Saturday, September 18

What is a DANA, how climate change affects you, and how to mitigate flash floods

The strong storms that have hit a large part of the center and east of the peninsula in recent days have caused significant flooding in various parts of the provinces of Toledo, Castellón and Tarragona. This Thursday significant rains are still expected in the Balearic Islands, Catalonia, Aragon and Navarra. The meteorologist at the Danish Meteorological Center and researcher at the Complutense University of Madrid, Daniel Santos, and the expert in hydrology and professor of Hydraulic Engineering at the Polytechnic University of Madrid, Luis Mediero, explain what DANA consists of, what consequences it may have to produce and also which tools can be used in eroded or flood-prone areas.

Spain already touches 12,000 kilometers of areas at risk of flooding, which the climate crisis threatens to worsen

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What is a DANA?

DANA is the acronym for Isolated High Level Depression. This meteorological phenomenon usually occurs with the arrival of autumn, when a cold air pocket is isolated, static, and a hot and humid air pocket rises and collides, unleashing a virulent precipitation.

“Internationally, there is a similar concept called cut-off low. It is a system at high levels of the atmosphere that is isolated from movement; In other words, if the wind moves that system, the normal thing is that it continues to advance, but when it is isolated from the flow in the upper part of the atmosphere, what happens is that it stays quiet. He stands still. By being isolated, it may or may not generate very heavy rainfall, but that depends on what happens at low levels, “explains meteorologist Daniel Santos. Not whenever a DANA occurs, torrential rains occur, this depends on what happens in the lower parts. “If there is enough food to activate precipitation or storms, what has happened these days will happen,” adds Santos.

Can these meteorological phenomena and their consequences be foreseeable?

Yes and no. What can be predicted is that a DANA will occur, but not what consequences it may have. Understand consequences in the form of rains, storms and floods.

“Weather predictions are made using computer models that then analyze the predictors. Human predictors analyze those predictions and they are the ones who, checking – not just the models, but the latest radar data – get an adaptation to say that this DANA is going to occur within two, three, four, five or six days, “says the expert, who assures that these phenomena” are predicted quite well. ”

“The consequences are another thing. That this depression was going to be isolated and that the models and predictors detected it is perfectly known. The consequences that this has in terms of rainfall is something else. The phenomenon can be predicted. The consequences depend on what happens at low levels “, adds the researcher.

What happens at the lower levels when there is torrential rain?

That there is humidity, hot air and rising winds. “The moment the air rises it meets DANA and that combination of factors generates evolution clouds, high clouds, which later generate storms. With the presence of DANA at the top, all of this is like you’ve set the pot to cook. and you have the cover up and there comes a time when it starts to bubble and it starts to rain, and it rains very hard, “exemplifies the meteorologist.

How does climate change influence the formation of DANA?

It is not clear. What the scientists do confirm is that extreme phenomena, such as these torrential rains, multiply in scenarios in which climate change has occurred. “When attribution studies are done, extreme precipitation is quite difficult to map directly to climate change. We know that it has a responsibility in the increase in frequency, but we cannot say that this DANA has been caused by climate change, but rather that climate change helps an increase in DANAs and quite possibly an increase in the virulence of precipitation “, Santos points out.

“If you model and use a scenario in which there have been no greenhouse gas effects, DANAs appear five times in ten years, for example, and with greenhouse gases they appear eight times in ten years,” he adds.

The hydrology expert, Luis Mediero, also considers that the effect of global warming makes these pockets of cold air more frequent to become independent. “Climate change tends to weaken the jet stream, so it is most likely that these DANA-like events and at this time in late summer and early fall are becoming more frequent,” says the professor. “Where there is clear evidence is in the increase in temperatures. The higher the temperature the air is capable of storing more water vapor and more humidity, that means that these storms are going to be more virulent.”

Why on this occasion have storms of this type also affected the center of the peninsula and not only the Levante?

In Santos’ words, DANA can be located anywhere and if it finds those hot, humid pockets, storms can occur. “As long as there are those precursors that feed and generate those clouds, in the end, it does not have to fall only in the east. There have to be those additional ingredients to the DANA, ingredients at low levels. As there were, it can fall in Portugal, in Extremadura, in Toledo, in the Levant or in Zaragoza “. Locating the precursors would be “very difficult.” The exact location of a storm is something very complex due to the small dimensions of the same and the speed at which they are activated. “We can see it almost in the last moments because it is a very small and local phenomenon.”

Does climate change influence the fact that this year DANA occurred without even ending August?

“All this begins to happen when we make the transition between summer and autumn. Whether it is in August or early September is not climatically significant. In fact, for climatologists, autumn began on September 1,” says Santos.

Can the floods that wash away streets and houses be foreseen?

“Yes, you can”, defends Mediero. And he explains that the Ministry for Ecological Transition maps these flood zones. “In recent years, if this cartography is consulted, in most cases these floods occur at points that have already been identified as territories that could potentially end up flooded.”

Why is Spain vulnerable to flooding?

Because of its climate, because of soil erosion, because of having built in flood-prone areas. “Soil erosion is also important. On the one hand, when a storm falls there is a part of the rainwater that does not flood and is absorbed by the ground. The less vegetation the soil has, the lower the proportion of rainwater will be. absorbed by the ground and more prone to flooding. The more wooded an area is, the tree itself absorbs a part of the water and the soil, in this way, is less erodible than bare soil “, explains Mediero.

“An arid area with bare soil will be more prone to erosion. When the water carries sediment, a greater volume of the mass is produced that moves and that, together with the dragging of solids such as tree trunks, when reaching a narrowing of the channel like a bridge, it can block it and thus increase the water level or even break bridges, producing a flood wave greater than the natural downstream “, he adds.

What solutions are there?

There are different solutions. From applying mitigation measures, to displacing an entire population, something that does not usually happen and which is only used in extreme situations. The aim is to create green cities with trees, permeable soil or green roofs so that the volume of rain that is transformed into runoff decreases.

“In some cases that is not enough, especially for storms like the ones we have had these days. The second step is to apply mitigation measures in buildings, that is, what we do is protect the building, for example, by putting some portable floodgates. in the urban estate or in the house itself, in such a way that water is prevented from entering the house, “says Mediero, who has participated in the launch of a platform called Saferplaces This is used to assess which areas are most prone to flooding and you have the option of applying mitigation measures building by building.

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