Isahaya Bay Research Articles

諫早湾内の塩淡成層構造に及ぼす南北排水門からの排水の影響評価

The flow processes of drain from the north and south gates of the Isahaya reclamation dike and its contribution on salinity stratification in Isahaya Bay have been investigated by using a 3-D flow simulation model. The low salinity water mass by the drain is seen in the inner area of Isahaya Bay. It spreads thinly to the central area of Isahaya Bay and flows out to the Ariake Sea along the Shimabara Peninsula in about one tidal cycle. In a flood period, the freshwater from the drain gates decreased the average salinity in Isahaya Bay by about 0.6 PSU. On the other hand, the inflow of low salinity water from the Ariake Sea caused the decrease of about 8 PSU. The inflow of low salinity water from the Ariake Sea plays a dominant role in the salinity stratification in Isahaya Bay.

農用車両走行による諫早湾干拓土壌の構造と力学特性の変化

農用車両走行による諫早湾干拓土壌の構造と力学特性の変化

諫早湾口北部周辺の流動特性について

Ten years or more have passed since large-scale poor cultured laver harvests which arose in the head of Ariake Bay in 2000 winter.However, except for cultured laver, fish catches including a bivalve has decreased and does not show even a sign of recovery. When the drainage gate is opened over a long period of time, it is thought indispensable towards the Ariake Bay reproduction to investigate the change and influence of the aquatic environment of Isahaya Bay and Ariake Bay before and after opening the drainage gate.In this study, the field observation on flow regime at the northern mouth of Isahaya Bay was carried out under the stratified and mixing condition twice in 2010. As a result, structure of cavity flow appeared at the mouth of Isahaya Bay in all condition of both flood and ebb tide. Now, various flow regime has produced on various conditions at the northern mouth of Isahaya Bay, if potential flow will be recovered by opening the gate, it is expected flow regime changes a lot.

諫早湾潮止め後10年間の有明海における主な底生動物相の変化

諫早湾潮止め後10年間の有明海における主な底生動物相の変化

Study on water quality analysis in the reservoir of Isahaya Bay land reclamation project

Study on water quality analysis in the reservoir of Isahaya Bay land reclamation project

Changes of tides in Isahaya Bay due to a barrier

A regional ocean tide model was designed to cover an area that is broad in scope and size, yet also provide a high degree of resolution for scientific and engineering interests. Using this model study, the changes in the tidal regime in the Ariake Sea after the construction of the dike in the Isahaya Bay were simulated in terms of the tide, tidal current, bottom shear stress, tidal energy dissipation and transport potential and free oscillation mode. The simulation results indicated the impact of the dike construction on the tidal regime by showing the general decreases in the tide, tidal current, the capacities of sediment transport and free oscillation period due to the construction of the dike.

The Wind Response Analysis of Low Salinity Water Mass in Isahaya Bay

The mechanism of formation and the wind response of low salinity water mass in Isahaya Bay have been investigated by using the 3-D flow simulation model incorporating time-varying wind stress on the water surface. The northerly wind and the southerly wind are prevailing in Isahaya Bay and are repeated at the period of several days. The analytical results show that the change of the prevailing wind direction induces the appearance and disappearance of low salinity water mass. When the southerly wind blows, the low salinity water mass near the water surface is swept out Isahaya Bay and the salinity stratification is gone with the wind. On the other hand, the inflow of low salinity water mass to Isahaya Bay is accelerated by the northerly wind and the salinity stratification is formed.

Generation Mechanism of Upwelling current with "Aoshio" occurrence in summer of 2008 in Ariake Sea

Aoshio (the anoxic blue-green turbid water) was first observed in Isahaya Bay, Ariake Sea in August 15, 2008. Field data analysis and numerical experiments were made to clarify the generation mechanism of this Aoshio in Isahaya Bay. The field observations showed that the occurrence of Aoshio was associated with three characteristic phenomena, the intrusion of high density water (HDW) into the lower layer at neap tide just before the occurrence of Aoshio, rapid decrease of dissolved oxygen(DO) concentration, leading to the formation of anoxic water during the HDW intrusion and transportation of this anoxic water to the head of Isahaya bay due to south-west wind-induced current, followed by the vertical mixing at around spring tide. Numerical experiments could reproduce these phenomena with high accuracy and the generation mechanism was confirmed by the numerical experiments.

Numerical Simulation of Sediment Transport in Ariake Bay

Sediment transport in Ariake Bay is simulated by FVCOM-based numerical model. The sediment flux toward the head of the bay in upper Ariake Bay is reproduced by the model. Variations of sediment flux due to the growth of Chikugo River.'s plume and passing typhoon is also discussed. In addition, the effect of Isahaya Bay\. reclamation on the sediment transport process is discussed. Sediment flux around Isahaya Bay is significantly decreased and it may be the cause of the increasing transparency in lower Ariake Bay after 1990's.

An Analysis of Material Circulations in Sediment behind the Drain Gates of Isahaya Reclamation Dike

The environmental characteristics in seabed sediment behind the north and south drain gates of Isahaya Bay were investigated by sampling the undisturbed sediment cores. The vertical distributions of TOC, DO, AVS, NH4-N, NO3-N and PO4-P were obtained. The material circulations in the sediment at two sites were analyzed by using a sediment model. The model parameters were adjusted so that the analytical results agree well with the observed ones. The model analysis revealed that the fluxes of organic material and oxygen consumption to the seabed surface at St.2 were two times larger than those at St.1, respectively. These differences at St.1 and St.2 may be attributed to the current in Isahaya Bay.

Environmental Impact Assessment of the Reclamation Project in Isahaya Bay, Nagasaki, Japan

Environmental Impact Assessment of the Reclamation Project in Isahaya Bay, Nagasaki, Japan

諫早湾におけるラジウムの放射能比・底生有孔虫群集・重金属濃度の分布

諫早湾におけるラジウムの放射能比・底生有孔虫群集・重金属濃度の分布

Environmental Characteristics of Seabed Sediment in Isahaya Bay in Summer 2009

The environmental characteristics of seabed sediment in Isahaya Bay were investigated comprehensively in summer of 2009. The sediment was composed of the grain size from medium silt to fine silt. The value of IL was totally higher than 10 % and especially the organic pollution advanced near the both drain gates of the reclamation dike. The value of AVS was larger than the standard of water for fisheries and the sediment at the head of the bay was in the anaerobic condition. The velocity of the sediment oxygen consumption took the largest value near the south drain gate. The relationships of AVS, NH4-N, NO2-N+NO3-N and PO4-P versus ORP revealed that the seabed sediment had been in sulfate reduction and PO4-P had been released in the sediment.

Anthropogenic decline of the peculiar fauna of estuarine mudflats in Japan

Mudflats are formed in the upper littoral zones in estuaries, especially in the innermost part of macro-tidal embayments (upper reaches of estuaries), providing a habitat for peculiar fauna and flora. The peculiar fauna and flora of estuarine mudflats appear to have been extirpated in many estuaries in Japan, along with the mudflats that have been damaged or destroyed by thoughtless and drastic coastal reclamation. At present, Ariake Bay, which constitutes the largest area of mudflats remaining in Japan, also has the greatest variety of species. The mudflat-specific species are almost or completely restricted to Ariake Bay, while populations of the same (or closely related) species are distributed along Asian continental coasts, which suggests they are continental relicts. Some of the mudflat-specific species now restricted to Ariake Bay (e.g.,the bivalve, Tegillarca granosa, the polychaete, Hediste japonica, the salt marsh plant, Suaeda japonica) previously enjoyed a wide distribution in Japan. Even within Ariake Bay, huge mudflats in Isahaya Bay were recently lost to a reclamation project there. The high productivity of benthic organisms in mudflats seems to benefit carnivores such as birds and fishes (e.g., Japanese eel). Conservation and restoration of mudflats are needed to conserve endangered species and to maintain traditional human fisheries and culture. From this aspect, restoration of the largest mudflats in Isahaya Bay in Ariake Bay is highly desirable.

Influence of Fresh Water's Inflow upon Water Quality Dynamics in Isahaya Bay

Influence of Fresh Water's Inflow upon Water Quality Dynamics in Isahaya Bay

An Experiment for Preventing Mass Mortality of Cultured Short-neck Clams Due to Hypoxia Formation During Summer in Isahaya Bay

An Experiment for Preventing Mass Mortality of Cultured Short-neck Clams Due to Hypoxia Formation During Summer in Isahaya Bay

Environment-Friendly Agriculture in the reclaimed land of Isahaya Bay

Environment-Friendly Agriculture in the reclaimed land of Isahaya Bay

Numerical Experiments on Features of Nonlinear Tide and Its Influences on Sediment Transport in the Ariake Sea and the Yatsushiro Sea

We conducted numerical experiments on nonlinear tide and fine sediment transport in the Ariake Sea and the Yatsushiro Sea by Delft3D model. As a result of the present research the followings are clarified: i) Amplitude of S2 tide correlates inversely with M2 tide; ii) Maximum velocity of nonlinear tidal current is around 20cm/s near bay mouth area of both bays; iii) In the case excluding tidal flat sedimentation is enhanced; and iv) Variation of nodal factor f for M2 tide can change erosion/sedimentation characteristics around mouth of Isahaya Bay.

Perturbation of regional ocean tides due to coastal dikes

The tidal regime modeling system for ocean tides in the seas bordering the Korean Peninsula is designed to cover an area that is broad in scope and size, yet provide a high degree of resolution in coastal development areas, including the Saemangeum area in the eastern Yellow Sea and the Ariake Sea in Japan, where serious environmental problems have occurred after the completion of interior tidal dikes. With this simulation system, we have estimated the changes in tidal regime due to barriers at Saemangeum and Isahaya Bay in the Ariake Sea. Some results in terms of perturbations in tidal elevations due to the construction of coastal dikes are presented and discussed.

Changes of air temperature and meteorological environment at Isahaya Bay reclamation area

Changes of air temperature and meteorological environment at Isahaya Bay reclamation area