2.2.3. Index of Priority

In addition to establishing anti-erosion and coastal flooding measures, work must be prioritized on the basis of resources and needs, and a chronological list of operations must be drawn up. The index of priority for each cell is defined as:

$$IP = \sum\_{i} V\_{M\_i} \times \sum\_{i} V\_{SE\_{i'}} \tag{3}$$

where the morphological vulnerability index (*VM*) is the sum of the erosive tendency index and the risk of coastal flooding index; the socioeconomic vulnerability index (*VSE*) is the sum of six indexes specifying the coast value (Table 1). The scale used for every index goes from 1 to 4 (min *IP* = 12, max *IP* = 192). The minimum and maximum value of each index are based on the lowest and highest qualitative functional response among all the analysed cells with respect to the specific aspect.


**Table 1.** Description of terms VM and VSE in Equation (3).

Note that the index of priority is equivalent to the "risk" defined by Benassai [64] as the product between morphological vulnerability and socioeconomic vulnerability.

#### **3. The Venetian Littoral**

#### *3.1. Description of the Area and Assessment of Sediment Budget*

The Coastal Plan of the Veneto Region (IT) [23] (carried out according to the methods described in the previous Section) investigates the Venetian coastline (Figure 5), which is 160 km long and faces the Northern Adriatic Sea. The coastline's borders are the mouth of the River Tagliamento to the North and the mouth of the River Po di Goro to the South. It is subdivided into two provinces and ten coastal municipalities.

The Adriatic Sea is rectangular-shaped, is about 750 km long and 200 km wide, and is connected to the Mediterranean Sea at its Southern end by the Strait of Otranto, which is about 80 km wide. Its depth is rather limited in the Northern part, where the bottom descends south-eastwards with a 1 in 1000 slope. The Adriatic coast is plagued by a combination of high waves and storm surges, which are responsible for the flooding of coastal areas, in particular, Venice and its lagoon. The highest surge was on 4 November 1966 when the sea level rose approximately 180 cm above the mean sea level (MSL) and persisted above the 100 cm mark for more than 15 h (Canestrelli [65]).

The North Adriatic Sea is characterized by two main wind (and correspondingly wave) regimes, which are primarily influenced by local orography. The prevailing winds along the Venetian coastline are the Bora and the Scirocco, which blow from the North-East and South-East respectively.

The Venetian coastline is characterized by low beaches, lagoons (i.e., Caorle, Venice and Po River Delta) and the mouths of seven rivers: the Tagliamento, Livenza, Piave, Sile, Brenta, Adige, and Po.

Along the 100 km stretch of coast from the mouth of the River Tagliamento to the Porto Caleri inlet [23] lie a vast number of areas with a high tourist value (e.g., Bibione, Caorle, Jesolo, Lido di Venezia, Sottomarina). Many of them are protected by coastal structures (e.g., groins, seawalls, breakwaters), and few are free of urban settlements (e.g., Valle Vecchia). Only a few, mainly discontinuous, dune systems can be found along the coast because they were destroyed at various times in the past.

The remaining Venetian littoral comprises the Po Delta, which covers 610 km2 and has 60 km of coast stretching from the Porto Caleri inlet to the mouth of the River Po di Goro. The active river branches of the River Po are (from North to South) Po di Maistra, Po di Pila, Po di Tolle, Po di Gnocca and Po di Goro. The coastal fringe is characterized by a sequence of low sandy and vulnerable barrier islands, beaches and spits that separate lagoons, fishing valleys, bays, tidal flats and marshes from the sea. Inland, ground elevation is almost completely below sea level (locally −2.5/−3.0 m.s.l.), and consequently the risk of coastal flooding is very high. The morphological characteristics of the Po Delta make it Italy's largest wetland, as well as particularly unstable and very fragile when subjected to human pressure.

The Venetian coast is subdivided into 20 homogeneous littoral cells separated by inlets or the mouths of rivers (from North to South) Tagliamento, Bocca di Porto Baseleghe, Bocca di Falconera, Livenza, Piave, Sile, Bocca di Lido, Bocca di Malamocco, Bocca di Chioggia, Brenta, Adige, Bocca di Caleri, Bocca di

Porto Levante, Po di Maistra, Busa Tramontana, Busa Dritta, Busa di Scirocco, Busa Storiona, Po di Tolle, Po di Gnocca, and Po di Goro. These 22 limits are shown in Figure 5 and listed in Table 2.

**Figure 5.** Venetian littoral and its subdivision into coastal cells.


**Table 2.** Littoral cell boundary, sediment diameter (d50, mm) sampled at different depths and evaluated with the Dean rule (dEQ, mm).

The plan [23] is based on the information and data available for the Venetian littoral; they comprise offshore wave characteristics, sediment grain size, topographic and bathymetric surveys over a range of time (bathymetry: 2005, 2007/2008, 2010, 2012/2014, DTM: 2008, 2012/2013), subsidence rate (1992–2000 and 2002–2010), shoreline position (1983, 2000, 2003, 2012), flooding risk maps (from 2007/60/EC directive) and a catalogue of existing shore protection structures and nourishment/dredging carried out.

Nearshore wave conditions were evaluated using the SWAN model (Simulating WAve Nearshore, [66]), developed by Delft University of Technology (NL) and based on offshore wave data. Unfortunately, a spatially refined evaluation of the offshore wave statistics obtained by oceanographic models was not available, as it is still an ongoing project. Wave information was therefore obtained by existing WAM simulations forced by data from the European Centre for Medium-Range Weather Forecasts (ECMWF) between June 1992 and December 2008, and were restricted to two points in the Northern Adriatic sea (P1: Longitude 13◦00 Latitude 45◦00 , P2: Longitude 13◦00 Latitude 45◦30 , wave roses in Figure 6).

**Figure 6.** Offshore wave climate in the Northern Adriatic Sea: point P1 (left), point P2 (right).

The nearshore wave climate (−10 m depth) was obtained with the SWAN model. The SWAN transforms the directional wave spectrum, which cannot be fully described by a single small plot inside a regional map. For this reason, the energy has been integrated in frequency and its directional distribution is given in the figure. For instance, Figure 7 presents the energy distribution for the point P1 and P2, whose wave climate is fully characterised by the wind rose in Figure 6. Figure 8 presents the energy distribution obtained through the SWAN model, propagating the waves from P1 and P2. The Northern part of the Venetian littoral is mainly subject to waves from the South-East, with the Scirocco blowing along the main axis of the basin and acting on a much longer fetch in this zone. Due to shoreline orientation, the Northern part of the Po Delta is exposed to the Bora, which causes high waves, although fetch is limited, with wave periods ranging between 5 and 7 s. In the Southern part of the Po Delta, sea conditions are governed by both Bora and Scirocco wind and waves. Figure 8 clearly shows that the Po Delta is characterized by larger wave energy.

**Figure 7.** Offshore Energy polar plot relative to point P1 (left), point P2 (right) summarising the wave rose in Figure 6.

Sediment surveys for the Northern Adriatic Sea have been re-organized and stored in a Coastal GIS. Table 2 shows the average sediment size for each littoral cell at different depths. In general, sediment on the Veneto coast is fine sand, with grain diameter ranging between 0.12 and 0.25 mm. As expected, grain size is coarser near the shoreline and decreases seawards. Deviations occur is some places, e.g., RO8, where rivers transport fine sediment that may deposit in the nearshore zone.

Dean [67] proposed an equilibrium profile, *y = Ax2/3*, giving a relationship between water depth (*y*) and the distance from the shoreline (*x*) via parameter *A* that, according to Hanson and Kraus [68] (*A* = 0.41d500.94 for d50 <0.4 mm), is a function of the median diameter d50 (mm). The formulation can be inverted to start from the bathymetric profile, so that parameter *A* can be adapted, and the corresponding "equilibrium diameter" *dEQ* can be found*.* The result for each littoral cell is shown in the last column of Table 2 and can be compared with actual grain size at different depths. The value of *dEQ* is very useful since it provides an average bed-profile shape immediately.

Assessment of the river sediment transport is complicated by the almost complete absence of systematic hydrographic surveys. Therefore, numerical models based on what little information was available were used instead. In order to evaluate river sediment discharge at the mouth, Lanzoni [69] proposed a one-dimensional numerical model using topographic surveys, the annual hydrological regime, and a medium grain size. Considering steady forcing conditions, the model estimates a "formative discharge" that produces the river topography observed and the corresponding sediment transport capacity. This approach was applied to the main rivers in the Veneto Region (Tagliamento, Piave, Brenta, Adige, and Po).

**Figure 8.** Nearshore wave climate (energy polar plot) for the Venetian littoral. The scale of the energy plot is common to the three images, allowing a qualitative comparison.

The rate of long-shore sediment transport was based on the local computation of the wave and currents for each wave state with the formula proposed by Bijker [70], following the procedure pointed out in [71], integrating across the profile and averaging. Results are shown in Figure 9 and the method is described in [23]. The spatial pattern of the simulated net transport contains divergence and convergence areas that separate areas with oppositely directed net sediment fluxes. Divergence points are located in front of the mouths of the two main rivers (Adige and Po). Convergence points are at Bocca di Lido and Bocca di Caleri. The latter, placed between cell RO1 and RO2, is a highly persistent point of convergence for net transport and thus, as observed, a deposition area (volume ~100,000 m3/year). The most dynamic zone is the Po Delta area, which has a symmetric morphology with a divergence net sediment transport of ~200,000 m3/year.

**Figure 9.** Long-shore sediment transport for the Venetian littoral.

It was evaluated, by comparing few methods [72], that the cross-shore sediment transport *QCR* was heading offshore and approximately equal to 1 m3/km/year, except where the cell boundary element is a river mouth, where an additional contribution is assessed to simulate the sediment plume losses.

Subsidence along the coast is associated with natural causes related to the area's geological history (e.g., sediment consolidation) and with anthropogenic activities, mainly fluid withdrawal. An innovative technique called Advanced Differential Interferometric SAR (A-DInSAR [73,74]) was applied in order to measure the deformation of the Earth's surface. The subsidence in the Northern part is equal to 1–2 mm/year and is mainly related to natural causes. The subsidence in the Po Delta is much larger and ranges from 3–5 mm/year, with it being linked to both natural and anthropogenic causes.

Accumulation and erosion were measured up to the depth of closure to compare successive bathymetric profiles and calculate the volume of accretion and erosion. Details and methodology are given in [75].

After the evaluation of every source term, the sediment balance for each littoral cell was calculated with Equation (1). Each homogenous littoral cell was divided into two parts in order to better appreciate the coastal processes involved. Note that the long-shore sediment transport in the middle of the cell must be the same for each semi-cell. The long-shore sediment transport at the boundary between adjacent cells may differ on account of potential depositional or erosive areas surrounding inlets or river mouths.

The balance is solved by using a compensation of error technique based on a matrix of a priori uncertainties. The accuracy of each variable is weighted with a specific coefficient and the mass unbalance for each cell is subdivided among the estimated variables forming the budget based on their weight. The final sediment budget is summarized in Table 3: 19 semi-cells (total length ~58 km) have a depositional behaviour with a volume >10,000 m3/year; 15 semi-cells (total length ~62 km) have an erosive behaviour with a volume ≤10,000 m3/year; and 6 semi-cells (total length ~19 km) are almost stable, with a volume in the range of ±10,000 m3/year.


**Table 3.** Sediment balance (thousand m3/year).

#### *3.2. Design Phase*

Based on the sediment balance and on the criteria used to select the mitigation options (see Section 2), a coastal management proposal for every stretch of coast was carried out. For the sake of brevity, only a summary of the mitigation measures is presented here, and a detailed example of the RO1 cell is described in the next paragraph.

In the Northern zone, the goal is to "hold" the shoreline position and to protect shore-based activities using protection measures with minimum impact (e.g., avoiding seawalls, detached barriers, etc.). Management in this zone includes building a series of groins in cells VE4, VE5 and VE10 and adding large nourishments to cells VE4, VE7, VE8 and VE10 (volume = 3,650,000 m3). The global volume of sand needed for maintenance is approximately 385,000 m3/year.

A "Building with Nature" methodology is applied to the Po Delta area. Localized sand nourishment and dune reinforcement are nature-based defences that provide several ecosystem services, including flood/erosion risk mitigation and environmental conservation. The volume of sand needed for maintenance (nourishments and dunes) is approximately 145,000 m3/year.

A scheduled monitoring program was established across the entire Regional littoral in order to collect the data and information necessary.

The proposed index of prioritization was also applied to each cell to assess a chronological list of the operations based on the available resources. Figure 10 (top) shows the morphological and socioeconomic vulnerabilities along the littoral. The indexes reflect the urban essence of the area between the mouths of the rivers Tagliamento and Adige (where the main tourist activities—5,000,000 visitor/month in summer—are concentrated); the main cultural heritage sites (Venice, Caorle); and the natural essence of the Po Delta (an area with one of Italy's highest environmental values).

**Figure 10.** Vulnerability and index of priority.

Figure 10 (bottom) shows the index of priority, with the circles highlighting the 5 stretches of coast with the highest index of priority. For each stretch, the main issues are presented below.


#### *3.3. Example for RO1 Littoral Cell*

The Rosolina coastline (RO1 cell) is 8 km long and the normal shoreline direction is 80◦ N. The economy in the northern and central part is based mainly on tourism (1,100,000 visitors in 2016) and fishing in the backshore Caleri lagoon. The lagoon and the Southern part of the cell are major environmental areas, protected and designated as Natura 2000 sites (SCI IT3270017 and SPA IT3270023).

The cell is delimited to the North by the mouth of the River Adige which, in the final stretch, flows parallel to the beach and is confined by a weak dike that closes an old branch of the river mouth. The sediment budget analysis (Table 3) put the total fluvial sediment transport at ~265,000 m3/year, of which ~145,000 m3/year was directed toward the cell being studied (the remaining volume is directed northward toward the adjacent cell VE10). The fine sediment is lost and only ~45,000 m3/year contributes to cell advancement. Until 2007, there was a narrow beach on the sea side of the weak dike, but it has now completely disappeared, as a result of the imbalance between potential long-shore transport (~65,000 m3/year) and the actual river contribution. The crest height of this dike is very low (+1.5 m above sea level) and some waves overtop it, creating a depositional area inside the river mouth and obstructing river outflow.

Further South along the right side of the mouth of the River Adige, the beach is protected by a system of 5 groins and a detached submerged breakwater. Erosion (~13,000 m3/year) also predominates in this area due to the limited river sediment supply mentioned above, therefore an insufficient volume of 30,000 m<sup>3</sup> is nourished every year in a bid to balance the long-shore transport directed to the Southern semi-cell (~88,000 m3/year). The interaction between nearshore hydrodynamics and the submerged barrier caused the formation of a deep channel (~3 m) in the breakwater's seaward zone. The channel obstructs natural circulation and sand deposition from the River Adige towards the beach.

No structures were built in the southern semi-cell, as it is characterized by deposition phenomena (shoreline accretion equal to 4 m/year, volume of accretion equal to ~15,000 m3/year) since long-shore sediment transport in the Southern boundary of RO1 is reduced to ~65,000 m3/year.

The seabed appears steeper in the northern and central part than in the southern part (Caleri inlet). The −5 m isobath is 400 m from the shoreline in the northern zone, while it is 1000 m from the shoreline in the southern zone (Figure 11, top).

The Caleri inlet is a convergence point between two adjacent cells since the sand comes from the River Adige to the North and from cell RO2 to the South, making the inlet a potential dredging area (available volume ~140,000 m3/year).

The mitigation planned (Figure 11, bottom) follows on from the criteria in Figure 3. Given the large LST and the significant divergence of the LST along the northern semi-cell, the existing groins are considered appropriate, with them being reinforced and their number slightly increased. The annual nourishment volume is obtained from the sediment balance results.

Sediment resources can be derived from dredging the Caleri inlet, and it is sufficient to provide sediment elsewhere, too. A cautious dredging of only half of the forecasted annual increase in stock volume is addressed in the initial plan, and the monitoring programme will check the actual potential.

In practice, mitigation measures involve:


**Figure 11.** Topo-bathymetry (top) and adopted mitigation for the RO1 cell.
