Abstract

Abstract. 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.

Highlights

  • This paper reviews current understanding of the factors that are responsible for the damaging floods affecting the Venice city centre and for their future evolution

  • 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

  • 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

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Summary

Introduction

This paper reviews current understanding of the factors that are responsible for the damaging floods affecting the Venice city centre and for their future evolution. In the northern Adriatic Sea, given the relatively small importance of tidal excursions (about 1 m) compared to the local water depth (average depth of about 35 m), the effect of tides on the storm surge propagation has been neglected for a long time in the prediction practice with hydrodynamic models where only the meteorological forcing was used, and the astronomical tide was either added to the model results to get the actual prediction or subtracted from the observations for model validation Examples of this approach and of its success are Lionello et al (2006b), Bajo et al (2007), and Mel and Lionello (2014) among many others. An example of such simulations can be found in Appendix B, and it shows that in numerical simulations nonlinear interactions are lower than 5 % at the peak of the water height

A description of the largest past events
The propagation of the sea level signal in the interior of the lagoon
Characteristics of cyclones producing storm surges and floods of Venice
Links to large-scale patterns
The role of solar cycles in extreme floods
Past evolution and recent trends of floods and extreme sea levels
Future evolution of extreme water heights
Findings
Conclusions and outlook
Full Text
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