**6. Conclusions**

148 Hydrodynamics – Natural Water Bodies

As expected, oxygen values averaged at each section (Fig. 15) increase, proceeding from internal to external points. At P1 and P2 profiles, photosynthesis produces maximum values in a 60 cm surface layer. At the P3 point (internal but near the mouth), a strong influence of external sea water on bottom layers is confirmed, which shows the same oxygen value, while at surface layers values are typical of internal waters. No information about plume dispersion could be obtained at external points where oxygen distribution is characterised by classic coastal sea profiles with oxygen decreasing values in the direction of deeper layers

Results of simulated salinity concentration (Fig. 16), similar to those presented in Fig. 4B, indicate a northerly oriented freshwater dispersion, different from the case analysed in Fig. 4B, which presents in the first phases a less oriented dispersion plume and during the following times (hour 15 – 18) a prevailing orientation to the southern coastal zone. In the actual case, the plume is west bounded by the continuous breakwaters, this means that the geometry is well reproduced in the model, and is dispersed to the north, for the effect of the

where photosynthesis is low and bacterial consumption increases.

wind, which was negligible in the previous examined condition.

Fig. 16. Simulation of the freshwater plume dispersion during experiment II.

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A freshwater dispersion plume in the sea has been described in depth in the present paper with the aim of producing a 3D numerical model and with the validation of two field campaigns carried out in different conditions. The investigated area concerns the coastal zone near Cesenatico (Adriatic Sea, Italy). The fresh water is dispersed by the canal harbour mouth into the open sea.

The model shows good performance in the application here presented, which is characterised by the presence of complex sea structures, requiring a very detailed and small mesh dimension in the geometry description.

Field data were acquired during two field campaigns and are of different typology: surface lagrangian paths, acquired by innovative properly designed drifters (in both campaigns); vertical profiles of temperature and salinity and dissolved oxygen acquired by a multiparameter probe in properly defined fixed points (in the second campaign). During the first campaign the hydrodynamic was driven only by the tidal oscillation and during the second also by surface wind, the tested conditions were therefore different and interesting for understanding the complex dynamics.

Comparison between model results and measurements are good for the surface hydrodynamic description and for the areal and vertical distribution of concentration, in particular, the resulting salinity values compared with experimental data have shown a surprisingly good agreement.

During the second experiment the presence of biological aggregates and foams was observed on the sea surface interested by the plume. The presence of biological traces in sea areas interested by freshwater dispersion is a well known phenomenon.

Vertical measurement of thermoaline parameters shows appreciable variations on salinity vertical distribution in the southern zone, where measured values appear very similar in the south near mouth and offshore sea. On the contrary, at the northern zone the vertical profiles present a salinitydistribution which reveals the arrival in the surface layers of volumes coming from the mouth section enriched by internal freshwater. A difference of 2 g/l between bottom and surface layers with thermocline from depth of 60 to 120 cm is registered. Similar behaviour was observed for temperature. In fact in the north the temperature profile presents lower values in surface layers (25.6°C) than in the underlying thermocline (26.4 °C), but inversion does not interrupt stratification which is maintained by variation in density. At thermocline depths a temperature decrease is appreciable due to the colder masses stored at the bottom of the harbour canal.

The points, interested by the dispersion plume, showed a pH vertical profile similar to temperature profile. Low pH values usually indicate biological organic substance degradation or nitrification phenomena typically active in waters of internal channels receiving wastewater. In N1 near the mouth point, higher values are confined in a 1 metre thickness layer, sited at a 1 metre depth. On this layer lower pH values confirm the presence of a plume conditioned by freshwater also indicated by a lower temperature.

The methodology proposed in this paper appears to be useful and accurate enough to simulate the dynamics of the freshwater dispersion at the investigated scale.

The results here presented are original and have allowed a general comprehension of the thermoaline and hydrodynamic assessment of the dispersion area.

The model now validated can in the future be applied to investigate the dispersion in other meteo climatic conditions, tides and other canal mouth geometries.

Freshwater Dispersion Plume in the Sea: Dynamic Description and Case Study 151

Jouanneau J. M. and Latouche C. (1982). Estimation of fluxes to the ocean from mega tidal

Kourafalou V.H., Lee T.N., Oey L.-Y. and Wang J.D. (1996). The fate of river discharge on

Lamberti A., Archetti R., Kramer M., Paphitis D., Mosso C., Di Risio M. (2005). European

Liu S.K. and Leendertse J.J. (1978). Multidimensional numerical modelling of estuaries and

Mestres M., Sierra J.P., Sánchez-Arcilla A. (2007). Factors influencing the spreading of a low-

Mancini M.L. (2008). Wastewater finishing by combined algal and bacterial biomass in a

Mancini M.L. (2009). Wastewater discharge and thermoaline conditioning in south

Mestres M., Sierra J.P., Sanchez Arcilla, A., Del Rio, J.G., Wolf T., Rodriguez A. and Ouillon

Molleri G. S. F., De M. Novo E. M. L., Kampel M. (2010). Space-time variability of the

Nezlin G.P and DiGiacomo P.M. Satellite ocean color observations of stormwater runoff

Ogston A. S., Cacchione D. A., Sternberg R. W., Kineke G. C. (2000). Observations of storm

O'Donnell J. (1990). The formation and fate of a river plume: a numerical model. *J. Phys.* 

Sherwin T. J., Jonas P. J. C. and Sharp C. Subduction and dispersion of a buoyant effluent

Siegel H., Gerth M. and Mutze A. (1999). Dynamics of the Oder River plume in the southern

*Continental Shelf Research*, 25,(14), 1692-1711. ISSN: 0278-4343.

23:29. ISSN: 0018-8158.

1978. ISSN: 0065-2768.

Vol. 52.(10-11), 841 - 866 ISSN 0378-3839.

*ROMA, ANDIS*, 2008, pp. 50/1 - 50/8

(3-4). 342-352. ISSN: 0278-4343.

10-14 November 2009. vol. 1, 143/1 - 143/7.

*Scientia Marina*. 67 (4). 379 – 391. ISSN 0214-8358.

shelf. *Continental Shelf Research*, 20, (16), 2141-2162.

*Oceanogr*. 20, 551-569. ISSN: 1520-0485.

1997. ISSN 0025-326X.

(1143 – 1159). ISSN: 0278-4343.

0148-0227.

4343.

estuaries under moderate climates and the problems they present. *Hydrobiologia*, 91.

the continental shelf, 2. Transport of coastal low-salinity waters under realistic wind and tidal forcing. *Journal of Geophysical Research* 101 (1996), 3435–3455. ISSN

experience of low crested structures for coastal management. *Coastal Engineering*.

coastal seas, *Advances in Hydroscience*, Vol. 11, Academic Press, New York (USA),

discharge river plume. *Continental Shelf Research*, 27, (16-15), 2116-2134. ISSN: 0278-

tidal flow channel. Modeling and field experiences in Cesenatico. *International Symposium on Sanitary and Environmental Engineering-SIDISA 08 -Proceedings,* 

Cesenatico (I) coastal area near breakwaters. *Proceedings of the Ninth International Conference on the Mediterranean Coastal Environment.-MEDCOAST 09.* Sochi-Russia.

S. (2003). Modelling of the Ebro River plume. Validation with field observations.

Amazon River plume based on satellite ocean color. Continental Shelf Research 30

plumes along the San Pedro Shelf (southern California) during 1997–2003. (2005).

and river flood-driven sediment transport on the northern California continental

plume in a stratified English bay. *Marine Pollution Bulletin*, Vol. 34, No. 10, 827-839,

Baltic Sea: Satellite data and numerical modelling. *Continental Shelf Research* 19
