Understanding the factors that drive extreme water levels is key to an accurate assessment of flood hazard. The city of Venice has always been affected by flooding due to extreme water levels. In this study, we examine the factors driving and influencing extreme water levels in the Venice lagoon, aiming at deriving accurate extreme water level estimates in the Venice lagoon.
Due to the shallowness of the Venice lagoon, extreme water levels are influenced by both atmospheric forcing (surge) and water level of the lagoon (tide and bottom level) and interactions between these two. Furthermore, these extreme water levels have been changing over time due to variations in the bottom level. These variations are reportedly due to local (anthropogenic and natural) subsidence and sea level rise. In this study we resort to the available long-term water level observations of the Punta della Salute tide-gauge. Given the effects of subsidence and sea level rise in these data, we start by homogenizing the data by removing these trends and jumps from the time-series. Using the homogenized time-series, we study the influence of the dependence between tide and surge components on the extreme water level estimates. Finally, we quantify the effect in the estimates of modelling this dependence in the extreme value models.
To homogenize the data and better understand the underlying trends, a time-series analysis was performed on the time-series of water level observations. Mann-Kendall tests for monotonic trend were performed, followed by an analysis using changepoint detection methods. Changepoint detection was performed using the RHtest and BEAST methods on the Punta della Salute time-series as well as time-series from neighbouring tide-gauge stations. Ultimately trend decomposition using the BEAST method was used to detrend and homogenize the Punta della Salute time-series.
After detrending, the tide and surge components were separated using tidal harmonic analysis and reconstruction. The relationship of these now separated components was quantified during extreme water levels using the Pearson r correlation and the Kendall τ rank correlation. This relationship between tide and surge was described using copulas to estimate extreme water levels. Different copula variants were evaluated and extreme water level estimates derived using copulas that describe dependence were compared to extreme water level estimates using a copula that describes tide and surge as independent components. Lastly, these were compared to those derived from univariate extreme value analysis to assess the influence of separation of tide and surge components combined with copulas as opposed to a more traditional univariate extreme value analysis.
The main conclusions of this study are as follows.
• The water level observations of the Punta della Salute tide-gauge are indeed affected by jumps and trends due to subsidence and sea level rise. These can be successfully removed using the applied techniques.
• There is a clear dependence between tide and surge in the Venice lagoon, with lower tide levels leading to higher surge levels. The non-inclusion of this dependence (by assuming independence) in the combined analysis of tide and surge signals to drive total extreme water levels leads to an overestimation of the total water level extremes.
• Extreme water level estimates from the combined analysis of the tidal and surge signal are higher, but compatible with those from the analysis of the total water level signal (without separation of tidal and surge signal). This gives confidence in the combined analysis accounting for the dependence between the signals and allowing for a further application of the models to account for projected climate changes.
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What long ago started as a small fishing village, seeking refuge from the Romans, slowly evolved into the city of Venice that we know today. With its unique location came unique problems, most of which were related to the interplay between Venice and its lagoon. By severe measures in the past it has continued to withstand the test of time.
In the decades to come, Venice, once again, has found itself in a difficult situation. Like has been done in the past, drastic measures are required to deal with the current and upcoming difficulties threatening the survival of Venice. These difficulties range from over-tourism to sea-level rise and the subsiding of the city. Acting like the Magistrato alle Acque acted in the past, extreme visions where laid out as possible solutions to these threats.
A workshop week with focus on interdisciplinary design formed the basis for two extreme visions which are laid out in this report. With the aim of answering the main research question: How do flood defense systems influence the spatial aspects of the territory in the context of a high dynamic landscape in the Anthropocene?
The plan for the Perfect Lagoon is one of these, which has focuses on tackling all of the current and upcoming problems were the emphasis lies on preserving and perfecting the lagoon using the building with nature philosophy, while also saving the city from drowning. Preservation is done by solving the sediment budget problems. Due to the constantly eroding system, salt marshes and land is slowly disappearing.
In this plan, drastic actions will be taken to counteract the constant erosion as well as the effect that sea level rise will have on this unique estuary. Drastic measures like redirecting rivers and re-purposing the MOSE contribute towards this goal.
After preservation comes restoration as one of the goals is to restore and increase ecological value, restoration of salt marshes and removal of negative influences like pollution.
As a second vision, the plan of the Symbiotic System deals with the same problems but here the emphasis lies on interconnectedness of Veneto. More attention is paid to mass tourism. The plan aims to turn Venice into a modern interconnected metropolitan area. The city and the lagoon will be treated as two separated system where the focus lies completely on the city of Venice. The lagoon will be left to its own devices in order to find a new, still unknown, equilibrium.
These visions are then further worked out and explained, and for both visions, technical design are made to, step-by-step, bring these visions closer to reality. From these visions along with their technical design we can conclude that flood defence systems have a major influence in the spacial aspects of the territory. Not only in its primary function, but more importantly in the secondary functions. Both primary and secondary functions can be used to create a paradigm shift for the territory. Using the multidisciplinary approach, an integral design can be made for the flood defence, in which the opportunities in a territory can be maximized. ... Read more