**3. Physical features of the case study**

The study site of the present survey is the pulmonary system of Cesenatico canal harbour (Northern Adriatic Sea, Italy) and the near coastal zone (Mancini, 2009). An aerial view of the canal harbour in Cesenatico is shown in Fig. 1A. During summer and in dry weather conditions the main part of discharged freshwater comes from waste water treatment plants (WWTP) and from drainage pipe systems. Treated and untreated wastewaters reach the canal with high hourly variations and the sea outlet provides regulated discharge into sea according to unsteady tidal flow. Generally the harbour canal, having a very low ground slope, guarantees a good thermoaline turbulent mixing, for the entire water column, if the basin structure presents a high pulmonary surface/wastewater loading ratio. Small estuaries with higher ground slope, receiving sea water by tidal oscillations within a few hundred metres from the coast-line and high flow rate coming from WWTP, present flow primarily directed to the sea where freshwater continuously flows on the surface. Cesenatico canal harbour basin reveals a condition where the pulmonary area and freshwater input from inland (300.000 EI, Equivalent Inhabitants) produce in the canal mouth vertical profiles of velocity, turbulent thermoaline mixing and depth of the overflowing layer daily and hourly varying in function of tidal type and phase (Bragadin et al., 2009). The investigated area is characterized by a low constant slope of the bottom and starts from the canal harbour mouth towards to a 2000 m radius. The near coastal zone is characterized by submerged breakwaters in a northerly direction and by emerged breakwaters in a southerly direction. The discharging plume area and its vertical profiles of

with horizontal and vertical exchange eddy diffusivities Dx Dy and Ee were defined in a similar way to exchange mass coefficients. So energy sourcing into the grid is detected in the function of strain tensions induced by vertical velocity and the term of energy dissipated by shear stress at the bottom is calculated from energy in flux direction in lower level S and Chézy shear coefficient C. In the surface layer a wind effect generating turbulence by waves is considered in the higher part of the water column. Supposing constant motion for the sea,

*<sup>u</sup> S Le*

*e* 2 *<sup>e</sup> D a*

9 4 5.610 *E u t w*

where *u* is the mean velocity, De the dissipation coefficient, a2 a constant parameter taking account of energy transfer from high to small turbulence conditions and *uw* the wind

The study site of the present survey is the pulmonary system of Cesenatico canal harbour (Northern Adriatic Sea, Italy) and the near coastal zone (Mancini, 2009). An aerial view of the canal harbour in Cesenatico is shown in Fig. 1A. During summer and in dry weather conditions the main part of discharged freshwater comes from waste water treatment plants (WWTP) and from drainage pipe systems. Treated and untreated wastewaters reach the canal with high hourly variations and the sea outlet provides regulated discharge into sea according to unsteady tidal flow. Generally the harbour canal, having a very low ground slope, guarantees a good thermoaline turbulent mixing, for the entire water column, if the basin structure presents a high pulmonary surface/wastewater loading ratio. Small estuaries with higher ground slope, receiving sea water by tidal oscillations within a few hundred metres from the coast-line and high flow rate coming from WWTP, present flow primarily directed to the sea where freshwater continuously flows on the surface. Cesenatico canal harbour basin reveals a condition where the pulmonary area and freshwater input from inland (300.000 EI, Equivalent Inhabitants) produce in the canal mouth vertical profiles of velocity, turbulent thermoaline mixing and depth of the overflowing layer daily and hourly varying in function of tidal type and phase (Bragadin et al., 2009). The investigated area is characterized by a low constant slope of the bottom and starts from the canal harbour mouth towards to a 2000 m radius. The near coastal zone is characterized by submerged breakwaters in a northerly direction and by emerged breakwaters in a southerly direction. The discharging plume area and its vertical profiles of

The model equations are discretized and solved into a finite-difference formulation.

velocity.

**3. Physical features of the case study** 

2

(16)

*<sup>L</sup>* (17)

*<sup>U</sup> S g <sup>C</sup>* (18)

(19)

*z* 

> 3 2

3 2

*Et* is the total energy generated for surface units in the function of wind velocity

thermoaline and quality parameters appear strongly conditioned by external waves and currents also in dry weather conditions. Recent monitoring investigations (Mancini, 2009) reveal vertical parameter profiles at mouth varying from an initial uniform vertical profile, also in correspondence with low tidal outgoing ranges. During these conditions, reinforced afternoon wind generates significant sea waves, which oppose the surface freshwater flow. On the contrary, during nightly major outgoing tidal phases, strong stratification is maintained in the mouth zone, as in the dispersion plume area facing the breakwaters. Morphologic, hydraulic and water quality measurements have been executed into the transition estuary of the harbour canal and near the mouth of adjacent breakwaters (a view of the breakwater is shown in Fig. 2B).

B **DEPTH (cm)**

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340

3,0

20,0

Fig. 1. A) Aerial view of Cesenatico. The bullets represent the position of the sample in Fig. 1B: from the city centre to the sea, respectively: Mariner Museum, Garibaldi Bridge, Vincian Ports and sea mouth. B) Salinity and oxygen profiles inside the harbour canal in the position plotted in nearby Fig. 1A.

Freshwater Dispersion Plume in the Sea: Dynamic Description and Case Study 135

and Garibaldi Bridge are located in the canal, approx. 500 m from the mouth, the Vincian Ports are located 50 m from the sea mouth. In the last tract close to the mouth a partial mixing with sea water is permitted, that increases salinity, oxygen and pH, according to the

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,

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

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

external sea conditions.

have been carried out.

fixed investigated points.

Other research activities, carried out on freshwater-sea water balance for the volumes of channels of the Cesenatico Port Canal system, (Mancini, 2008) indicated that wastewater, discharged in the internal zone, has a hydraulic retention time before sea dispersion, ranging between 1 and 3 days. As a consequence, there is a freshwater storage in the most internal parts of the channel during incoming tidal phases; on the other side during the outgoing tidal phases, there is the outflow of the main part of the internal storage freshwater. Fig. 2B shows monitoring data in correspondence with an internal channel section at 4 Km from the mouth. The selected section balance between saline waters introduced by tidal oscillations combined with wastewater flux incoming from inland maintains the salinity range within a 0-3 g/l. The figure also plots oxygen and redox, showing similar behaviour.

A

Fig. 2. A) Cesenatico northern coastal area characterized by submerged breakwaters. B) Daily behaviour of physical-chemical parameters measured in the internal section of the channel limiting transition volumes.

Before the outfall these outgoing volumes flow through the historical tract of the harbour which presents depths varying between 3 to 5 m. In this tract a typical overflowing volume upon the static higher density deep layers (Fig. 1B) can be observed. The Marinery Museum

Other research activities, carried out on freshwater-sea water balance for the volumes of channels of the Cesenatico Port Canal system, (Mancini, 2008) indicated that wastewater, discharged in the internal zone, has a hydraulic retention time before sea dispersion, ranging between 1 and 3 days. As a consequence, there is a freshwater storage in the most internal parts of the channel during incoming tidal phases; on the other side during the outgoing tidal phases, there is the outflow of the main part of the internal storage freshwater. Fig. 2B shows monitoring data in correspondence with an internal channel section at 4 Km from the mouth. The selected section balance between saline waters introduced by tidal oscillations combined with wastewater flux incoming from inland maintains the salinity range within a 0-3 g/l. The figure also plots oxygen and redox,

A

**OXYGEN - SALINITY - REDOX**  Visdomina bridge - measurements 19.07.07

redox

B

salinity

0,0 2,0 4,0 6,0 8,0 10,0 12,0 14,0 16,0 18,0 20,0

oxygen mg/l - salinity g/l

0.00 2.24 4.48 7.12 9.36 12.00 14.24 16.48 19.12 21.36 0.00 hours

oxygen

Before the outfall these outgoing volumes flow through the historical tract of the harbour which presents depths varying between 3 to 5 m. In this tract a typical overflowing volume upon the static higher density deep layers (Fig. 1B) can be observed. The Marinery Museum

Fig. 2. A) Cesenatico northern coastal area characterized by submerged breakwaters. B) Daily behaviour of physical-chemical parameters measured in the internal section of the

showing similar behaviour.

channel limiting transition volumes.

0,0 4,0 8,0 12,0 16,0 20,0 24,0 28,0 32,0 36,0 40,0

redox mV

and Garibaldi Bridge are located in the canal, approx. 500 m from the mouth, the Vincian Ports are located 50 m from the sea mouth. In the last tract close to the mouth a partial mixing with sea water is permitted, that increases salinity, oxygen and pH, according to the external sea conditions.
