Fortnightly and monthly variability of the exchange through the Strait of Gibraltar

The fortnightly and monthly variability of the exchange through the Strait of Gibraltar has been studied from two simultaneous five-month long moored datasets, at Camarinal Sill and the East Section. The study focuses on the M_sf and M_m tidal components and their role for the subinertial exchange. A significant monthly signal is observed in the upper layer transport. Also, a significant fortnightly signal is observed in the lower layer transport, which minimum (maximum flow toward the Atlantic) takes place approximately on spring tides. In consequence the net transport has both signals, with maximum taking place during neap tides and a small monthly inequality. Fortnightly and monthly variability in the interface depth is also observed at Camarinal Sill, the interface being deeper on neap and shallower on spring tides. At the East Section the interface depth signals are not significant.The subinertial variability of the transports is separated in two contributions. The first one is called quasistatic transport and arises from the subinertial fluctuations of currents. The second contribution, called tidally rectified transports, arise from the non-linear correlation of currents and interface depth at tidal frequencies. The tidally rectified transports are important at Camarinal but not at the East Section. An apparent contradiction between the fortnightly signals of the subinertial currents and subinertial transports is resolved when the fortnightly signal of the tidally rectified transports are considered. The fortnightly signal of the quasistatic and tidally rectified transports mutually cancel in the upper layer, but not in the lower layer where the rectified transports dominate. A simple model for the spring-tide mixing forcing accounts for the fortnightly variability of the lower layer quasistatic transports but underestimates it for the upper layer. Finally, the observed lower layer transport is compatible with the hydraulic control condition at Camarinal Sill except for certain periods during intense spring tides.     

长江口水动力及污水稀释扩散模拟

利用Delft3D数学模型对长江口水动力条件、上海市现有及拟建排放口污水排放的稀释扩散场进行了模拟。模型利用潮汐数据和排放口附近的现场实测数据进行了率定和验证，分别对丰水期、枯水期的大、小潮情况下的流速场、潮位场及污水扩散场进行了研究。结果表明：长江口的水动力条件有利于污水的稀释扩散；现状排放量对长江口水环境影响不大；但如果污水在排放前不进行一定的处理，规划排放量排人长江口将会严重恶化其水质，特别是枯水期小潮时最为严重。建议根据长江口和杭州湾的环境容量，结合排放口的稀释扩散能力，合理确定污水排放方案。