Kyushu University Academic Staff Educational and Research Activities Database
List of Presentations
Soichi Yamaguchi Last modified date:2019.07.23

Assistant Professor / Fluid Environmental Research / Department of Advanced Environmental Science and Engineering / Faculty of Engineering Sciences


Presentations
1. Yuichi Hayami, Soichi Yamaguchi, Naoki Fujii, The mechanism of the progress of hypoxia in Ariake Sea without the increase of terrestrial load, ECSA 57: Changing estuaries, coasts and shelf systems - Diverse threats and opportunities, 2018.09, In the inner area of Ariake Sea, Japan, serious hypoxia occurred every summer and generated massive kill of bivalves. So, it became a big social problem. Since the macrobenthos in this area was richer at the end of 1980s than now, it was supposed that the hypoxia progressed in these 30 years. However, as the terrestrial load of organic matter and nutrients did not increase, it was unknown why the hypoxia progressed. Our recent studies clarified that there were 3 mechanisms that generated the long term progress of hypoxia. The first one was the increase of organic matter from 1970s to early 90s. The box model analysis of COD showed that it was generated by the net ecosystem production. It would be due to the decrease of bivalves and enhancement of the water residence time. The second one was the influence of the sea dike construction in Isahaya Bay which is a sub-basin of Ariake Sea in 1997. The sea dike construction weakened the tidal current between the inner area of Ariake Sea and the bay mouth. Therefore, the vertical mixing in this area decreased and the estuarine circulation was enhanced. The stratification in the inner area became stronger as the enhancement of the intrusion of the dense water in the bottom layer caused by the estuarine circulation. The third one was the long term change in tidal amplitude including 18.6 years lunar nodal cycle. There was an inter-decadal scale decrease of tidal amplitude in outside of the bay. Also, the tidal amplitude of M2 tides decreased from late 1990s to 2000s due to the lunar nodal cycle. So, the tidal amplitude in 2007 was lowest in these 50 years. As the tidal amplitude decreased, the tidal mixing weakened and the stratification was enhanced. It generated further progress of hypoxia..
2. Soichi Yamaguchi, Naoya. Minamiura, Mechanism of red tide outbreaks in winter in the western area of the inner Ariake Sea, ECSA 57: Changing estuaries, coasts and shelf systems - Diverse threats and opportunities, 2018.09, Aquaculture of fishery resources has been conducted in coastal waters all over the world. In Ariake sea, a region of freshwater influence (ROFI) in Japan, aquaculture of Porphyra yezoensis, namely ”Nori” has been conducted in great magnitude during autumn and winter.
Red tide outbreaks by several phytoplankton species frequently occur during the aquaculture season, which causes color bleaching of the nori due to depletion of nutrients in water column and prevents stable production. One of the characteristics of red tide occurrences is for the red tides to be concentrated in western area of the inner Ariake Sea independently of the phytoplankton species. The mechanism for this is poorly understood. The objective of the present study is to clarify factors for the concentration of red tide outbreaks in the western area of the inner Ariake Sea using numerical simulation and the observed data.
The results of numerical simulation revealed that the sea water residence time was relatively long in the western area of the inner Ariake Sea compared to those in other areas. There was a high correlative relationship between red tide index (cell number × duration time) and the water residence time. On the other hand, growth rate of a representative diatom : Eucampia zodiacus calculated by the observed nutrient concentration and transparency hardly has a correlation with the red tide index. Finally, the impact of nori aquaculture on the water residence time and transparency was evaluated by the numerical simulation with/without the hydrodynamic resistance of the aquaculture facilities. The results of numerical simulations inferred that the aquaculture increased the water residence time in the western area of the inner Ariake Sea and therefore made the hydrographic environment more suitable for the red tide outbreaks. Sustainable practices of the aquaculture is necessary for the future stable production.
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3. SEASONAL CHANGE IN TIDAL CURRENT ENERGY DUE TO TIDE-INDUCED EDDY IN GOTO ISLANDS, JAPAN, [URL].
4. Mechanism of red tide outbreaks in winter in Ariake Sea, [URL].
5. Long term variation in hypoxia in the inner area of Ariake Sea.
6. VARIATION OF DO CONCENTRATION IN COLD-DOME WATER IN ARIAKE SEA.
7. METHOD VALIDATION FOR REDUCING PRESSURE GRADIENT FORCE ERROR IN COASTAL OCEAN MODELING
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8. 山口 創一, Tidal Current Energy Potential in Nearshore Waters of Western Japan, International conference on ocean energy 2016, 2016.02, [URL], Much attention has been paid to tidal renewable energy application in Japan since the earthquake in 2011. This study estimates the tidal current energy potential in nearshore waters of western Japan by unstructured-grid finite volume coastal ocean model..
9. Soichi Yamaguchi, Yusaku kyozuka, TIDAL CURRENT ENERGY MAP AROUND KYUSHU-OKINAWA REGION, JAPAN, Grand renewable energy 2014, 2014.07, Numerical simulation is conducted for obtaining tidal current energy map around Kyushu- Okinawa region, Japan, using finite volume coastal ocean model:FVCOM. Calculation domain covers whole area of Kyushu-Okinawa region and consists of unstructured triangular meshes which can have high resolution near coasts with complicated geometry and low resolution in offshore sea area, so that the present model continuously can calculate the detail distribution of tidal current from offshore area to near coasts with computational efficiency. Simulation results are validated with tide-gauge data observed at coasts over the Kyushu-Okinawa region. The present model shows good agreement with the observed. High tidal current was detected in narrow channels such as those in Goto islands, Kanmon strait and Ariake Sea, exceeding 3.0 m sec-1. Based on the calculated current field, tidal energy map is also obtained. This map can be used for estimating electric power gain and selecting the location best suited to tidal energy plant..
10. Numerical simulation is conducted for obtaining tidal current energy map around Okinawa-Kyushu region, Japan, using finite volume coastal ocean model:FVCOM. Calculation domain covers whole area of Kyushu- Okinawa region and consists of unstructured triangular meshes which can have high resolution near coasts with complicated geometry and low resolution in offshore sea area, so that the present model continuously can calculate the detail distribution of tidal current from offshore area to near coasts with computational efficiency. Simulation results are validated with tide-gauge data observed at coasts over the Kyushu- Okinawa region. The present model shows good agreement with the observed. High tidal current was detected in narrow channels such as those in Goto islands, Kanmon strait and Ariake Sea, exceeding 3.0 m sec-1. Based on the calculated current field, tidal energy map is also obtained. This map can be used for estimating electric power and selecting the adequate location for tidal energy plant..
11. Red tides frequently occur in Isahaya Bay, Ariake Sea every summer, especially much attention is paid to the red tide dominated by Chattonella spp. (C. antiqua and C. marina) because it leads to the death of livings in the sea. One of the characteristic phenomena related to the red tide outbreak is the vertical mixing of water column before its occurrence due to meteorological disturbance such as typhoon arrival. In the present study, the author conducted biweekly field measurements on nutrient dynamics in Isahaya Bay before/after the typhoon arrival in summer 2005. Meteorological disturbance by the typhoon caused complete vertical mixing of water column and resuspension of bottom sediment in large quantity. Dissolved inorganic nitrogen (DIN) and phosphorus (DIP) concentration greatly increased after the disturbance, especially by a factor of about 6.4 in vertically averaged DIN concentration. Nutrient budget analysis showed that the nutrient supply from the bottom sediment including pore water and resuspended sediments was dominant for the increase. Estimated nutrient release rate in the present study was more than two times the rate obtained by laboratory experiments that previously conducted using undisturbed bottom sediment. It is important to take into account the effect of bottom sediments dynamics on the nutrient environment in evaluating the flux between water and seabed..
12. Huge tidal flats with tidal creeks develop around rivers in Ariake Sea. River water that contains anthropogenic pollutants flows into the inner sea area over complicated geometry of the tidal flats. Numerical simulation for the Ariake Sea, up to date, has applied low-resolution numerical model incapable of representing the geometry in detail. The present study applied a finite volume coastal ocean model (FVCOM) with high resolution capable of representing such complex geometry. The model results have been validated against extensive observations and showed that the complex geometry of the tidal flats with tidal creeks had great effects on the dynamics of river water. Also shown is the dependence of the model accuracy on the model resolution around rivers by comparing with coarse resolution models, and its effects on the calculated dynamics of river waters., [URL].
13. Generation Mechanism of Upwelling current with “Aoshio” occurrence in summer of 2008 in Ariake Sea.
14. Proposal of open bondary condition for the numerical simulation of Ariake Sea.
15. Reproduction and its formation mechanism of cold-dome in Ariake sea in summer.
16. Study on the formation mechanism of density stratification in Isahaya Bay.
17. Cell division of bloom-forming dinoflagellate Akashiwo sanguinea in the Ariake Sea, Japan.
18. A NUMERICAL SIMULATION OF PHYSICAL ENVIRONMENTS IN THE ARIAKE SEA INCLUDING THE EFFECTS OF THE TIDAL FLAT.
19. Variation of Water Quality due to the location of a Mega-Float in Tokyo Bay.
20. A Numerical Study of Physical Environments in the Ariake Sea.
21. A NUMERICAL MODEL FOR WATER QUALITY AROUND THE MEGA-FLOAT IINCLUDING THE EFFECTS OF MARINE MUSSEL IN TOKYO BAY.
22. Variability of Water Quality Due to the Location of a Maga-Float in Tokyo Bay.