摘要：Floods in the Venice city centre result from the superposition of severalfactors: astronomical tides; seiches; and atmospherically forcedfluctuations, which include storm surges, meteotsunamis, and surges causedby atmospheric planetary waves. All these factors can contribute to positivewater height anomalies individually and can increase the probability ofextreme events when they act constructively. The largest extreme waterheights are mostly caused by the storm surges produced by the sirocco winds,leading to a characteristic seasonal cycle, with the largest and mostfrequent events occurring from November to March. Storm surges can beproduced by cyclones whose centres are located either north or south of theAlps. Historically, the most intense events have been produced bycyclogenesis in the western Mediterranean, to the west of the maincyclogenetic area of the Mediterranean region in the Gulf of Genoa. Only asmall fraction of the inter-annual variability in extreme water heights isdescribed by fluctuations in the dominant patterns of atmosphericcirculation variability over the Euro-Atlantic sector. Therefore, decadalfluctuations in water height extremes remain largely unexplained. Inparticular, the effect of the 11-year solar cycle does not appear to besteadily present if more than 100 years of observations areconsidered. The historic increase in the frequency of floods since themid-19th century is explained by relative mean sea level rise.Analogously, future regional relative mean sea level rise will be the mostimportant driver of increasing duration and intensity of Venice floodsthrough this century, overcompensating for the small projected decrease inmarine storminess. The future increase in extreme water heights covers awide range, largely reflecting the highly uncertain mass contributions tofuture mean sea level rise from the melting of Antarctica and Greenlandice sheets, especially towards the end of the century. For a high-emissionscenario (RCP8.5), the magnitude of 1-in-100-year water height values at thenorthern Adriatic coast is projected to increase by 26–35 cm by 2050 and by53–171 cm by 2100 with respect to the present value and is subject tocontinued 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 thefuture increase in extreme water heights. This analysis shows the need foradaptive long-term planning of coastal defences using flexible solutionsthat are appropriate across the large range of plausible future water heightextremes.
