Geomorphological Effects of Land Reclamation on the Coastal Plain East of the Venice Lagoon (Italy) ^†

Abstract

The present study aimed at identifying the transformations that occurred since the 1600s in a low-lying territory located east of the Venice Lagoon. The environmental modifications were examined in a GIS environment through the interpretation of multidisciplinary data. The analysis of historical maps realized from 1641 to 1943, as well as aerial photographs and satellite images taken in the last few decades, was fundamental to reconstruct the evolution of the study area mainly in relation to anthropogenic processes. Over the past few centuries, various attempts of land reclamation partially or totally failed. However, even if an overall general decrease in both the total pond surface area and the marsh extension has been observed since the 19th century, the territory appeared entirely dry back only in the 1940s owing to the efficacy of the main most recent works.

1. Introduction

According to the Ramsar Convention [1], wetlands are areas where the land is covered by water or the water table is at or near the surface of the land. Depending on their position, wetlands can be characterized by freshwater, brackish water or saltwater; consequently, they can include a wide variety of inland and coastal habitats. They can be permanent or temporary and can have natural or anthropogenic origin. Despite their importance in supporting biodiversity and in providing valuable ecosystem services [2,3,4] and benefits to society and economy [5,6,7], wetlands are generally considered as unhealthy and unproductive territories and, consequently, they are often converted into areas more useful for human activities [8]. Land reclamation, which is carried out in many parts of the world, e.g., refs. [9,10,11,12,13,14,15,16,17], to adapt the territory to human needs for several purposes [18], is one of the main causes responsible for irreversible modifications to wetlands.

In the past few centuries, before urbanization and industrialization, the main purpose of reclamation was to improve land and make it suitable for agriculture [14,19]. In more recent times, it has been also aimed at creating new areas for other different economic and societal purposes, such as urban and industrial development, the construction of infrastructures, and recreation and tourism activities [16,20,21,22,23,24,25]. It was estimated that around 11% of the global population (around 680 million people) lives in low elevation coastal zones (i.e., land less than 10 m above sea level) [26]. However, as this value is expected to grow and exceed one billion by 2050 under all Shared Socioeconomic Pathways [27], coastal land reclamation is considered the main solution to create new spaces for urban development [10].

As coastal areas support a wide variety of human activities [28] and coastal ecosystems provide a number of goods and services [29], in the last few decades, particular attention has been paid to saltwater wetland reclamation and its effects on the environment, such as habitat loss and the degeneration of coastal ecosystems [30,31], declines in biodiversity and ecosystem services [31,32], modifications in local morphology and hydrodynamics [33], increase in soil and water pollution [34], and rising water consumption by accelerating surface water cycling through evapotranspiration processes [35]. Conversely, freshwater wetlands have received relatively less attention. In addition, wetlands located in fluvial or deltaic environments have been reclaimed over time as well. Consequently, on the one hand, reclamation has increased the amount of usable land, but on the other hand, it has often produced serious long- and short-term impacts on the environment—in particular the disruption of ecosystems and the loss of natural habitats and biodiversity. Fluvial ecosystems develop in response to dynamic processes that occur in a multi-dimensional framework, also including interactions between the channel and the riparian/floodplain system [36]. These processes significantly affect the evolution of the perifluvial zones and lead to the formation of wetlands, which are considered important biodiversity hotspots [37] and are among the most biologically productive habitats [38]. Reclamation in fluvial environments is generally carried out by enclosing submerged areas with walls, by draining and/or filling them with high-quality materials (mainly soil having variable grain size, from gravel to clay) or through interventions on the fluvial network and the realization of hydraulic works, e.g., [39,40,41,42,43].

In the present paper, an example of freshwater wetland reclamation carried out in the last four hundred centuries in the inner coastal plain east of the Venice Lagoon (northern Adriatic Sea, Italy) (Figure 1) is discussed. In particular, the purpose of our study was to identify the profound land transformations produced by the human activities carried out since the 17th century to reclaim this low-lying territory. The interpretation of the results in the light of the well-documented natural and anthropogenic processes allowed understanding the response of the environment to these actions. Modifications to ecosystems were also briefly considered.

In the literature, various studies on historical coastal wetland reclamation exist; however, they have often different objectives, cover shorter periods or are focused on diverse environmental features. For example, ref. [41] analyzed two examples of Medieval land reclamation in Holland and in the Po Valley, mainly focusing on social consequences, whereas [18] compared the evolution of two wetland landscapes from the Bronze Age to the present (the Pontine marshes in Italy and the Onlanden in the Netherlands) through a triangular model. Ref. [42] reconstructed land reclamation activities carried out in the Volturno delta plain (Italy) in the last 150 years, and the resulting landscape transformations, by analyzing historical maps; however, they mainly focused on drainage network and coastline changes and on land use and land cover modifications. Similarly, ref. [44] described and quantified land reclamation occurred in Ria Formosa (Portugal) since the end of the 19th century through the analysis of historical maps and aerial photographs, which made it possible to recognize land use variations; therefore, land reclamation was considered as a function of the dynamics of economic activities.

Currently, the area under investigation is not reached by tides and coastal flooding. However, in the past, it was impaired by natural phenomena mainly related to fluvial processes, which were responsible for the formation of freshwater marshes. Historical information about human interventions carried out to reclaim the investigated territory was previously published by various authors. However, to the best of our knowledge, researches aimed at identifying in detail the subsequent phases of conversion of a territory from a wetland into a dry land in a time span of four centuries, also including the analysis of a number of historical maps realized from the Modern Age to the present, have not been published yet. This approach made it possible to obtain both qualitative and quantitative results. A similar method was used by [39] to investigate the evolution of the Leyre river mouth (Arcachon Bay, France), but it was applied to a salt/brackish environment.

2. Study Area

The coastal plain extending east of the Venice Lagoon, between the Sile and the Livenza rivers, is a low-lying territory characterized by the presence of both agricultural areas and small urban centers. In the last few decades, the pressure of urbanization and tourism is significantly increased on the coastal stretch. The present setting of the territory is the result of radical transformations produced over the last centuries by several human interventions, first of all diversions of the lower courses of the Piave and the Sile rivers, that were mainly aimed at preventing the progressive sedimentary infilling of the Venice Lagoon, which was caused by the solid transport carried by these rivers [45,46], and at draining the wetlands that were historically present in this coastal region. In particular, the formation of large marshes was favored by various factors, e.g., the frequent occurrence of river floods [47], the low land elevation (commonly below mean sea level), and the very gentle slope of the coastal plain (about 0.05%) [48]. Consequently, in the last five centuries, several land reclamation activities were carried out to drain the swamps, and canals and sluices with embankments were created to prevent flooding [49]. As a result, the present coastal plain is crossed by a dense network of canals and ditches, and pumping stations keep this area dry. Soils characterized by fine-grained sediments and frequently containing organic matter are common; they have poor drainage and sometimes are affected by salinization [48,50]. Land reclamation has also favored the compaction of these deposits and the dehydration and oxidation of the organic material, thus triggering an increase in the local subsidence rates [51] and the consequent further decrease in ground elevation, particularly in the low-lying areas that were previously characterized by marsh conditions. At present, the investigated territory is affected by significant subsidence [52] on the order of 4–6 mm/yr [53]. According to [54], land reclamation carried out in the first half of the 20th century has likely contributed to this phenomenon. Over time, the effects of subsidence have been exacerbated by the construction of river embankments, which have prevented sediment deposition on the flood plain [55].

During the Holocene, the inner zones of the investigated territory were mainly shaped by fluvial dynamics, whereas the evolution of the southern stretch, also including the coastal area, was influenced by both marine and fluvial processes. In the inner zones, which are of interest for this study, silty and clayey sediments, also including organic material, are common in low-lying areas, while sandy deposits characterize the old fluvial ridges formed by abandoned courses of the Piave and the Livenza river systems. These ridges are quite clearly recognizable, as they are 1–2 m higher than the surrounding zones [48] and, owing to their elevation, they have been historically considered preferred sites for urbanization and farming. In addition, their coarser grain size favored drainage, thus offering more healthy environmental conditions.

In the southern stretch, evidence of ancient sandy beach ridges [56,57,58,59] testify to the local coastline progradation that occurred during the Holocene [60,61,62,63]. Fine-grained sediments containing organic matter are common in the old inter-ridge depressions [59]. Although the ridges have not preserved their original relief and morphology owing to the extensive reclamation works and other human interventions (e.g., landfills, land smoothing) carried out to make these area suitable for farming, weak ground undulations are locally present [59].

In our study, we focused our attention on a selected territory (i.e., the Consorzio Ongaro Inferiore 1° Bacino), which is located between the lower courses of the Piave and the Livenza rivers and has an extension of about 1224 km² (Figure 1). It was selected because in the past few centuries, it was significantly impacted by both natural and the above-mentioned anthropogenic processes. At present, much of the Consorzio’s land is set aside for agricultural use, and only small areas are urbanized. The most important urban area located in this territory is a small town named Eraclea, also including some hamlets. Ground elevation usually ranges from −1.5 to 0 m a.s.l.; lower values (down to 2.5 m b.s.l.) locally occur in small depressions, whereas higher elevation (up to 2 m a.s.l.) is measured close to the Piave River banks [64]. Depending on their origin, soils have different composition [48]. Fluvial ridges, which are evidences of abandoned watercourses, and crevasse-splay deposits are composed of sand, silty sand and sandy silt, whereas at the transition between fluvial ridges and nearby lower zones, sediments show higher grain-size variability. Areas with lower elevation are generally characterized by clay and clayey silt; in addition, significant amounts of organic matter (sapric soil materials) are also locally observed and indicate the location of old freshwater marshes. These deposits have low permeability, very slow drainage, and low oxygen availability. Similar deposits are also present in the southern part of the territory, but they differ from the previous ones mainly because they locally contain fragments of shells. In particular, the presence of bivalve mollusks belonging to the genus Cardium indicates that their evolution was also significantly conditioned by marine processes.

3. Materials and Methods

The environmental modifications that took place in the last few centuries were identified through both the joint interpretation of existing multidisciplinary data and information and the results obtained from new investigations, which were based on photo-interpretation, analysis of historical maps, and in situ surveys.

The QGIS 3.16 software was used to store and compare data, to georeference historical maps and photographs, to analyze spatial information, and to create evolutionary maps of the study area.

In particular, the availability of historical maps, showing the real conditions of the region at different times, made the precise reconstruction of the evolution of the territory starting from the first half of the 17th century possible. Notwithstanding their different accuracy, the old maps provided valuable information about the morphological and environmental characteristics of the area in the past. However, only the most detailed and reliable maps (i.e., 1641, 1776, 1831, 1833, 1891–1892 and 1943), also characterized by low distortion and better resolution, were georeferenced, digitized, and compared (Table 1), whereas the others were mainly considered to gain possible additional information about the evolution of the territory. The choice to digitize only the maps having the above-mentioned specific characteristics was made to reduce possible inaccuracies deriving from the limitations of the tools and methods used in the past to produce these documents. The georeferencing process was carried out using the coordinate reference system for Italy “Gauss-Boaga East” of the digital topographic maps at the 1:5000 scale, which are available in shapefile format in the “Regione del Veneto” (Veneto Region) Geo Data Portal [65].

In order to georeference the historical maps, first we identified all the features that were recognizable on both the old and the current topographic maps, and then we selected them as references to locate the ground control points (GCPs). We used, on average, 30 well-distributed ground control points (GCPs) per map. The 1641 map was characterized by a relatively lower quality, so it was georeferenced by using the Thin Plate Spline transformation. Conversely, the other maps were georeferenced by using the polynomial algorhithms, in particular the second-order polynomial, because they were less distorted and more detailed. The GIS project was set accordingly to the Monte Mario/Italy Zone 2 (East) EPSG: 3004 reference system. Channels, marshes, and ponds, which were identified on each map, were manually outlined and measured. Then, they were overlapped and compared to define their variations over time.

Aerial photographs taken in 1983 and 1999 (available at the aerial photo library of the “Regione del Veneto” geo data portal [66], Regione del Veneto—L.R. n. 28/76—Formazione della Carta Tecnica Regionale) (Table 2) and satellite terrain images taken until 2024 (from the Google Earth Pro 7.3.6 software) documented recent significant transformations, mostly resulting from urbanization and other anthropogenic interventions, and variations in land use. These photographs were chosen owing to their good quality and resolution that also gave the possibility of clearly identifying traces of abandoned sinuous fluvial channels, old drainage canals, and former marsh areas, thus providing further information about the planform morphology and the real position of the channels identified on the historical maps. In addition, satellite images gave evidence of the present setting of the territory. On both aerial and satellite images, it was also possible to identify evidence of Holocene ancient coastal beach ridges. We used the polynomial algorithms to georeference the aerial photographs; on average, 20 GPCs were selected.

Information about the geomorphological characteristics of the investigated area (also including evidence of ancient features) were obtained from ref. [67].

Sedimentological and stratigraphic data, which were useful to better define the evolution of the study area, were available through the data service “Webgis Geologia—Piani Acque e geologia” of the Metropolitan City of Venice [68]. In particular, only data derived from cores realized in areas not particularly modified by human interventions (e.g., away from buildings, streets, and other infrastructures) were selected to avoid them being altered by anthropogenic activities. Important information about soil characteristics and distribution was provided by ref. [48] and by ref. [69]. They improved the knowledge of the local deposits in relation to the different past environmental conditions and ref. [48] allowed confirming the position of ponds, ridges and channels that were recognized on the historical maps.

Table 1. Historical maps that were digitized and compared.

Title	Year	Color	Source
Mappa progettuale del territorio costiero compreso tra Piave e Livenza (Map of the coastal territory between the Piave and the Livenza rivers)	1641	Color	[70]
Map realized by Trevisan	1776	BW	[71]
Topografia della Provincia di Venezia pubblicata da Luigi Forti (Topographic map of the Venetian territory published by Luigi Forti)	1831	BW	[72]
Carta topografica del Regno Lombardo Veneto construtta sopra misure astronomico-trigonometriche ed incisa a Milano nell’Istituto Geografico Militare dell’I.R. Stato Maggiore Generale austriaco (Topographic map of the Lombardy-Veneto Kingdom, obtained from astronomical-trigonometric measurements and engraved in Milan in the Military Geographical Institute of the IR Austrian General Staff).	1833	BW	[73]
IGMI Topographic maps, 1:25,000 scale	1891–1892	BW	CNR-ISMAR
Topographic maps, 1:50,000 scale, Sheets 39-II, 52-I, 52-IV	1943	Color	[74]

Table 1. Historical maps that were digitized and compared.

Title	Year	Color	Source
Mappa progettuale del territorio costiero compreso tra Piave e Livenza (Map of the coastal territory between the Piave and the Livenza rivers)	1641	Color	[70]
Map realized by Trevisan	1776	BW	[71]
Topografia della Provincia di Venezia pubblicata da Luigi Forti (Topographic map of the Venetian territory published by Luigi Forti)	1831	BW	[72]
Carta topografica del Regno Lombardo Veneto construtta sopra misure astronomico-trigonometriche ed incisa a Milano nell’Istituto Geografico Militare dell’I.R. Stato Maggiore Generale austriaco (Topographic map of the Lombardy-Veneto Kingdom, obtained from astronomical-trigonometric measurements and engraved in Milan in the Military Geographical Institute of the IR Austrian General Staff).	1833	BW	[73]
IGMI Topographic maps, 1:25,000 scale	1891–1892	BW	CNR-ISMAR
Topographic maps, 1:50,000 scale, Sheets 39-II, 52-I, 52-IV	1943	Color	[74]

In situ surveys were mainly carried out to validate already existing data and information and to check the present environmental conditions, including the presence of native vegetation communities.

Finally, the geomorphological changes were identified and interpreted in the light of the well-documented natural and anthropogenic processes occurred before, thus demonstrating their effects on the environment.

Table 2. Aerial photographs that were analyzed to detect recent transformations and to identify traces of abandoned channels and former marsh areas.

Date	Source	Format	Altitude (m)	Color
17 March 1983	Veneto Region	Digital aerial photos	2600	BW
19 March 1999	Veneto Region	Digital aerial photos	2500	BW

Table 2. Aerial photographs that were analyzed to detect recent transformations and to identify traces of abandoned channels and former marsh areas.

Date	Source	Format	Altitude (m)	Color
17 March 1983	Veneto Region	Digital aerial photos	2600	BW
19 March 1999	Veneto Region	Digital aerial photos	2500	BW

4. Results

Historical documents provided a lot of detailed information about the evolution of the territory, which were very useful to have a complete view of processes and events responsible for the transformations that occurred in the last few centuries. In particular, the analysis of historical maps made possible the precise identification of the real conditions of the territory at different times since the 16th century.

In the 1500s, the study area was characterized by the presence of extensive wetlands, including marshes and ponds, crossed by channels. The major watercourses formed fluvial ridges, and very small urban settlements and arboreal vegetation were present in zones with higher elevation.

The conditions that occurred in the 16th century were clearly documented on the available cartography realized in the 17th century [49]. A detailed historical map, dating back to 1641 (Table 1), showed that in the mid-1600s, the territory was almost completely characterized by the presence of wetlands that covered about 114 km²; they also included some ponds having various extensions and covering a total area of 24 km² (Figure 2). Some fluvial ridges, formed by ancient channels of the Piave River system, were the only zones that rose above the marshes. Streams, mainly having a NE–SW orientation, crossed the wetlands. They were more frequent in the central–southern sector of the investigated area. Some of them supplied the ponds with water.

Environmental conditions occurring in the 18th century were mainly detected through the analysis of the map realized by Trevisan in 1776 (Table 1, Figure 3). The comparison with the 1641 map highlighted some variations. Indeed, even if the marshlands still covered about 112 km², the total pond surface area appeared to be significantly increased (i.e., 31 km²). In addition, the channel network was characterized by a smaller number of streams, which were more frequent in the southwestern sector. Only some well-developed fluvial ridges, formed by ancient channels of the Piave River system, but different from those observed in the 1641 map, rose above the marshes.

Analysis of other historical maps, which were not georeferenced, added details about the conditions existing at the end of the 18th century. One of them was the Von Zach’s map, realized between 1792 and 1805, that provided useful information about land use. It showed a territory almost entirely covered by marshlands. Urbanization and agricultural activities were observed only on fluvial ridges, which were also characterized by the presence of orchards, vines, arable lands, and woods.

Evidence of the first important attempts of reclamation, based on better-organized interventions, was observed in the 1833 map (Table 1, Figure 4), showing the presence of some drainage canals in the northeastern sector of the investigated area. On the other hand, on this map, ponds were not clearly distinguishable. Conversely, ponds were easily identified in the 1831 map (Table 1, Figure 5) that, differently from the 1833 cartographic document, did not display drainage canals. The analysis of the 1831 map revealed that wetlands covered about 95 km² and ponds about 16 km².

A clear decrease in pond and marsh extensions was detected at the end of the 19th century. Indeed, the 1891–1892 map (Table 1, Figure 6) showed that wetlands covered 88 km², whereas ponds covered about 2 km². The digging of a number of ditches, which crossed the entire area and were cut taking into account the local topographic characteristics, was noteworthy.

Even if reclamation works were completed after the Second World War, the territory appeared to be almost entirely dry back at the beginning of the 1940s. This condition was clearly evident in the 1943 map (Table 1, Figure 7), which displayed a dense network of drainage canals with pumping stations and the absence of marshes and ponds, which were replaced by cultivated lands with farmhouses.

Over time, land use has also changed along with the environmental modifications that occurred in the last centuries. In the 1700s, in less wet areas, only a few cereals and poor-quality fodder of permanent pasture were produced, whereas reeds and straw to be used for bedding or as heating fuel were harvested from marshes [47]. In the mid-1700s, rice cultivation was introduced; it became a real treasure in a short time. In the subsequent centuries, reeds and straw were also used to build a type of peasant’s house, which was locally named “casone”. In the 1930s, land productivity was very satisfying [75]. Oat, wheat, clover, corn, vines, mulberry trees, and industrial crops, such as tobacco, beetroots and oilseed plants, were cultivated. In 1966, a devastating flooding caused loss of life, extensive damage to buildings and infrastructures, and the destruction of crops and livestock. After this event, interventions were carried out to improve drainage conditions, and new irrigation systems were realized [75]. As a result, corn cultivation became prevailing. At present, in the lower Piave River coastal plain crop rotation is common, mainly including corn, cereals and soy [48]. Sugar beet and rise cultivations are also locally present as well as horticultural crops and orchards.

In the last few decades, some agricultural zones have been urbanized and, locally, industrial areas have also developed (Figure 8). Therefore, after land reclamation, territories intended for farming have decreased. At present, more than 80% percent of the investigated territory is used for agricultural purposes, and built-up areas are mostly located close to the main road junctions. Since the second half of the 20th century, the shape and extension of the plots of farmlands that still exist have not significantly changed. The most noteworthy modifications were generally observed on the ditch network, as shown in Figure 9. In particular, the number of ditches was locally reduced, some channels were rectified (Figure 10), and new irrigation systems were applied. In addition, a decrease in riparian plant communities along channels and canals was also noticed. At present, spontaneous vegetation can be found only in a few small areas.

Sedimentological and stratigraphic data well document the environmental changes that occurred in the last few centuries. At present, clayey silt and silty clay, also including peat and organic matter in general, characterize most of the investigated area and testify the ancient existence of marshes and ponds, whereas sandy deposits perfectly correspond to old fluvial ridges. The latter are progressively replaced by silty sand and sandy silt away from the position of the abandoned channels. Locally, sediments composed of sand and sandy silt and covering marsh deposits indicate river overflow and embankment failure.

As regards natural vegetation, a few native flora species only grow along some channels [48]. On the other hand, riparian vegetation is almost completely absent along the drainage canals, owing to the continuous maintenance and cleaning works carried out to keep the entire water network efficient. In addition, pesticide use and the introduction of invasive alien species have reduced biodiversity [69]. Consequently, at present, there is no evidence of the oak–hornbeam lowland forest—and related vegetation—that, in the past, was a widespread climax community in the alluvial plain of the Veneto and Friuli Venezia Giulia regions. Hedges composed by shrubs and trees, which were common in rural areas until the 1950s, are also missing [69]. In agreement with [48], Phragmites australis, Typha sp.pl., Caltha palustris, and Sparganium erectum are common close to the channel banks, whereas riparian zones are characterized by tree and shrub species, mainly Salix alba, Alnus glutinosa, and Populus nigra, together with exotic and naturalized species, such as Robinia pseudoacacia, Ailanthus altissima, and Amorpha fruticosa.

5. Discussion

In the last millennia, river avulsions and flooding, mainly related to the courses of the Piave River system, have largely influenced the evolution of the investigated area, thus producing serious impacts on local populations.

Testimonies provided by Latin authors suggest that in Roman times, the present coastal plain was a lagoon characterized by islands and marshes, which were largely affected by tides. Islands were chosen by local communities as preferred locations in order to escape Barbarian invasions [76]. Since that period, inhabitants have always taken various measures to protect the territory from damages caused by inundations, to reclaim wetland, and to maintain navigable the lagoon channels [77]. However, these conditions significantly changed at the end of the 6th century, when a catastrophic flood caused the formation of two new Piave River courses flowing toward the investigated coastal area [47]. This event was responsible for the deposition of abundant alluvial sediments that transformed the lagoon into a marshland and the islands into peninsulas [76]. In such conditions, maritime trade became more difficult and the vulnerability to enemy attack increased; therefore, the territory began to depopulate, the works carried out to reclaim the marshlands were abandoned, and the area underwent rapid degradation [47]. In addition, it must be considered that the preservation of reclaimed lands require continuous monitoring and interventions and, in that period, in Italy, the available means and resources were too scarce to guarantee their maintenance in the long term, particularly after the fall of the Roman Empire and the consequent lack of a centralized power, i.e., a condition that favored the use of heterogeneous technological methods [78]. Various authors came to the same conclusion when they analyzed the historical evolution of reclaimed areas, such as the Pontine marshes in the Middle Ages [18] or territories in the Po Plain [41].

On the other hand, the frequent river floods that occurred in the subsequent centuries in the lower Piave River plain favored the accumulation of considerable amounts of sediments close to the riverbed. They resulted in well-developed fluvial ridges that, between the 12th and the 13th centuries, were chosen as preferred sites for urbanization and farming activities [47,79] because they guaranteed protection from inundations owing to their higher elevations. Malarial marshes were common as well.

From the beginning of the 15th century, the ancient Republic of Venice (i.e., the Serenissima) carried out a number of hydraulic works to avoid the lagoon swamping, such as the Piave and Sile river diversions, the digging of other canals, and the construction of embankments.

Until the 17th century, the Piave River mouth was located close to the eastern lagoon inlet (Figure 11), whereas the Sile River flowed into the lagoon. Consequently, as their sediment discharge could favor the infilling of the Venice Lagoon, both their lower courses were diverted eastward. First, a new canal was dug about 13 km east of the Piave River (named “Taglio del Re” and completed in 1579) to divert the Piave River flow when it was in spate, but this solution did not produce positive effects on the preservation of the lagoon, and the nearby territories continued being inundated when the river was in flood. In the first half of the 16th century, an embankment was also built along the right side of the Piave River [47,70]. As a consequence, more severe inundations started affecting the territories located east, adjacent to its left side, thus creating new freshwater marshes.

As the realization of the “Taglio del Re” did not give the expected results, in the middle of the 17th century the Piave River was diverted eastward again through the digging of a new canal east of the previous one (Figure 11) to preserve the lagoon and to avoid the inundation of the nearby coastal plain, too. According to the project, fluvial water was channeled into the Livenza River, but before flowing into the Adriatic Sea close to Porto S. Margherita, it had to inundate the nearby marshlands (corresponding to the study area), thus forming a lake [47,70]. The works started in 1641 and ended in 1664. Artificial levees were built to contain the water, but embankment failures frequently occurred. Finally, in 1683 owing to a levee breach, a new Piave River mouth naturally formed at Cortellazzo (i.e., the present river mouth location), following the land slope best conditions. Therefore, Figure 2 shows the characteristics of the territory before the Piave River diversion: it was a wide wetland, also including some ponds and crossed by channels, in which a few fluvial ridges were the only areas arising above the marshes.

In the second half of the 17th century, the Sile River, which previously flowed into the lagoon, was diverted eastward too, and its water was channeled into the old Piave riverbed (Figure 11) [47]. As it is a resurgent river, the new mouth did not represent a threat for the lagoon. Consequently, since the 17th century, the events related to the fluvial dynamics of the Piave and the Livenza rivers were those mainly responsible for the evolution of the investigated area.

The modifications that were recognized in the 18th century by comparing the 1641 and 1776 maps were probably caused by the human interventions carried out in the second half of the 17th century to divert the Piave River eastward, also including the formation of the Piave River lake. The disappearance of the lake due to the 1683 embankment failure was likely the main factor responsible for the worsening of the already existing wetland conditions, resulting in the formation of larger ponds and a simplification of the channel network (Figure 3).

It is evident that the human activities carried out on the fluvial network in the 1500s and 1600s did not bring benefits but rather increased the degradation of the territory by leading to frequent flooding. The same results were observed in the Po Valley when, in the past few centuries, the rivers were constrained by embankments and new canals were dug [80]. Owing to the ineffectiveness of the drainage systems, some territories remained submerged for long periods of time, thus creating unhealthy conditions [41]. On the other hand, ref. [81] observed that some correctly planned human interventions carried out in the Po Plain between the 16th and the 17th centuries brought benefits and transformed flooded land into cultivable fields. They consisted of embanking the watercourses responsible for inundations and reducing their hydrometric levels to elevations that were lower than those of the territories they passed through. However, owing to the required costs, probably the described method was not widely used to reclaim wetlands, and this could be one of the main causes of failure of some attempts of land drainage in our study area.

In the investigated territory, the first appreciable results of land reclamation occurred in the 19th century, which were mostly due to the intensification of the reclamation works [49]. The Republic of Venice did not support the interventions that were carried out until the end of the 18th century for fear that the implementation of drainage systems could harm the lagoon [79].

The first land reclamation consortia formed in the 1500s to organize land drainage activities, but they mainly developed from the 17th century to the beginning of the 1900s [47], thus bringing improvements to the reclamation works. Consequently, better results were achieved.

The first significant canalization works and the construction of motor-driven pumping stations began in the first half of the 19th century together with the growth of the land reclamation consortia that had to manage the water drainage system. The decrease in marsh and pond extensions observed in the 1830s (Figure 4 and Figure 5) is clear evidence of the first positive effects of these interventions and the use of new methodologies. Nevertheless, many difficulties continued to be encountered during land reclamation. For example, the pumping stations did not always operate efficiently, and diseases, such as malaria and pellagra, were frequently major public health problems.

Owing to the development of a dense well-organized canal network and the building of new more efficient pumping stations, at the end of the 19th century, ponds were almost completely dried up, as shown on the 1891–1892 map (Figure 6). The major effects were observed in the northeastern and southwestern zones, which is likely because they were characterized by both the presence of a greater number of canals and higher elevations due to the closeness to the fluvial ridges of the Livenza and Piave rivers, respectively. In the past, riversides were often considered suitable sites for starting land reclamation [82]. In the investigated area, the digging of drainage canals and the construction of sluices and embankments to avoid the possible effects of high tides were carried out in areas close to sea level. The best results were achieved in territories lying at about (or more than) 1 m a.s.l., where the new reached conditions were able to support agricultural activities. Lands at 0.30–1 m a.s.l. were more suitable for meadows and pastures, whereas marshes persisted in areas below mean sea level [47]. However, the available tools and resources were still too scarce to overcome any unforeseen events.

Until the beginning of the 20th century, part of the territory was still characterized by wetlands and ponds, mainly from autumn to spring, owing to the difficulties in keeping this area dry due to technological limitations and the scarce availability of resources. Indeed, the works were carried out by using shovels and wheelbarrows. Significant support was given by the adoption of the animal-powered Decauville systems (i.e., narrow-gauge rails). Therefore, lands underwent reclamation only before seeding [47]. In addition, during the First World War, the environmental conditions worsened, particularly in 1917 because of the destruction of the pumping stations [75]. However, after the war, new pumping stations were built, thus improving land reclamation again. This improvement must be set in a wider historical context, as after the unification of Italy and the First World War, most of the Italian wetlands underwent extensive and massive land reclamation works (in Italy named “bonifica integrale”) [18] that were responsible for an intense landscape transformation, including the creation of new and more healthy lands to be mainly used for urbanization, farming and industrial activities. This new condition, which occurred in the 1940s, is evident in Figure 7: the absence of marshes and ponds replaced by a dense network of drainage canals with pumping stations demonstrated how the last human interventions were able to completely artificialize the water flows and, consequently, modify the environment.

As regards land use, in the 1700s, the presence of extensive wetlands made the territory highly unhealthy and undermined land productivity, which was very scarce or absent [47]. Agricultural activities were made easier later by the realization of the drainage canal network and the construction of sluices. Drainage canals and sluices also favored water replacement, which was useful for fishing and for improving rise cultivation.

Overall, in the last few centuries, reclamation works, modifications of the riverbeds and their levees, the digging of new canals, and anthropization have significantly altered the floodplain and irreversibly modified the original environment. On the one hand, land reclamation has made productive previously unusable lands, but on the other hand, it has modified or destroyed native habitats. As agricultural activities have prevented the evolution of the natural lowland vegetation, channels play a fundamental role in preserving what still remains of plant richness and diversity [83,84,85,86]. However, the ordinary management practices that are carried out to facilitate the flow, thus avoiding waterlogging and guaranteeing the rapid removal of water from land, often hinder the development of the typical riparian vegetation. As a result, along the drainage canals, riparian vegetation is almost completely absent and, at present, native plant communities are monotonous and fairly poor in species.

6. Conclusions

The impact of land reclamation on the evolution of a low-lying territory located east of the Venice Lagoon was investigated in the long term (last four centuries). The research was performed in a GIS environment through the analysis and interpretation of historical maps, aerial photographs and other multidisciplinary data. Evolutionary maps, useful to visualize and compare the different environmental conditions that occurred over time and to quantify the effects of land reclamation, were also produced. Cause–effect relationships among landscape modifications, natural processes and human activities were distinctly recognized. Most of the detected environmental changes were mainly due to the anthropogenic interventions carried out to preserve both the lagoon from the infilling produced by the river sediment discharge and the lower Piave River coastal plain from flooding. Even if since the 16th century, various attempts of land reclamation were carried out, the investigated area displayed the typical characteristics of a wetland until the 19th century because the interventions were not always correctly planned or, in other cases, the available means and resources were too scarce to counteract the effects of natural processes, which also played a significant part in the landscape evolution. However, the first appreciable results of human activities were observed in the 19th century as a consequence of the intensification of reclamation works along with the development of land reclamation consortia in charge of their organization. Anyway, the territory appeared to be entirely dry back in the 1940s, thus proving the efficacy of human interventions carried out after the First World War, which consisted in the building of new pumping stations and in the digging of a number of drainage ditches crossing the entire area. At present, this low-lying territory is mainly used for farming purposes and only partially urbanized.

Modifications on the drainage network and variations in land use (e.g., urbanization of agricultural areas) are the most significant changes that have been detected in the last few decades. However, the anthropogenic activities that were carried out to counteract the effects of natural events and to transform an unhealthy wet territory into cultivable fields radically modified not only the landscape but also the original ecosystems. Agricultural activities have prevented the evolution of natural lowland vegetation and pesticide use as well as the introduction of invasive alien species has reduced biodiversity. In this context, channels and canals could play a fundamental role in preserving plant richness and diversity, but the ordinary management practices, which are carried out to facilitate the flow, often hinder the development of the typical riparian vegetation, which results in communities that are poor in species.