Teatini, Pietro

We present the result of a test aimed at evaluating the capability of RADARSAT-2 and COSMO-SkyMed to map the natural subsidence and ground movements induced by anthropogenic activities in the historical center of Venice. Firstly, ground movements have been retrieved at quite long- and short-term by the Persistent Scattered Interferometry (PSI) on 2008-2015 RADARSAT-2 and 2013-2015 COSMO-SkyMed image stacks, respectively. Secondly, PSI has been calibrated at regional scale using the records of permanent GPS stations. Thirdly, considering that over the last two decades “in the historical center of Venice” natural land movements are primarily ascribed to long-term processes, and those induced by human activities act at short-term, we have properly resampled 83-month RADARSAT-2 C-band and 27-month COSMO-SkyMed X-band interferometric products by a common grid and processed the outcome to estimate the two components of the displacements. Results show that the average natural subsidence is generally in the range of 0.9 – 1.1 mm/yr and the anthropogenic ground movements are up to 2 mm/yr.

ABSTRACT - This paper describes the studies performed and the results obtained under the framework of the Project "Sistema Lagunare Veneziano", Research Line 2. 7. A short review of the land subsidence evolution is first presented, including the more recent period (1950 - 1970) of human induced land sinking. The altimetrica! situation of the entire venetian territory is then analysed in detail for the fast twenty years through the comparison of two precise regional levelling carried out in 1973 and 1993. This twenty-year period is quite significant to evaluate the present trend of land subsidence after the cessation of human-induced process measured in 1973 and completely verified in 1975. Different behaviours in altimetrica! evolution have clearly come out. A ground stability belongs to the zones in mainland, Venice and its surroundings, while a certain subsidence stili occurs at the northern and southern extremities of the lagoon's edge and along the littoral where the natural compaction of recent deposi ts is more active than elsewhere and the rate of natural subsidence is greater. The degenerative state of the venetian littoral strips has worsen in the recent past by land sinking. In the fast twenty years the relatively slight subsidence has helped in permanently establishing the de-stabilisation of the submerged littoral zone that is evidenced by the deepening of the near shore bottom slope. In order to analyse in-depth the subsidence process, more theoretical studi es have been performed.New appropriate and complex statistica/ methodologies use.fùl in formulating a judgement on the linearity of the dependence between groundwater withdrawal and land subsidence were elaborated. A new 3-D non linear .finite element model was developed for the simulation and prediction of land subsidence due to intensive groundwater exploitation. The model is based on a quasi 3-D approach for flow and makes use of a 1-D vertical compaction model. The overall modelling approach, solved with an origina! block SOR iterative procedure, is applied to the regional aquifer-aquitard system underlying the venetian lagoon. The flow . and consolidation models, suitably calibrated, have allowed to well reproduce thephenomenon. SOMMARIO Questo lavoro presenta la sintesi degli studi effettuati in tema di subsidenza nell'ambito della linea di ricerca 2. 7 del Progetto Sistema Lagunare Veneziano, prima fase. I risultati qui descritti si basano sostanzialmente sull'analisi comparata delle altimetrie 1973 e 1993 risultanti dalle livellazioni di alta precisione (effettuate con gli stessi criteri dal CNR, ISDGM Venezia) che partendo dai capisaldi base di Rua di Feletto e Treviso hanno quotato 208 capisaldi stabiliti lungo le direttrici Conegliano-Treviso-Venezia e circumlagunare. L'analisi relativa all'arco di tempo ventennale ha pennesso di confennare la stabilità delle zone di terraferma, nonché del centro storico di Venezia. Le area più subsidenti sono risultate essere quelle alle estremità lagunari nord e sud e lungo i litorali. L'esame di alcuni profili batimetrici ha evidenziato che in corrispondenza delle zone di maggior perdita altimetrica del litorale emerso si è verificata w1a certa destabilizzazione del litorale sommerso con un incremento della pendenza media. Ai rilievi di campagna sono stati affiancati studi più teorici. L'interdipendenza esistente tra depressurizzazione del sistema acquifero e subsidenza è stato analizzato con i più recenti criteri statistici evidenziando una relazione lineare tra la causa e l'effetto. Il fenomeno è stato simulato con un nuovo modello matematico tridimensionale non lineare agli elementi finiti fonnato da un modulo idrologico "quasi 3-D" ed un modulo di consolidazione monodimensionale verticale in cui le caratteristiche idrogeologiche degli aquitard dipendono dalla tensione intergranulare efficace. La non-linearità è stata risolta con uno schema iterativo che, per la particolare stmttura del modello, è equivalente ad uno schema di sovrarilassamento a blocchi. La calibrazione del modello ha consentito di riprodurre il fenomeno in modo soddisfacente dal 1930 al 1973, durante il periodo più critico per il sistema lagunare

Understanding the hydrogeological processes is critical for a sound management of groundwater resources in costal areas. Here lie majority of human settlements, industrial production, and fish farming. Human pressure on the coastland environment is constantly increasing, and many studies predict a rising of seawater level in the next 50 years raging from few cm up to several tens of cm, with expected threatening consequences (e.g., Carbognin et al., 2009). If these are common characteristics of most costal areas, wetlands, lagoons and estuaries also have often unique flora and fauna depending on the groundwater-surface water processes. The hydrologic setting of the transitional environments is complicated by their Late Quaternary subsoil architecture. The deposits represents the transition through the fluvial in tide-dominated depositional systems triggered by the sea level changes. In particular, in the Venice area numerous geomorphological features representing i.e. fluvial paleoriver beds, ancient tidal channels, and paleobeach ridges occur (Tosi et al., 2009). These features are generally filled by sandy deposits and can be considered preferential path for the groundwater flow, both in the horizontal and vertical directions. In order to have a better understanding of the hydrogeological setting of these areas, and also to produce more useful models, it is crucial to acquire information both inland and within the lagoon or wetland, covering both its permanent wet and tidal areas. Acquiring information that can be used to model the groundwater processes of these areas is often logistically challenging and therefore expensive and slow. This applies both to punctual, invasive and direct measurements such as depth to groundwater table and salinity from boreholes, to non invasive, area covering, indirect data such as resistivity or seismic investigations. Apart from the logistics, in many cases the quality of the data reflects the spatial and or temporal alternation of dry land and ponds-marshes-surface water in general. Airborne electromagnetics (AEM) can greatly improve the data quality and coverage in such areas, while cutting significantly the acquisition costs. Its direct output is geoelectrical cross sections or maps that are then used as input for hydrogeological models. The application of AEM for groundwater monitoring and modeling has been steadily rising in the past decade, due to parallel developments of better AEM systems and processing, e.g. inversion methodologies. However, so far there have been extremely limited attempts of applying AEM to areas such as lagoons, wetlands, rivers or bays. This manuscript shows that AEM can produce quantitative results useful for groundwater modeling also in these areas, presenting the results of a survey carried out in the central and southern sectors of the Venice Lagoon, Italy, by the SkyTEM system. We present some of the inversion outcome as horizontal average resistivity maps at different depth intervals and cross sections obtained by SkyTEM application in the two areas where different hydrogeological processes are under investigation.

The southern catchment of the Venice watershed (Italy) is threatened by shallow aquifer and soil salinization. The saltwater may extend inland up to 20 km from the Adriatic coastline and deepen down to some tens of meters. Here, saltwater contamination is driven by other forcing factors than excessive pumping, such as ground elevation, buried geological structures, tide encroachment along watercourses, climate and tide conditions, and drainage practices implemented in reclaimed areas. This work aims to outline a conceptual model of the saltwater contamination highlighting the mechanisms driving the saltwater-freshwater exchanges. Results show that the fresh/salt-water interface depth varies from 1 to 30 m below the ground level and exhibits a significant, mainly seasonal, time variation. The dynamics of the soil salinization process is especially sensitive to changes in river (Brenta, Bacchiglione, Adige, Gorzone) discharges, groundwater and channel levels, which are regulated by a number of pumping stations, and climate conditions. Relict geomorphological features, filled with high permeability sediments, act as preferred pathways for groundwater flow and solute transport. In fact they provide a hydraulic connection between freshwater aquifers and sea, possibly facilitating saltwater intrusion landward or, conversely, acting as reservoir of freshwater provided by precipitation, irrigation, percolation through channel beds.

Le livellazioni geometriche sono state nel secolo scorso l’unico metodo di rilievo altimetrico che abbia consentito di misurare con precisione l'entità della subsidenza “moderna” dell’area costiera nord adriatica. Solo alla fine degli anni 1990 è stata istallata una rete per misure GPS in differenziale (DGPS) e in continuo (CGPS). Nell’ultimo decennio inoltre, l'utilizzo del radar ad apertura sintetica (SAR) su vettori satellitari ha consentito lo sviluppo e l’affinamento dell’analisi interferometrica differenziale (InSAR) e dell’analisi interferometrica su riflettori persistenti (IPTA) che si sono dimostrati di estrema efficacia per lo studio dei movimenti verticali del suolo. Nel caso della pianura costiera Veneta, sono stati utilizzati i satelliti ERS-1/2 ed ENVISAT dell’Agenzia Spaziale Europea, rispettivamente per il periodo 1992-2005 e 2003-2009, ed il satellite TerraSAR-X dell’Agenzia Spaziale Tedesca, per il biennio 2008-2009. Oggi si dispone di una densità di dati SAR che, data la risoluzione spaziale dei satelliti tra 20 e 3 m, è maggiore di circa 2 ordini di grandezza nelle l’analisi a scala regionale e più di 3 ordini per analisi locali rispetto alle misure tradizionali su capisaldi. Ciò ha permesso la mappatura dei movimenti del suolo a scala “regionale” (100×100 km2), locale (10×10 km2) e puntale al livello di singole strutture. Le serie di dati SAR sono stati calibrati e validati con le misure altimetriche di livellazione, DGPS e CGPS nella rete di monitoraggio ISES-IRMA. Grazie all’elevata densità di informazioni, all’ottima risoluzione spaziale e accuratezza verticale millimetrica del monitoraggio SAR è emersa una dinamica verticali del territorio costiero Veneto diversa da quanto ottenibile utilizzando le sole tecniche di livellazione tradizionale. L’immagine attuale indica che il processo subsidenziale si esplica con una forte variabilità spaziale, sia a scala regionale che locale. L’analisi integrata dei dati altimetrici e delle numerose nuove informazioni sul sottosuolo, recentemente acquisite nell’ambito di una serie di ricerche condotte dagli Autori, ha permesso la caratterizzazione delle componenti dei movimenti verticali del suolo della pianura costiera Veneta in funzione della profondità alla quale agiscono e la loro distribuzione areale.

The salt water contamination process in the Venice watershed between the southern edge of the lagoon of Venice and the Adige River extends up to 20 km inland from the coast. The depth of the fresh/salt-water interface varied from 2 to 30 m below the ground level and exhibits a significant, mainly seasonal, time variation and its dynamics is especially sensitive to the riverbed seepage of sea water, encroaching from river mouths.

Remote-sensing techniques represent suitable tools for geological cartography and sustainable management of natural resources. Spaceborne Synthetic Aperture Radar (SAR) systems offer the possibility, through differential SAR interferometry (DInSAR), to map surface displacements at mm to cm resolution. Spectacular results are obtained for geophysical sciences with earthquake displacement, volcano deformation, glacier dynamics and land subsidence being mapped. With regard to land subsidence, SAR interferometry exhibits complementary characteristics to the levelling surveys, because it has the capability to map large urban areas at low cost and high spatial resolution. The high precision levelling surveys, on the other hand, are used outside of the cities and to set up a reference point for the SAR subsidence values. In the case of the south-eastern Veneto region, where till to 2000 high precision levelling surveys are available only (i) around the Lagoon margin and along few lines from (ii) Venezia to Rua di Feletto (Treviso), from (iii) Mestre to Padova, (iv) form Padova to Rovigo, and (v) from Chioggia to Adria, SAR interferometry has the capability to monitor the vertical displacements of all the built-up or sparsely vegetated areas (i.e. where stable structures permit the formation of a coherent phase signal over time) not fully covered with levelling results. DInSAR makes use of two SAR images acquired from slightly different orbit configurations and at different times to exploit the phase difference of the signals. The phase signal derived from an interferometric image pair relates both to topography and line-of-sight surface movement between the acquisitions, with atmospheric phase distortions, signal noise and inaccuracy in the orbit determination as main error sources. The basic idea of DInSAR is to subtract the topography related phase (for instance simulated from a Digital Elevation Model) from the interferogram to derive a displacement map. In this study a time series of six interferometric radar images of the European Remote Sensing Satellites ERS-1 and ERS-2 from 1993 to 2000 was used. In order to generate a single subsidence map with reduced errors, the interferometric radar images were combined. In the following, displacement in the vertical direction was assumed, the land subsidence map was transformed to the Italian cartographic system with a spatial resolution of 30 m, and the pixel corresponding to the benchmark Nodale 63 (ex 24’) in Treviso (that has been already considered stable for the levelling surveys) was considered the stable reference. The high accuracy of the 1993-2000 DInSAR survey is confirmed by a quantitative validation with the 1993-2000 levelling data. For the 87 benchmarks where values from both surveying techniques are available a zero average difference of the vertical displacement velocity is prescribed and a standard deviation of 0.9 mm/year is found; the minimum and maximum differences are –3.1 mm/year and +2.5 mm/year, respectively. From this number and previous works we conclude that the accuracy of the SAR interferometric subsidence rates is on the order ±1 mm/year, which is also the expected accuracy of the levelling surveys. An example of the DInSAR capability to monitor the evolution of land subsidence in small urban area is Abano – Montegrotto Terme where clearly appears the influence of the drastic reduction of groundwater pumping used for geothermal purpose. DInSAR results match very well the available leveling data. DInSAR mapping in large cities such as Padova, Treviso, and Mestre is given. These applications indicates the potentiality of the radar satellite analysis in built-up areas characterized by a general stability, with displacement rate within the range 1 mm/year. DInSAR use looks very useful to perform detailed investigations in coastal areas too. E.g., Bibione – Lignano, Chioggia, Jesolo and Caorle reveal the presence of a significant seaward gradient in land subsidence practically impossible to be detected by other traditional monitoring techniques like geometric leveling and DGPS. Hence, the proposed methodology provides a fundamental support for coastal management projects and environmental risk analyses. SAR interferometry has been applied in other minor cities of the Veneto plain characterized by very different conditions form the displacement point of view. Castelfranco and Conegliano located close to the Alpine foothills appear quite stable; land settlement in Rovigo appears very changeable with high values (2-3 mm/year) downtown and lower rate (about 1 mm/year) in the outskirts; in Cavarzere, and Oderzo, San Donà the subsidence velocity ranges between 3 and 4 mm/year. The more critical situation is found in Adria, Taglio di Po, and Portogruaro where the sinking velocity reaches 5 mm/year in large portion of these towns. Although in 2000 the old leveling network has been extended within the framework of the ISES Project (the present overall length is about 1000 km), a detailed monitoring of several villages is impossible though leveling and DGPS. Because in these urban areas the hydrogeological risk is high due to the presence of rivers and the critical elevation, since they often lie below the mean sea level, the areally distribution of the subsidence rate has to be well known. Results shown by the present work prove that this integration in the knowledge of the subsidence process can be efficiently obtained by DInSAR. This technique represents an effective methodology for both a back-analysis during the last decade and the future monitoring in the portion of territory covered by large and small built-up areas.

Relative sea level rise (RSLR), that has been occurred along the entire coastal areas of the Northern Adriatic Sea, includes land subsidence, both natural and man-induced, and eustacy. Their combined effect has produced relative ground settlements ranging from centimetres to meters. RSLR represents one of the geologic hazards threatening the low-lying coast. Recent progresses made in understanding these two processes are presented. Synthetic Aperture Radar (SAR) interferometry has significantly improved the knowledge of actual land subsidence. In particular, comprehensive maps of the vertical displacements occurred over the period 1992-2009 in the region between Venice and Ravenna reveal a significant spatial variability, ranging from a slight 1 to 2 mm•yr-1 uplift, to a serious subsidence of more than 15 mm•yr-1. The availability of tide gauge data in Trieste, Venice, and Ravenna allows accurate assessment and meaningful observations on sea level change. The period 1896-2006 is characterized by an average rise of 1.2±0.1 mm•yr-1. The analyses here performed show that a time series at least 50 yr long must be used to obtain statistically significant results and reliable trend, due to the 7-8 year pseudo-cyclicity, recorded at many Mediterranean coastal stations. In Venice and Ravenna the influence of land subsidence on the RSLR amounts to 57% and 85%, respectively. This percentage has been estimated in 95% at the Po Delta.

Within the Co.Ri.La. project 3.2 on Hydrodynamics and Morphology, interferometric radar images from the ERS-1/2 satellites from 1993 to 2000 have been used to generate a subsidence map of the Lagoon of Venice. The SAR interferometric land subsidence map pictures very well the different displacement rates of the various compartments of the Lagoon. The SAR interferometric displacement map is discussed in comparison to the levelling results available through the ISES project. A geocoded thematic map of the Lagoon of Venice at 30 m resolution is also presented.

Lagoon environments are very important for groundwater modeling in costal areas, they are delicate and in rapid evolution due to global climatic changes. Airborne electromagnetics (AEM)is a very valuable methodology that can provide high density, high quality data to produce 3D hydrogeological models to depths in excess of hundred meters below surface water column. We present the results from the SkyTEM Venice lagoon survey of 2009, integrated with data from very high resolution seismic survey. The AEM data results enhance greatly the understanding of the hydrogeology and surface-groundwater interactions in the lagoon area, where indirect measurements abound but wells are missing. For example, there is clear evidence of fresh water aquifers underneath the central part of the lagoon, at depth of about 40 m. The near surface part of the AEM data compare well with seismic data, showing that main reflectors come from the interface between the superficial Late Pleistocene looser, saline water saturated sediments and the deeper, more compact and fresher Holocene sediments. There is also clear evidence of submarine groundwater discharge in the lagoon, of paleorivers, and a possible indication of gas seepage trough shallow sediments. Seismic and AEM provide complimentary datasets to discriminate between pore water salinity, lithology and gas. Seismic horizons can actually be included during inversion of AEM data, producing more robust output. AEM data from the southern part of the survey that crosses the shore line and continued also onshore allow a clear mapping of the saline water intrusion inland, and highlight the relationship between pore water salinity of the lagoon sediments and spatial distribution of salt marshes. The latter seem to act like salt sinks, increasing sediments electrical conductivity.