**4. Plume modelling on Cesenatico (Italy) discharging area**

The hydrodynamic model described above has been used to simulate the evolution of the plume originating from the freshwater discharge from the harbour canal. The bathymetry and the geometry of the breakwaters and structures were modelled by a small mesh dimension. The regular mesh, shown in Fig. 3, was made by 144x170 grid points, with cell dimensions 12 m x 12 m. On the same figure it is possible to recognize the shoreline, the harbour canal (points P2, P3 and P4) and coastal defence structures, parallel to the shoreline. The points on the same Fig. 3 are the profile points where measurements, later described, have been carried out.

Fig. 3. Investigation area, calculation mesh utilized in model simulations and location of the fixed investigated points.

Freshwater Dispersion Plume in the Sea: Dynamic Description and Case Study 137

A typical summer condition is shown in Fig. 4, the hydrodynamic and dispersion is forced by the freshwater outflow and by the tidal excursion at the offshore boundary. Unfortunately field data are not available for this condition, but only for different scenarios

Fig. 4A presents the results of a simulation carried out in the absence of coastal surface current and wind velocity lower than 1 knot. Simulation conditions are representative of the cycle of freshwater outfall in which tide, according with internal basin storage volumes, provides outgoing velocity from the channel mouth starting from 10.00 a.m. and ending 18 hours later at 4.00 a.m. The physical feature of the presented simulation is characterized by a first low decreasing tidal phase and low outgoing velocity typical of the last summer

Fig. 4B. Sea water level at the offshore boundary during simulation with results in Fig. 3.

Here, in the early afternoon, variations in salinity and phytoplankton biomass are limited and restricted to the near mouth area and the surface thermoaline profile could be conditioned by wind coastal waves. Evening and nightly scenarios show static conditions for coastal sea with very low current and undefined direction, while the most part of freshwater accumulated in the internal basin is outfalled from the mouth according to the maximum tidal decreasing phase. Thermoaline stratification is guaranteed, such as in internal harbour section as in the receiver coastal sea. The simulation period shown in Fig. 4 (12h-15h-18h) covers the main decreasing tidal phase, when most freshwater, coming from WWTP and confined into the internal channel according with tidal phase, is completely discharged through the harbour canal. Evident stratification conditions are represented in coastal sea away from the breakwaters, such as in the north and south zones. The maximum decrease in sea salinity concentrations is evaluated in 7-8 g/l within the south breakwater confined shore area near the south embankment. In this zone, water volumes flowing through restricted breakwater mouths permit higher incoming surface velocity and low depth permits near the beach vertical mixing and a more homogeneous areal distribution.

<sup>0</sup> <sup>5</sup> <sup>10</sup> <sup>15</sup> <sup>20</sup> <sup>25</sup> -25

time [hours]

periods. The tidal excursion at several tidal phases is shown in Fig. 4B.

commented in the later sections.





swl [cm]

Fig. 4A. Example of simulation results of the freshwater plume dispersion in different tidal phases plotted in Fig. 4B.

[m]

[m]

200 400 600 800 1000 1200 1400 1600

200 400 600 800 1000 1200 1400 1600

200 400 600 800 1000 1200 1400 1600

[m]

[m]

18h

[m]

12h

6h

[m]

200 400 600 800 1000 1200 1400 1600

200 400 600 800 1000 1200 1400 1600

200 400 600 800 1000 1200 1400 1600

[m]

[m]

15h

[m]

9h

3h

[m]

[m]

[m]

Fig. 4A. Example of simulation results of the freshwater plume dispersion in different tidal

phases plotted in Fig. 4B.

A typical summer condition is shown in Fig. 4, the hydrodynamic and dispersion is forced by the freshwater outflow and by the tidal excursion at the offshore boundary. Unfortunately field data are not available for this condition, but only for different scenarios commented in the later sections.

Fig. 4A presents the results of a simulation carried out in the absence of coastal surface current and wind velocity lower than 1 knot. Simulation conditions are representative of the cycle of freshwater outfall in which tide, according with internal basin storage volumes, provides outgoing velocity from the channel mouth starting from 10.00 a.m. and ending 18 hours later at 4.00 a.m. The physical feature of the presented simulation is characterized by a first low decreasing tidal phase and low outgoing velocity typical of the last summer periods. The tidal excursion at several tidal phases is shown in Fig. 4B.

Fig. 4B. Sea water level at the offshore boundary during simulation with results in Fig. 3.

Here, in the early afternoon, variations in salinity and phytoplankton biomass are limited and restricted to the near mouth area and the surface thermoaline profile could be conditioned by wind coastal waves. Evening and nightly scenarios show static conditions for coastal sea with very low current and undefined direction, while the most part of freshwater accumulated in the internal basin is outfalled from the mouth according to the maximum tidal decreasing phase. Thermoaline stratification is guaranteed, such as in internal harbour section as in the receiver coastal sea. The simulation period shown in Fig. 4 (12h-15h-18h) covers the main decreasing tidal phase, when most freshwater, coming from WWTP and confined into the internal channel according with tidal phase, is completely discharged through the harbour canal. Evident stratification conditions are represented in coastal sea away from the breakwaters, such as in the north and south zones. The maximum decrease in sea salinity concentrations is evaluated in 7-8 g/l within the south breakwater confined shore area near the south embankment. In this zone, water volumes flowing through restricted breakwater mouths permit higher incoming surface velocity and low depth permits near the beach vertical mixing and a more homogeneous areal distribution.

Freshwater Dispersion Plume in the Sea: Dynamic Description and Case Study 139

wave height HS was lower than 0.3 m for the whole day. The measured sea water level and wave conditions on the day of the experiment are plotted in Fig. 6A. The weather conditions were very mild, without wind and with ascending tide, so we had the opportunity to monitor a condition driven only by the tidal excursion. Figure 6B shows the swl during the experiment and the contemporary velocity and direction of the drifters launched 1 km

A

B Fig. 6. A) Measured swl (top panel), significant wave height (HS), direction and period (TP). B), drifters' velocity (top panel), direction (central panel) and contemporary swl (bottom

Clusters of three drifters were launched simultaneously at the offshore boundary. The launch position of the drifters is the offshore location in Fig. 7A. The first cluster was launched at about 9:00 a.m. just offshore from the harbour breakwaters, at a distance of 1.2 km from the beach, the second cluster was launched one hour later offshore from the northern beach and the last cluster was launched at 11:00 am offshore from the southern beach. The velocity and direction of the drifters during the experiment is plotted in Fig. 6B. The mean drifter velocity during the experiments was 0.18 m/s, with a direction

offshore from the Cesenatico harbour canal.

panel).

perpendicular to the beach.

The results also reveal different effects on plume areal dispersion and on thermoaline profiles between zones confined by continuous breakwaters (north shore) and by discontinuous breakwater (south shore). Comparing salinity vertical distribution in internal and external points of the north continue breakwaters, under a surface layer (50-60 cm) almost corresponding to breakwater submergence (Lamberti et al., 2005), differences in salinity and oxygen profiles become significant. Freshwater dispersion appears obstructed in the internal north confined area because continuous breakwaters produce a "wall effect" for incoming plume with mass exchange reduced for deep layers. Here, in the absence of north directed sea currents, flows are allowed only from north-south boundary mouths with vertical mixing limited to the surface layer.
