There are lots of dredged hollow places in Osaka Bay, which had been dredged for reclamation at industrial areas in 1960's.and 1970's.The dredged hollows make water movement stagnant and water quality worth.Dissolved oxygen is deficient near the bottom. In recent years, several times of “blue tide” which are the oxidation of hydrogen sulphide following the upwelling of anoxic water have been observed.It is not clear how much contribution such hollow places makes to declining water quality in the head of the bay. Field surveys of currents and water quality are carried out. An average current velocity is 4 cm/s in the hollows as same as that of the enclosed port area. There is not a clear difference between the water quality in the hollow places and that around the places

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Distributions of dissolved oxygen and phosphorus concentrations were measured in a harbor area along the north coast of Osaka Bay. An anoxic water mass existed within a shallow depression in Koshienhama canal, where surfacing of anoxic water frequently occurs. Bottom anoxic water appeared beneath the surface chlorophyll maximum layer in a channel where sewage flows into. Total phosphorus concentration is higher in the upper layer than that in the lower layer, in contrast to total nitrogen concentrations. This suggests desorption of phosphorus from the bottom sediment. To pretend generation of hypoxic water, a countermeasure which uses buoyancy of river water was considered.

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港内に大きな埋立地と複雑な水路をもつ大阪湾奥部北部港湾域において流動観測を実施した. 港湾域の沖合の流動構造と同様に, 港内では, 上げ潮時には西から東へ, 下げ潮時には東から西への流れが生起される. 南側開口部では, 潮流の影響を受け, 西から東へと時間的なずれを生じながら, 全層で流出入することが明らかになった. また, 並行して実施した3次元流動シミュレーションの結果, 港内の流動は多層構造をもち, 淡水流入とそれによる密度成層が港内の流動構造に大きな影響を与えていることがわかった. 最後に, 3次元流動水質モデルを用い, これら港内の流動がDOの収支にどのような影響を及ぼしているかについて評価した。

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　都市沿岸域では長年にわたり水質改善施策が講じられてきたが，港湾域のような閉鎖性の高い海域では，いまだ十分な水質の改善はみられない．このような高閉鎖性海域の水質改善には，その海域の特性を捉えた水環境の改善施策や技術開発が必要と言える．本研究では，新たな改善策として，発電所などの沿岸事業場からの排水を利用した流動制御の可能性について定量的な検討を行った．大阪湾奥部に位置する発電所を例に，取放水の方法や水量を変えた流動シミュレーションを実施した結果，対象とした港湾域の流動は潮流に強く支配され，大きな流況改善効果は認められなかったが，残差流系を考慮した取放水方法により，港湾スケールの水交換を促進させることが可能であることが示唆された．

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Water mass which is deficient in oxygen spreads in the Port of Amagasaki-Nishinomiya -Ashiya in the head of Osaka Bay from April to November. In the past two years field survey of dissolved oxygen has been carried out in the port, but the detailed dynamics of the oxygen-deficient water is not clear totally in the head of the port. In the present paper a new field survey of DO and nutrients is conducted in small water areas around the reclaimed islands in the head of the port. The water areas are divided into two types, that is, the small and enclosed areas in the head and the other area in the port. In the head of the port excess production of phytoplankton increases, and the great deal of organic nutrients makes the concentration of DO very low in spring. This oxygen-deficient water spreads out to the other water area in early summer. The behavior of the oxygen-deficient water in each area is linked together in high summer.

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Water near sea bottom in the head of a bay into which a lot of nutrients flow becomes deficient in oxygen during summer. The characteristics of the anoxic water are different between in the whole bay and at the enclosed area in the head of the bay. The vertical difference of temperature and DO (dissolved oxygen) in a bottom layer are correlative in the whole bay, and not so at the enclosed area. It is because there is excess consumption of oxygen at the enclosed area, which is influenced well on by the water temperature under a thermocline. In the present paper the factors which have influence on the development and decline of the anoxic water are discussed. When the anoxic water mass develops early in summer, the development of that depends on the temperature of the middle layer close under the thermocline. The higher temperature leads more consumption of oxygen in the process of the degradation of detritus. This consumption of oxygen is more important factor of the dynamics of anoxic water at the enclosed area than in the whole bay.

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沿岸域における流動の再現性を向上すべく，流動モデルを高度化する1つの方法としてデータ同化を用いた．データ同化には，海洋モデルや大気モデルでは比較的実施例の多い4次元変分データ同化法（4D-Var）を採用し，密度差が大きく，潮汐が流動を支配する沿岸域での4D-Var の利用可能性，再現性の向上効果を検討した．HF 海洋レーダによる表面流速データならびに大阪湾水質定点自動観測システムによる水温，塩分，流速の4次元データの同化により，表面流速分布および密度構造の再現性が改善し，特に密度躍層の水深の再現が明瞭に改善することが認められた．また，密度成層の改善が水質予測にも効果をもたらすことが明らかとなった．ただし，同化ウィンドウが1時間とかなり短いことなど，4D-Var の沿岸域への適用には依然課題が残っている．一方で，今回の計算結果からは，明石海峡西流時に海峡付近の流動において課題を残していることなど，4D-Var により，モデルや計算条件における課題の抽出が可能であることが示唆された．

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