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On 18 September 2020, medicane Ianos hit the western coast of Greece resulting in flooding and severe damage at several coastal locations. In this work, we aim at evaluating its impact on sea conditions and the associated uncertainty through the use of an ensemble of numerical simulations. We applied a coupled wave-current model to an unstructured mesh representing the whole Mediterranean Sea, with a grid resolution increasing in the Ionian Sea along the cyclone path and the landfall area. To investigate the uncertainty of modelling sea levels and waves for such an intense event, we performed a multimodel ensemble of ocean simulations using several coarse (10 km) and high-resolution (2 km) meteorological forcings from different mesoscale models. The performance of the ocean and wave models was evaluated against observations retrieved from fixed monitoring stations and satellites. All model runs emphasized the occurrence of severe sea conditions along the cyclone path and at the coast. Due to the rugged and complex coastline, extreme sea levels are localised at specific coastal sites. However, numerical results show a large spread of the simulated sea conditions for both the sea level and waves highlighting the large uncertainty in simulating this kind of extreme event. The multi-model / multi-physics approach allows us to assess how the uncertainty propagates from meteorological to ocean variables and the subsequent coastal impact. The ensemble mean and standard deviation were combined to prove the hazard scenarios of the potential impact of such an extreme event to be used in a flood risk management plan.

Flooding causes serious impacts on the old town of Venice, its residents, and its cultural heritage. Despite this existence-defining condition, limited scientific knowledge on flood risk of the old town of Venice is available to support decisions to mitigate existing and future flood impacts. Therefore, this study proposes a risk assessment framework to provide a methodical and flexible instrument for decision-making for flood risk management in Venice. We first use a state-of-the-art hydrodynamic urban model to identify the hazard characteristics inside the city of Venice. Exposure, vulnerability, and corresponding damage are then modeled by a multi-parametric, micro-scale damage model which is adapted to the specific context of Venice with its dense urban structure and high risk awareness. Furthermore, a set of individual protection scenarios are implemented to account for possible variability in flood preparedness of the residents. This developed risk assessment framework was tested for the flood event of 12 November 2019 and proved able to reproduce flood characteristics and resulting damage well. A scenario analysis based on a meteorological event like 12 November 2019 was conducted to derive flood damage estimates for the year 2060 for a set of sea level rise scenarios in combination with a (partially) functioning storm surge barrier, the Modulo Sperimentale Elettromeccanico (MOSE). The analysis suggests that a functioning MOSE barrier could prevent flood damage for the considered storm event and sea level scenarios almost entirely. A partially closed MOSE barrier (open Lido inlet) could reduce the damage by up to 34 % for optimistic sea level rise prognoses. However, damage could be 10 % to 600 % higher in 2060 compared to 2019 for a partial closure of the storm surge barrier, depending on different levels of individual protection.

On 12 November 2019, an exceptional flood event occurred in Venice, second only to the one that had occurred on 4 November 1966. The event caused the flooding of about 90% of the streets in the historic centre and the sea level reached a value of 189 cm compared to the local mean-sea-level datum. Subsequent analysis of the meteorological event highlighted the contributions at different temporal and spatial scales. A sub-synoptic cyclone, centred in the Tyrrhenian Sea, caused a strong Sirocco wind along the entire Adriatic basin, with a fairly typical atmospheric configuration. However, embedded in the first cyclone, a second meso-beta scale cyclone developed and moved in the north-westward direction over the Adriatic Sea along the Italian coast. This cyclone had a speed of about 12 m/s, very close to the speed of the shallow water waves for the depth of the northern Adriatic basin. The perturbation then triggered a Proudman resonance, as confirmed by the numerical simulations, and caused a meteotsunami-like wave that affected the north-western coasts of the Adriatic Sea. Through model simulations, we have estimated that the mesoscale cyclone contributed about 40 cm, of which about 40% may be attributed to the air-pressure forcing, amplified through the Proudman resonance, and the rest to the wind forcing influencing both the open Adriatic Sea and the shallow Venetian lagoon. Finally, we have also analysed the propagation and transformation of the perturbation upon its entrance into the Venetian lagoon. This work is part of the COST action CA19109 MEDCYCLONES (European Network for Mediterranean Cyclones in weather and climate) and the Interreg Italy-Croatia STREAM project (Strategic development of flood management, project ID 10249186).

The Black Sea is one of the largest enclosed seas in the world. More than 160 million people live in its coastal areas and large rivers flow into it, as the Danube, the Dniester, the Dnieper and the Don. The Danube river, located on the western coast, is the most important European waterway whose catchment area is shared by more than fifteen countries. Black sea coastal waters are also often characterized by high suspended sediment concentrations derived from coastal runoff and local sediment re-suspension from the river sources. Measuring coastal sediment transport in field situations is notoriously difficult and generally subject to great uncertainty, and model estimates are often used in practice. The specific objectives of the current study are to understand the present state of the western Black Sea, and to study current and wave dynamics with the assistance of state-of-the-art numerical models for simulating waves, currents, and their interactions with the sediment loads. We used the hydrodynamic model named SHYFEM, hard coupled with the Wavewatch III wave model for the current study, while the coupled Sedtrans05 model will be used in the future for sediment modeling. The numerical models are applied on an unstructured grid representing the western Black Sea and the Danube Delta through the use of triangular elements of different shapes and sizes.The simulations of the coupled SHYFEM-WW3 were compared with the significant wave heights (Hs) measured by the Jason-3 and Sentinel-3. Despite some discrepancies, SHYFEM-WW3 provides good agreements with the altimeter data. The preliminary comparison done on the basin scale application show better performance of the model for the western area (Correlation coefficient = 0.89 in the western Black sea, while Correlation coefficient = 0.81 in the basin scale). Next steps will be dedicated to a coastal application on the western Black Sea coupling circulation, waves and sediment transport.All of the modeling activities are developed within the framework and funded by the Horizon 2020 DOORS (Developing Optimal and Open Research support for the Black sea) project, (Grant no. 101000518) https://www.doorsblacksea.eu/.

Ocean model performances are highly related to the vertical coordinate system implemented. We study geo-potential (or z-) coordinates and we focus on the numerical treatment of the moving free surface. Typically z-coordinate models are coded with a surface layer with varying but not-vanishing thickness, which limits the vertical resolution in areas with high tidal range. We propose a z-coordinate algorithm that, thanks to the insertion and removal of surface layers, can deal with an arbitrary large tidal oscillation independently of the vertical resolution. The algorithm is based on a classical two steps procedure used in numerical simulations with moving boundaries (grid movement followed by a grid topology change) which leads to a stable and accurate numerical discretization. With ad-hoc treatment of advection terms at non-conformal edges that may appear due to insertion/removal operations, mass conservation and tracer constancy are preserved. This algorithm can be reverted, in the particular case when all layers are moving, to other surface-following z-coordinates, such as z-star. With a simple truncation error analysis and realistic numerical experiments, we show the performances of z-coordinates with surface layer insertion/removal in coastal environments.

This paper analyses the variability of the sea level barotropic components in the Mediterranean Sea and their reproduction using a hydrodynamic model with and without data assimilation. The impact of data assimilation is considered both in reanalysis and short-forecast simulations. We used a two-dimensional finite element model paired with an ensemble Kalman filter, which assimilated hourly sea level data from 50 stations in the Mediterranean basin. The results brought about a significant improvement given by data assimilation in the reanalysis of the astronomical tide, the surge, and the barotropic total sea level, even in coastal areas and far from the assimilated stations (e.g. the southeastern Mediterranean Sea). As with the reanalysis simulations, the forecast simulations, which start from analysis states, improve, especially on the first day (37 % average error reduction) and when seiche oscillations are triggered. Since seiches are free barotropic oscillations that depend only on the initial state, their reproduction improves very effectively with data assimilation. Finally, we estimate the periods and the energy of these oscillations by means of spectral analysis, both in the Adriatic Sea, where they have been extensively studied, and in the Mediterranean Sea, where the present documentation is scarce. While the periods are well reproduced by the model even without data assimilation, their energy shows a good improvement when using it.

A new model for transport and fate of chemicals in the aquatic environment is presented. The tool, named ChemicalDrift, is integrated into the open-source Lagrangian framework OpenDrift and is hereby presented for organic compounds. The supported chemical processes include the degradation, the volatilization, and the partitioning between the different phases that a target chemical can be associated with in the aquatic environment, e.g. dissolved, bound to suspended particles, or deposited to the seabed sediments. The dependencies of the chemical processes on changes in temperature, salinity, and particle concentration are formulated and implemented. The chemical-fate modelling is combined with wide support for hydrodynamics by the integration within the Lagrangian framework which provides e.g. advection by ocean currents, diffusion, wind-induced turbulent mixing, and Stokes drift generated by waves. A flexible interface compatible with a wide range of available metocean data is made accessible by the integration, making the tool easily adaptable to different spatio-temporal scales and fit for modelling of complex coastal regions. Further inherent capabilities of the Lagrangian approach include the seamless tracking and separation of multiple sources, e.g. pollutants emitted from ships or from rivers or water treatment plants. Specific interfaces to a dataset produced by a model of emissions from shipping and to an unstructured-grid oceanographic model of the Adriatic Sea are provided. The model includes a database of chemical parameters for a set of poly-aromatic hydrocarbons and a database of emission factors for different chemicals found in discharged waters from sulfur emission abatement systems in marine vessels. A post-processing tool for generating mean concentrations of a target chemical, over customizable spatio-temporal grids, is provided. Model development and simulation results demonstrating the functionalities of the model are presented, while tuning of parameters, validation, and reporting of numerical results are planned as future activities. The ChemicalDrift model flexibility, functionalities, and potential are demonstrated through a selection of examples, introducing the model as a freely available and open-source tool for chemical fate and transport that can be applied to assess the risks of contamination by organic pollutants in the aquatic environment.

A new model for transport and fate of chemicals in the aquatic environment is presented. The tool, named ChemicalDrift, is integrated in the open-source Lagrangian framework OpenDrift, and is hereby presented for organic compounds. The supported chemical processes include the degradation, the volatilization, and the partitioning between the different phases that a target chemical can be associated to in the aquatic environment, e.g. dissolved, bound to suspended particles, or deposited to the seabed sediments. The dependencies of the chemical processes on changes of temperature, salinity, and particle concentration, are formulated and implemented. The chemical fate modelling is combined with wide support for hydrodynamics by the integration within the Lagrangian framework which provides, e.g., advection by ocean currents, diffusion, wind induced turbulent mixing, and Stokes drift generated by waves. A powerful interface to a wide range of available metocean data is made available by the integration, making the tool flexible and adaptable to different spatio-temporal scales and fit for modelling of complex coastal regions. Further inherent capabilities of the Lagrangian approach include the seamless tracking and separation of multiple sources as, e.g., pollutants emitted from ships or from rivers or water treatment plants. Specific interfaces to a dataset produced by a model of emissions from shipping, and to an unstructured-grid oceanographic model of the Adriatic Sea, are provided. The model includes a database of chemical parameters for a set of poly-aromatic hydrocarbons, and a database of emission factors for different chemicals found in discharged waters from sulphur emission abatements systems in marine vessels. A post-processing tool for generating mean concentrations of a target chemical, over customizable spatio-temporal grids, is provided. Model development and functional testing are presented, while tuning of parameters, validation, and reporting of numerical results, are planned as future activities. The ChemicalDrift model flexibility, functionalities, and potential, are demonstrated through a selection of examples, introducing the model as a valuable tool for chemical fate and transport, that can be applied to assessment of the risks of contamination by organic pollutants in the aquatic environment.

This paper analyses the variability of the sea level barotropic components in the Mediterranean Sea and their reproduction using a hydrodynamic model with and without data assimilation. The impact of data assimilation is considered both in reanalysis and short-forecast simulations. We used a two-dimensional finite element model paired with an ensemble Kalman filter, which assimilated hourly sea level data from 50 stations in the Mediterranean basin. The results brought about a significant improvement given by data assimilation in the reanalysis of the astronomical tide, the surge, and the barotropic total sea level, even in coastal areas and far from the assimilated stations (e.g. the southeastern Mediterranean Sea). As with the reanalysis simulations, the forecast simulations, which start from analysis states, improve, especially on the first day (37 % average error reduction) and when seiche oscillations are triggered. Since seiches are free barotropic oscillations that depend only on the initial state, their reproduction improves very effectively with data assimilation. Finally, we estimate the periods and the energy of these oscillations by means of spectral analysis, both in the Adriatic Sea, where they have been extensively studied, and in the Mediterranean Sea, where the present documentation is scarce. While the periods are well reproduced by the model even without data assimilation, their energy shows a good improvement when using it.

The correct reproduction of sea-level dynamics is crucial for forecasting floods and managing the associated risk. On the other hand, sea-level monitoring through observations can provide a description only of past events and it is challenging and costly, both of time and money. In this context, oceanographic models are increasingly used to describe the sea dynamics, providing a spatial/temporal extension to the observations. The best solution, which merges the observation accuracy and the model spatial/temporal resolution, is the data assimilation analysis, which is particularly important in coastal regions with scarce monitoring resources. In this study, we investigate the benefits of assimilating sparse observations from tide gauges in an unstructured hydrodynamic model for simulating the sea level in the Mediterranean Sea. We use the Ensemble Kalman filter, both to obtain an analysis of the past and to produce accurate forecasts. In the analysis we tested the assimilation in storm-surge simulations, only-tide simulations, and total-level simulations, using the observations in the stations. The results of storm-surge simulations were compared with those of total-level simulations, by adding the tide obtained from harmonic analysis of the observations. RMSE and correlation show improvements for all the components of the sea level and all the stations considered (not assimilated). The averaged-over-station RMSE reduces from 9.1 to 3.4 cm for the total level. The greatest improvements happen when the model without assimilation, due to an error of the wind-pressure forcing, did not reproduce some barotropic free modes of oscillation triggered by an initial surge. The preliminary forecast simulations of storm surge show improvements due to the data assimilation extending up to 5 days of forecasting. Even in this case, the longer improvements seem to happen when a free mode of oscillation is triggered. The results of this study will be used to improve the sea level forecasting system in the Adriatic Sea, developed within the framework of the Interreg Italy-Croatia STREAM project (Strategic development of flood management, project ID 10249186).

The Mediterranean basin occasionally hosts small intense vortices that evolve in tropical-like cyclones, also called “medicanes”. Although they are more intense over the sea, their landfall may be associated with destructive extreme events, such as heavy precipitation, windstorms, flooding, and marine storminess. On 18 September 2020, medicane Ianos hit the western coast of Greece resulting in flooding and severe damages at specific coastal locations. In this work, we aim at evaluating the impact of medicane Ianos on the sea state and water level through the use of numerical simulations. We applied a coupled wave-current model to an unstructured mesh representing the whole Mediterranean Sea, with a grid resolution varying from 15 km in the open sea to 2 km along the predetermined cyclone path, and up to 500 m along the landfall area (the western Greek coast). In order to investigate the uncertainty of the ocean model derived by the atmospheric modelling of such an intense event, we performed an ensemble of simulations using several coarse (10 km) and high-resolution (2 km) meteorological forcings from different mesoscale models. Also, results obtained using ERA5 reanalysis or IFS analysis are considered as a benchmark. The multi-model approach allows us to assess how the uncertainty propagates from meteorological fields to the ocean quantities and the subsequent coastal impact. The model performance was evaluated against observations retrieved from fixed monitoring stations and satellites. The numerical results show a large spread of the simulated sea conditions. Due to the rugged and complex coastline, extreme sea levels are localized at specific coastal sites. The ensemble results were combined for proving a set of indicators of the potential impact of such an intense event, in order to assess the effectiveness of this multi-model ensemble approach. This work is part of the COST action CA19109 MEDCYCLONES (European Network for Mediterranean Cyclones in weather and climate) and of the Interreg Italy-Croatia STREAM project (Strategic development of flood management, project ID 10249186).

Full comprehension of the dynamics of hazardous sea levels is indispensable for assessing and managing coastal flood risk, especially under a changing climate. The 12 November 2019 devastating flood in the historical city of Venice (Italy) stimulated new investigations of the coastal flooding problem from different perspectives and timescales. Here Venice is used as a paradigm for coastal flood risk, due to the complexity of its flood dynamics facing those of many other locations worldwide. Spectral decomposition was applied to the long-term 1872–2019 sea-level time series in order to investigate the relative importance of different drivers of coastal flooding and their temporal changes. Moreover, a multivariate analysis via copulas provided statistical models indispensable for correctly understanding and reproducing the interactions between the variables at play. While storm surges are the main drivers of the most extreme events, tides and long-term forcings associated with planetary atmospheric waves and seasonal to inter-annual oscillations are predominant in determining recurrent nuisance flooding. The non-stationary analysis revealed a positive trend in the intensity of the non-tidal contribution to extreme sea levels in the last three decades, which, along with relative sea-level rise, contributed to an increase in the frequency of floods in Venice.

Extreme sea levels at the coast result from the combination of astronomical tides with atmospherically forced fluctuations at multiple time scales. Seiches, river floods, waves, inter-annual and inter-decadal dynamics and relative sea-level rise can also contribute to the total sea level. While tides are usually well described and predicted, the effect of the different atmospheric contributions to the sea level and their trends are still not well understood. Meso-scale atmospheric disturbances, synoptic-scale phenomena and planetary atmospheric waves (PAW) act at different temporal and spatial scales and thus generate sea-level disturbances at different frequencies. In this study, we analyze the 1872-2019 sea-level time series in Venice (northern Adriatic Sea, Italy) to investigate the relative role of the different driving factors in the extreme sea levels distribution. The adopted approach consists in 1) isolating the different contributions to the sea level by applying least-squares fitting and Fourier decomposition; 2) performing a multivariate statistical analysis which enables the dependencies among driving factors and their joint probability of occurrence to be described; 3) analyzing temporal changes in extreme sea levels and extrapolating possible future tendencies. The results highlight the fact that the most extreme sea levels are mainly dominated by the non-tidal residual, while the tide plays a secondary role. The non-tidal residual of the extreme sea levels is attributed mostly to PAW surge and storm surge, with the latter component becoming dominant for the most extreme events. The results of temporal evolution analysis confirm previous studies according to which the relative sea-level rise is the major driver of the increase in the frequency of floods in Venice over the last century. However, also long term variability in the storm activity impacted the frequency and intensity of extreme sea levels and have contributed to an increase of floods in Venice during the fall and winter months of the last three decades.

Saltwater intrusion (SWI) in deltas, estuaries, and other coastal areas represents a pressing problem affecting the sustainability of freshwater resources. Observations and numerical experiments are used to investigate SWI in the surface waters of the Italian's largest river delta (the Po Delta) under low discharge conditions. The model successfully reproduced the observed salinity variations along main river branches and in the delta lagoons and investigated fresh and saltwater dynamics during the entire summer 2017. The numerical model is used to explore the hydrological response of the delta system to climate change. The relative effect of controlling factors on saltwater penetration, such as sea-level rise (SLR), air-sea fluxes, and reduction of river discharge, was evaluated. Results suggest that changes driven by climate and SLR will exacerbate SWI in the surface waters of the Po Delta system. Extent of SWI in the major river branch is projected to increase up to 80% further into the river. Persistence of SWI at the same site is expected to increase 100% longer. Moreover, additional synthetic simulations were used to investigate the sensitivity of SWI to several river discharges and relative SLR values, highlighting the effects of the two factors in the delta. Such results show different combinations of river discharge and relative SLR values that represent possible scenarios, and are important for managing SWI in river-sea systems affected by both climatic and anthropogenic factors such as discharge changes from dam construction, land use, and irrigation.

This paper reviews the state of the art in storm surge forecasting and its particular application in the northern Adriatic Sea. The city of Venice already depends on operational storm surge forecasting systems to warn the population and economy of imminent flood threats, as well as help to protect the extensive cultural heritage. This will be more important in the future, with the new mobile barriers called MOSE (MOdulo Sperimentale Elettromeccanico, Experimental Electromechanical Module) that will be completed by 2021. The barriers will depend on accurate storm surge forecasting to control their operation. In this paper, the physics behind the flooding of Venice is discussed, and the state of the art of storm surge forecasting in Europe is reviewed. The challenges for the surge forecasting systems are analyzed, especially in view of uncertainty. This includes consideration of selected historic extreme events that were particularly difficult to forecast. Four potential improvements are identified: (1) improve meteorological forecasts, (2) develop ensemble forecasting, (3) assimilation of water level measurements and (4) develop a multimodel approach.

Monitoring networks aims at capturing the spatial and temporal variability of one or several environmental variables in a specific environment. The optimal placement of sensors in an ocean or coastal observatory should maximize the amount of collected information and minimize the development and operational costs for the whole monitoring network. In this study, the problem of the design and optimization of ocean monitoring networks is tackled throughout the implementation of data assimilation techniques in the Shallow water HYdrodynamic Finite Element Model (SHYFEM). Two data assimilation methods – nudging and ensemble square root filter – have been applied and tested in the Lagoon of Venice (Italy), where an extensive water level monitoring network exists. A total of 29 tide gauge stations were available, and the assimilation of the observations results in an improvement of the performance of the SHYFEM model, which went from an initial root mean square error (RMSE) on the water level of 5.8 cm to a final value of about 2.1 and 3.2 cm for each of the two data assimilation methods. In the monitoring network optimization procedure, by excluding just one tide gauge at a time and always the station that contributes less to the improvement of the RMSE, a minimum number of tide gauges can be found that still allow for a successful description of the water level variability. Both data assimilation methods allow identifying the number of stations and their distribution that correctly represent the state variable in the investigated system. However, the more advanced ensemble square root filter has the benefit of keeping a physically and mass-conservative solution of the governing equations, which results in a better reproduction of the hydrodynamics over the whole system. In the case of the Lagoon of Venice, we found that, with the help of a process-based and observation-driven numerical model, two-thirds of the monitoring network can be dismissed. In this way, if some of the stations must be decommissioned due to a lack of funding, an a priori choice can be made, and the importance of a single monitoring site can be evaluated. The developed procedure may also be applied to the continuous monitoring of other ocean variables, like sea temperature and salinity.

Floods in the Venice city centre result from the superposition of several factors: astronomical tides; seiches; and atmospherically forced fluctuations, which include storm surges, meteotsunamis, and surges caused by atmospheric planetary waves. All these factors can contribute to positive water height anomalies individually and can increase the probability of extreme events when they act constructively. The largest extreme water heights are mostly caused by the storm surges produced by the sirocco winds, leading to a characteristic seasonal cycle, with the largest and most frequent events occurring from November to March. Storm surges can be produced by cyclones whose centres are located either north or south of the Alps. Historically, the most intense events have been produced by cyclogenesis in the western Mediterranean, to the west of the main cyclogenetic area of the Mediterranean region in the Gulf of Genoa. Only a small fraction of the inter-annual variability in extreme water heights is described by fluctuations in the dominant patterns of atmospheric circulation variability over the Euro-Atlantic sector. Therefore, decadal fluctuations in water height extremes remain largely unexplained. In particular, the effect of the 11-year solar cycle does not appear to be steadily present if more than 100 years of observations are considered. The historic increase in the frequency of floods since the mid-19th century is explained by relative mean sea level rise. Analogously, future regional relative mean sea level rise will be the most important driver of increasing duration and intensity of Venice floods through this century, overcompensating for the small projected decrease in marine storminess. The future increase in extreme water heights covers a wide range, largely reflecting the highly uncertain mass contributions to future mean sea level rise from the melting of Antarctica and Greenland ice sheets, especially towards the end of the century. For a high-emission scenario (RCP8.5), the magnitude of 1-in-100-year water height values at the northern Adriatic coast is projected to increase by 26–35 cm by 2050 and by 53–171 cm by 2100 with respect to the present value and is subject to continued increase thereafter. For a moderate-emission scenario (RCP4.5), these values are 12–17 cm by 2050 and 24–56 cm by 2100. Local subsidence (which is not included in these estimates) will further contribute to the future increase in extreme water heights. This analysis shows the need for adaptive long-term planning of coastal defences using flexible solutions that are appropriate across the large range of plausible future water height extremes.

Floods in the Venice city centre result from the superposition of several factors: astronomical tides, seiches and atmospherically forced fluctuations, which include storm surges, meteotsunamis, and surges caused by planetary waves. All these factors can contribute to positive sea-level anomalies individually and can also result in extreme sea-level events when they act constructively. The largest extreme sea level events have been mostly caused by storm surges produced by the Sirocco winds. This leads to a characteristic seasonal cycle, with the largest and most frequent events occurring from November to March. Storm surges can be produced by cyclones whose centers are located either north or south of the Alps. The most intense historical events have been produced by cyclogenesis in the western Mediterranean, to the west of the main cyclogenetic area of the Mediterranean region in the Gulf of Genoa. Only a small fraction of the interannual variability of extreme sea levels is described by fluctuations in the dominant patterns of atmospheric circulation variability over the Euro-Atlantic sector. Therefore, decadal fluctuations of sea-level extremes remain largely unexplained. In particular, the effect of the 11-year solar cycle appears to be small, non-stationary or masked by other factors. The historic increase in the frequency of extreme sea levels since the mid 19th Century is explained by relative sea level rise, with no long term trend in the intensity of the atmospheric forcing. Analogously, future regional relative mean sea level rise will be the most important driver of increasing duration and intensity of Venice floods through this century, overwhelming the small decrease in marine storminess projected during the 21 century. Consequently, the future increase of extreme sea levels covers a large range, partly reflecting the highly uncertain mass contributions to future mean sea level rise from the melting of Antarctica and Greenland ice-sheets, especially towards the end of the century. In conclusion, for a high emission scenario the magnitude of 1-in-100 year sea level events at the North Adriatic coast is projected to increase up to 65 % and 160 % in 2050 and 2100, respectively, with respect to the present value, and subject to continued increase thereafter. Local subsidence can further contribute to the future increase of extreme sea levels. This analysis shows the need for adaptive planning of coastal defenses with solutions that can be adopted to face the large range of plausible future sea-level extremes.

The City of Venice and the surrounding lagoonal ecosystem are highly vulnerable to variations in relative sea level. In the past ~150 years, this was characterized by a secular linear trend of about 2.5 mm/year resulting from the combined contributions of vertical land movement and sea-level rise. This literature review reassesses and synthesizes the progress achieved in understanding, estimating and predicting the individual contributions to local relative sea level, with focus on the most recent publications. The current best estimate of historical sea-level rise in Venice, based on tide-gauge data after removal of subsidence effects, is 1.23 ± 0.13 mm/year (period from 1872 to 2019). Subsidence thus contributed to about half of the observed relative sea-level rise over the same period. A higher – yet more uncertain – rate of sea-level rise is observed during recent decades, estimated from tide-gauge data to be about 2.76 ± 1.75 mm/year in the period 1993–2019 for the climatic component alone. An unresolved issue is the contrast between the observational capacity of tide gauges and satellite altimetry, with the latter tool not covering the Venice Lagoon. Water mass exchanges through the Gibraltar Strait currently constitute a source of substantial uncertainty for estimating future deviations of the Mediterranean mean sea-level trend from the global-mean value. Subsidence and regional atmospheric and oceanic circulation mechanisms can deviate Venetian relative sea-level trends from the global mean values for several decades. Regional processes will likely continue to determine significant interannual and interdecadal variability of Venetian sea level with magnitude comparable to that observed in the past, as well as non-negligible differential trends. Our estimate of the likely range of mean sea-level rise in Venice by 2100 due to climate change is presently estimated between 11 and 110 centimetres. An improbable yet possible high-end scenario linked to strong ice-sheet melting yields about 170 centimetres of mean sea-level rise in Venice by 2100. Projections of natural and human induced vertical land motions are currently not available, but historical evidence demonstrates that they can produce a significant contribution to the relative sea-level rise in Venice, further increasing the hazard posed by climatically-induced sea-level changes.

On 12 November 2019, an exceptional flood event occurred in Venice, second only to the one that occurred on 4 November 1966. The maximum recorded sea level value of 189 cm above local datum resulted in the flooding of more than 85% of the pedestrian surface of the historical city. Moreover, with four extremely high tides since 11 November 2019, this has been the worst week for flooding in Venice ever since 1872, when official statistics were first produced. The event that struck Venice and the northern Adriatic Sea on 12 November 2019, although having certain conditions seemingly typical of the events that cause exceptional high waters, also had some peculiar characteristics not observed before and therefore it requires an in-depth analysis. Several factors made this event exceptional: an in-phase timing of the peak of the storm surge and the astronomical tide; an anomalously high monthly mean sea level in the Adriatic Sea induced by a steady low-pressure and wind systems over the Mediterranean Sea associated with large-scale low-frequency atmospheric dynamics; a deep low-pressure system over the central-southern Tyrrhenian Sea that generated strong sirocco (south-easterly) winds along the main axis of the Adriatic Sea pushing the waters towards north; a fast-moving local depression - and the associated wind perturbation - travelling in the north-westward direction along the Italian coast that may have forced long ocean waves (e.g., edge wave); and very strong winds (100 km h-1 on average, with gusts reaching 110 km h-1) over the Lagoon of Venice which led to a further rise in water levels and damage to the historic city. In this study, a large set of available observations and the high-resolution numerical simulations are used to quantify the influence of these drivers on the peak flood event and to investigate the peculiar weather and sea conditions over the Mediterranean Sea during the Venice floods of November 2019.

Addressing coastal risks related to sea storms requires an integrative approach which combines monitoring stations, forecasting models, early warning systems and coastal management and planning. Such great effort is sometimes possible only through transnational cooperation, which becomes thus vital to face effectively and promptly these marine events which are responsible for several damages impacting on the environment and citizens' life. Here we present a shared and interoperable system to allow a better exchange and elaboration of information related to sea storms among countries. The proposed Integrated Web System (IWS) is a combination of a common data system for sharing ocean observations and forecasts, a multi-model ensemble system, a geoportal and interactive geo-visualization tools to make results available to the general public. Multi-model ensemble mean and spread for sea level height and wave characteristics are used to describe three different sea condition scenarios. IWS is designed to provide sea state information required for issuing coastal risk alerts over the analysed region, as well as for being easily integrated into existing local early warning systems. This study describes the application of the developed system to the exceptional storm event of 29th of October 2018, that caused severe flooding and damages to coastal infrastructures in the Adriatic Sea. The forecasted ensemble products were successfully compared with in situ observations. The hazards estimated by integrating IWS results into existing early warning systems were confirmed by documented storm impacts along the coast of Slovenia, Emilia-Romagna and the City of Venice. For the investigated event, the most severe simulated scenario resulted to provide a realistic and conservative estimation of the peak storm conditions to be used in coastal risk management.

The lagoons of the Po River delta are potentially exposed to complex mixtures of contaminants, nevertheless, there is a substantial lack of information about the biological effects of these contaminants in the Po delta lagoons. These environments are highly dynamic and the interactions between chemical and environmental stressors could prevent the proper identification of biological effects and their causes. In this study, we aimed to disentangle such interactions focusing on Manila clams, previously exposed to six lagoons of the Po delta, adopting three complementary tools: a) the detailed description via modelling techniques of lagoon dynamics for salinity and water temperature; b) the response sensitivity of a number of target genes (ahr, cyp4, ρ-gst, σ-gst, hsp22, hsp70, hsp90, ikb, dbh, ach, cat, Mn-sod, Cu/Zn-sod, cyp-a, flp, grx, TrxP) investigated in clam digestive glands by Real Time PCR; and c) the relevance of DNA adducts determined in clams as markers of exposure to genotoxic chemicals. The lagoons showed specific dynamics, and two of them (Marinetta and Canarin) could induce osmotic stress. A group of genes (ahr, cyp4, Mn-sod, σ-gst, hsp-22, cyp-a, TrxP) seemed to be associated with overall lagoon characteristics as may be described by salinity and its variations. Lagoon modelling and a second group of genes (hsp70, hsp90, cat, ikb, ach, grx, Cu/Zn-sod) also suggested that moderate increases of river discharge may imply worse exposure conditions. Oxidative stress seemed to be associated with such events but it was slightly evident also under normal exposure conditions. DNA adduct formation was mainly associated with overwhelmed antioxidant defences (e.g. low Cu/Zn-sod) or seemingly with their lack of response in due time. In Po delta lagoons, Manila clam can be affected by chemical and environmental factors which can contribute to induce oxidative stress, DNA adduct formation and, ultimately, to affect clam condition and health.

In this study we investigated the tidal dynamics in a system of inter-connected land-locked basins formed by the Mediterranean, the Marmara, the Black and the Azov seas (MMBA system). Through the application of an unstructured grid hydrodynamic model to a unique domain representing the whole MMBA system, we simulated the tidal propagation and transformation inside each basin and in the straits connecting them. The model performance was evaluated against amplitudes and phases of major tidal constituents from 77 tidal gauges. The numerical results provided a description of the characteristics of the principal semi-diurnal, diurnal and long-term tides over the entire system. Even if the narrow straits act as a barrier for the tidal sea surface oscillations, our numerical results demonstrated that the along-strait interface slope produces water fluxes between the adjacent basins of the same order of magnitude of the climatological transports estimated by several authors. The long-term tidal modulations of the water exchange between the Mediterranean and the Black seas resulted to be non negligible and may partially explain the monthly and fortnightly flow variability observed in the Dardanelles and Bosphorus straits.

In the Adriatic Sea, massive rainfall events are causing flooding of rivers and streams, with severe consequences on the environment. The consequent bacterial contamination of bathing water poses public health risks besides damaging tourism and the economy. This study was conducted in the framework of WATERCARE, an EU Interreg Italy-Croatia Project, which aims at reducing the impact of microbial contamination on Adriatic bathing water due to heavy rainfall events drained in the local sewage network and; enhancing the quality of local waters; and providing support for the decision-making processes regarding the management of bathing water in line with EU regulations. The study involved the development of an innovative water quality integrated system that helps meet these objectives. It consists of four components: a real time hydro-meteorological monitoring system; an autosampler to collect freshwater samples during and after significant rainfall events; a forecast system to simulate the dispersion of pollutants in seawater; and a real-time alert system that can predict the potential ecological risk from the microbial contamination of seawater. The system was developed and tested at a pilot site (Fano, Italy). These preliminary results will be used to develop guidelines for urban wastewater and coastal system quality assessments to contribute to develop policy actions and final governance decisions.

On 12 November 2019, an exceptional flood event took place in Venice, second only to the one that occurred on 4 November 1966. Moreover, with four extremely high tides since 11 November 2019, this was the worst week for flooding in Venice since the beginning of sea level records (1872). The event that struck Venice and the northern Adriatic Sea on 12 November 2019, although having certain conditions seemingly typical of the events causing exceptional high waters, had some peculiar characteristics not observed before, which deserved an in-depth analysis. Several factors made this event exceptional: the in-phase timing between the peak of the storm surge and the astronomical tide; a deep low-pressure cyclone over the central-southern Tyrrhenian Sea that generated strong Sirocco (south-easterly) winds along the main axis of the Adriatic Sea, pushing waters to the north; a fast-moving local depression – and the associated wind perturbation – travelling in the north-westward direction over the Adriatic Sea along the Italian coast, generating a meteotsunami; very strong winds (28 m s − 1 on average with 31 m s − 1 gusts) over the Lagoon of Venice, which led to a rise in water levels and damages to the historic city; and an anomalously high monthly mean sea level in the Adriatic Sea, induced by a standing low-pressure and wind systems over the Mediterranean Sea, that was associated with large-scale low-frequency atmospheric dynamics. In this study, the large set of available observations and high-resolution numerical simulations have been used to quantify the contribution of the mentioned drivers to the peak of the flood event and to investigate the peculiar weather and sea conditions over the Mediterranean Sea during the Venice floods of November 2019.

We analyse high rate (20 Hz) altimeter derived sea level information, both from Synthetic Aperture Radar (SAR) altimetry missions and Low Resolution Mode missions, in an inner sea and coastal environment. They are compared versus measured and high resolution model data. After a compact description of the relevant physical processes affecting the local sea level values, we focus on three specific passes on the Adriatic Sea, East of Italy. Our main finding is that altimeters and models provide both valuable, partially overlapping, but complementary, information to be exploited in parallel with a double purpose: to progressively approach the truth and to ensure the parallel improvement of both sources. From altimeter data there is a very strong suggestion that the spatial, and implicitly temporal, variability of the fields is much higher than shown by models. The reasons are discussed.

The aim of this study was to investigate the sediment dynamics in the largest lagoon in Europe (Curonian Lagoon, Lithuania) through the analysis of in situ data and the application of a sediment transport model. This approach allowed to identify the propagation pathway of the riverine suspended sediments, to map erosion-accumulation zones in the lagoon and calculate the sediment budget over a 13-year-long simulation. Sampled suspended sediment concentration data are important for understanding the characteristics of the riverine and lagoon sediments, and show that the suspended organic matter plays a crucial role on the sediment dynamics for this coastal system. The numerical experiments carried out to study sediment dynamics gave satisfactory results and the possibility to get a holistic view of the system. The applied sediment transport model with a new formula for settling velocity was used to estimate the patterns of the suspended sediments and the seasonal and spatial sediment distribution in the whole river&ndash, lagoon&ndash, sea system. The numerical model also allowed understanding the sensitivity of the system to strong wind events and the presence of ice. The results reveal that during extreme storm events, more than 11.4 &times, 106 kg of sediments are washed out of the system. Scenarios without ice cover indicate that the lagoon would have much higher suspended sediment concentrations in the winter season comparing with the present situation with ice. The results of an analysis of a long-term (13 years) simulation demonstrate that on average, 62% of the riverine sediments are trapped inside the lagoon, with a marked spatially varying distribution of accumulation zones.

Este artículo contiene 30 páginas, 14 figuras., The Mediterranean community represented in this paper is the result of more than 30 years of EU and nationally funded coordination, which has led to key contributions in science concepts and operational initiatives. Together with the establishment of operational services, the community has coordinated with universities, research centers, research infrastructures and private companies to implement advanced multi-platform and integrated observing and forecasting systems that facilitate the advancement of operational services, scientific achievements and mission-oriented innovation. Thus, the community can respond to societal challenges and stakeholders needs, developing a variety of fit-for-purpose services such as the Copernicus Marine Service. The combination of state-of-the-art observations and forecasting provides new opportunities for downstreamservices in response to the needs of the heavily populatedMediterranean coastal areas and to climate change. The challenge over the next decade is to sustain ocean observations within the research community, to monitor the variability at small scales, e.g., the mesoscale/submesoscale, to resolve the sub-basin/seasonal and inter-annual variability in the circulation, and thus establish the decadal variability, understand and correct the model-associated biases and to enhance model-data integration and ensemble forecasting for uncertainty estimation. Better knowledge and understanding of the level ofMediterranean variability will enable a subsequent evaluation of the impacts and mitigation of the effect of human activities and climate change on the biodiversity and the ecosystem, which will support environmental assessments and decisions. Further challenges include extending the science-based added-value products into societal relevant downstream services and engaging with communities to build initiatives that will contribute to the 2030 Agenda and more specifically to SDG14 and the UN’s Decade of Ocean Science for sustainable development, by this contributing to bridge the science-policy gap. The Mediterranean observing and forecasting capacity was built on the basis of community best practices in monitoring and modeling, and can serve as a basis for the development of an integrated global ocean observing system.

Satellite imagery provides evidence of complex mixing dynamics in the coastal zone in front of multiple mouth deltas. One peculiar feature, identified in front of the Po Delta (Italy), consists in warmer water bulges present in some periods in the coastal zone between the river mouths. Such features are evident during both high and low river discharge. Through an integrated approach based on the analysis of satellite imagery, in situ field data and a high-resolution oceanographic model, representing the whole river-delta-sea system, we investigated the relative contribution of the different forcing in controlling coastal mixing of riverine waters. The results evidence that the occurrence of these warmer saltier water bulges is due to upwelling induced by the combined action of tides and wind regimes aligned along coastline. Winds from the land and along the coast drive the upwelling through the well-known mechanism described by Ekman. The presence of river discharge enhance the water column stratification, creating the conditions in which tidal action follows the tidal straining theory. Both processes are identified in modelling results. The occurrence of these localized coastal waters with peculiar thermohaline characteristics, detectable on satellite imagery of the area, can be relevant in the definition of the freshwater areas of influence and the mechanisms of riverine water mixing in the near coastal zone. This can shed some light, eventually, on characterizing the sediment dynamics, as well as the thermohaline properties of waters in the area, and also to identify eventual impacts on the local ecosystems and fishery.

The aim of this study was to investigate the variability of the water exchanges in the Curonian Lagoon based on the hydraulic regime and the atmospheric forcings. A finite element hydrodynamic model has been applied to the Curonian Lagoon to simulate the circulation patterns for ten years. With the help of a transport-diffusion model the salinity distribution and the renewal times of the Curonian Lagoon have been investigated when forced by river runoff, wind and Baltic Sea level fluctuations. The hydrodynamic model has been validated using in situ salinity measurements. Model results show that the variability depends mainly on seasonal changes in hydrographic forcing and on the dominant wind regimes that prevail over the Curonian Lagoon. Exchanges between the southern and the northern part of the lagoon are mostly depended on the wind forcing and are much less influenced by the river discharge. However, when looking at the water renewal time, the most important factor is the river discharge into the lagoon. Other physical forcings are only marginally determining the renewal time, and not even ice cover is able to influence it. Even if ice cover is strongly inhibiting the exchanges between southern and northern lagoon, it is basically not able to change the absolute value of the renewal times.

In this paper the problem of the optimisation of at-sea campaigns is tackled throughout the implementation of a near real-time decision support system (DSS) for short-term Lagrangian observing platforms. The DSS consisted of two modules: a real-time coastal-ocean forecasting system (CFS), including both hydrodynamic and particle-tracking numerical models, and a module to provide optimised deployment solutions for a set of Lagrangian current-meter buoys. The system was calibrated for a shallow water environment – the Cabras Lagoon in the western Mediterranean Sea. The DSS was then applied to provide in situ optimised measurement strategies during a Lagrangian survey campaign. The different solutions provided by the DSS were verified by real-time measurements and the system was established to be efficient in reducing the cost–benefit ratio and favouring long records and near-synoptic and non-redundant observations.