Water pollution mechanisms caused by bottom sediments in limnology and coastal waters and methods for improving water environments

本研究探讨了不同水域底部沉积物造成水污染的机制，并探讨了改善这些水域水环境的方法。主要研究结果总结如下。从理论和实验两方面研究了底泥与上覆水之间扩散传质的影响。建立了光滑河床、粗糙河床和矩形凹区湍流三种泥沙/水界面垂直扩散通量的计算模型。此外，为了研究这些床型上的流动结构，利用PIV测量了凹处的湍流流动，并采用小波变换和适当的正交分解进行了分析。从实验和理论两方面研究了恒定浮力倾斜边界产生的羽流。建立了假设自相似流动结构的理论模型。成功地得到了浮力场和速度场的解。分析结果与实验结果吻合较好。分析使我们能够定量地估计水库中羽流引起的水质迁移。为了弄清河口营养盐的循环规律，在大阪湾地区进行了野外观测。结果表明，上游供给的总磷和总氮有20%沉降到下游，大部分有机物从河口沉降到lOknroflshore地区。人们已经认识到，反硝化现象在河口氮的去除中起着重要作用。通过野外资料调查了舞科滩涂的潮流特征。由于压力梯度的影响，东向流较为突出，对输沙起主要作用。在此基础上，提出了一种模拟倾斜床剖面爬坡的数值方法。

This research deals with mechanisms of water pollution caused by bottom sediments in various waters and examines methods for improving the water environments in these waters. Main results are summarized as follows.The effect of diffusional mass transfer between bottom sediment and overlying water were investigated theoretically and experimentally. The models to estimate the vertical diflusive flux at the sediment/water interface were formulated for smooth bed, rough bed and turbulent flow in a rectangular concave. Moreover, to investigate flow structures over such bed forms, the turbulent flows in the concave were measured by using a PIV and analyzed by employing a wavelet transform and a proper orthogonal decomposition.A plume generated from a sloping boundary of constant buoyancy was investigated experimentally and theoretically. A theoretical model was developed assuming a self-similar flow structure. Solutions for buoyancy and velocity fields were successfully obtained. The agreement between the analysis and experiments was quite satisfactory. The analysis enables us to quantitatively estimate water quality transport caused by the plume in a reservoir.To clarify the cycling of nutrient salts in river estuary, field observations were carried out in Osaka Bay area. Results show that 20% of the amounts ofTP and TN supplied from the upstream have settled in the lower reaches of the river, and that most of the organic substances have settled in the area of lOknroflshore from the river mouth. It has been understood that the denitrification phenomenon is playing an important role of a removal of nitrogen in the estuary.The characteristic of tidal current at Maiko beach was investigated through field data. The eastern direction flow due to the pressure gradient was prominent there and it had a major effect to sediment transport. Then, A numerical method to simulate a run-up over a sloping bed profile was proposed.