A map of the study area is shown in Fig. 2. The S. Jacinto outfall is located off the Portuguese west coast near the Aveiro estuary. The total length of the outfall, including the diffuser, is 3378 m (the first 3135 m section having a diameter of 1600 mm and the last 243 m section having a diameter of 1200 mm). The diffuser consisting of 72 ports alternating on each side, nominally 0.175 m in diameter, is 332.5 m long. Currently, only the last 20 of the 72 ports are working in a length of 98.2 m. These are discharging upwards at an angle of 30° to the horizontal axis, being the port height about 1.3 m. The outfall has a true bearing direction of 290° and is discharging at a varying depth approximately between 14 and 17 m.
The sea floor near the diffuser has a moderately sloping, sandy bottom with isobaths oriented parallel to the coastline. In that area the coastline itself runs at about a 200° angle with respect to true north. Flow variation through the outfall in question is not typical of WWTPs since the effluent is mainly of industrial origin. Effluent flowrate ranges most frequently between 0.6-0.8 m3/s.
During the campaign the discharge remained fairly constant, with an average flowrate of approximately 0.61 m3/s.
The remainder of the chapter is outlined as follows. Section 2 describes the AUV near field monitoring mission to S. Jacinto outfall, focusing on the near-field transects and the ambient conditions. Data processing and results in terms of plume dispersion and dilution estimation are presented in Section 3. Finally, a brief summary of the main conclusions is presented in Section 4.
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