistocene Ostracoda from the Takatsukayama Member of the Meimi Formation, Hyogo Prefecture, western Japan: significance of the occurrence of Sinocytheridea impressa

Thirty-seven ostracode species are identified from 15 samples from lithological Units II and III at the type locality of the middle Pleistocene (Marine Isotope Stage 11: ca. 0.41 Ma) Takatsukayama Member of the Meimi Formation, Hyogo Prefecture, western Japan. Three ostracode biofacies are distinguished on the basis of Q-mode cluster analysis. The depositional environments of each biofacies can be inferred as bay sandy coast (BC), enclosed inner bay (IB) and middle bay influenced by freshwater inflows from rivers on the Harima Plain (CB). A rapid increase in water depth from bay coast to middle bay occurred at the deposition of the middle part of Unit II of the Takatsukayama Member. Sinocytheridea impressa (Brady), which probably disappeared or rapidly decreased from paleo-Osaka Bay at about 0.3–0.35 Ma, is abundant at the study site. One of the reasons for the disappearance or rapid decrease of this species is likely that paleo-Osaka Bay changed from a low-salinity, enclosed nutrient-rich and highly turbid bay to a moderate salinity, ventilated bay with good circulation, together with the vicariance event suggested by previous studies. This change was probably linked with both widespread inundation in the neighboring paleo-Harima-nada Bay and the development of large-scale tidal currents from the Akashi Strait connecting both bays. Thereafter, Neomonoceratina delicata Ishizaki and Kato replaced S. impressa and became the dominant taxon in paleo-Osaka Bay. Sinocytheridea impressa which lived in paleo-Osaka and Harima-nada Bays has larger valves than any specimens of this species from Recent seas. The cause of this large size in the past might have been an abundant food supply or dissolved chemicals from freshwater inflows into paleo-Osaka and Harima-nada Bays. Alternatively, whether large-valve populations occurred as a result of genetic changes in relation to the geographical isolation or not, natural selection operated in favor of increased valve size in paleo-Osaka and Harima Bays for reasons that are as yet uncertain.

架橋下通航における操船者の行動に関する研究-II : 操船行動モデルの汎用化

A navigation officer makes a voyage plan before departing. Especially, he makes a passage plan for approaching and passing when the ship will pass through a channel or strait. Navigation route is made by a regulation for safety navigation in the congested sea area and the Maritime Traffic Safety Law sets up the Traffic Route in the main narrow channel. In recent years, the huge bridge has been built in strait, such as the Akashi strait, the Kurushima strait and the Bisan-seto strait. Although it had various discussions on the influence of the bridge pier in the narrow channel, the bridge pier has always been arranged out of the Traffic Route. In this research, we investigated by the questionnaire how the selection action consciousness of a navigation officer's planning route would change when obstacles, such as a bridge pier, had been arranged at the strait in which a vessel can navigate freely. The action of a navigation officer was modeled by the result and this model can be carried out the reappearance of the navigation routing in arbitrary strait width and bridge pier intervals. By combining this routing model and an evaluation index, it becomes possible to evaluate the influence of an arrangement of the bridge pier to marine traffic in a narrow channel.

FEASIBILITY STUDY ON ENVIRONMENTAL IMPACT ASSESSMENT FOR HYPOTHETICAL FLOW CONTROL PROJECTS USING FAZZY SET THEORY

The concept of ‘environmental standard’ has been applied to coastal environmental impact assessment, EIA. The standard values have been determined for each water quality and for each divided coastal area. The achievement of improving water qualities depends on whether the values estimated by EIA are larger than the standard values or not. The uncertainly and complexity of the natural system, however, make it difficult for traditional approaches to come up with simple answers. To facilitate this integrative approach the basic concept of fuzzy set theory is introduced. In addition, the integrated evaluation function is also introduced. In the present study, the hypothetical Akashi Strait breakwater project is evaluated to test the usefulness of EIA used as a case study to examine the usefulness of this methodology: various models and data set are assembled and analyzed. Water qualities of COD, total nitrogen and total phosphorus are used to make EIA in Osaka Bay.

Derivation of near-source ground motions of the 1995 Kobe (Hyogo-ken Nanbu) earthquake from vibration records of the Akashi Kaikyo Bridge and its implications

According to the results of the previous inversion studies, the main ruptures of the 1995 Kobe earthquake occurred on two faults, namely the Nojima and Suma faults, which have a step-over about 4 km wide in the Akashi strait. However, the possibility of the rupture in the step-over itself has not been clarified yet due to the unavailability of near-epicentre seismic data. To this unavailability, free-field ground motions at the piers of a suspension bridge located in the step-over are derived from the vibration records of its towers. The ground motions are successfully derived within 0.6–1.2 Hz, where the transfer functions are stable and the influences of the cable vibrations are negligible. The preliminary analysis of the obtained ground motions shows that although the ruptures of the main faults could simulate the early parts of the ground motions, they fail to recover a major pulse afterwards. This suggests that the step-over might have had a delayed rupture.

Vertical Changes of Holocene Ostracodes in Bore Hole Cores from off Kobe, Related to the Opening of Straits and Relative Sea-Level Change in Western Japan.

Fossil ostracodes were studied in two bore hole cores (OB-1 and OB-2) from Osaka Bay off Kobe in western Japan to examine vertical changes in ostracode faunas. Fifty-two species were identified in 56 sediment samples. Based on numerical analyses of these ostracodes, the samples examined are grouped into five biofacies: BM (brackish mud), SM (shallow mud), SS (shallow sandy mud), DS (deep sandy mud) and DM (deep mud). These biofacies are distributed in upward sequence in both cores. From ca. 11, 000-10, 000cal yrs BP, the predominant species living were those dominant in enclosed inner brackish bays at water depths of 2-7m. Inner to middle, enclosed shallow muddy bay species increased at the study sites after the opening of the Akashi Strait (ca. 9, 700cal yrs BP). Ostracode assemblages changed ca. 8, 000cal yrs BP, when Osaka and Harima-nada Bays were connected with Mizushima-nada Bay. Tidal currents from the Akashi Strait have since been strong. As a result, specimens of intertidal to shoreface species were transported with sandy sediments into deeper Osaka Bay off Kobe. Thus, their individual numbers increased rapidly. A peak in the number of intertidal to shoreface species and deep mud-dwellers occurred around 5, 500cal yrs BP, suggesting that tidal currents from the Akashi Strait and paleo-water depth reached their maxima at this time (ca. 33m). These species then decreased with time, with the start of a regression. After ca. 2, 000-1, 600cal yrs BP, mud-dwellers inhabiting enclosed, organic matter-rich bays were abundant. Their numbers increased further as coarse materials from the Akashi Strait decreased and the Yodo River began to bring flood clays into the study sites. Both “form A” and “form M” of Bicornucythere bisanensis are present in both core samples. “Form M” first invaded the study sites through the Kitan Strait to the south. “Form A” came to Osaka Bay off Kobe, ca. 8, 350-8, 000cal yrs BP, and has since become the dominant of the two forms.

Seismic investigation of an active fault off Kobe: Another disaster in the making?

On 17 January 1995 a magnitude 7.2 earthquake struck the Kinki district of southwestern Japan. This earthquake, called the Hyogo‐ken Nanbu earthquake (but most familiarly known as the Kobe earthquake), killed more than 6000 people, injured about 40 000 people, and destroyed about 100 000 houses. The hypocenter was about 16 km beneath the Akashi Strait between Akashi City and the northern Awaji Island (Figure 1). Its focal mechanism is right‐lateral slip, trending northeast‐southwest, which is produced by a regional compressive stress field that trends east‐west. The aftershocks were aligned northeast‐southwest from north of Kobe to the northern Awaji Island. Surface ruptures appeared along the Nojima Fault on the northwest coast of Awaji Island, but no surface ruptures were observed in the Kobe area.