The exercise is purely speculative, but not entirely unrealistic. What would happen if the worst meteorological phenomenon of the century in Spain, which wreaked chaos and destruction in several regions, especially Valencia, causing at least 158 ​​deaths and a still unknown number of missing people, happened in Portugal? When asked, the experts interviewed by The Observer are categorical: “Let’s hope it never happens to us, it’s better.”

The numbers, in addition to the images that arrive of cars swept away by turbulent waters or piled up under the mud, collapsed bridges and destroyed houses, people trying to save themselves by clinging to small bushes or trees, speak for themselves. Let’s look at the municipality of Chiva, about 30 kilometers from Valencia, one of the areas where the floods devastated everything. In this place it rained the equivalent of an entire year in just eight hours: 491 liters per square meter (mm). It is more than double the maximum ever recorded in mainland Portugal, 220 mm in Penhas da Saúde (Covilhã), in 1997.

Now in Lisbon, if it rains about 50 mm for an hour – which will give rise to a red alert, the highest on the Civil Protection scale – flooding is guaranteed in several areas, such as Baixa, Paço de Arcos or Algés. This is stated by Pedro Miranda, researcher at the Dom Luiz Institute and expert in extreme weather phenomena, stressing that breaking records like those in Spain would be a “shame.” “Our street drainage system does not allow it to drain that quickly in one hour”says.

Despite this, Portugal is currently much better prepared to face situations of heavy rains and floods than a decade ago, says climatologist Carlos da Câmara, also from the Dom Luís Institute, but points out that “there is still much to do. do” and that this implies, on the one hand, structural changes, such as the works that are being carried out to build drainage systems in cities such as Lisbon and Porto.

In the capital, for example, the plan foresees the opening of two tunnels: one of five kilometers between Campolide and Santa Apolónia and another of one kilometer that goes from Chelas to Beato. “It will drastically minimize” the effects of flooding, highlights engineer Mineiro Aires. It must be taken into account, however, that in a situation as extreme as that of Valencia it may not be enough. In this sense, he makes a comparison with earthquakes: “If we have an earthquake of violence and with an absolutely exceptional combination of efforts, It is also very difficult for even anti-seismic structures to resist.”

The phenomenon that devastated Valencia could also occur in Portugal

The process that gave rise to the Valencia storm is known in Spain as DANA (Isolated depression at high levelsin free translation Isolated depression at high levels). Around here the most common term is cold drop. This meteorological phenomenon occurs when a pocket of cold air separates from a polar mass in the upper layers of the atmosphere. From that moment on, and upon coming into contact with hot air masses on the surface, it follows its own path, often erratic, with torrential rains and cyclonic winds, as the Observer already described in this Explainer.

The phenomenon, which has also reached Portugal (there are several districts in yellow and the Algarve in orange), although much weaker, is not rare in the country. “We have had similar situations here, but not as frequently as it happens in the southeast of Spain.on the Mediterranean coast, especially at the beginning of autumn, when the sea is still relatively warm,” begins Pedro Miranda, professor of meteorology at the Faculty of Sciences of the University of Lisbon and expert in extreme weather phenomena.

The climatologist evokes the memory of the storm that, in the 90s, “ravaged” the Monchique mountain range, causing serious floods and landslides that devastated everything in its path. In about six hours it rained 260 liters per square meter (mm). “There were no deaths, because at that time it was a rural area, without large buildings. Valencia is the third largest city in Spain,” he highlights. But there are more examples: in 2010, in Madeira, a storm caused a flood in a mountainous area that flooded the rivers until they flowed into the sea near Funchal and caused 80 deaths.

Pedro Miranda highlights that the topography of Valencia “helped start the storm.” “It was formed in the Mediterranean, but it only started to rain when it was forced to ‘climb’ the slopes of Spain, where it poured all the water it had.” In Portugal, with many mountainous areas and steep slopes near the coast, the ingredients are also brought together to favor phenomena of this type.

In our country, the most suitable areas are precisely the southern coasts, since the masses of warm water are the source of energy that fuels this type of storms. Climatologist Carlos da Câmara points out, however, that Cities such as Lisbon, Porto, Coimbra, where flooding is typical due to Mondego, or Águeda, which also experiences frequent flooding, would be some of those that may be at greatest risk.. Ponte da Barca, Caminha or Ponte de Lima are also candidates to suffer this type of storm.

The climatologist also says that with climate change these phenomena, whether cold drops or others that generate intense rain, will be more frequent. Moving on to a comparison, use the example of the game of rigged dice to get a certain number (imagine six) more often. “If I roll a die and it comes up a six, I won’t say it’s because the die is rigged. What I’m saying is that if I use it, the probability of it happening will be greater,” he says. Climate change is not much different, as it increases the warming of oceans and seas, such as the Mediterranean, which has been “extremely hot.” “The data is being skewed when we compare it with the situation 50 years ago,” he points out.

What would happen if it rained as much in Portugal as in Valencia?

The Valencian Community was, by far, the most affected by the cold drop. This Thursday, more than 360,000 people did not have access to drinking water in the region, where even a prison served as a refuge for the most affected populations. The news that in some municipalities of Valencia it rained as much in one day as it usually rains in an entire year in the region (500 mm) made headlines in the newspapers. The figure is “brutal”, but it can be difficult to visualize.

Carlos da Câmara therefore proposes a seemingly unusual comparison: a kitchen of about ten square meters where, for every square meter, 100 bottles are poured. “That’s a total of a thousand bottles of water in your kitchen alone. The water would reach the ceiling very easily.“he explains. “A person always thinks that a millimeter is a little and even a liter is a little, but think about bottles and you will see that 500 liters is brutal,” he highlights.

Spain suffered the worst flooding in decades after torrential flooding occurred in the eastern province of #Valencia. The death toll increases and people remain missing
These images from the American Landsat-8 satellite illustrate the magnitude of the disaster, with images… pic.twitter.com/yG9kJuYZwg

— ESA Spain (@esa_es) October 31, 2024

Therefore, a situation like that of Valencia in Portugal would be equally catastrophic. “Indeed, If in Lisbon we had even 300 millimeters in four hours, it would be a shame, we would have floods everywhere.”says Pedro Miranda. The climatologist has doubts that even spending “fortunes” on the construction of structures such as canals or tunnels is possible to deal with such a scenario. “They are always sized by a [valor] historical. I have my reservations that they work 300 millimeters in an area like Lisbon,” he admits.

Carlos da Câmara highlights that in terms of planning and drainage systems, many things have changed in recent years: “We are more aware and beginning to realize that the only way to deal with extremes – we cannot overcome them – is to mitigate. For the climatologist, this implies working in two areas: civic education of the population and structural changes. This is where the tunnel systems being built in Porto and Lisbon come into play.

The tunnel plan designed 100 years ago

It is called the Lisbon General Drainage Plan. It is described by the Chamber as the “invisible work that prepares the city for the future” and one of the main objectives is precisely “to significantly reduce flooding and the consequent social and economic costs.” It is an old dream, which has passed through six presidents of the capital municipality over the last two decades.

The plan, which suffered several delays and public tenders, foresees the construction of two drainage tunnels that will start from the two highest points of the city to drain water from intense rains to the Tagus River. The first tunnel, with 17 thousand cubic meters of capacity, will run between Campolide and Santa Apolónia and the second between Chelas and Beato. The mayor of Lisbon, Carlos Moedas, even stated that if the drainage tunnels had already existed at the time of the 2022 floods, in which a person who was in a basement died, “the floods would not have happened.”

Work on the first tunnel. It started in December of last year. and those of the second approximately a month later. It is expected that In two years both projects will be completed.says engineer Mineiro Aires, who follows the progress of the work.

Construction of the tunnel between Campolide and Santa Apolónia began. #Lisbon #PlanoGeralDrenagem ???? pic.twitter.com/YfPEdwDPys

— Lisbon (@CamaraLisboa) December 4, 2023

Tunnels alone don’t solve anything. They are the “back column,” says Mineiro Aires, explaining that the ribs and vertebrae are also necessary: ​​“The superficial connections inside, to collect water and intercept the basins.” For all these reasons, it is a “complex work”, as well as expensive. The cost of the tunnels is 130 million euros.s, but the total of the Lisbon Drainage Plan is 250 million euros.

According to Mineiro Aires, some unforeseen events have arisen during the project that could cause delays. “A lot of archeology is being found, which forces us to stop and evaluate. There is also the problem of contaminated soils, which are often detected late,” he says. The studies carried out promise a solution for 100 years that During this period, around twenty serious floods could be avoided, which would generate losses in the order of hundreds of millions of euros..

METROEven this plan, like any other, would have difficulties in effectiveness in a situation like the one experienced in Spain.. “The situation in Valencia is an absolutely extraordinary situation. When we talk about rainfall of around 500 liters per square meter, it is an unimaginable amount of water,” highlights Mineiro Aires.

“In the face of normal events, with this 100-year return period, Lisbon will be quite well protected. But, if something bigger than that happens, what is being done may not be able to provide an answer on its own,” he admits. Mineiro Aires affirms that the work of engineers is to find a balance so that the works are viable and guarantee adequate coverage for the most expected risks. “Those that are more exceptional, it is impossible. Engineering has this art, it has this beauty, which is the balance between the desirable, the possible and the best,” he concludes.

In Algés there is also a system of gates operating on the river to prevent floods such as those that caused a fatality in December of last year.