Statistical characterization of spatio-temporal sediment dynamics in the Venice lagoon

Characterizing the dynamics of suspended sediment is crucial when investigating the long-term evolution of tidal landscapes. Here we apply a widely tested mathematical model which describes the dynamics of cohesive and noncohesive sediments, driven by the combined effect of tidal currents and wind waves, using 1 year long time series of observed water levels and wind data from the Venice lagoon. The spatiotemporal evolution of the computed suspended sediment concentration (SSC) is analyzed on the basis of the \u201cpeak over threshold\u201d theory. Our analysis suggests that events characterized by high SSC can be modeled as a marked Poisson process over most of the lagoon. The interarrival time between two consecutive over threshold events, the intensity of peak excesses, and the duration are found to be exponentially distributed random variables over most of tidal flats. Our study suggests that intensity and duration of over threshold events are temporally correlated, while almost no correlation exists between interarrival times and both durations and intensities. The benthic vegetation colonizing the central southern part of the Venice lagoon is found to exert a crucial role on sediment dynamics: vegetation locally decreases the frequency of significant resuspension events by affecting patiotemporal patterns of SSCs also in adjacent areas. Spatial patterns of the mean interarrival of over threshold SSC events are found to be less heterogeneous than the corresponding patterns of mean interarrivals of over threshold bottom shear stress events because of the role of advection/dispersion processes in mixing suspended sediments within the lagoon. Implications for long-term morphodynamic modeling of tidal environments are discussed

Dataset of wind setup in a regulated Venice lagoon

This data article includes the dataset of wind setup in the Venice lagoon computed by means of a 2-D hydrodynamic model. The capability of the model to reproduce the hydrodynamic regime of the lagoon has been extensively investigated, with particular attention to the calibration of the wind shear stress at the water surface, in order to precisely characterize the contribute of wind setup on the water level estimation inside the lagoon. We analyze the wind setup induced considering all the reliable wind speeds (with step of 1 m/s) and wind directions (with step of 30) potentially blowing over the Venice lagoon, comparing the results obtained considering the present not-regulated configuration of the lagoon (pre-Mo.S.E. scenario) to the regulated configuration (post- Mo.S.E. scenario), which refers to the hydrodynamic regime when theMo.S.E. movable barriers will be operational. The analysis shows that the wind setup significantly increases when the gates at the three inlets of the Venice lagoon are regulated, up to exceeding four times the pre-Mo.S.E. scenario.We deem this result is of paramount importance for the management of the Mo.S.E. barriers and for the definition of their operating strategy aiming at preventing the flooding at all the urban settlements of the lagoon

An eco-geomorphic model of tidal channel initiation and elaboration in progressive marsh accretional contexts

The formation and evolution of tidal networks have been described through various theories which mostly assume that tidal network development results from erosional processes, therefore emphasizing the chief role of external forcing triggering channel net erosion such as tidal currents. In contrast, in the present contribution we explore the influence of sediment supply in governing tidal channel initiation and further elaboration using an ecogeomorphic modeling framework. This deliberate choice of environmental conditions allows for the investigation of tidal network growth and development in different sedimentary contexts and provides evidences for the occurrence of both erosional and depositional channel-forming processes. Results show that these two mechanisms in reality coexist but act at different time scales: channel initiation stems from erosional processes, while channel elaboration mostly results from depositional processes. Furthermore, analyses suggest that tidal network ontogeny is accelerated as the marsh accretional activity increases, revealing the high magnitude and prevalence of the depositional processes in governing the morphodynamic evolution of the tidal network. On a second stage, we analyze the role of different initial topographic configurations in driving the development of tidal networks. Results point out an increase in network complexity over highly perturbed initial topographic surfaces, highlighting the legacy of initial conditions on channel morphological properties. Lastly, the consideration that landscape evolution depends significantly on the parameterization of the vegetation biomass distribution suggests that the claim to use uncalibrated models for vegetation dynamics is still questionable when studying real cases

Multipurpose Use of Artificial Channel Networks for Flood Risk Reduction: The Case of the Waterway Padova\u2013Venice (Italy)

Many rivers are increasingly threatened by extreme floods, and effective strategies for flood risk mitigation are difficult to pursue, especially in highly urbanized areas. A flexible and multipurpose use of the complex networks of artificial channels that typically cross these regions can play a role in flood risk mitigation. A relevant example concerns the possible completion of a waterway from Padova to the Venice Lagoon, in North-Eastern Italy. Once completed, the waterway can boost shipping (which is considerably more climate and environment friendly than road transport), can lead to a urban re-composition of the territory and, serving as a diversion canal for the Brenta River, can reduce hydraulic hazard as well. The goal of the present work was to assess this last point. To this purpose, the 2DEF hydrodynamic model was used to reproduce the complex Brenta\u2013Bacchiglione river network. This network includes river reaches, diversion canals, bed sills, pump stations, and control structures that assures the proper operation of the system in case of flood events. The mixed Eulerian\u2013Lagrangian, semi-implicit formulation of the model provided accurate and computationally efficient results for subcritical regimes. The model results showed that the waterway can divert a significant part of the Brenta floodwaters toward the Venice Lagoon, thus reducing flood hazard in the Brenta River downstream of Padova. The benefits also extend to the Bacchiglione River, whose floodwaters can be diverted into the Brenta River through an existing flood canal; indeed, the waterway withdrawal produces a drawdown profile in the Brenta River that allows diverting larger flow rates from the Bacchiglione River as well. Finally, by conveying the sediment-laden floodwaters of the Brenta River within the Venice Lagoon, the waterway could contribute to counteract the generalized erosion affecting the lagoon

Optimal floodgate operation for river flood management: The case study of Padova (Italy)

Study region: A large, densely populated area nearby Padova (Veneto Region, Italy) is exposed to floods owing to the Brenta-Bacchiglione river network, which is formed by two main rivers and by a set of interconnected channels, control structures and pump stations. Study focus: The Brenta and Bacchiglione rivers suffer from an increasing pressure in terms of flood events, especially for urban sprawl, anthropogenic modifications of drainage networks, and climate change. Finding and implementing effective remedies is hard in developed countries due to the presence of several constraints. Optimal flood management in complex river networks is then a way to reduce flood hazard, at a relatively low cost compared to structural measures. Hence, optimal operation rules for floodgates at an existing control structure are searched for to control the upstream water level and to divert a proper amount of the Bacchiglione discharge into the Brenta River. The operation rules have been endorsed by the Civil Engineering Department in charge of flood management and have been implemented in the flood forecasting Early Warning System of the Regional Civil Protection Office. New hydrological insights: The proper operation of control structures allows reducing flood risk by balancing the water discharge in the river networks. The engagement of end-users proves beneficial as it fosters exchange of knowledge and allows for the effective adoption of research outcomes in decision making

Toward coherent space–time mapping of seagrass cover from satellite data: an example of a Mediterranean lagoon

Seagrass meadows are a highly productive and economically important shallow coastal habitat. Their sensitivity to natural and anthropogenic disturbances, combined with their importance for local biodiversity, carbon stocks, and sediment dynamics, motivate a frequent monitoring of their distribution. However, generating time series of seagrass cover from field observations is costly, and mapping methods based on remote sensing require restrictive conditions on seabed visibility, limiting the frequency of observations. In this contribution, we examine the effect of accounting for environmental factors, such as the bathymetry and median grain size (D50) of the substrate as well as the coordinates of known seagrass patches, on the performance of a random forest (RF) classifier used to determine seagrass cover. Using 148 Landsat images of the Venice Lagoon (Italy) between 1999 and 2020, we trained an RF classifier with only spectral features from Landsat images and seagrass surveys from 2002 and 2017. Then, by adding the features above and applying a time-based correction to predictions, we created multiple RF models with different feature combinations. We tested the quality of the resulting seagrass cover predictions from each model against field surveys, showing that bathymetry, D50, and coordinates of known patches exert an influence that is dependent on the training Landsat image and seagrass survey chosen. In models trained on a survey from 2017, where using only spectral features causes predictions to overestimate seagrass surface area, no significant change in model performance was observed. Conversely, in models trained on a survey from 2002, the addition of the out-of-image features and particularly coordinates of known vegetated patches greatly improves the predictive capacity of the model, while still allowing the detection of seagrass beds absent in the reference field survey. Applying a time-based correction eliminates small temporal variations in predictions, improving predictions that performed well before correction. We conclude that accounting for the coordinates of known seagrass patches, together with applying a time-based correction, has the most potential to produce reliable frequent predictions of seagrass cover. While this case study alone is insufficient to explain how geographic location information influences the classification process, we suggest that it is linked to the inherent spatial auto-correlation of seagrass meadow distribution. In the interest of improving remote-sensing classification and particularly to develop our capacity to map vegetation across time, we identify this phenomenon as warranting further research.</p

Benefici dell\u2019idrovia Padova-Venezia sulla riduzione della pericolosit\ue0 idraulica nel bacino Brenta-Bacchiglione

-La gestione del nodo di Voltabarozzo (Padova) non pu\uf2 prescindere dalle condizioni del fiume Brenta -L\u2019idrovia Padova-Venezia per ridurre la pericolosit\ue0 idraulica nel Bacchiglione-Brenta -L\u2019idrovia per ridare flessibilit\ue0 di manovra al nodo idraulico di Voltabarozz

Minimization, equivalence and fate of the hydrogeological hazard: a new criterion to manage the flood threats applied to the Padova hydraulic node

Until two centuries ago, the Bacchiglione river crossed the city of Padova and the entire neighborhoods were threatened by its floods. In order to solve this problem, in 1830 the Austrian Government established the construction of a new channel, the Scaricatore channel, diverting the Bacchiglione river outside Padova, up to the Roncajette river at Voltabarozzo. However, this artificial channel was not able to convey the maximum flow rates of the Bacchiglione. Therefore, in the 1930s, according to the project of Luigi Gasparini, the Scaricatore was enlarged and the new San Gregorio channel was digged with the aim of diverting the water from the Voltabarozzo node toward the Piovego channel and then to the Brenta river, in order to reduce the hydraulic threat in the Roncajette. The water discharged through both the San Gregorio and the Roncajette is controlled by two different systems of sluice gates, located at the Voltabarozzo node. The management of these gates is actually manual, based on the water level gauged in some sections of the Bacchiglione and the Brenta. In the present study we first estimated the flow capacity of the San Gregorio channel, depending on the water level both at Voltabarozzo and in the Brenta. The target of this work is to develop an automatic procedure to define the optimal maneuvers at the gates, according to a criterion that consists in the "minimization, equivalence and fate" of the hydraulic hazard. Five hazard classes have been identified, related to the water level in the Scaricatore and to the flow rates both in the Brenta and in the Roncajette, as result of the regulations at the gates. The definition of these classes has been strongly related to the knowledge of the geomorphological and hydrogeological features of the territory: the key elements we considered are the spatial and temporal propagation mechanisms of the flood waves, the characterization of overflow-collapse triggering mechanisms of the banks and the assessment of the threat to infrastructure and private buildings. The procedure has been tested over several events, by means of a 2-D shallow water numerical model, based on a finite-element technique where the domain is discretized with triangular elements. Results show that the automatic procedure can be a powerful tool to manage the Padova node during the floods of the Bacchiglione river. Moreover, this procedure allows to reproduce the regulations at the gates carried out in the past floods events, when the regulations have not been recorded yet. Finally, the procedure has been used to evaluate the effects of some hydraulic interventions, such as the rise of the levees at some locations and the construction of the waterway "Idrovia Padova - Venezia", which should divert part of the flow rates of the Brenta toward the Venice lagoon, just downstream the confluence with the San Gregorio - Piovego system. Criteria and strategies implemented here for the Voltabarozzo node can be applied to other regulation structures, in order to improve the hydraulic hazard mitigation and decision making process