Increasing geohydrological instability in a valley of the Italian Central Alps: a study in the Anthropocene

ABSTRACT Riverbed deepening, waterway shortening, and forced narrowing are among the most relevant consequences of the anthropic disturbance of alpine rivers over the last few centuries. A map was generated through a detailed study of the Adda Valley, a famous valley in the Central Alps, northern Italy. This study was conducted to evaluate the anthropization status, geomorphological patterns (in terms of altimetric and planimetric riverbed variation), evolutionary trends, and flood hazards of the river over the last 150 years. The map presents the expansion of urban and industrial areas that occurred before and after one of the most devastating floods in Adda Valley. Supporting both the identification of potentially vulnerable areas and the possible effects of damaging floods, the map provides information for improved evaluation of the vulnerability of bottom valley areas after anthropogenic modification and for identifying the damaging effects of past flood events, which could recur.

To establish new thriving settlements, fluvial rectification was common throughout the Western world in the nineteenth century, not only along major rivers, but also in alpine streams (Agnoletti et al., 2011;Comiti, 2012;Liébault & Piégay, 2002).The proximity to streams meant richer soil, which is ideal for agricultural societies; however, such areas can become hazardous during flooding (Balica et al., 2014;Capelli et al., 1997;Dai Prà & Allegri, 2017), which is relatively common in alpine valleys (Turconi et al., 2014).Depending on both natural and anthropogenic factors, flood hazards are influenced by the geomorphology and width of the valley itself (Camarasa-Belmonte & Soriano-García, 2012;Guerriero et al., 2018;Petroselli et al., 2020).Another critical factor is the quantity and quality of bridges and other river-crossing infrastructure (Horacio et al., 2019).Fluvial rectification, once considered an efficient flood protection action, is now known to increase the impact of 'flood power' (Thompson & Croke, 2013).Thus, to support the effective implementation of river management actions, it is essential to possess a detailed multi-temporal cartography of the planimetric and altimetric evolution of the riverbed (Langhammer & Vajskebr, 2003).
River management should be based on an integrated analysis of the status of a watercourse, in which subsequent planning and development actions must be rooted (Jun et al., 2020;Li et al., 2020).Integrated river basin management (IRBM) is considered the best starting point for integrating the management of water quantity and quality for both ground and surface water (European Union, 2000).Within IRBM, the impacts of both natural and anthropogenic components are considered at different levels to guide the adoption of specific policies related to landscape planning and resource utilization (Zúñiga et al., 2020).Land use and land and vegetation management are key factors in water resource planning and management (Apollonio et al., 2016;Ribas Palom et al., 2017).A highly participative approach, involving all major stakeholders, achieves a balance between economic development and the resulting impacts on river basin natural resources.A good IRBM typically implies a participatory and comprehensive approach.
The aim of this study is to evaluate the level of anthropogenic impact on the valley floor of the Adda River valley (Valtellina, Lombardy).In Italy, valley floors are often characterized by a thriving economy (Bramanti & Ricci, 2020) and the Valtellina valley floor is a highly productive area in the region.Detailed analysis of the level of anthropic impact is provided for Pian della Selvetta, a sector of the Adda River plain approximately 10 km upstream of the inlet into Como Lake.This area was chosen as the focus of this work for two main reasons: (1) it perfectly exemplifies the issues associated with highly artificialized water streams, and (2) severe damage occurred in the area during the July 1987 Adda alluvial event (Figure 1).Therefore, this mapping study focuses on the evolution of anthropic impacts in Pian della Selvetta.
This study was based on an analysis of urban expansion and anthropized area evolution, consistent with the current literature (Nirupama & Simonovic, 2007).It also focused on the historical evolution of the planimetric course (Roccati et al., 2019;Visser, 2014) and sediment granulometry of the riverbed, the presence of hydraulic works, evolutionary tendencies from a morphological perspective, and historical land use changes in the fluvial corridor (Hooke, 2006).Overall, this analysis provides a better understanding of the framework underlying the anthropic and natural transformations that have occurred in the area, which is highly affected by often-damaging flood events.

General setting
Pian della Selvetta is an alluvial plain in the southwest of Valtellina, a valley located to the northeast of Como Lake (Lombardy, northern Italy), through which the Adda River flows (Figure 2).The north-south boundaries of Pian della Selvetta include all areas below 270 m.a.s.l., while the east-west delimitation extends from the Colorina to the Ardenno municipalities.The total area covers 172.5 ha.

Geomorphology
Pian della Selvetta, similar to the rest of the Adda Valley, is a typical alpine alluvial plain set in a morphological landscape designed mainly by glacial and river erosion processes (Bollati et al., 2017).It is also influenced by the Periadriatic Fault System (or Insubric Line), an important fault that developed along the lower sector of the Valtellina northern slope, marking the contact between the Austroalpine and Pennidic Domains and the Southern Alps Domain and representing the separation between the Swiss Alps and Southern Alps (Montrasio et al., 2012).
The Valtellina landscape shows the effects of glacier shaping in its secondary valleys (Bini et al., 1996).Moreover, the debuttressing that developed when the glaciers receded after the Würm Ice Age provoked several instability processes along the slopes [landslides, gravitational slope deformations (Dramis, 1984), counter-slopes, and trenches], especially in the medium-high Adda Basin (Panizza, 1995).
The glacial morphology in the lower valleys was obliterated by superficial water erosion.The valley bottoms show a considerable amount of alluvial material, developed on the terraces and alluvial planes.At the beginning of the twentieth century, the Adda River had a more sinuous flow with wide meanders that dug through the fluvio-glacial deposit; this space is now occupied by urban and productive areas.
The glacial morphology and lithological-structural characteristics of the valley, as well as regional faults, such as the Tonale and Giudicarie Lines, have contributed to the hydrographic system.Occasional active and quiescent landslides, as well as gravitational slope deformations, characterize Valtellina and Pian della Selvetta.
Pian della Selvetta is geologically similar to the rest of Valtellina area, but its urban and anthropic aspects make the area more interesting.Since the first half of the twentieth century, it has been a thriving rural area.Therefore, in 1850, the Adda River was rectified and its shore artificialized.However, geological contours do not exist for either the historical natural riverbed or the artificialized riverbed.The Pian della Selvetta is almost wholly composed of alluvial deposits and fans, making it extremely vulnerable to potentially disastrous floods.The 1987 alluvial event completely submerged Pian della Selvetta (Figure 3) with a mixture of water and deposits with heights varying from 4.34 to 5.00 m above ground level (Maraga & Turitto, 1998).

Climatology
Valtellina has a continental climate, in which January is the coldest month and July is the hottest.The average annual precipitation is variable; within a few kilometers, cumulative annual amounts fluctuated between 726 mm/year (Tirano, at the valley bottom) and 1715 mm/year (Scais, Orobie Alps).In Pian della Selvetta, the annual precipitation varies from 1200 to 1400 mm/year.Several studies have demonstrated that the number of extreme precipitation events is increasing in northern Italy (Caporali et al., 2021).This trend was confirmed for the Lombardy Region (Todeschini, 2012).Extreme convective precipitation events, which are increasing, generally occur in the summer or early autumn (Brugnara & Maugeri, 2019).

Socio-economic assets
The Lombardy Region is one of Italy's richest areas.In 2019, the Lombardy Region's gross domestic product (GDP) alone accounted for over 22% of Italy's national GDP, and the same occurred in 2020 (EURO-STAT, 2020).This makes Lombardy the top region in Italy in terms of economic income.Therefore, the Lombardy Region, including Valtellina, is among the richest areas in Italy, and its development severely affects the Italian economy.
Alpine river basins also play a central role in electricity production in Italy.Almost 20% of electricity generated in Italy is green energy, and 42% of this green energy is hydroelectric power (ENEL Green Power, 2021a).Most of these hydroelectric power plants are located in the Alps, and 661 are found in the Lombardy Region.This region is ranked second in terms of hydroelectric energy production (ENEL Green Power, 2021b).Regarding population density, the Lombardy Region is once again the top Italian region, with 436 people per km 2 , almost twice the Italian average (188.43people per km 2 ) (EUROSTAT, 2021).

Land use
Land use evolution was analyzed by comparing COR-INE land cover (CLC) data for 1954, 1999, 2007, and 2015.Europe developed the CLC project to detect and monitor land use characteristics for the classification and denomination of land use through a specific, informative system organized across several hierarchical layers.The third and fourth CLC classification levels were used as the main map.Some simplifications to the original classifications were made, eventually obtaining 13 macro-categories: cultivated areas (comprising the 'agricultural areas' and 'green and permanent pastures' classes), productive areas (comprising the 'industrial areas,' 'dumps and degraded areas,' 'quarries,' and 'service and technological areas,' classes), residential areas (comprising the 'residential areas' and 'urban greenery' classes), main roads, alluvial deposits, shrubs and bushes, woods, and stream networks.Variation in the compositions of these categories in the different census  datasets highlighted the evolution and transformation of the area, and especially the growth of anthropized areas.Along with the land use study, variations in the populations of the main municipalities of Pian della Selvetta from 1951 to 2021 were analyzed.

Adda River hydraulic works analysis
An analysis of the impact of hydraulic works was based on the official cadastre data of hydraulic works.The cadastre details existing hydraulic works, including their shape, dimensions, and conservation status (bank defense, embankment, crossing, etc.).In this study, the earliest document found on regimentation interventions in the Adda water stream was a chorography named 'Representation of the channeling works of Adda River.'This chorography was attached to an 1850 rectification plan redacted by the 'Imperial Regia Direzione Generale Delle Pubbliche Costruzioni Di Milano' (Maraga & Turitto, 1998).The Adda River was subdivided into 31 stretches for the hydraulic works impact analysis (CNR Ge.Se.Flu Project, 2014).Stretches located in Pian della Selvetta were identified by the numbers 6, 7, and 8 on the main map.Based on the same data, the artificiality was evaluated for each stretch.Scale analysis, based on a homogeneous geomorphological stretch classification, allowed us to assess the overall artificial state of the water course.
Table 1 reports the topological elements found in the cadastre and cartographic projects, with an explanation of the abbreviations.It was assumed that every anthropic action considerably impacted the naturalness of the riverbed.Accordingly, the overall consistency of the riverbed acquired from the cadastre was reported, as well as the number of studies and the longitudinal influence of their development, within the geomorphological context of the Adda River (Table 2).
The artificiality definition process was based on the 'Hydromorphologic Evaluation System and Water Flows Monitoring' (IDRAIM) Project and on other reference studies (Rinaldi et al., 2014).This study examined longitudinal and transvers works and embankments along the river.The presence of bridges represents a disturbance to the naturalness of the riverbed and an obstruction to solid material transfer.The road network along the river was included in this study for further conditioning.In particular, the presence of roads, their position, and the presence of embankments were considered, depending on the presence of continuous hydraulic works or levees near the river.
To evaluate riverbank naturalness, the length occupied by hydraulic works of any type was compared to the total riverbank length, resulting in a percentage.It seemed appropriate to add a '> 66%' class to the original IDRAIM 'longitudinal works presence' classification, to better suit the Adda River results.The data were converted into a map using a graduated color scale (Table 3).
The three classes of transverse works were based on the IDRAIM project illustrated guidelines.By analyzing the alteration works for their impact on solid material transportation, a classification was proposed, as shown in Table 4. Crossing works were classified according to Table 5.The stretch scores were summed to obtain an artificiality class, with four class levels, as reported in Table 6.
Based on the historical evolution of the Adda riverbed, longitudinal riverbed variation was assessed for two different periods: (1) 1954-2015 to identify the overall evolution along with land use, and (2) 1999-2015 to exclude the effects of the 1987 flood on riverbed evolution.The evolution of the Adda River during these periods was reconstructed using cartographies and aerial photographs, and mediumterm (50-100 years) and short-term (10-20 years) planimetric variations were evaluated.Medium-and short-term altimetry variations were similarly reconstructed, at the stretch scale.

Historical major alluvial events
Flood events in the Adda River have been documented since 1300 (Govi et al., 1998).Over 90 great floods have been recorded, of which 12 were recorded between 1950 and 2019 (Abbate et al., 2021).A previous study reported 10 flood events observed between 1844 and 1987 (Tropeano & Turconi, 1999).The flood events were classified into different levels of severity, depending on their documented effects (Table 7).A retrospective historical study was performed through the analysis of archive documents, newspaper articles, technical reports, and iconographic materials, to analyze the flood events that have affected the Adda River valley.This analysis allowed the structure of a specific database with detailed information on the hydrographic rods on which geohydrological instability processes occurred.The identified information was stored in a database concerning critical flood events, the recurrence of torrential landslide processes, and destructive cases.Critical flooding events were identified based on landslides concomitant with the flood event, widespread torrential processes, the extent of the flooded area of the main valley floor, bank erosion, damage to built-up areas and infrastructure, and possible fatalities.To examine the data objectively, scores were given for each of the categories (geohydrological processes and damage), the sums of which generated the severity scores attributed to each of the events.

Effects of the 1987 flood
In 1987, a CNR-IRPI field survey recorded and mapped the effects of a catastrophic alluvial event in Adda Basin.The severe summer event of 1987 in Valtellina occurred in two phases: (1) an enormous flood of the Adda Valley bottom (July 18-21) (Luino, 2005), and (2) deep cracks were surveyed on the right side of Mount Zandila (July 25).The cracks widened to 900 m on 26 and 27 July, following a semi-circular shape and delimiting the detrital mass mobilized by an ancient landslide.In the early morning of July 28, a rock mass with a volume of 34 million cubic meters was released.This movement was referred to as the 'great Val Pola landslide.' The accumulation blocked the valley, with estimated heights of 90 m at some points (Luino, 2005).The landslide event resulted in 53 fatalities, 600 evacuations, and damage equivalent to over 2 billion euros (Alexander, 1988).Both the landslide and the flood damage contributed to the creation of the specific Law 102/90 (also known as the 'Valtellina Law'), which finalized the 'structural interventions plan' for Adda Valley.Evidence of the 1987 flood was synthesized in n.18 maps, published in a CNR-IRPI Internal Report (Govi et al., 1998;Govi & Turitto, 1992).
Recently, these maps were integrated in an investigation of original aerial and terrestrial photos and were digitalized using QGIS 3.10.1.The conversion into digital data made more accurate and wider analysis possible, including a new study focused on the evolution of the territory.Six digital layers were created: (1) polygons concerning the different deposits (gravelly, sandy, and silty), (2) points representing the different heights reached both by the deposits (gravelly, gravelly-sandy, sandy, siltysandy, and silty) and the water, (3) bridges destroyed by the flow, (4) slope erosion, (5) areas submerged by water, and (6) artificial banks destroyed by flooding.

Data collection
The data used in this study are presented in Table 8.Comparisons of the CLC maps, hydraulic works cadastre, ground-level effects of alluvial events, and hydrographic network transformation analysis were utilized to trace the natural and anthropic evolution of the study area.These transformations were related to the increase in vulnerability of urbanized areas and the mitigation works built over decades to reduce flood damage.

Land use and territorial evolution
The results clarify an ongoing tendency to expand both residential and industrial areas, with annexed roads and services; from 1954 to 2015, urban areas increased by 7.1% and industrial areas by 18.5%, at the expense of natural areas (Figure 4).According to the ISTAT data, the area recorded an overall growth of 575 inhabitants between 1951 and 2021, with a peak in the 1970s.Of the four main settlements considered, Berbenno had the highest population growth.Currently, 58% of the land in the area is anthropized (32% agriculture, 18% industrial, and 8% residential).

River artificiality and evolutionary trends
Tables 3 and 4 show that 70% of the Adda River is highly artificialized, and only 2% was classified as having low artificiality.Pian della Selvetta exemplifies this situation.This heavy artificialization, consequent to important territorial anthropization, highlights a vulnerable region whose safety depends on fluvial work maintenance.During 1954During -2015During and 1999During -2015, the Adda riverbed shrunk.

Planimetrical and morphological trends
The Adda River segments in Pian della Selvetta (stretches 6, 7, and 8) showed no significant variation,

Altimetry variations
A medium-term negative sediment balance (−2 m) is distinguishable, but short-term progression is more difficult to define.The altimetric variation suggests a balance near equilibrium, while the overall narrowing indicates that the actual sediment volumes are smaller than they were some decades ago.This, along with the reduction of fluvial bars, indicates a negative sediment balance, not as pronounced as that at the medium-term scale.

Most impactful documented flood events
The chronological record of torrential floods in Valtellina shows oscillatory behavior and consists of 475 events that occurred along 136 waterways from 1400 to the present.Of these 475 events, 197 have occurred in the past century.A peak in the number of events occurred in the mid-1800s, as confirmed by paleoclimatological records for the Southern Alps (Joannin et al., 2014); although the decadal average number of events was 10, in the decade 1840-1850, at least 60 events were recorded in the study area.Some of the most severe events occurred on 28 September 1885 (with six fatalities) (De Bernardi, 1982); 21-22 August 1911 (with 14 fatalities) (Palladino et al., 2011); 26 September 1927 (with five fatalities) (Giandotti,  1928); and 18 July 1987 (with 52 fatalities) (Luino, 2005).The 1987 flood event was chosen for analysis because of the number of casualties and amount of recorded damage, the extensive availability of field data, and the associated anthropization trends.Numerous landslides and floods, as well as intensive erosion by the Adda River, have occurred over the entire Valtellina region.Many tributaries of the Adda River have flooded inhabited areas and transported detritus downstream.In Val Pola, a huge rock landslide (Crosta et al., 2004), which blocked the valley and the Adda River outflow for some time, destroyed four inhabited areas, claiming 29 victims.Several landslides have also occurred in the tributary basins in the middle and low Valtellina, claiming 23 victims.The damage to the valley floor was severe, causing widespread damage to houses, roads, railways, mitigation works, and other infrastructure.Pian della Selvetta was one of the most damaged areas: the Adda River broke through its artificial banks, flooding almost all the rural fields and partly following its ancient riverbed (Figure 6).This was also the area in which the highest hydrometric levels and deposits were recorded.
On the night of July 18, the Adda River began to erode the ground in front of the artificial embankment in Pian della Selvetta, undermining the concrete plate.However, a siphoning in the field plan broke the artificial bank.When the floodwater opened breaches through both the railway and highway, the flatland of Pian della Selvetta was flooded.During the flood peak, approximately 10 km 2 was submerged, storing an approximate total volume of 28 × 10 6 m 3 of water.The silty-sandy deposits were, on average, 40 cm thick, and the thickest points reached 1 m (Govi et al., 1998).Since 1987, six more major flood events triggered by precipitation have occurred in the Adda River Basin, affecting areas of between 300 and 2000 km 2 (Abbate et al., 2021).

Discussion
This study describes the morphological evolution and transformation of the Adda River in its final stretch, particularly in Pian della Selvetta, where important flooding events have occurred.The plane-altimetric transformations of this watercourse, triggered by anthropic activities, present many similarities to several other alpine valleys (Scorpio et al., 2018).Since the second half of the twentieth century, also known as the post-war (WWII) 'economic boom,' intensified economic interest in such areas has led to the expansion of newly developed urban and industrial centers.Consequently, riverbank areas have been extensively occupied.The flooding of primary and secondary watercourses prompted the building of defensive and hydraulic regulation structures.This choice justified further extension of the anthropized areas, and anthropogenic systems and infrastructure progressively altered the natural courses of the water bodies.The main observed morphological effects included the shortening of watercourses, decreases in lateral spreading, the acceleration of flood waves, and the progressive deepening of riverbeds.Fully fledged geomorphological metamorphosis can occur in such a transformation framework (Maraga et al., 2015).
In the past few decades, flooding protection works have demonstrated local or regional disadvantages, inducing further structural development, which in turn, compromised the state and natural balance of the overall hydrographic network.Over the last 50-70 years, there have been significant changes in both the natural and anthropic contexts.Therefore, this study aimed to evaluate these aspects of the periodicity of natural flood events in the Adda River.The results demonstrated that the entire Valtellina basin was extremely artificialized.The ancient unicursal course of the Adda River, with its islands in the middle and upper sections and braided channels in the Pian della Selvetta section, has been severely altered by an anthropization process that is still progressing.
It is evident that the trends of land use change and anthropization sprawl, often accompanied by the growth of the resident population, are factors that increase vulnerability.The expansion of industrial areas in proximity to the Adda River is still visible today.The hydrographic course cannot be perceived in some densely constructed sections, and the presence of critical hydroelectric works intercept the hydrographic network, often further modifying its water regime.These conditions are visible predominantly in the Pian della Selvetta area of the Adda River, which, owing to its pseudo-flat morphology, has always been considered suitable for residential, industrial, and service-related purposes, similar to other alpine valley floor areas (Luino et al., 2002(Luino et al., , 2018)).
From a civil protection perspective, it is essential to emphasize that in such contexts, people's safety is directly linked to the effectiveness and efficiency of the artificial banks and river rectification systems.In 1987, the most disastrous flooding event occurred, as an embankment break where the former 1850 riverbed diverged strongly from the current artificial flow, and the artificial bank was probably relatively weak (Figure 7).Based on the observed disadvantages, and considering the vast number of rectification and defense works along the watercourses, it would be desirable to implement appropriate preventive structural health monitoring actions (Audisio & Turconi, 2011;Luino et al., 2014;Turconi et al., 2022).
An assessment of the artificialization of the Adda River and other major watercourses in the Italian alpine regions is currently ongoing.This assessment will allow the estimation of current geomorphological and environmental trends, providing improved management and operational guidelines at the regional scale (Rinaldi et al., 2014;Surian et al., 2009).Ideally, good maintenance of hydraulic works should be combined with efficient land use plans.Reducing residential and industrial areas prone to flooding would reduce the existing risk, which can be quantified as the product of land vulnerability and expected process intensity (Turconi et al., 2022).However, as it would be almost impossible to relocate existing settlements, the adequate maintenance of artificial levees, banks, and hazard mitigation works is essential to reduce the level of risk.In addition, the deepening (more than 4 m in some stretches) and narrowing (several tens of meters) of the Adda riverbed lead to a decrease in the transport of solid material, resulting in erosive dynamics in downstream stretches that undermine the stability of artificial banks and increase the structural failure risk of crossing structures and transverse and longitudinal works along the watercourse (CNR Ge.Se.Flu Project, 2014).
The misperception of safety, induced by the presence of conditions that have been consolidated over several decades by artificial works, has wrongly justified the expansion of settlements in riverine areas.The results of this study demonstrate the relevance of the census and maintenance of hydraulic works, as well as that of the assessment of morphological and anthropization trends of rivers.

Conclusions
This study aimed to interpret the consequences of anthropogenic and natural variations along stretches of an intensely modified alpine valley floor over centuries.The analysis has been extended to the last 150 years.In particular, it focuses on interpreting floodinduced effects in fluvial and perifluvial areas that have been highly transformed by anthropogenic action.
The main map describes the morphological evolution, land use changes, and land transformation in the Pian della Selvetta, which has historically been vulnerable to flooding.The presence of buildings, artifacts, and anthropized areas generates an increase in vulnerable regions, which also triggers a drastic reduction in drainage disposal capacity, owing to the presence of extensively sealed sectors.
The massive works built along the Adda River over the centuries, aimed initially at risk-mitigation, and the massive works intended for the exploitation of hydroelectrical power (dams) have contributed to the geomorphological transformation of the riverbed.In addition, there have been further interventions in the river area, such as progressive narrowing through channelization and the construction of numerous crossing works with bank reinforcement.This has resulted in a drastic shortening of river paths and a significant reduction in river sections, as well as the deepening of the river network, disruption to the propagation of stone sediments (resulting in the instability of the works themselves), ecosystem discontinuity between the main watercourse and its tributaries, and imbalances in the groundwater level.These effects have only been quantified through careful analytical reconstruction of the natural and anthropogenic transformation of the area over a significant period using terrestrial and aerial imagery, mapping, site surveys, and unpublished documents.
Synthesis maps, such as the one proposed, facilitate knowledge of the territory and its transformation, allow the interpretation of potentially damaging geomorphological processes, and provide indicators of river behavior (such as the possible reactivation of river corridors).This makes it possible to effectively predict the consequences of flood events in areas with different vulnerability characteristics, including the type, size, location, and efficiency conditions of hydraulic works.Often, the presence of hydraulic works provides a false sense of security to residents, as occurred after the 1987 flood, during which a breach in the artificial embankment occurred precisely at an ancient bend in the Adda River.During analysis, it is advisable to evaluate whether the regulatory works of a watercourse intercept obsolete river courses, which naturally possess draining debris mattresses consisting of alluvial lithoid material, along which surface and sub-aerial waters may easily find preferential routes of flow.

Software
Spatial analyses of the morphometric indices were conducted using QGIS 3.10.1 with the geographic resources analysis support system GRASS GIS 7.

Figure 2 .
Figure 2. Satellite view of the area under study, located in the Lombardy Region (N Italy).

Figure 3 .
Figure 3. Aerial orthophoto comparison between 1987 and 2020.New industrial settlements, as well as new residential areas, were built in the area that, in the 1987 orthophoto, were clearly submerged by water and mud.
presence of consolidation fluvial weirs (≤ 1 every 200 m), or presence of open retention fluvial bridle Mid-low slope riverbeds (≤ 1%) Low presence of fluvial weirs or crossbars (≤ 1 every 1000 m) C High slope riverbeds (> 1%) High presence of consolidation fluvial weirs (> 1 every 200 m), or presence of one or more retention fluvial weirs Mid-low slope riverbeds (≤ 1%) High presence of fluvial weirs or crossbars (> 1 every 1000 m)Table 5. Classification of crossing works in three classes and their characteristics.Class Characteristics A No works B Low presence of crossing works (≤ 1 every 1000 m) C High presence of crossing works (>1 every 1000 m) owing to the 1850 river rectification.For the rest of the Adda River, ortho-photographic and cartographic documents from 1954 to 2014 highlighted significant variations.The Adda riverbed is characterized by relevant narrowing.As shown in Figure5, 64% of the stretches reported significant decreases in width from 1954 to 2015, while only 10% of the stretches increased.Similar data for 1999-2015 shows that 53% of the stretches suffered a relevant narrowing, while only 4% increased their width.The 1987 flood clearly alleviated the narrowing process; fluvial wandering completely disappeared and fluvial bars were reduced.The overall degree of morphological alteration in the Adda River is very high for most sections (22 out of 31), and the remaining stretches show alterations ranging from high to intermediate (CNR Ge.Se.Flu Project, 2014).

Figure 7 .
Figure 7. Photographic documentation of an embankment break that occurred in 1987 along the Adda river during a flood event.

Table 2 .
Overall envelope of longitudinal hydraulic works, sorted by type, and overall numerical consistency of transverse hydraulic works and crossing works, reported in a cadastre, detailing riverbed artificiality interferes levels.

Table 3 .
Evaluation score weight distributions for different riverbed artificiality elements.LEN = length.

Table 4 .
Classification of transverse works in three classes and their characteristics.

Table 6 .
Artificiality class attribution, based on scores obtained by separate components and number of Adda River stretches in each class.

Table 7 .
Major flood events in the Adda River Basin over the past two centuries.N.A.:Not available.

Table 8 .
Data sources, including their basic description, year, scale, or detail.