Variabilità di facies sedimentarie in meandri fluviali e tidali: esempi dai depositi Olocenici della Pianura Veneta

Nelle piane costiere, i depositi di point-bar originati dai canali meandriformi fluviali e tidali ospitano la maggior parte degli acquiferi superficiali, il cui sfruttamento ha contribuito allo sviluppo socioeconomico di queste zone. Tuttavia, lo sfruttamento eccessivo di questi acquiferi insieme all’intensa urbanizzazione ha favorito l'intrusione di acqua marina e la contaminazione degli acquiferi stessi da parte di sostanze inquinanti. Una conoscenza dettagliata dei depositi di point-bar è essenziale per comprendere al meglio il flusso negli acquiferi e per una loro migliore gestione. Attraverso un approccio multidisciplinare che combina analisi statistiche e l’analisi di immagini satellitari, dati sedimentologici e indagini geofisiche, il presente lavoro analizza i depositi olocenici derivanti dall’evoluzione di canali meandriformi fluviali e tidali della Pianura Veneta (Italia). La Pianura Veneta è l’ambiente ideale per studiare questi depositi in quanto consente di combinare lo studio dell’evoluzione in pianta dei meandri con dati di sottosuolo. Le analisi hanno permesso di caratterizzare i depositi di canali meandriformi fluviali e tidali dalla scala della point-bar a quella dell’intero canale e forniscono dati qualitativi e quantitativi per la modellazione del flusso negli acquiferi. In particolare, i risultati: i) hanno contribuito ad arricchire le conoscenze attuali fornendo spunti di riflessione rispetto a quanto descritto finora sull'eterogeneità litologica interna dei depositi di point-bar e sullo sviluppo di meandri a partire da canali rettilinei; ii) hanno permesso di sviluppare un nuovo approccio quantitativo per studiare la variabilità delle proprietà dei sedimenti all'interno dei corpi di point-bar; iii) hanno evidenziato l'impatto che le morfologie della piana alluvionale e le eterogeneità del substrato hanno sull'evoluzione morfodinamica dei fiumi meandriformi.Along the coastal plains, point-bar deposits originating from tidal and fluvial meandering channels host most of the surficial aquifers, which are the major source of groundwater for the socio-economic development of these areas. However, overexploitation of these aquifers has favoured seawater intrusion, and urbanisation has led to pollution of the aquifers. Detailed knowledge of point-bar deposits hosting these aquifers is crucial to gaining a better understanding of subsurface pathways for groundwater flows and providing insights into aquifer management. Through a multidisciplinary approach, which combines remote-sensing analysis, sedimentary-core data, geophysical surveys, literature review, and statistical analyses, the present work analyses Holocene deposits from fluvial and tidal meandering channels of the Venetian Plain (Italy). The Venetian Plain provides a unique setting to study fluvial and tidal meandering channel deposits since commonly allows combining planform evidence with subsurface data. The results allow the characterisation of fluvial and tidal meandering channels from the bar the channel-belt scale and provide qualitative and quantitative data that can fruitfully contribute to model groundwater flows. Specifically, results: i) contribute to enriching current knowledge by providing insights to challenge the paradigms related to the internal lithological heterogeneity of point-bar deposits, along with the development of meander bends from straight channels; ii) allow to set up a new quantitative approach to efficiently study sediment-property variability within point-bar bodies; iii) highlight the impact of floodplain morphologies and substrate heterogeneities on the morphodynamic evolution of meandering rivers

Correlazione tra architettura deposizionale 3D ed evoluzione morfodinamica di meandri tidali: un esempio dal sottosuolo della Laguna di Venezia (Italia)

Depositional products of fluvial meandering channels are largely investigated, because of their widespread distribution in densely populated alluvial and coastal plains and their sedimentological characteristics as hydrocarbon or water reservoirs. Although their importance in the exchanges with open marine water bodies, meandering tidal channels are relatively unexplored, and their sedimentological features are commonly investigated on the basis of facies models built up for their fluvial counterparts. The Venice Lagoon represents an excellent environment where it’s possible to deepen our knowledge on morphodynamics of meandering tidal channels. Through integration between high-resolution geophysical and core data collected in the northern part of the Venice Lagoon, this thesis focused on a detailed reconstruction of a meandering tidal paleochannel, and aims to depict its morphodynamic evolution. This work allowed to build up a high-resolution architectural 3D model, and to analyse the depositional geometries of three main tidal point bars. Only one of these three bars is entirely visible. This bar is characterized by a peculiar planform evolution, with a dramatically increase in the radius of curvature starting from an initial cuspate geometry. Additionally, the study of some minor channels (e.g. chute channels) allowed to depict stratal geometries arising from the interaction between rates of lateral migration and vertical accretion. This work allowed to understand distinctive features of tidal meanders, which can be used to detect the tidal origin of laterally accreting deposits in the fossil recor

Geophysical and Sedimentological Investigations Integrate Remote-Sensing Data to Depict Geometry of Fluvial Sedimentary Bodies: An Example from Holocene Point-Bar Deposits of the Venetian Plain (Italy)

Over the past few millennia, meandering fluvial channels drained coastal landscapes accumulating sedimentary successions that today are permeable pathways. Propagation of pollutants, agricultural exploitation and sand liquefaction are the main processes of environmental interest affecting these sedimentary bodies. The characterization of these bodies is thus of utmost general interest. In this study, we particularly highlight the contribution of noninvasive (remote and ground-based) investigation techniques, and the case study focuses on a late Holocene meander bend of the southern Venetian Plain (Northeast Italy). Electromagnetic induction (EMI) investigations, conducted with great care in terms of sonde stability and positioning, allowed the reconstruction of the electrical conductivity 3D structure of the shallow subsurface, revealing that the paleochannel ranges in depth between 0.8 and 5.4 m, and defines an almost 260 m-wide point bar. The electrical conductivity maps derived from EMI at different depths define an arcuate morphology indicating that bar accretion started from an already sinuous channel. Sedimentary cores ensure local ground-truth and help define the evolution of the channel bend. This paper shows that the combination of well-conceived and carefully performed inverted geophysical surveys, remote sensing and direct investigations provides evidence of the evolution of recent shallow sedimentary structures with unprecedented detail

From electromagnetic to sediment textural maps: an integrated approach to unravel the intra-point-bar variability of sediment properties

Sedimentary sand bodies that originated by the evolution of meandering channels such as point bars, host surficial aquifers, providing preferential pathways for groundwater flow in mud-dominated floodplains. These aquifers are a major source of freshwater, yet they can suffer from different environmental issues such as pollution and salt-water intrusion, which can critically damage agriculture and socio-economical activities. Understanding the internal architecture of point-bar bodies is essential to plan proper aquifer management and address environmental issues in coastal plains.To provide a spatially extensive, cost-effective characterization of point-bar body architecture, this work proposes a multidisciplinary approach to investigate geophysical and sedimentological data through statistical analyses, with an application to two palaeo-meanders in the Venetian Plain, Italy. In this setting, point-bar deposits mainly consist of fine to coarse sand and display lower electromagnetic conductivity values compared to the surrounding muddy overbank deposits. Statistical multivariate analyses on sedimentological and geophysical properties overcome the qualitative direct dependence of electromagnetic conductivity on sediment grain size alone, highlighting a stronger relationship with sediment sorting. By calibrating this relationship on a reasonable number of samples representative of the different depositional bodies, this methodology finally provides maps of sediment textural properties at the meander scale

Reconstructing sediment distribution in meandering river deposits through a simplified numerical modelling approach, with applications to the Holocene deposits of the Venetian Plain (Italy)

Many present-day alluvial floodplains display traces of abandoned meandering channel belts developed during the past millennia (i.e. mid to late Holocene). Deposits of these ancient rivers represent preferential pathways for groundwater flows and related environmental issues, such as contaminant propagation or saltwater intrusion in coastal areas. However, since formative bankfull flows in such old and abandoned routes are hard to estimate, fossil meanders have not been commonly addressed by morphodynamic numerical models, and most of them have been investigated following classical sedimentological approaches based mainly on punctual data derived from sedimentary cores. This study aims at investigating the sediment distribution within different fossil bends on the southern Venetian Plain (NE Italy), and relating such distribution to numerically modelled bed shear stresses used herein as a proxy of sediment sorting patterns. For this purpose, formative flows in the studied palaeomeanders are first inferred from measured sediment grain size and estimates of bend widths. Then, shear stress distributions are computed along the studied palaeobends using a 2D linearized model. Model results are finally compared with conductivity distributions gauged directly in the field through electromagnetic induction investigations in the frequency domain. Our results suggest significant correlations between shear stress distributions and sediment sorting estimated from conductivity data. We deem that the integration between sedimentological reconstructions and state-of-the-art numerical modelling can provide a solid contribution to predicting the spatial distribution of sediment properties within ancient meandering channel belts, with relevant implications for the understanding of shallow aquifer dynamics and soil management

From electromagnetic to sediment textural maps: an integrated approach to unravel the intra-point-bar variability of sediment properties

Sedimentary sand bodies that originated by the evolution of meandering channels, such as point bars, host surficial aquifers, providing preferential pathways for groundwater flow in mud-dominated floodplains. These aquifers are a major source of freshwater, yet they can suffer from different environmental issues such as pollution and salt-water intrusion, which can critically damage agriculture and socio-economical activities. Understanding the internal architecture of point-bar bodies is essential to plan proper aquifer management and address environmental issues in coastal plains.To provide a spatially-extensive, cost-effective characterisation of point-bar body architecture, this work proposes a multidisciplinary approach to investigate geophysical and sedimentological data through statistical analyses, with an application to two paleo-meanders in the Venetian Plain, Italy. In this setting, point-bar deposits mainly consist of fine to coarse sand and display lower electromagnetic conductivity values compared to the surrounding muddy overbank deposits. Statistical multivariate analyses on sedimentological and geophysical properties overcome the qualitative direct dependence of electromagnetic conductivity on sediment grain size alone, highlighting a stronger relationship with sediment sorting. By calibrating this relationship on a reasonable number of samples representative of the different depositional bodies, this methodology finally provides maps of sediment textural properties at the meander scale