10 - Georg Umgiesser, "The impact of operating the mobile barriers in Venice (MOSE) under climate change" (2020)
4. Discussion
Without operating the mobile gates Venice will be flooded in the future quite heavily. As can be seen in Fig. 5, with the REF scenario (no closures) the city of Venice will experience flooding for more than 1000 h when the sea level rises for 50 cm.
When taking into account closures at the inlets, the above results show that the mobile barriers in Venice (MOSE) will actually be able to defend the city from high water in the near future. Even with a SLR of 50 cm the time (average per year) the safeguarding level will be ex-ceeded is only 18 h (in case a security increment is used for the forecasts of 10 cm). In the worst case (using the possibly wrong forecast levels without modification), this time rises to 100 h per year. Since forecast models are expected to improve in the future, the time of flooding should be lower than this number.
Clearly, these findings do not apply to a SLR above a certain level. With a SLR of 100 cm the total time of flooding per year is close to 1000 h. In this case the MOSE will certainly not be able to defend the city of Venice from high tides any more.
However, the time of flooding is only one part of the story. If we look at the number of closures the story changes. For a SLR of 50 cm, as shown in the results above, the gates will have to be closed between 300 and 430 times a year, this is one closure per day on average. This is an incredibly high number of closures and the mobile barriers were not planned for this frequency of closures. What concerns the total time the barriers are closed, with 50 cm SLR this time ranges between 1400 and 1800 h. The frequent and long closures will have a negative effect on the shipping that has to go through the inlets to reach the industrial and touristic port. Since Lido has no sluice gates to let the passenger ships pass, all traffic in case of closures has to go through the central Malamocco inlet, which is equipped with a sluice gate. It remains to be seen if only one sluice gate is able to handle the whole ship traffic, both industrial and touristic one.
From Fig. 3 it is interesting to note that the scenarios of using forecasted values (FOR) and using observed values (OBS) are very si-milar in the number of total closures, only scenario SEC (forecast with security increment) shows a higher number of closures. Moreover, for very high values of SLR all three curves give basically the same answer. Therefore, for the sake of statistics it is really not important to distin-guish between the two scenarios FOR and OBS, and observational va-lues can be directly used for this kind of study.
The number of closures decreases from a maximum value of 500 (FOR and OBS) to a nearly constant value of 210 closures per year, equal for all scenarios. This is because the water level outside the la-goon is so high, that it is nearly always above the value of the safe-guarding level. Since the average sea level is presently around 30 cm above datum, a SLR of 80 cm will bring the mean sea level to 110 cm, equal to the safeguarding level. Moreover, because the tidal amplitude during spring tide is 50 cm, a SLR of 130 cm will bring even the low tide above the safeguarding level of 110 cm. In this case, the closing and opening procedure breaks down and cannot be applied anymore. During the short periods where the water in the lagoon is higher than in the sea, the barriers are opened, just to be closed again after a short period.
The same can be seen when looking at the total time of closure. For very high SLR, the lagoon is nearly all the time closed (8200 h of 8760 total hours in a year). Even more interesting, with a value of 75 cm of SLR the lagoon is closed 4400 h (all scenarios have similar values). This means that with a SLR of 75 cm the lagoon starts to be longer closed than open. In this case we must speak about opening the lagoon, and not so much any more about closing the lagoon.
However, the average fluxes through the inlets show another point. With increasing levels of SLR, the fluxes, at first, start to rise. The ex-planation to this surprising tendency is that with higher water levels, the section of the openings is bigger and also friction inside the inlets is lower. Therefore, more water can enter or exit the lagoon more easily than before. This effect can be clearly seen by looking at the REF case (no closures) in Fig. 6. On the other side, when operating the gates, the number of closures is small and does not really influence the water budget of the lagoon. The maximum of fluxes is achieved between 20 and 30 cm SLR (with the closing active). After this point, the discharges start decreasing. This means that higher levels of SLR and smaller va-lues of friction are now offset by the increasing number of closures that have to be carried out. At 40 cm SLR the discharges are back to present values. After this point exchanges are strongly affected by the closure of the inlets, and continue to decrease until about 150 cm SLR, where they stabilize at a rate of 160 m3/s. This is what can be called the metabolic exchange value that is always maintained, determined by the closing procedure used in this article.
It is interesting that for small values of SLR there is a positive effect what concerns the exchange rates and water renewal capacity of the lagoon, even if the lagoon is being closed during high tides. This is
certainly something that could not have been anticipated by just looking at the increasing number of closures. At a SLR of 50 cm the lagoon has to be closed about once a day (350 closures correspond to approximately a 50 fold increase with respect to the present numbers). However, the exchanges through the inlets at these levels of SLR do not yet really feel this number of closures and the reduction through the inlets is only around 10 % compared to the present fluxes.
Finally, the results also show the relative ineffectiveness of partial closures. If it is decided to only close Lido, or leave Lido open, a small reduction of water level can be found inside the lagoon and at the city of Venice. Moreover, as Fig. 7 shows, for storm surges that are higher than 130 cm, nearly always the water levels inside the lagoon exceed the safeguarding level, and flooding of the city will happen. Relying on partial closures is therefore not a viable strategy, and the gates should be operated always together in order to guarantee a water level below 110 cm. There might be situations where using partial closures could be beneficial (e.g., artificially enhancing the circulation), but certainly not for the storm surge protection of the city.
Other studies have been carried out looking at the impact of the MOSE on various aspects. One study (Ghezzo, Guerzoni, Cucco, & Umgiesser, 2010) looked at the influence of the construction works on the internal circulation and the exchange capabilities (water renewal times) of the lagoon. In another article Bellafiore, Ghezzo, Tagliapietra, and Umgiesser (2014) studied the effect of the mobile barrier closures on the salt marshes and their survival. The impact of the closures on the oxygen levels was also studied (Melaku Canu, Umgiesser, & Solidoro, 2001). Other points to be studied are the microbiological pollution in the lagoon and the impact the MOSE has on these parameters. More-over, all these studies only looked at single events and did not explore longer time periods and different levels of SLR.
Recently, two articles (Del Bello, 2018; Reimann, Vafeidis, Brown, Hinkel, & Tol, 2018) also commented on the effects that the closure could have on the ecosystem. Both articles express the opinion that a high frequency of closures could be harmful for the Venice ecosystem. However, these aspects remain to be studied more thoroughly.
One might ask what the options for the city of Venice are. At this point is quite difficult to say. One proposed solution (Gambolati & Teatini, 2013) is to raise the city by pumping water into the under-ground. The study declares the possibility to raise the city of Venice by about 30 cm permanently. Another obvious possibility is to perma-nently separate the lagoon from the sea by building static barriers.