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MWWD 2008 5th International Conference on Marine Waste Water Discharges and Coastal Environment Cavtat Croatia, Oct. 2

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MWWD 2008 5th International Conference on Marine Waste Water Discharges and Coastal Environment Cavtat Croatia, Oct. 2

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    1. MWWD 2008 – 5th International Conference on Marine Waste Water Discharges and Coastal Environment Cavtat (Croatia), Oct. 27-31, 2008 Mirella Di Giovanni, Salvatore Nicosia, Enrico Napoli, Gaspare Viviani, Giuseppe Ciraolo Interpreting and modelling field data for wastewater dispersion into sea through dimensional analysis

    2. The area investigated: 25 measuring stations distributed over 1 km2 sea surface in front of the city Di Giovanni et al., DIIAA Palermo (Italia) 2

    3. The urban area, source of the wastewater discharge- still raw at the time (year 2005) Di Giovanni et al., DIIAA Palermo (Italia) 3

    4. Among the outputs of the Department’s research: thematic maps as overall pictorial view of the seawater quality Di Giovanni et al., DIIAA Palermo (Italia) 4

    5. The hydraulic behaviour of water discharged into sea: “plume” or “jet” at the issue. By definition… Di Giovanni et al., DIIAA Palermo (Italia) 5 a simple jet is driven into the marine environment by the initial value of its momentum at the outfall only; a simple plume, instead, has no initial momentum, but moves into sea due to its tendency to buoyancy.

    6. The physical quantities playing a role in jet dynamics are assumed to be 5: A, the cross-sectional area of the jet; u, the time-averaged jet velocity in the axial direction; ?, the density of the fluid discharged; q, volume per unit time or volume flux of the jet; m, the specific momentum flux. Di Giovanni et al., DIIAA Palermo (Italia) 6

    7. (?) A physical quantity is called specific when its ordinary definition is modified dividing it by the fluid density, ?. Di Giovanni et al., DIIAA Palermo (Italia) 7

    8. Three relationships link together the factors of major importance For the mass flux of the jet, which is the mass of fluid passing a jet cross section per unit time: For the momentum flux, which is the amount of momentum passing a jet cross section per unit time: For the buoyancy flux ? the buoyant or submerged weight of the fluid passing through a cross section per unit time: Di Giovanni et al., DIIAA Palermo (Italia) 8

    9. The initial values of - specific mass flux or “volume flux” q; - specific momentum flux, m; - and specific buoyancy flux, ?… Di Giovanni et al., DIIAA Palermo (Italia) 9

    10. Almost all of the properties of turbulent jets that are of importance to engineers can be deduced… Di Giovanni et al., DIIAA Palermo (Italia) 10

    11. Example: the jet mean axial velocity, um , depends on Q, M, and the ratio of the distance from the outfall (z) to the characteristic length scale lQ Di Giovanni et al., DIIAA Palermo (Italia) 11

    12. The same procedure could be applied e.g. with the aim to find a relationship for … the volume flux q at any distance along the trajectory, or the mean concentration C of a substance of interest (tracer).

    13. Using this definition… Di Giovanni et al., DIIAA Palermo (Italia) 13

    14. Summary of analytical solutions for simple jets and plumes Di Giovanni et al., DIIAA Palermo (Italia) 14

    15. The basis for an analytical model, summarized

    16. Outfall confi_ guration and discharge data

    17. Predicting the wastewater dilution into the marine environment In this case study, the initial value of momentum was very low (? 410 N), so it was possible to define the discharge as a simple plume.

    18. Once defined the behaviour of the fluid discharged into the sea… … under the assumption of Gaussian distribution of concentration across the plume axis it was possible to calculate the value of the most important variables characterizing the plume, such as velocity, buoyancy and concentration of any tracer released within wastewater, aiming at predicting the dilution. Salinity - a conservative tracer - was chosen as indicator for calculations.

    19. The calculations, 1 Local salinity value in the plume ? weighted average between the wastewater and the seawater entrained. The plume we are dealing with originates from a surface port; thus the formula for the rate of flow becomes where: the cross section A is replaced by the product of distance from plume axis, x, times the sea depth, H; bu is the spreading coefficient, the parameter typical of the Gaussian concentration distribution.

    20. The calculations, 2 For the spreading coefficient empirical studies suggest to use the relationship The solution of the integral above, then, yields q(z,x): After these steps, salinity can be calculated at any point of measure:

    21. Flow chart of the calculations

    22. Verifying the results

    25. Concluding remarks

    26. Acknowledgements

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