Sedimentation strategies provide effective but limited mitigation of relative sea-level rise in the Mekong delta

The Mekong delta is experiencing rapid environmental change due to anthropogenic activities causing accelerated subsidence, sea-level rise and sediment starvation. Consequentially, the delta is rapidly losing elevation relative to sea level. Designating specific areas for sedimentation is a suggested strategy to encourage elevation-building with nature in deltas. We combined projections of extraction-induced subsidence, natural compaction and global sea-level rise with new projections of fluvial sediment delivery to evaluate the potential effectiveness of sedimentation strategies in the Mekong delta to 2050. Our results reveal that with current rates of subsidence and sediment starvation, fluvial sediments alone can only preserve elevation locally, even under optimistic assumptions, and organic sedimentation could potentially assume a larger role. While sedimentation strategies alone have limited effectiveness in the present context, combined with enhanced organic matter retention and interventions reducing anthropogenic-accelerated subsidence, they can considerably delay future relative sea-level rise, buying the delta crucial time to adapt.

Exploring the partial use of the Mo.S.E. system as effective adaptation to rising flood frequency of Venice

The Venice lagoon (Italy) is particularly vulnerable to the impact of subsidence and sea level rise driven by climate change. Some structural measures have been adopted over time to protect Venice from flooding, among which a system of flap gates (Experimental Electromechanical Module, Mo.S.E., system) has been operational in the testing phase since October 2020. However, relative sea level rise and wind set-up pose relevant management challenges, as a frequent closing of the lagoon would have negative impacts on flushing capacity, the fishing industry, and port activities. Here, the focus is on the hydrodynamic effects of a partial closure of the Mo.S.E. barriers that, compared to closing all the three inlets of the lagoon, could play a role in reducing the economic and environmental impacts of the Mo.S.E. system. The main goal is to identify the flooding events that can be counteracted by closing only the Lido inlet, which is the closest to the city of Venice. Based on the tidal and meteorological dataset collected in the period 2000–2019, a robust modelling exercise identifies a linear relationship between tidal range and reduction of the sea level peaks, which results in the protection of all urban settlements within the lagoon from two-thirds of the flooding events up to a relative sea level rise of +0.4 m.

Sea-level rise in Venice: historic and future trends (review article)

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 an average rate of relative sea-level rise 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 quantification, understanding and prediction of the individual contributions to local relative sea level, with a focus on the most recent studies. Subsidence contributed to about half of the historical relative sea-level rise in Venice. The current best estimate of the average rate of sea-level rise during the observational period from 1872 to 2019 based on tide-gauge data after removal of subsidence effects is 1.23 ± 0.13 mm/year. A higher – but more uncertain – rate of sea-level rise is observed for more recent years. Between 1993 and 2019, an average change of about +2.76 ± 1.75 mm/year is estimated from tide-gauge data after removal of subsidence. Unfortunately, satellite altimetry does not provide reliable sea-level data within the Venice Lagoon. Local sea-level changes in Venice closely depend on sea-level variations in the Adriatic Sea, which in turn are linked to sea-level variations in the Mediterranean Sea. Water mass exchange through the Strait of Gibraltar and its drivers currently constitute a source of substantial uncertainty for estimating future deviations of the Mediterranean mean sea-level trend from the global-mean value. Regional atmospheric and oceanic processes will likely contribute significant interannual and interdecadal future variability in Venetian sea level with a magnitude comparable to that observed in the past. On the basis of regional projections of sea-level rise and an understanding of the local and regional processes affecting relative sea-level trends in Venice, the likely range of atmospherically corrected relative sea-level rise in Venice by 2100 ranges between 32 and 62 cm for the RCP2.6 scenario and between 58 and 110 cm for the RCP8.5 scenario, respectively. A plausible but unlikely high-end scenario linked to strong ice-sheet melting yields about 180 cm of relative sea-level rise in Venice by 2100. Projections of human-induced vertical land motions are currently not available, but historical evidence demonstrates that they have the potential to produce a significant contribution to the relative sea-level rise in Venice, exacerbating the hazard posed by climatically induced sea-level changes.

Venice flooding and sea level: past evolution, present issues, and future projections (introduction to the special issue)

Venice is an iconic place and a paradigm of huge historical and cultural values at risk. The frequency of the flooding of the city centre has dramatically increased in recent decades, and this threat is expected to continue to grow – and even accelerate – through this century. This special issue is a collection of three review articles addressing different and complementary aspects of the hazards causing the floods of Venice, namely (1) the relative sea level rise, (2) the occurrence of extreme water heights, and (3) the prediction of extreme water heights and floods. It emerges that the effect of compound events poses critical challenges to the forecast of floods, particularly from the perspective of effectively operating the new mobile barriers (Modulo Sperimentale Elettromeccanico – MoSE) in Venice and that the relative sea level rise is the key factor determining the future growth of the flood hazard, so that the present defence strategy is likely to become inadequate within this century under a high-emission scenario. Two strands of research are needed in the future. First, there is a need to better understand and reduce the uncertainty of the future evolution of the relative sea level and its extremes at Venice. However, this uncertainty might not be substantially reduced in the near future, reflecting the uncertain anthropogenic emissions and structural model features. Hence, complementary adaptive planning strategies appropriate for conditions of uncertainty should be explored and developed in the future.

Exploring the partial use of the Mo.S.E. system as effective adaptation to rising flood frequency of Venice

In times of climate change the impact of coastal hazards should be mitigated by identifying and implementing effective adaptation strategies, encompassing a balanced mix of structural and non-structural measures based on high level scientific knowledge. Due to its hydro-geological features, the Venice lagoon (Italy) is particularly vulnerable to climate change. Some structural measures have been adopted over time to protect Venice from flooding, among which a system of flap gates (Mo.S.E. system) has been operational under testing phase since October 2020. However, relative sea level rise and wind setup pose relevant management challenges, as a frequent closing of the lagoon would have negative impacts on flushing capacity, fishing industry and port activities. Hence, optimal operation rules for the existing control structure are searched to anticipate and to adapt to a possible acceleration of sea level rise induced by climate change. Here, the focus is on the hydrodynamic effects of a partial closure of the Mo.S.E. barriers that, with respect to closing all the three inlets of the Lagoon, could play a role in reducing the economic and environmental impacts of the Mo.S.E. system. The main goal is to identify the flooding events that can be counteracted by closing only the Lido inlet, which is the closest to the city of Venice. Based on the tidal and meteorological dataset collected in the period 2000–2019, a robust modelling exercise indicates that the closing of the Lido inlet only would protect the Venice lagoon from two third of the flooding events up to a relative sea level rise of +0.4 m.