An Integrated Approach to Study the Morphodynamics of the Lignano Tidal Inlet

Journal of Marine Science and Engineering

Article An Integrated Approach to Study the Morphodynamics of the Lignano Tidal Inlet

Marco Petti, Silvia Bosa , Sara Pascolo * and Erika Uliana

Dipartimento Politecnico di Ingegneria e Architettura, University of Udine, 33100 Udine, Italy; [email protected] (M.P.); [email protected] (S.B.); [email protected] (E.U.) * Correspondence: [email protected]; Tel.: +39-0432-558713

Received: 23 December 2019; Accepted: 22 January 2020; Published: 24 January 2020

Abstract: The morphological evolution of a tidal inlet is the combined result of tides and wind waves, which interact in a non-linear manner and over very different time-scales. Likewise, the presence of maritime structures built in the vicinity of the tidal inlet, for coastal or port defense or to stabilize the inlet itself, can greatly affect this dynamic equilibrium, changing erosional and depositional patterns of the adjacent shoreline. In this study, the narrowing phenomenon of the Lignano tidal inlet subsequent to the construction of the related port, is examined through an integrated approach in order to propose and verify a possible form of evolution. This approach is the result of the combination of three methods: the historical reconstruction of the shifting of the coastline, an empirical scheme which describes the qualitative morphology of a mixed-energy tidal inlet, and a process-based morphodynamic modeling, which adopts a bi-dimensional depth averaged (2DH) approach. The application of numerical modeling has required the definition of a reduced input set of data representing an average year, in particular for wind and tidal conditions, including the meteorological component. The magnitude and the directions of the simulated dominant sediment transport are coherent with real processes both from a qualitative and a quantitative point of view.

Keywords: tidal inlet; morphological evolution; sediment transport; wave-current interaction; wind-tide input

1. Introduction Tidal inlets are important connections between the open sea and back-barrier sheltered basins, lagoons or estuaries. Morphodynamic processes involving their formation, maintenance and evolution are particularly complex, due to the strong interplay between tidal currents and wind waves. These two dominant factors and their non-linear interaction determine the magnitude and the direction of longshore and cross-shore sediment transport to a large extent, that lead to erosion and deposition mechanisms, and thus to the morphology of the seabed and the coastline [1]. The relative role and the predominance of tidal currents on the wave motion or vice versa, can influence the shape of the barrier islands separated by tidal inlets, together with the growth of the flood and the ebb tidal delta [2–5]. The complex dynamism governing these environments makes them potentially unstable over time-scales which can vary differently from years to decades or centuries [6,7]. Tidal inlets act primarily as hydraulic regulators of water exchange between the sea and the back-barrier basin; they also function as both source and sinks of sediment in this manner governing the morphological, ecological and environmental balance of the adjacent coastal system [8]. Moreover, in many cases, they represent preferential waterways towards harbor sites located at the back-barrier basin, increasing the socio-economical value of these natural connections [9]. For these reasons, the morphological evolution of tidal inlets is the focus of a decadal detailed research, which has

J. Mar. Sci. Eng. 2020, 8, 77; doi:10.3390/jmse8020077 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2020, 8, 77 2 of 21 attempted to understand the main processes governing their behavior and their consequent prediction over a medium or long term. The relationship between the hydrodynamic regime and the morphological evolution of tidal inlets and of the nearby coastline has been thoroughly examined. Hayes [2] provided a pioneering classification scheme which linked the qualitative barrier island morphology to the combination of the mean tidal range and the mean wave height in the area immediately outside the inlet. In particular, different energy fields have been defined depending on which forcing prevails; this makes it possible to designate the coasts as mostly dominated by waves or by tides or as mixed energy environments if they are equally affected by both waves and tides. Further works have improved this initial analysis taking into account other factors, such as the tidal prism and emphasizing the importance of the ratio between tidal range and wave height, particularly along coastlines with moderate wave energy [10–12]. In these contexts, Mulhern et al. [12] have verified that the relative effects of tide and waves are of extreme importance, but the utility of the energy regime classification in predicting and constraining island shape is more limited. In this sense, the scheme developed by Hayes [2] can be a valid descriptive reference providing useful information on the typical morphology of tidal inlets, but it needs to be supplemented by more complete physically-based approaches. To this purpose, process-based morphodynamic modeling has shown it is capable of providing better insight into the respective roles of tides and waves in driving morphological changes of tidal inlets and to predict a realistic evolution of the coastline and of the bed morphology. Several modeling systems have been applied to analyze the interaction between currents, waves, and bathymetry in coastal environments numerically [13–22]. The most widely used methodology consists of coupling different modules: a hydrodynamic model based on the classical shallow water equations, a wind wave model to generate or propagate wave fields nearshore and a morphodynamic model to compute sediment transport and bed evolution [7,13,15–17,19,20,22]. The main differences between the performed applications concern: the choice to adopt the full 3D (tri-dimensional) or the 2DH approach, the simulated time-scale, and the consequent definition of the representative input boundary conditions in terms of tide and waves. The use of a 2DH approach instead of a 3D one is preferable and more appropriate if the longshore processes are dominant over cross-shore mechanisms [7,13], which require the computing of the variability of hydrodynamic components along the vertical axis. This implies a great computational effort, especially when considering large domains. The time-scale of the simulations is very important, in order to define a proper representation of the input conditions, especially over medium-long periods varying from years to decades. A limited number of forcings has to be considered to avoid excessive computation times and this leads to the definition of a reduced set of tidal and wave conditions that accurately approximate the morphological evolution over a time period lasting longer than a storm [23,24]. The selection of the representative conditions and their sequencing depends on which phenomena prevail; since waves and tides evolve over different time scales, the task of recognizing the correct reduced input set is not an easy challenge. For this reason, in several applications the tide and the wave motion have been simulated separately [25–28], refer to simplified imposed wave conditions [8,15,29] or short recorded time series [30]. Finally, given the complexity and uncertainties derived from all the necessary simplifications, many studies are based on idealized configurations of the basin-tidal-delta system, reproducing a simple geometry with an initial regular bathymetry [14,17,19,21,26]. Bed morphology turns out to be a dominant controlling variable [17,26], especially when a morphological factor is applied to accelerate or upscale the changes in the seabed [27,31,32]. This means that the results of long-term morphodynamic modeling can be very different according to the initial bathymetry of the domain. The Lignano tidal inlet belongs to the Marano and Grado lagoon system, in the northern Adriatic Sea, Italy. The complex dynamism of this transitional area is highlighted by significant morphological changes that have occurred over the last few decades, during which a narrowing trend has been outlined. Petti et al. [9] provided an accurate historical reconstruction of the coastline evolution J. Mar. Sci. Eng. 2020, 8, 77 3 of 21 J. Mar. Sci. Eng. 2020, 8, 77 3 of 21 pointing out that the protective pier of the port besidebeside the inlet has greatly aaffectedffected the border of the Lignano barrier spit. Moreover, some preliminarypreliminary numerical simulations were conducted on two configurations,configurations, beforebefore andand afterafter the the construction construction of of the the port port respectively. respectively. Beyond Beyond uncertainties uncertainties due due to theto the scarce scarce bathymetric bathymetric information information preceding preceding the the construction construction of of the the port, port, the the comparativecomparative analysisanalysis of the results seems to confirmconfirm that the pier is oneone of the main causes of the deposition mechanism, whichwhich isis leading leading to to progressive progressive narrowing narrowing of the of inlet. the Theinlet. simulations The simulations have been have performed been performed selecting someselecting tide andsome wind tide events and which wind can events be considered which asca representativen be considered of the morphodynamicsas representative involving of the themorphodynamics nearshore of the involving Lignano the barrier nearshore spit over of the an averageLignano year. barrier These spit conditions over an average have been year. defined These accordingconditions to have the resultsbeen defined of a numerical according application to the results carried of a out numerical by Petti etapplication al. [22] to studycarried the out longshore by Petti sedimentet al. [22] transportto study the in thelongshore same area. sediment transport in the same area. Based on the considerations made above, the present work aims to improve and completecomplete the previous studystudy of the narrowing phenomenon which is affecting affecting the Lignano tidal inlet. In In particular, particular, a possible equilibrium evolutionary formform ofof thethe barrier spitspit is proposed. This hypothesis is suggested and subsequently verified verified by by means means of ofan an integrated integrated approach approach which which combines combines three three methods. methods. The Thehistorical historical reconstruction reconstruction of the of themain main morphologi morphologicalcal changes changes is isinterpreted interpreted in in the the light light of of the theoretical-experimental classificationclassification suggestedsuggested byby HayesHayes [[2];2]; thisthis hashas allowedallowed usus toto make a realistic prediction of thethe morphologicalmorphological evolution. Then a 2DH process-based model is applied in order to validate the empirical scheme andand to prove thethe morphologicalmorphological trendtrend ofof thethe narrowingnarrowing condition.condition. The chosen time-scale remains remains the the average average year year and and the the reduced reduced input input set set of of hydrodynamic hydrodynamic conditions conditions is isbetter better defined defined taking taking into into account account the the simultaneo simultaneousus presence presence of of tides tides and and wind wind waves which are equally important in the morphologicalmorphological evolutionevolution ofof thisthis context.context. The arrangement of the paper is asas follows:follows: Section 22 presentspresents aa briefbrief descriptiondescription ofof thethe studystudy site withwith thethe historicalhistorical analysisanalysis andand thethe morphologicalmorphological characterization;characterization; in Section3 3 the the adopted adopted numerical model and the computational domains are described and Section4 4 reports reports thethe reducedreduced setset inputinput conditions.conditions. Finally, Finally, Section 55 showsshows thethe resultsresults obtained,obtained, followedfollowed byby thethe relative relative discussion discussion and conclusions.

2. Study Site and Historical Analysis The Marano and Grado lagoon is a transitionaltransitional environment located in the north-east of Italy (Figure(Figure1 1a,b),a,b), betweenbetween thethe riverriver mouthsmouths ofof thethe TagliamentoTagliamento riverriver inin thethe westwest andand thetheIsonzo Isonzo riverriver inin the east.east.

(a) (b)

Figure 1. Venice lagoonlagoon andand MaranoMarano and and Grado Grado lagoon lagoon depicted depicted (a ()a in) in the the European European context context and and (b )(b in) thein the northern northern Adriatic Adriatic Sea. Sea.

The lagoonlagoon spreads spreads over over an an area area of aboutof about 160 km1602, km and2, it and extends it extends for 32 kmfor alongside 32 km alongside the coastline the withcoastline an average with an width average of 5 width km [20 of,33 5]. km It is [20,33]. one of theIt is largest one of lagoonsthe largest in Italy, lagoons second in Italy, only tosecond the Venice only lagoon,to the Venice and it lagoon, is connected and it to is the connected Adriatic Seato the through Adriatic six Sea tidal through inlets (Figure six tidal2a), inlets from which(Figure a 2a), network from which a network of channels branches out towards the inner part of the lagoon with progressively

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J. Mar. Sci. Eng. 2020, 8, 77 4 of 21 ofJ. Mar. channels Sci. Eng.branches 2020, 8, 77 out towards the inner part of the lagoon with progressively shallower depths.4 of 21 As depicted in Figure2b, the Lignano tidal inlet is the most western inlet, and it separates the Lignano shallower depths. As depicted in Figure 2b, the Lignano tidal inlet is the most western inlet, and it barriershallower spit depths. and the As Marinetta depicted barrier in Figure island. 2b, the Lignano tidal inlet is the most western inlet, and it separates the Lignano barrier spit and the Marinetta barrier island.

(a) (b) Figure 2. Lignano tidal inlet: (a) within the Marano and Grado lagoon context and (b) its detail picture. Figure 2. LignanoLignano tidal tidal inlet: ( (aa)) within within the the Marano Marano and and Gr Gradoado lagoon lagoon context and ( b) its its detail detail picture. picture.

The Lignano tidal inlet is undergoing a progressive narrowing phenomenon, which is due to The Lignano tidal inlet is undergoing a progressive narrowing phenomenon, which is due to the the formation and growth over time of a sand deposit on the external side of the protective pier of formation and growth over time of a sand deposit on the external side of the protective pier of Marina Marina Punta Faro port. In order to determine the causes of the inlet shrinking, an in-depth historical Punta Faro port. In order to determine the causes of the inlet shrinking, an in-depth historical reconstruction has been carried out by Petti et al. [9] with a focus on the morphological evolution of reconstruction has been carried out by Petti et al. [9] with a focus on the morphological evolution of the easternmost part of Lignano Sabbiadoro barrier spit. the easternmost part of Lignano Sabbiadoro barrier spit. Figure3 shows that until the beginning of the 1980s, when the Marina Punta Faro port was built, Figure 3 shows that until the beginning of the 1980s, when the Marina Punta Faro port was built, the width of the inlet was close to 660 m; after the construction of the protective pier, it was reduced to the width of the inlet was close to 660 m; after the construction of the protective pier, it was reduced approximately 500 m. to approximately 500 m.

Figure 3. Detailed pictures of the Lignano tidal inlet: the current sand deposit delineated by the dotted Figure 3. DetailedDetailed pictures pictures of the Lignano tidal inlet: th thee current current sand sand deposit deposit delineated by the dotted line, the old and the current lighthouses in the circles and width of the inlet section over the years. line, the old and the current lighthouseslighthouses in the circlescircles andand widthwidth ofof thethe inletinlet sectionsection overover thethe years.years. Since 1988 the increase of the sand deposit, delineated by the dotted line, has led to a Since 19881988 the the increase increase of theof sandthe sand deposit, deposit, delineated delineated by the dottedby the line, dotted has ledline, to ahas considerable led to a considerable narrowing of the inlet, which has been further reduced to 300 m with an overall loss of narrowingconsiderable of narrowing the inlet, which of the has inle beent, which further has reduced been further to 300 reduced m with anto overall300 m with loss ofan 55%overall compared loss of 55% compared to the 1988 width. The rapid expansion of this deposit is also proven by the need to to55% the compared 1988 width. to the The 1988 rapid width. expansion The rapid of expansion this deposit of this is also deposit proven is also by theproven need by to the move need the to move the lighthouse further towards the center of the inlet, to adequately signal the navigation lighthousemove the lighthouse further towards further the towards center of the the inlet,center to of adequately the inlet, signalto adequately the navigation signal channel the navigation entering channel entering the lagoon basin. The old red lighthouse, one of the most representative historical symbols of Lignano Sabbiadoro, dates back to 1928. However, from the early 2000s, its functionality rapidly ceased, and this subsequently required the construction of the new current lighthouse, which is located about 140 m from the previous one.

J. Mar. Sci. Eng. 2020, 8, 77 5 of 21 the lagoon basin. The old red lighthouse, one of the most representative historical symbols of Lignano Sabbiadoro, dates back to 1928. However, from the early 2000s, its functionality rapidly ceased, and this subsequently required the construction of the new current lighthouse, which is located about 140 m J. Mar. Sci. Eng. 2020, 8, 77 5 of 21 from the previous one. The historical reconstructionreconstruction ofof the the morphological morphological development development of of the the sand sand deposit deposit provided provided by byPetti Petti et al.et al. [9] [9] is basedis based on on a varietya variety of of cartographic, cartographic, aerial aerial and and satellite satellite images images of of thisthis area.area. The ﬁrstfirst available image, dated 1891, 1891, highlights highlights the the rounded rounded shape shape of of the the spit spit end end of of the the Lignano Lignano barrier, barrier, which remained almost unchanged even after a seriesseries of groynesgroynes waswas builtbuilt inin 1938.1938. The coastline substantially coincided with that of 1969, reported in Figure3 3.. In 1976 the construction of the Marina Punta Faro port began, involving the north-east part of the barrier which has thus lost its equilibrium roun roundedded border. During the years after 1991, when the harbor waswas completed, completed, no otherno other structures structures were builtwere in built this area;in this however, area; thehowever, shoreline the progressively shoreline progressivelymoved from the moved pier towardsfrom the the pier center towards of the the inlet, center as of shown the inlet, in Figure as shown4. in Figure 4.

Figure 4.4.Historical Historical reconstruction reconstruction of the of sand the deposit sand fromdepo 1988sit tofrom 2019: 1988 border to lines 2019: of theborder progressive lines growth.of the progressive growth. This picture has been realized by drawing the borders of the barrier spit end obtained by PettiThis et al. [picture9] on the has same been figure, realized in di ffbyerent drawing and separate the borders georeferenced of the barrier images spit and end then obtained supplementing by Petti etthem al. [9] with on additionalthe same figure, available in different years. The and lines separate were georeferenced qualitatively identified images and visually, then supplementing delimiting the thememerging with areas additional of the available sand deposit years. from The the lines submerged were qualitatively ones. The identified temporal evolutionvisually, delimiting highlights the progressiveemerging areas rounding of the sand of the deposit barrier from spit end,the subm as iferged to reconstitute ones. The a temporal shape similar evolution to the highlights original onethe withoutprogressive the port.rounding The accumulationof the barrier spit of sediments end, as if startedto reconstitute from the a southernshape similar part ofto thethe portoriginal and one has withoutdeveloped the towards port. The the accumulation north-east and of sediments the lagoon started side, suggesting from the southern that the sandpart comesof the port from and the has sea developedside. The areatowards of the the surfaces north-east enclosed and the by lagoon the contours side, suggesting has been that plotted the insand Figure comes5 to from obtain the thesea side.growth The trend area of of the the deposit surfaces on enclosed the horizontal by the plane. contours has been plotted in Figure 5 to obtain the growthObserving trend of thisthe deposit curve, it on can the be horizontal stated that plane. until 2000 the increase in the deposit area follows an almost linear trend; from 2000 to 2017 the curve is characterized by a sharp rise of its medium slope as evidence of a marked enlargement of the deposit extension. Finally, from 2017 this growth seems to have reached an upper limit, and there has even been a slight reduction in the last 2 years. This evidence suggests that the construction of the pier has altered the morphological equilibrium of the Lignano tidal inlet determining the development of the rapid and progressive sand deposit, which has led to the cross section narrowing over the past 30 years due to the reformation of a newly rounded barrier spit. This evolution can be explained referring to the main morphological characteristics of the different typologies of tidal inlets classified by Hayes [2]. According to this scheme, the energy regime that governs the coastal area in front of the Marano and Grado lagoon can

Figure 5. Historical reconstruction of the sand deposit from 1988 to 2019: computed area of horizontal surfaces enclosed by the lines for each year.

J. Mar. Sci. Eng. 2020, 8, 77 5 of 21

The historical reconstruction of the morphological development of the sand deposit provided by Petti et al. [9] is based on a variety of cartographic, aerial and satellite images of this area. The first available image, dated 1891, highlights the rounded shape of the spit end of the Lignano barrier, which remained almost unchanged even after a series of groynes was built in 1938. The coastline substantially coincided with that of 1969, reported in Figure 3. In 1976 the construction of the Marina Punta Faro port began, involving the north-east part of the barrier which has thus lost its equilibrium rounded border. During the years after 1991, when the harbor was completed, no other structures were built in this area; however, the shoreline progressively moved from the pier towards the center of the inlet, as shown in Figure 4.

Figure 4. Historical reconstruction of the sand deposit from 1988 to 2019: border lines of the progressive growth.

J. Mar. Sci. Eng. 2020, 8, 77 6 of 21 This picture has been realized by drawing the borders of the barrier spit end obtained by Petti et al. [9] on the same figure, in different and separate georeferenced images and then supplementing thembe with defined additional through available the mean years. tidal rangeThe lines and we there mean qualitatively wave height. identified The meanvisually, tidal delimiting range, given the by emergingthe diff erenceareas of between the sand the deposit mean from high the water subm levelerged (MHWL) ones. The and temporal the mean evolution low water highlights level (MLWL) the is progressiveabout 0.76 rounding m as results of the from barrier the analysis spit end, carried as if outto reconstitute by Petti et al. a shape [20,34 ].similar The mean to the wave original height one close withoutto the the Lignano port. The tidal accumulation inlet is about of 0.6 sediments m as deduced started from from a 30-year-long the southern wave part simulationof the port (1964–1995)and has developedcarriedJ. Mar. Sci. outtowards Eng. by 2020 Benatazzo , the8, 77 north-east et al. [ 35and] who the lagoon reproduced side, thesuggesting present averagethat the wavesand comes climate from of the the Adriatic sea6 of 21 side.Sea. The According area of the to surfaces the graph enclosed developed by the by Hayescontours [2, 12has] withbeen theseplotted two in values, Figure the5 to energy obtain regimethe Observing this curve, it can be stated that until 2000 the increase in the deposit area follows an growthresults trend mixed of the and deposit tide-dominated. on the horizontal plane. almost linear trend; from 2000 to 2017 the curve is characterized by a sharp rise of its medium slope as evidence of a marked enlargement of the deposit extension. Finally, from 2017 this growth seems to have reached an upper limit, and there has even been a slight reduction in the last 2 years. This evidence suggests that the construction of the pier has altered the morphological equilibrium of the Lignano tidal inlet determining the development of the rapid and progressive sand deposit, which has led to the cross section narrowing over the past 30 years due to the reformation of a newly rounded barrier spit. This evolution can be explained referring to the main morphological characteristics of the different typologies of tidal inlets classified by Hayes [2]. According to this scheme, the energy regime that governs the coastal area in front of the Marano and Grado lagoon can be defined through the mean tidal range and the mean wave height. The mean tidal range, given by the difference between the mean high water level (MHWL) and the mean low water level (MLWL) is about 0.76 m as results from the analysis carried out by Petti et al. [20,34]. The mean wave height close to the Lignano tidal inlet is about 0.6 m as deduced from a 30-year-long wave simulation (1964– 1995)FigureFigure carried 5. Historical 5. Historicalout by reconstruction Benatazzo reconstruction et of al.the of [35] thesand sandwho deposit deposit reproduced from from 1988 1988 tothe 2019: to present 2019: computed computed average area area waveof horizontal of horizontalclimate of the Adriaticsurfacessurfaces Sea.enclosed enclosed According by the by thelines to linesthe for graph foreach each year. developed year. by Hayes [2,12] with these two values, the energy regime results mixed and tide-dominated. Following a rigorous classification, the lagoon of Marano and Grado is a microtidal environment. Following a rigorous classification, the lagoon of Marano and Grado is a microtidal However,environment. the northernHowever, Adriatic the northern Sea is Adriatic characterized Sea is bycharacterized shallow waters by shallow that strongly waters attenuatethat strongly the waveattenuate motion the throughwave motion bottom through friction bottom dissipations friction [36 ,dissipations37]. As a consequence, [36,37]. As a the consequence, resulting mean the waveresulting height mean has wave a low height average has value, a low thus average requiring value, a thus smaller requiring tidal range a smaller to produce tidal range tidal-influenced to produce morphologytidal-influenced [2]. morphology This means [2]. that This tidal means processes that ti aredal stillprocesses predominant are still andpredominant lead to a and morphology lead to a typicalmorphology of mesotidal typical environments,of mesotidal environments, which are characterized which are by characterized short, stunted by barrier short, islandsstunted andbarrier the abundanceislands and of the tidal abundance inlets, as of the tidal case inlets, of Marano as the andcase Grado of Marano lagoon. and Grado lagoon. Generally,Generally, mesotidalmesotidal barrierbarrier islandsislands havehave aa drumstickdrumstick shape,shape, withwith thethe fatfat partpart ofof thethe drumstickdrumstick beingbeing locatedlocated onon thethe updriftupdrift sideside ofof thethe barrierbarrier [[2–5],2–5], asas illustratedillustrated inin FigureFigure6 a.6a.

(a) (b)

Figure 6. (a) Morphological model of a typical mesotidal inlet with drumstick shape barrier islands; Figure 6. (a) Morphological model of a typical mesotidal inlet with drumstick shape barrier islands; (b) hypothetical equilibrium shoreline according to the scheme (a) and comparison of the equilibrium (b) hypothetical equilibrium shoreline according to the scheme (a) and comparison of the equilibrium coastline in 1938 and the current one. coastline in 1938 and the current one.

The comparison between this morphological model and the historical evolution of the Lignano barrier spit, suggests a possible new equilibrium shape, depicted in Figure 6b. The red shoreline recreates a rounded and broad shape similar to the one preceding the construction of the port, and which follows the curvature imposed by the pier. It is likely that the new equilibrium profile will be close to the current one, according to the growth curve of Figure 5. In order to verify this hypothesis, a morphodynamic model is applied, taking into account the equally important role of tides and waves and their non-linear interaction in defining dominant sediment transport directions and magnitude.

J. Mar. Sci. Eng. 2020, 8, 77 7 of 21

The comparison between this morphological model and the historical evolution of the Lignano barrier spit, suggests a possible new equilibrium shape, depicted in Figure6b. The red shoreline recreates a rounded and broad shape similar to the one preceding the construction of the port, and which follows the curvature imposed by the pier. It is likely that the new equilibrium proﬁle will be close to the current one, according to the growth curve of Figure5. In order to verify this hypothesis, a morphodynamic model is applied, taking into account the equally important role of tides and waves and their non-linear interaction in deﬁning dominant sediment transport directions and magnitude.

3. Numerical Model and Computational Domains The simulations of the morphodynamic evolution of the analyzed area have been carried out using a numerical model, which couples a morphological-hydrodynamic model and a fully spectral wind-wave model [20]. The hydrodynamic model is based on classical 2DH shallow water equations, which include: turbulent fluxes treated according to the Smagorinsky approach as in the study of Pascolo et al. [38], the forcings due to wind shear stress and radiation stress, and the mean bottom shear stress, which is influenced by both current and wave motion as pointed out by Soulsby [39,40]. The resulting differential equation system is integrated with a finite volume method which is able to represent phenomena such as the propagation of water front over a dry bed, which regularly occurs in lagoons and coastal environments at each tidal cycle, with the submergence and emergence of saltmarshes and beaches. As also confirmed by the historical analysis of the terminal part of the Lignano barrier spit, sediment transport plays an important role in the morphodynamic changes of transitional environments like a lagoon tidal inlet. The finer sediments are usually transported in suspension, while the coarser ones move close to the bottom. In literature, the bed load study is typically carried out through an equilibrium approach [29,31,41] while for the suspended one both equilibrium and non-equilibrium approaches are used [30,42–44]. Numerical comparisons conducted on laboratory tests seem to indicate the non-equilibrium approach as more accurate [42]. However, this kind of approach requires knowledge of the form of the sediment concentration distribution over the water depth. When working on complex environments, the calibration of these parameters can be extremely difficult, and in this way a simpler approach would be preferable. Following these considerations, in the present study both bed and suspended loads have been estimated through an equilibrium approach, according to which sediment flux is equal to the maximum flux compatible with the current, with instant adaptation to any change in the hydrodynamic conditions. In particular, van Rijn formulae have been adopted for both bed and suspended loads, considering the effects of both current and wave motion [45,46]. The sum of bed and suspended load present in Exner equation, is responsible for the bed level change, which has been associated with the hydrodynamic model to obtain a morphological-hydrodynamic model. The fully spectral wind-wave model used to compute the wave parameters is SWAN (Simulating WAves Nearshore), an open source third generation model [47], which solves the wave action density balance equation, applying a finite difference method. The morphological-hydrodynamic model and SWAN interact through water depth, bottom height, currents, wave field and radiation stresses. The two models run separately one after each other and exchange the results of the former as input for the latter and vice versa [20]. Each model runs for 1200 s and it works on its own computational grid. The results are interpolated on an exchange grid, to be transferred from one model to the other. The whole process is managed by a main program that runs the two models alternatively until the end of the simulation. Based on the characteristics of the models described above, two different computational structured grids have been realized: one for the wave generation model and the other for the morphological-hydrodynamic model. The wave generation process is a physical phenomenon, which involves the whole Adriatic Sea, and for this reason the wider the generation area, the more accurate the wave motion representation is. J. Mar. Sci. Eng. 2020, 8, 77 8 of 21

With this in mind, this mesh should cover a large area, which should also be represented in a suﬃciently detailed manner. Therefore, to keep the computational times compatible with our purposes, it has been decided to discretize the northern Adriatic Sea, covering an area which extends for 130 km in the north-south direction and 166 km in the east-west direction. The resulting mesh has approximately

J.300,000 Mar. Sci. squared Eng. 2020, elements 8, 77 with size of 250 m (Figure7a). 8 of 21

(a) (b)

Figure 7. Computational domains: ( (aa)) the the regular regular grid grid of the spectral model covering the northern Adriatic Sea; (b) the morphological-hydrodynamicmorphological-hydrodynamic mesh.mesh.

The computationalcomputational grid grid for thefor morphological-hydrodynamicthe morphological-hydrodynamic modelshould model cover should a considerably cover a considerablysmaller area insmaller order area to allow in order the useto allow of denser the use cells of anddenser hence cells ensuring and hence a higher ensuring resolution a higher of resolutionthe hydrodynamic of the hydrodynamic variables. For thisvariables. reason, For in thisthis work reason, the morphological-hydrodynamicin this work the morphological- grid hydrodynamicreproduces part grid of the reproduces Marano and part Grado of the lagoon Marano and and a portion Grado oflagoon the sea and in fronta portion of the of coast the sea of Lignano in front (Figureof the coast7b). Thisof Lignano smaller (Figure domain 7b). has This been smalle discretizedr domain with has approximately been discretized 200,000 with quadrangularapproximately 200,000irregular quadrangular elements with irregular sides ranging elements from with 0.8 sides m toranging 375 m, from where 0.8 them to smallest 375 m, where have been the smallest used to haveguarantee been aused particular to guarantee detailed a particular representation detailed of the representation Lignano inlet of area. the Lignano inlet area. The bedbed elevationelevation of of both both the the computational computational grids gr hasids beenhas been assigned assigned as in [9as,22 in] combining[9,22] combining several severaldatasets, datasets, dating between dating between 2000 and 2000 2017. and This 2017. was This a necessary was a necessary choice due choice to the due lack to the of suitable lack of suitable surveys surveysrelating relating to the periods to the periods immediately immediately preceding preceding and following and following the construction the construction of the Marinaof the Marina Punta PuntaFaro port. Faro port. Regarding the the boundary boundary conditions conditions assigned assigned to to the the morphological-hydrodynamic morphological-hydrodynamic mesh, mesh, a reflectivea reflective condition condition has has been been applied applied to to all all land land boundaries, boundaries, while while on on the the seaside seaside a a tidal boundary condition has been used. Considering Considering that that in in the the Adriatic Adriatic Sea Sea the the tide tide moves moves counter counter clockwise clockwise [48], [48], the samesame tidaltidal oscillationoscillation has has been been assigned assigned to to the the “east “east side” side” and an “westd “west side” side” boundaries boundaries (Figure (Figure7b), with7b), with a time a shifttime of shift about of 50 about minutes, 50 minutes, as deduced as fromdeduced preliminary from preliminary simulations simulations aimed at determining aimed at determiningthe tidal propagation the tidal speed. propagation For the speed. spectral For grid, the outflow spectral boundary grid, outflow conditions boundary have beenconditions imposed have at beenthe southern imposed boundary, at the southern allowing boundary, the waves allowing to freely the leave waves the to domain. freely leave the domain. Manning’s coefficients coefficients used used to to evaluate evaluate current current bed bed shear shear stress stress have have been been assigned assigned as in asPetti in etPetti al. et[20] al. [for20 ]the for theportion portion of mesh of mesh that that describ describeses the the lagoon, lagoon, while while for for the the sea sea side side a Manning’s coefficientcoefficient of 0.0250.025 ss/m/m11/3/3 has been applied. According According to to measurement campaigns carried out by ARPA FVGFVG [[49],49], thethe fractionfraction of of cohesive cohesive sediment sediment tends tends to to zero zero in in the the tidal tidal inlet, inlet, which which is theis the focus focus of ofthis this study, study, and and also also on theon seaside.the seaside. Hence, Hence, in this in workthis work only granularonly granular sediments sediments have beenhave used, been withused, a withmean diametera mean ofdiameter 200 µm. Thisof 200 value μm. has This been obtainedvalue has following been obtained granulometric following investigations granulometric carried investigationsout on a series ofcarried samples out taken on a alongseries theof samples Lignano taken littoral along by OGS the(National Lignano Institutelittoral by of OGS Oceanography (National Instituteand Experimental of Oceanography Geophysics) and [Experimental50]. Geophysics) [50].

4. Wind and Tide Characterization As stated above, the morphological evolution of the tidal inlet is strongly influenced by tides, wind waves and their mutual interaction. Tidal oscillations directly enter the morphological-hydrodynamic model as boundary conditions and indirectly into the spectral model since they affect water level and currents. On the other hand, wind energy input is fundamental in the wave generation process and acts as a drag force over the water surface in the morphological-hydrodynamic model.

J. Mar. Sci. Eng. 2020, 8, 77 9 of 21

4. Wind and Tide Characterization As stated above, the morphological evolution of the tidal inlet is strongly influenced by tides, wind waves and their mutual interaction. Tidal oscillations directly enter the morphological-hydrodynamic model as boundary conditions and indirectly into the spectral model since they affect water level and currents. On the other hand, wind energy input is fundamental in the wave generation process and actsJ. Mar. as Sci. a dragEng. 2020 force, 8, 77 over the water surface in the morphological-hydrodynamic model. 9 of 21 The simulation setup is defined selecting the forcings which govern the morphodynamic The simulation setup is defined selecting the forcings which govern the morphodynamic mechanisms that occur over an average year on the present configuration. This choice is based mechanisms that occur over an average year on the present configuration. This choice is based on the on the availability of the measured data. On the one hand there are no precise and extensive availability of the measured data. On the one hand there are no precise and extensive bathymetric bathymetric surveys carried out in the period immediately before or after the construction of the pier; surveys carried out in the period immediately before or after the construction of the pier; since the since the bed morphology is one of the main variables that greatly affects and controls the results, bed morphology is one of the main variables that greatly affects and controls the results, this work is this work is a first attempt to verify the current situation rather than the morphological evolution a first attempt to verify the current situation rather than the morphological evolution over a decadal over a decadal period. On the other hand, there are no extensive bathymetric surveys carried out period. On the other hand, there are no extensive bathymetric surveys carried out subsequently, with subsequently, with a relatively short time lag, through which the model can be validated. a relatively short time lag, through which the model can be validated. The choice of the most appropriate input data for the examined study therefore has been performed The choice of the most appropriate input data for the examined study therefore has been considering the volumes of sediments yearly deposited inside the Lignano canal, which is depicted performed considering the volumes of sediments yearly deposited inside the Lignano canal, which with a yellow dashed line in Figure8. is depicted with a yellow dashed line in Figure 8.

Figure 8.8. Position and shape of the Lignano canal.

This channel is used by vessels to move between the open sea and the harbor sites located within thethe lagoon.lagoon. For For this this reason, reason, in orderin order to guarantee to guarantee navigability, navigability, the channel the channel undergoes undergoes annual dredging annual interventions,dredging interventions, removing aremoving volume ofa volume approximately of approximately 30,000 m3 of 30,000 sediments m3 of whichsediments deposit which every deposit year insideevery year the terminal inside the part terminal of the seapart side of the channel sea side (the channel yellow (the fill in yellow Figure fill8). in Figure 8). InIn aa previousprevious numericalnumerical application,application, PettiPetti etet al.al. [[20]20] investigatedinvestigated thethe morphologicalmorphological evolutionevolution ofof thethe innerinner partpart ofof thethe MaranoMarano and Grado lagoon over an average year, forfor whichwhich aa sequencesequence ofof windswinds and tides acting over that time was reconstructed.reconstructed. The predicted seabed changes matched the annual dredged volumesvolumes inin thethe moremore criticalcritical branchesbranches ofof thethe lagoon.lagoon. This result confirmedconfirmed the validity of the approach, which linearly composes events and theirtheir induced eeffectsffects toto describe the morphodynamic global trend, followingfollowing aa criterioncriterion generallygenerally adoptedadopted inin maritimemaritime hydraulics.hydraulics. This approachapproach is is therefore therefore similarly similarly proposed proposed to study to study the morphological the morphological evolution evolution of the Lignano of the tidalLignano inlet. tidal The analysisinlet. The is thenanalysis extended is then to the extend northerned to Adriatic the northern Sea to derive Adriatic the corresponding Sea to derive wind the andcorresponding tide input data.wind and tide input data.

4.1. Wind Input Three anemometric stations, relatively close to the study area, have been considered: Lido Diga Sud, Lignano and Grado, reported in Figure 9a. More than one million items of data, measured from 1998 to 2017, have been analyzed in terms of wind speed and direction. Petti et al. [22] performed preliminary simulations aimed at studying the longshore transport in the Lignano nearshore and the morphological effects induced by the groynes built close to the Tagliamento river mouth. The need to sum up the collected wind data to derive an annual average anemometric characterization, has made it necessary to identify appropriate classes of both direction and speed. The measurements of the three anemometers were summarized by Petti et al. [22] to create a single directional frequency distribution representing the offshore annual average wind regime. This was done taking only those sectors that are not affected by the frictional interferences of buildings or dry land from each station.

J. Mar. Sci. Eng. 2020, 8, 77 10 of 21

4.1. Wind Input Three anemometric stations, relatively close to the study area, have been considered: Lido Diga Sud, Lignano and Grado, reported in Figure9a. More than one million items of data, measured from 1998 to 2017, have been analyzed in terms of wind speed and direction. Petti et al. [22] performed preliminary simulations aimed at studying the longshore transport in the Lignano nearshore and the morphological effects induced by the groynes built close to the Tagliamento river mouth. The need to sum up the collected wind data to derive an annual average anemometric characterization, has made it necessary to identify appropriate classes of both direction and speed. The measurements of the three anemometers were summarized by Petti et al. [22] to create a single directional frequency distribution representing the offshore annual average wind regime. This was done taking only those sectors that areJ. Mar. not Sci. aff Eng.ected 2020 by, 8 the, 77 frictional interferences of buildings or dry land from each station. 10 of 21

(a) (b)

FigureFigure 9. 9.Wind Wind speeds speeds higher higher than than 10 10 m m/s/s in in the the northern northern Adriatic Adriatic Sea: Sea: ( a(a)) polar polar chart chart of of the the relative relative frequenciesfrequencies according according to to the the data data of the of anemometricthe anemometric stations stations indicated indicated by triangles; by triangles; (b) duration (b) duration curve. curve. The results of the performed simulations showed that the sediment transport dynamics in the LignanoThe nearshore results of is the predominately performed simulations influenced by showed high intensity that the and sediment low frequent transport wind dynamics events instead in the ofLignano more frequent nearshore and is lower predominately intensity ones. influenced The same by happenshigh intensity inside and the Maranolow frequent and Grado wind lagoon events andinstead similar of more contexts frequent [20,34 and], for lower which intensity waves generated ones. The bysame wind happens speeds inside greater the than Marano 10 m /ands play Grado the dominantlagoon and role similar in sediment contexts transport [20,34], for mechanisms. which wave Followings generated these by considerationswind speeds greater and the than duration 10 m/s curveplay ofthe the dominant higher wind role velocitiesin sediment depicted transport in Figure mechanisms.9b, two classes Following have these been defined:considerations the first and from the 10duration to 14 m /curves and theof the second higher one wind from velocities 14 to 22 mdepict/s. Theed representativein Figure 9b, two velocities classes are have respectively been defined: 12 and the 17first m/ s,from identified 10 to as14 barycentricm/s and the ones second of the one areas from subtended 14 to 22 by m/s. theduration The representative curve. The relativevelocities polar are chart,respectively reported 12 in and Figure 17 9m/s,a, shows identified that the as two baryce mainntric blowing ones directionsof the areas are subtended 75 ◦N for theby Bora-Levantthe duration windcurve. and The 165 relative◦N for polar the Sirocco chart, wind.reported in Figure 9a, shows that the two main blowing directions are 75°NThe for duration the Bora-Levant of each windwind class,and 165°N over an for average the Sirocco year, wind. has been estimated on the above-defined curveThe and duration the summary of each is proposedwind class, in over Table an1. average year, has been estimated on the above-defined curve and the summary is proposed in Table 1. Table 1. Sequence of real and simulated winds representative of the average year and the relative morphologicalTable 1. Sequence factor of applied real and to eachsimulated class. winds representative of the average year and the relative morphological factor applied to each class. Wind Event Real Time Simulated Time Morphological Factor SiroccoWind 12 m Event/s Real 4 Time d Simulated 1 Time d Morphological4 Factor Bora/LevantSirocco 12 12 m/ sm/s 48 d 1 d 1 d 4 8 SiroccoBora/Levant 17 m/s 12 m/s 8 hd 1 d 8 h 8 1 Bora/LevantSirocco 17 17 m/ sm/s 40 8 h h 8 h 40 h 1 1 Bora/Levant 17 m/s 40 h 40 h 1

Furthermore, to avoid excessive computational times, different morphological factors have been assumed to amplify the volumes of erosion and deposition in particular during the Sirocco and Bora/Levant winds with intensity of 12 m/s. This choice is based on the linearity hypothesis of the applied methodology and on the morphological factor approach [7,31], which is the most common criterion to scale the calculated erosion and deposition by a constant factor. This factor can vary according to the different events considered as suggested by Walstra et al. [27]. The sequence of wind events is interspersed with calm periods lasting a few hours in order to allow the inertial decay of the phenomena from a hydraulic point of view and the consequent deposition of suspended material [27].

4.2. Tidal Input The defined wind series has to be combined with a representative tide; to study the morphodynamic processes governing the inner part of the Marano and Grado lagoon, an annual average tide was defined combining the three main harmonic components, solar semidiurnal, and

J. Mar. Sci. Eng. 2020, 8, 77 11 of 21

Furthermore, to avoid excessive computational times, diﬀerent morphological factors have been assumed to amplify the volumes of erosion and deposition in particular during the Sirocco and Bora/Levant winds with intensity of 12 m/s. This choice is based on the linearity hypothesis of the applied methodology and on the morphological factor approach [7,31], which is the most common criterion to scale the calculated erosion and deposition by a constant factor. This factor can vary according to the diﬀerent events considered as suggested by Walstra et al. [27]. The sequence of wind events is interspersed with calm periods lasting a few hours in order to allow the inertial decay of the phenomena from a hydraulic point of view and the consequent deposition of suspended material [27].

4.2. Tidal Input The defined wind series has to be combined with a representative tide; to study the morphodynamic J.processes Mar. Sci. Eng. governing 2020, 8, 77 the inner part of the Marano and Grado lagoon, an annual average tide11 of was 21 defined combining the three main harmonic components, solar semidiurnal, and lunar diurnal and lunarsemidiurnal diurnal respectively and semidiurnal [20]. The respectively resulting signal [20]. hasThe aresulting periodicity signal of about has a 13 periodicity days which of however about 13 is daysdifficult which to associate however with is difficult the wind to associate events defined with the in Sectionwind events 4.1. defined in section 4.1. For this reason and as an initial simple approach approach,, a sinusoidal function with a single harmonic component has has been chosen, having an amplitude eq equalual to ± 0.40 m. This This va valuelue corresponds to the ± average astronomicalastronomical tidal tidal range range in thein northernthe northern Adriatic Adriatic Sea, as Sea, reported as reported by Dorigo by [ 51Dorigo] and confirmed[51] and confirmedby a zero-crossing by a zero-crossing analysis, carriedanalysis, out carried over 24out years over 24 of recordingyears of recording of the Grado of the tideGrado gauge tide [gauge20,34]. [20,34].The resulting The resulting wind and wind tide and input tide conditions input conditions are shown are inshown Figure in 10 Figure. 10.

Figure 10. Sinusoidal tidal oscillation combined with the sequence of winds representative of an Figure 10. Sinusoidal tidal oscillation combined with the sequence of winds representative of an average year. average year. With this synthetic input set of forcings, the simulation of the average year has led to a sand deposit With this synthetic input set of forcings, the simulation of the average year has led to a sand in the Lignano canal of about 17,000 m3 (Table2). This volume is comparable with the annual dredged deposit in the Lignano canal of about 17000 m3 (Table 2). This volume is comparable with the annual material, but this result is not yet suﬃcient to state that the phenomenon is correctly interpreted by dredged material, but this result is not yet sufficient to state that the phenomenon is correctly the model. interpreted by the model. Table 2. Volume of sediments: dredged every year from the Lignano canal and deposited at the end of Tablethe numerical 2. Volume simulations of sediments: performed dredged respectively every year with from the the sinusoidal Lignano meancanal tideand anddeposited the reconstructed at the end oftide the in conjunctionnumerical simulations with the considered performed wind respecti events.vely with the sinusoidal mean tide and the reconstructed tide in conjunction with the considered wind events. Sediment Volume Sediment Volume Deposited at the End of The Numerical Simulations Dredged Every Year Sediment Volume SedimentAverage Volume Sinusoidal Deposited Tide at the End Reconstructed of The Numerical Tidal Oscillation Simulations Dredged Every Year Average Sinusoidal Tide Reconstructed Tidal Oscillation 30,000 m3 17,000 m3 29,000 m3 30,000 m3 17,000 m3 29,000 m3

The northern Adriatic Sea Sea is often subject to sea level rises due to the tidal meteorological component, inin particularparticular during during Sirocco Sirocco storms storms [52 ],[52], which which are also are often also associatedoften associated with low with pressure low pressure conditions. Petti et al. [9] have analyzed the water levels recorded during Sirocco and Bora/Levant events with maximum wind speeds of about 12 and 17 m/s. It has been noted that tidal oscillation undergoes strong variations: the mean water level considerably increases during Sirocco events while it decreases during Bora/Levant events. This is realistically due to the meteorological component linked to the drag effect of wind blowing on the sea surface and to the pressure differences between the southern part and the northern part of the Adriatic Sea during the Sirocco events. In fact, winds blowing from south-east, as in the Sirocco case, have a large fetch and they induce higher mean sea levels in the northernmost part of the Adriatic Sea. On the contrary, winds from north-east tend to accumulate water towards the western Venice lagoon, lowering the mean water level in front of the Marano and Grado lagoon. Based on these considerations, Petti et al. [9] have proposed a procedure to reconstruct tidal oscillations in conjunction with the highest wind events, thus also taking into account the meteorological component, comprehensive of both wind drag effects and pressure differences. The new sequence of winds and tides is depicted in Figure 11.

J. Mar. Sci. Eng. 2020, 8, 77 12 of 21 conditions. Petti et al. [9] have analyzed the water levels recorded during Sirocco and Bora/Levant events with maximum wind speeds of about 12 and 17 m/s. It has been noted that tidal oscillation undergoes strong variations: the mean water level considerably increases during Sirocco events while it decreases during Bora/Levant events. This is realistically due to the meteorological component linked to the drag effect of wind blowing on the sea surface and to the pressure differences between the southern part and the northern part of the Adriatic Sea during the Sirocco events. In fact, winds blowing from south-east, as in the Sirocco case, have a large fetch and they induce higher mean sea levels in the northernmost part of the Adriatic Sea. On the contrary, winds from north-east tend to accumulate water towards the western Venice lagoon, lowering the mean water level in front of the Marano and Grado lagoon. Based on these considerations, Petti et al. [9] have proposed a procedure to reconstruct tidal oscillations in conjunction with the highest wind events, thus also taking into account the meteorological component, comprehensive of both wind dragJ. Mar. e Sci.ffects Eng. and 2020 pressure, 8, 77 differences. The new sequence of winds and tides is depicted in Figure12 of11 21.

Figure 11. Reconstructed tidal oscillation, comprehensive of the meteorological component associated Figure 11. Reconstructed tidal oscillation, comprehensive of the meteorological component associated with the sequence of winds representative of an average year. with the sequence of winds representative of an average year. This new input set of winds and tides, representative of the average year, provides a sand deposit This new input set of winds and tides, representative of the average year, provides a sand of approximately 29,000 m3 inside the Lignano canal. The comparison with the previous result in deposit of approximately 29,000 m3 inside the Lignano canal. The comparison with the previous Table2 and the real dredged volume highlights that this sequence is more realistic than the previous result in Table 2 and the real dredged volume highlights that this sequence is more realistic than the one and it largely falls within the accuracy that can be achieved with this type of model. These results previous one and it largely falls within the accuracy that can be achieved with this type of model. confirm that the effects of an average year make it necessary to impose the tidal oscillations associated These results confirm that the effects of an average year make it necessary to impose the tidal with the wind events, as boundary conditions. oscillations associated with the wind events, as boundary conditions. 5. Results 5. Results The morphological evolution over an average year of the Lignano tidal inlet has been simulated, with theThe aim morphological to verify the evolution narrowing over process an average triggered year by of the the construction Lignano tidal of inlet Marina has Puntabeen simulated, Faro port. Inwith Figure the aim 12, theto verify bathymetric the narrowing comparison process is reported triggered between by the construction respectively of the Marina current Punta configuration Faro port. andIn Figure the depths 12, the resulting bathymetric after ancomparison average year. is repo Fourrted sections between have respectively been selected the current and the configuration relative cross widthand the has depths been resulting computed, after taking an average the same year. contour Four linesections as reference. have been selected and the relative cross widthThe has narrowing been computed, effect is moretaking evident the same on thecontour sea side line in as correspondence reference. with Sections3 and4, due to the deposition of sand on both the sides of the tidal inlet, while toward the lagoon this phenomenon is more contained. Section1 seems to be in countertrend, with a slight enlargement which is however coherent with the temporal evolution of the horizontal surface area in recent years, as shown in Figure4. Moreover, the greater deposit involving the external part of the tidal inlet, i.e., Sections3 and4 with respect to Sections1 and2, is confirmed by the fact that the southern branch of the port access canal is subjected to periodic dredging operations. This result suggests that most of the material comes from the sea and from the coast adjacent to the tidal inlet.

(a) (b)

Figure 12. Depth contours resulting (a) in the current configuration and (b) after the simulation of an average year. The cross width of the tidal inlet, pre- and post-simulation, is indicated along the four sections depicted with the yellow lines and the numbers in the circles.

The narrowing effect is more evident on the sea side in correspondence with sections 3 and 4, due to the deposition of sand on both the sides of the tidal inlet, while toward the lagoon this phenomenon is more contained. Section 1 seems to be in countertrend, with a slight enlargement which is however coherent with the temporal evolution of the horizontal surface area in recent years, as shown in Figure 4. Moreover, the greater deposit involving the external part of the tidal inlet, i.e. Sections 3 and 4 with respect to Sections 1 and 2, is confirmed by the fact that the southern branch of the port access canal is subjected to periodic dredging operations. This result suggests that most of the material comes from the sea and from the coast adjacent to the tidal inlet.

J. Mar. Sci. Eng. 2020, 8, 77 12 of 21

Figure 11. Reconstructed tidal oscillation, comprehensive of the meteorological component associated with the sequence of winds representative of an average year.

This new input set of winds and tides, representative of the average year, provides a sand deposit of approximately 29,000 m3 inside the Lignano canal. The comparison with the previous result in Table 2 and the real dredged volume highlights that this sequence is more realistic than the previous one and it largely falls within the accuracy that can be achieved with this type of model. These results confirm that the effects of an average year make it necessary to impose the tidal oscillations associated with the wind events, as boundary conditions.

5. Results The morphological evolution over an average year of the Lignano tidal inlet has been simulated, with the aim to verify the narrowing process triggered by the construction of Marina Punta Faro port. In Figure 12, the bathymetric comparison is reported between respectively the current configuration J.and Mar. the Sci. depths Eng. 2020 resulting, 8, 77 after an average year. Four sections have been selected and the relative13 cross of 21 width has been computed, taking the same contour line as reference.

(a) (b)

FigureFigure 12. 12.Depth Depth contours contours resulting resulting ( a(a)) in in the the current current conﬁguration configuration and and ( b(b)) after after the the simulation simulation of of an an averageaverage year. year. The The cross cross width width of of the the tidal tidal inlet, inlet, pre- pre- and and post-simulation, post-simulation, is is indicated indicated along along the the four four J. Mar.sections Sci.sections Eng. 2020 depicted depicted, 8, 77 with with the the yellow yellow lines lines and and the the numbers numbers in in the the circles. circles. 13 of 21

InInThe orderorder narrowing toto better better effect analyze analyze is thismore this morphodynamic evident morphodynamic on the process,sea sideprocess, elevenin correspondence eleven sections sections orthogonal with orthogonal sections to the Lignano 3to and the 4, Lignanoshorelinedue to shorelinethe have deposition been have considered: beenof sand considered: on the both first the sixthe werefirstsides si taken xof were the up takentidal to a inlet, limitup to distance whilea limit toward distance of about the of 650 lagoonabout m from 650 this mthephenomenon from coast, the the coast, last is more fivethe last crosscontained. five the cross inlet Section fromthe inlet 1 shore seems from to to shore.shore be in to Through countertrend, shore. Through these sectionswith these a slight thesections net enlargement sediment the net sedimentfluxeswhich over is fluxeshowever an average over coherent an year average havewith year the been temporalhave calculated been evolutio ca andlculated haven of and the been havehorizontal represented been representedsurface by curves area byin that recentcurves identify years, that identifypositiveas shown positive and in negative Figure and 4.negative directions. Moreover, dire These thections. greater directions These deposit directions indicate involving two indicate overall the tw external neto overall trajectories part net of trajectories ofthe the tidal qualitative inlet, of the i.e. qualitativesedimentSections transport3 sedimentand 4 with through transport respect the tothrough tidal Sections inlet, the 1 as tidaland depicted 2,inlet, is confirmed as in depicted Figure by 13 inthe. OnFigure fact the that external13. theOn southernthe sides external of branch the sides tidal of ofinlet,the the port the tidal net access inlet, flux canal of the sediments net is subjected flux isof directed sediments to periodic from is the dredgindirected outsideg fromoperations. towards the theoutside This inside resulttowards of the suggests lagoon, the inside asthat indicated mostof the of lagoon,bythe the material yellow as indicated comes arrows byfrom both the the near yellow sea the and arrows Lignano from both the barrier coastnear and theadjacent the Lignano Marinetta to the barrier tidal island. and inlet. Inthe the Marinetta deepest partisland. ofthe In theinlet, deepest the average part of direction the inlet, of the the av neterage sediment direction flux of crossing the net the sediment sections flux is from crossing the lagoon the sections towards is fromthe open the lagoon sea, as showntowards by the the open red arrows.sea, as shown by the red arrows.

FigureFigure 13. 13. GraphicalGraphical representation representation of ofthe the sediment sediment fluxes ﬂuxes through through the numbered the numbered sections sections in the in area the ofarea the of Lignano the Lignano inlet. inlet.The yellow The yellow arrows arrows indicate indicate flood ﬂood directions, directions, while while the red the arrows red arrows indicate indicate ebb directions.ebb directions.

MoreMore in detail, Figure 14 shows the numerical results results regarding the net volumes of sediments crossingcrossing the selected sections over an average year.

Figure 14. Net volumes of sediments crossing the sections on an average year. The positive volumes move from the outside towards the inside of the lagoon and vice versa.

J. Mar. Sci. Eng. 2020, 8, 77 13 of 21

In order to better analyze this morphodynamic process, eleven sections orthogonal to the Lignano shoreline have been considered: the first six were taken up to a limit distance of about 650 m from the coast, the last five cross the inlet from shore to shore. Through these sections the net sediment fluxes over an average year have been calculated and have been represented by curves that identify positive and negative directions. These directions indicate two overall net trajectories of the qualitative sediment transport through the tidal inlet, as depicted in Figure 13. On the external sides of the tidal inlet, the net flux of sediments is directed from the outside towards the inside of the lagoon, as indicated by the yellow arrows both near the Lignano barrier and the Marinetta island. In the deepest part of the inlet, the average direction of the net sediment flux crossing the sections is from the lagoon towards the open sea, as shown by the red arrows.

Figure 13. Graphical representation of the sediment fluxes through the numbered sections in the area of the Lignano inlet. The yellow arrows indicate flood directions, while the red arrows indicate ebb directions.

J. Mar.More Sci. Eng. in2020 detail,, 8, 77 Figure 14 shows the numerical results regarding the net volumes of sediments14 of 21 crossing the selected sections over an average year.

Figure 14. Net volumes of sediments crossing the sections on an average year. The positive volumes move from the outside towardstowards thethe insideinside ofof thethe lagoonlagoon andand vicevice versa.versa.

These volumes have been calculated starting from the area subtended by each portion of the curves, identified by a number and a letter. The positive volumes refer to flood directions that move from the outside towards the inside of the lagoon and vice versa. The difference between the volumes of one section and the next one can suggest whether there is an erosion or a deposition trend of sediments. If the difference is positive, a deposition of sediments occurs, on the contrary an erosion process is triggered within the area between the two selected sections. In this sense, it can be observed that the volume of sediments that progressively crosses the first four sections increases. The difference between Section 4 and 1 gives a total volume of about 37,000 m3 and it corresponds to the sandy material that the north-eastern part of the Lignano Sabbiadoro coast loses over an average year. This value is consistent with the experimental evidence of the erosion trend which involves the final part of Lignano barrier spit, requiring annual sand nourishments along the coastline between the transparent pier named “Terrazza a Mare” and the Red Lighthouse, as depicted in Figure 14. From Section 5a to Section 8a, the volumes decrease, confirming that the material tends to deposit near the Marina Punta Faro pier and that this process is triggered from the sea side towards the inside of the lagoon. This numerical result confirms the evolution of the shoreline depicted in Figure4. A further analysis has been carried out to understand how each simulated wind event contributes to the total net sediment volumes over an average year, and the results are reported in Table3. For all the considered sections, the Sirocco wind induces a longshore current which tends to move the material from the south-west to the north-east (values with the positive sign in Table3) bringing it closer to the Lignano inlet. Instead, in the case of the Bora/Levant, the relative longshore currents can have different net directions. Petti et al. [22] verified that the sediment transport induced by Bora/Levant winds along the coast of Lignano, near the Tagliamento river mouth, mainly moves from the north-east to the south-west, reshaping the shoreline profile. This is also the case of Section 1, which in fact has a negative sign in Table3, while in the next four sections, the sediment transport reverses its direction contributing in this manner to the supply of the deposit at the Marina Punta Faro pier. Then, Sections 6–11 show that the direction reverses again, favoring the ebb of sandy material from the lagoon to the open sea. The event that has the greatest influence on the total sediment volume is the Sirocco wind with a speed of 12 m/s, simulated for 24 h with a morphological factor equal to four. Also, the Sirocco with a wind speed of 17 m/s has an important effect on the total annual volume. J. Mar. Sci. Eng. 2020, 8, 77 15 of 21

Table 3. Sediment volumes crossing sections during each wind event of the simulation of an average year. The positive volumes represent ﬂood directions that move from the sea side towards the inside of the lagoon, the negative ones are ebb directions which move from the inside towards the outside.

Sediment Volumes Crossing the Sections (m3) 12 m/s 17 m/s Section Sirocco Bora/Levant Sirocco Bora/Levant Total (m3) 1 17,028 774 4296 828 +19,722 − − 2 20,373 1767 5164 302 +27,606 3 26,225 4979 6126 1741 +39,070 4 36,770 7896 8798 3544 +57,009 5 40,740 5431 10,727 1309 +58,207 6 19,144 7262 8449 5908 +14,423 − − 7 31,249 7774 9144 7241 +25,378 − − 8 23,259 10768 7604 9043 +11,052 − − 9 10,969 4834 4830 4518 +6447 − − 10 6790 2695 4233 3795 +4533 − − 11 5350 2502 2931 3765 +2014 J. Mar. Sci. Eng. 2020, 8, 77 − − 15 of 21

ComparedCompared to tothis this total total volume, volume, the the Bora/Levant Bora/Levant contribution contribution seems seems to be lessless significant,significant, despite despite the thefact fact that that the the windwind speedspeed of 12 m/s m/s is is simulated simulated for for 24 24 h hwith with a amorphological morphological factor factor equal equal to eight. to eight. In Inthis this way, way, it itcan can be be concluded concluded that SiroccoSirocco eventsevents seem seem to to determine determine the the overall overall handling handling of the of sandythe sandymaterial material in the in studythe study area, area, favoring favoring its displacement its displacement from thefrom Lignano the Lignano littoral littoral towards towards the Lignano the Lignanoinlet area. inlet area. AsAs further further proof proof that that the the numerical numerical model model is co isrrectly correctly interpreting interpreting the the narrowing narrowing phenomenon, phenomenon, FigureFigure 15 15shows shows the the profiles profiles of ofthe the selected selected section section (Figure (Figure 15a) 15 ina) inthe the current current configuration configuration and and that that obtainedobtained after after an anaverage average year. year.

(a) (b)

FigureFigure 15. 15.(a) (Selecteda) Selected section section for forthe thecomparison comparison between between (b) (theb) theprofiles proﬁles in the in thecurrent current configuration conﬁguration (black(black continuous continuous line), line), that that are are obtainedobtained afterafter an an average average year year (black (black dashed dashed line) line) and theand one the available one availablein 1961 in from 1961 Dorigo from Dorigo [51] (brown [51] (brown continuous continuous line). line).

TheThe comparison comparison between between the the two two profiles, proﬁles, before before and and after after the the simulation simulation of the of the average average year, year, pointspoints out out the the presence presence of ofa new a new sand sand deposit deposit on on the the Lignano Lignano side, side, highlighted highlighted by bythe the colored colored hatching.hatching. The The section section tends tends to change to change its itsshape, shape, showing showing deposits deposits in insome some places places and and erosions erosions in in others,others, but but overall overall it nevertheless it nevertheless maintains maintains its itsown own area. area. The The current current liqu liquidid section section measures measures 2718 2718 m3m and3 and at atthe the end end of ofthe the simulation simulation is isequal equal to to2682 2682 m m3. 3This. This suggests suggests that that a dynamic a dynamic equilibrium equilibrium conditioncondition seems seems to tobe bereached. reached. This occurs differently between the profile in the current configuration, in 2019, and the only available profile dating back to before the construction of the pier in a 1961 survey carried out by Dorigo [51] in the analyzed section. In this case significant differences can be observed: on the Lignano side there were no emerging parts, and the center of the tidal channel was slightly closer to the Marinetta island but substantially equidistant between the two sides. In the current configuration the center has moved a hundred meters close to the Marinetta island and has deepened by about 2 m. In addition to the depth data of 1961, Dorigo [51] reported the area of the liquid section depicted in Figure 15a and the flow velocities which were measured in 1952 and 1961 through the same section, during two tide cycles as having an average amplitude of about 105 cm. The area of the liquid section resulted equal to about 3438 m3 and 3909 m3 respectively, and therefore the section in the current configuration has decreased by approximately 21.9% and 31.3%. The volumes of water exchanged during the ebb and flood phases was computed by Dorigo [51] based on the available velocity data. The same volumes have been determined from the present simulation of an average year, considering the tidal oscillation during the Sirocco event with a wind speed of 12 m/s. In fact, this event has roughly the same minimum and maximum level as those measured in 1952 and 1961, as reported in Table 4 with the relative water volumes.

J. Mar. Sci. Eng. 2020, 8, 77 16 of 21

This occurs differently between the profile in the current configuration, in 2019, and the only available profile dating back to before the construction of the pier in a 1961 survey carried out by Dorigo [51] in the analyzed section. In this case significant differences can be observed: on the Lignano side there were no emerging parts, and the center of the tidal channel was slightly closer to the Marinetta island but substantially equidistant between the two sides. In the current configuration the center has moved a hundred meters close to the Marinetta island and has deepened by about 2 m. In addition to the depth data of 1961, Dorigo [51] reported the area of the liquid section depicted in Figure 15a and the flow velocities which were measured in 1952 and 1961 through the same section, during two tide cycles as having an average amplitude of about 105 cm. The area of the liquid section resulted equal to about 3438 m3 and 3909 m3 respectively, and therefore the section in the current configuration has decreased by approximately 21.9% and 31.3%. The volumes of water exchanged during the ebb and flood phases was computed by Dorigo [51] based on the available velocity data. The same volumes have been determined from the present simulation of an average year, considering the tidal oscillation during the Sirocco event with a wind speed of 12 m/s. In fact, this event has roughly the same minimum and maximum level as those measured in 1952 and 1961, as reported in Table4 with the relative water volumes.

Table4. Total water volumes exchanged through the section depicted in Figure 15a, during a semidiurnal tide, having the speciﬁed maximum and minimum level, computed by Dorigo [51] in 1952 and 1961 and that derived from the present simulations during the Sirocco event with a wind speed of 12 m/s.

Year Ebb Flood Maximum Minimum Total Water Minimum Maximum Total Water Tidal Level Tidal Level Volume ( Tidal Level Tidal Level Volume ( (cm) (cm) 106 m3)× (cm) (cm) 106 m3)× 1952 80 22 40.00 29 80 44.65 − − 1961 83 29 53.35 29 67 42.93 − − 2019 90 20 49.00 20 90 47.60 − −

The comparison shows that, despite the reduction of the cross section, the volume of water exchanged is very similar to that of the tidal inlet in its natural condition before the construction of the port. This further proves that the current conﬁguration can be considered close to a new equilibrium condition.

6. Discussion According to the qualitative classification provided by Hayes [2] based on a representative sample of examined tidal inlets, the energy regime of the Marano and Grado lagoon can be characterized as mixed and tide-dominated. This suggests that both wind waves and tides govern the main morphodynamic processes equally and that, although the definition is not strictly binding for the morphology of barrier islands [12], these have a drumstick shape, and rounded, large tips. This condition is very close to the one found in the Marano and Grado lagoon system, in particular in the Lignano tidal inlet, at least before the construction of the Marina Punta Faro port. As indicated in Section2, it seems reasonable to state that the geometric shape of the protection piers has strongly altered the profile of the terminal part of the Lignano barrier spit, which has thus lost its natural roundness. The historical analysis of the temporal evolution of the shoreline close to the pier originally provided by Petti et al. [9] and integrated in the present work as depicted in the new Figures4 and5, shows that the construction of the Marina Punta Faro has at least accentuated the subsequent deposition of the sandy material, which was already visible from the early 1980s. The morphological considerations that emerged from Hayes empirical scheme [2] have also made it possible to interpret the historical evolution in order to hypothesize a new conceptual form of equilibrium of the Lignano J. Mar. Sci. Eng. 2020, 8, 77 17 of 21 tidal inlet. Subsequent numerical investigations have been carried out to explore the phenomenon of the narrowing trend, effectively describing the direction and the source of the sediment transport. The numerical simulation setup has been finalized to represent both the tide and wave conditions that led to this rapid morphological evolution correctly. This currently represents a critical task because the two phenomena evolve on very different time scales and their interaction is not linear. Furthermore, the need to limit computational efforts necessarily requires some simplifications which therefore have direct effects on the results that can be obtained. In this sense, the present study differs from others that have treated only one of the two components, considering either waves [27] or tides [25,26,28] relevant to the morphological processes. Another valuable aspect is that the combination of tide, wind and their sequence have been established to simulate a morphologically significant period. Investigations on the long-term evolution are possible at the current state of art defining idealized domains [14,17,19,21,26], with a simplified and controlled bathymetry, and nevertheless are for purely qualitative purposes [16]. Since the aim of the present study is to examine the narrowing process of the Lignano tidal inlet from both a theoretical and a quantitative point of view, the average year has been chosen as the reference morphological period to be analyzed. This time frame is therefore very different from that of a single storm or at most a few months [8] and it required a preliminary analysis to reduce and combine the events [27]. The performed simulations confirmed that the selected limited number of events can accurately reproduce the overall morphodynamics governing the Lignano tidal inlet over an average year; in particular, the strongest wind conditions, and therefore the relative generated wave motion, have the greatest role in sediment transport mechanisms. This result agrees with the processes that also take place within the inner channels and tidal flats of the lagoon [20], where winds with speeds greater than 10 m/s trigger the main morphological changes. On the other hand, a decisive contribution is linked to the tidal meteorological component which can raise or lower the mean sea level depending on the wind direction. This phenomenon can be very significant in the Adriatic Sea, since it is an elongated and rather closed basin and it is affected by the pressure differences between north and south. In particular, the northern part is often subject to an increase in the mean sea level in conjunction with the Sirocco winds which act over a long fetch. On the other hand, the Bora wind generates a lowering of the level in the portion of the sea in front of the Marano and Grado lagoon due to the drag effect on the sea surface. These level differences, which have been reconstructed from the measured data, have proved to be decisive for transport mechanisms in the ebb tidal delta of Lignano, differently from the back-barrier basin which is mainly governed by the tidal astronomical component alone [20,34]. The investigation on the net sand fluxes across a series of sections located along the Lignano Sabbiadoro north-eastern coast and the relative tidal inlet, showed a trend which is very similar to real processes. In particular, two opposite directions of the dominant sediment transport have been highlighted: the first immediately nearshore, on both sides of the tidal inlet, entering the lagoon and the second at the center of the main ebb channel towards the open sea. This result is qualitatively coherent with the dominant transport directions described by Hayes [2] and Fitzgerald et al. [3] in the morphological model concerning mixed-energy tidal inlets, as reported in Figure6a. In this sense, the numerical modeling has been clearly supported and guided by the historical reconstruction combined with the empirical scheme; at the same time, it increased the value of the scheme itself, which provides a large-scale characterization of coastal environments, starting from some particular cases analyzed. In addition to the qualitative aspect, the net volumes of sand moved by longshore currents in the terminal part of the Lignano barrier spit correspond to those actually used in the annual nourishment interventions. Furthermore, the volumes of water exchanged through the tidal inlet, result as very similar to those measured in an “ante-operam” configuration, previous to the construction of the port. J. Mar. Sci. Eng. 2020, 8, 77 18 of 21

Generally, the total water volume exchanged by the lagoon with the open sea, during a characteristic tidal cycle (the tidal prism P) is related to the size of the tidal inlet, speciﬁcally the cross section area A, by means of the well-known equation [53,54]:

P = (A/k) (1/α) (1) where k and α are empirical parameters obtained from observational data. In particular, these coefficients can vary over a range of possible values depending on the morphological characteristics of the inlet, the frequency of the tidal forcing and the friction resistance to the flow [53,55]. The relationship (1) represents a simplified motion equation and it suggests the existence of a dynamic balance, in natural conditions, whereby the liquid area of the cross section adapts to the discharge through the section during a tide cycle. In particular, the tidal prism should not change if the tidal inlet is in a stable condition, that is, if its cross section area does not substantially change. Based on this consideration, the fact that the tidal prism computed in the current configuration results as similar to the one measured before the natural shape of the Lignano inlet was altered, suggests that a new equilibrium state seems to have been achieved. The coefficients k and α have certainly been modified during the transition phase between the construction of the pier and the current one. In particular, the parameter k is affected by the resistance to the flow, which can be affected by the pier, but it is also linked to the width of the section [53,55]. The great reduction of the tidal inlet width has consequently led to a reduction in the same coefficient. This explains why, even though the section area has decreased by approximately 30%, the prism has not been altered and therefore Equation (1) continues to hold. The estimation of the two parameters in the new present condition is beyond the scope of this study, therefore only qualitative assessments are provided in this regard. All these outcomes definitively corroborate the applied procedure and it proves that the numerical approach, with the support of historical research and theoretical models, can be a useful tool, even though it does not yet reproduce reality in detail [14,29]. The simulated events can describe the main erosive and depositional phenomena in a global manner, which is however consistent with the natural processes. However, in order to represent the morphology and the shape of the deposit close to the pier more accurately, as well as the ebb tidal delta, it would also be necessary to consider the slow but continuous action of the average tide. The longshore currents, generated by the drag effect of the wind and by the radiation stress forcings linked to the wave motion, transport the sediment toward the tidal inlet. This happens, as verified, during the most intense events, which occur over a couple of weeks of the year. In the remaining part of the time, the oscillating and repeated movement of the tide shapes and profiles the material deposited near the tidal inlet, leading to rounded equilibrium morphology. It is more difficult to represent this component, since it requires much greater computational times. In order to limit this effort, a high morphological factor should be applied, which can therefore result unbalanced compared to the other events. Moreover, this technique, that artificially increases the morphological changes of the bottom, might not be suitable to describe a phenomenon that occurs slowly over time.

7. Conclusions The present study has deeply examined by means of an integrated approach the narrowing phenomenon, which has changed the Lignano tidal inlet morphology in the last forty years. Three methods have been adopted and combined, in order to deﬁnitely verify the causes of the narrowing, to understand the deposit mechanisms that are reducing the inlet width, and ﬁnally to propose a new form of equilibrium of the Lignano barrier spit. The methodology considers the following steps: an in-depth analysis of the historical evolution of the coastline and in particular of the sand deposit close to the pier of the Marina Punta Faro port; a qualitative description of the morphodynamics of the Lignano tidal inlet based on the conceptual and empirical model of J. Mar. Sci. Eng. 2020, 8, 77 19 of 21

Hayes [2]; the numerical process-based approach to validate, also from a quantitative point of view, the hypothesis of evolution formulated through the first two steps. The historical synthesis and the empirical scheme, together with the main results provided by previous studies in the same or similar environments, supported the numerical setup based on the classic coupling between a 2DH morphodynamic model and a spectral model. In particular, the combined role of specific events of winds and tide has been fundamental to describe the morphological evolution over an annual time scale. Moreover, the meteorological component of tidal oscillations proved to be decisive in the analyzed case, given the particular shape of the Adriatic Sea and the consequent setup of sea level, due to pressure differences from north to south and wind drag effects. The results of the numerical modeling confirmed that the current state of the tidal inlet is very close to a new equilibrium condition. This has been demonstrated by the extent of the obtained narrowing, the solid volumes transported by the longshore currents directed from the outside towards the inner part of the lagoon and the shape of the cross section. In particular, the section area is preserved over an average year and it is linked to a tidal prism which results equal to that calculated in the previous equilibrium configuration before the construction of the port. The next step of this work will concern the possibility of estimating the new coefficients entering the relationship between the tidal prism and the inlet cross section area and the study of the evolution over a longer time than the average year, simulating events that have very different weights in the morphodynamic processes of an environment as complex as a tidal inlet.

Author Contributions: M.P., S.B., S.P. and E.U. contributed equally to this work in all its stages: conceptualization, methodology, data curation, formal analyses, writing, review and editing. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: This research was in part supported by the ‘Regione Autonoma Friuli Venezia Giulia, Direzione centrale infrastrutture, mobilità, pianificazione territoriale, lavori pubblici e edilizia’. Conflicts of Interest: The authors declare no conflict of interest.

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