ABSTRACT
ABSTRACT: Venice is at risk of being submerged for the combined effect of global warming and land subsidence, and the sea level rise (SLR). This combined effect, is known after the tide gauge record since 1872, but this period is too short to solve some critical uncertainties. Proxy data are necessary to get a long-term view of the sea level, going back to AD 1500. Written source from 1500 show an exponential increase of floods, suggesting the same trend for the SLR. A proxy of mixed biologic-documentary nature is the green belt of phototropic algae that grow along canals. Paintings by Veronese (1528-88), Canaletto (1697-1768) and Bellotto (1720-80), made with the ‘camera obscura’ as a technical tool, reproduced with high precision all details, including the algae belt on buildings. The comparison with the presentday level of the green belt gives a quantitative evaluation of the sea level rise that follows an exponential trend over the long-term (i.e. 500 yr). The pictorial documentation is rich for the XVIII century, but only one painting (dated 1571) for the previous period, so that a more robust statistical confirmation is needed. A novel type of proxy, taken from the evidence of urban archaeology, is the bottom step of the water stairs. Originally the palaces were elevated above the sea, and the stairs had a convenient number of steps to descend and reach boats. The bottom step was related to the upper front of the green algae belt used as an official reference, level nicknamed CM (i.e. ‘Comune Marino’ for Common Sea Level Mark). We can distinguish two types of external sea stairs: ‘Essential’ and ‘Monumental’ stairs. ‘Essential’ stairs had the lowest step above the algae belt level. They were safe and it was possible to jump from the boat to the steps, but at low tides they appeared truncated and elevated over the seawater. ‘Monumental’ stairs, especially in the Baroque period, had the bottom step below the algae belt level to appear completely visible and magnificent at any time, even at low tides. We investigated the stairs of all the palaces facing the Canal Grande, 82 in total, and we measured the depth of the bottom step making reference to the today green belt level. The combination of this novel proxy with the pictorial documentation and the tide gauge record, confirm the exponential sea level rise, in agreement with the observed frequency of the storm surges flooding the City. A discussion concerns the impact of the seawater on the Venice palaces that were built with waterproof basements and additional measures to be protected against splashing waves. After the sea level has exceeded the building safe threshold, salt water has been absorbed in the masonry and the cumulative effect NaCl cycles are destroying buildings and outdoor monuments. However, some alarming cases have been found even in indoor monuments apparently unreached by flooding waters. An example is the Cenotaph of Antonio Canova in the Frari Basilica, Venice. The monument is affected by dewing and severe deterioration that is increasing at impressive rate. Intensive instrumental monitoring has excluded condensation and roof percolation. It has been recognized that marine water and rain water are transported at underground level from the exterior to the interior, reach the Cenotaph basement and are then mobilized inside the marble statues by capillarity until they reach the surface by dewing. The combined effect of subsidence and sea level rise will increase the wetting frequency and will accelerate decay. The paper discusses the mitigation measures necessary for this monument, and for the whole City.
