The DANA (Isolated High Level Depression), which devastated the east and south of Spain last Tuesday, will go down in history as one of the worst storms the country has suffered. There is at least 158 dead (155 in the Valencian Community and three others in Castile-La Mancha and Andalusia) and the missing number in dozens. Floods are, after heatwaves, the second natural phenomenon that causes the most deaths. Our country has 25,000 kilometers of flood zones where 2.7 million people live, according to the Ministry of Ecological Transition and Demographic Challenge (MITECO).
Floods are a more or less frequent phenomenon. The data that evaluates its risk does so by evaluating a frequency of 10, 50, 100 and 500 years. These nearly three million people live in areas that can be flooded every fifty years. If we lower the temporal estimate, almost 500,000 citizens are established in areas that risk experiencing this situation every decade.
These figures only cover intercommunity basins, which are those that depend on the organization. If we add the others, we estimate that This number can reach nearly 700,000 inhabitants. The intercommunity basins with the most floodable kilometers are those of the Ebro (more than 3,500 kilometers), Júcar (more than 3,400), Duero (more than 3,200), Guadiana (1,800), Segura (1,800), Tajo (more than 1,600), Western Cantabria (1,200) and Guadalquivir (about 1,200 kilometers).
The company DOTGIS, dedicated to geospatial analysis, published a report in 2021 in which it calculates the most populated Spanish municipalities at risk of flooding. Four of them were in the Valencian Community: Massalfassar (55.1%), Quartell (54.5%), Benavites (51.5%) and Alfara del Patriarca (49.1%).
The other towns on the list are Deltebre and Sant Jaume d’Enveja (both in Tarragona). with 86.3% and 64.8% of the population at risk respectively. Salamanca has Castraz and La Maya with 69% and 67% of its inhabitants. Calatayud (Zaragoza) also appears in this decalogue with 57.7% and Betanzos (A Coruña) with 51.8%.
In this matter we cannot lose sight of the effect of the expansion of populations on the margins of river canals and boulevards. “We build more and more and if it is done in one way or another, anywhere, we end up being more vulnerable,” says Nieves Sánchez, president of the Official College of Geologists. The scientist also spoke about the investments required by these events. “The company is losing a lot of money by not doing things right.”
Andrés Díez, flood expert at the Geological and Mining Institute of Spain (IGME), explains that the urbanization of these enclaves influences the orography and infiltration capacity. To build in these riverbed environments, the land is usually leveled. This means that the buildings are located at lower levels in relation to the river and are more easily flooded. To illustrate this, the expert gives the example of Letur (Albacete), where two people died and five are missing because of this DANA.
The other aspect to evaluate is the reduction in soil infiltration. This urbanization is generally accompanied by the paving of the surface. In this way, the soil loses its ability to absorb water and, when it rains, “Everything that falls goes straight into the river”explains Díez.
When these phenomena occur, the liquid flow of the rivers does not enter the channel and use these flat areas to overflowexplains geologist David Uribelarrea, professor of geology at the Complutense University of Madrid (UCM). If the area is occupied, water can wash away everything in its path.
Uribelarrea, also an expert in river environments, also talks about the increase in urbanization over the last 60 years. On the 1956 aerial photograph In Spain, there are practically no buildings in these flood zones.. “People knew they were only useful for agriculture and not in their entirety.”
Díez agrees with his counterpart and points out that with this expansion of urbanization there is also the paradox that the most vulnerable buildings (due to their personal value) are those that are at greatest risk. “Whereas before you had a hayloft that, if it flooded, nothing would happen, now you have a retirement home. ». When constructing permanent buildings, in addition to increased flooding due to orographic change, the risk also increases.
Everything influences
Sánchez warns that any work will affect the land and the way water flows in extreme cases like this. “This diverts the course and generates a change of situation”he emphasizes. Naturally, once the canal overflows, it follows natural drainage routes, indicates the UCM professor. Typically it leaves large areas of shallow waterlogging.
However, if this water must pass through a built-up area, the flow space can be reduced by 80% or more. “The result is that the draft increases and especially the speed at which it unfolds”said Uribelarrea. The geologist adds that in addition, in situations like last Tuesday, the water is not clean. The magnitude of the storm causes soil loss, carries more sediment, and has more force.
The president of the Official College of Geologists explains that, to know this type of information, there is geological mapping. Sánchez also points out that projects should also be examined by those experts who could assess the risk of urban projects. MITECO has maps in which you can check flood zones over periods of 10, 50, 100 and 500 years in Spain, which, according to the IGME expert, must be taken into account before planning the works.
If the floodplain is already occupied, risk maps are the best allies, comments Uribelarrea. They are used to better plan the most dangerous areas, evacuation and access routes, as well as alarms for the population, he explains.
What happens to what is already built?
The UCM professor is clear and declares that, Once the land is developed, it is very complicated. What will work best is a good alarm system and evacuation protocol. However, he regrets that in Spain “there is no culture in this sense”.
Díez argues that these natural phenomena are inevitable, but that the damage can be minimized. The IGME expert tells some steps that can be taken to avoid disasters like that of last Tuesday evening. There are structural measures, such as the construction of dams, dams or dredging (cleaning and deepening) of rivers.
On the other hand, there are other options for imitating nature, he adds. One of them, already used in Central Europe, consists of building preferential overflow zones. In the upper parts of the rivers, land is established (it can be crops or meadows) of little value in which the water overflows and reaches the municipalities with less flow, he reports.
Regarding dams, the geologist warns that they can cause a false sense of security. These constructions absorb most of these storms and allow the rivers to flow peacefully. This can give confidence to the population and start building near the river banks. “Ultimately, a DANA like this comes in, exceeds its capacity and causes more damage than if the structure wasn’t there.”
In this sense, Sánchez defends that these areas must be maintained “under permanent observation”. The geologist invites us to evaluate these territories at all times, and not only when disasters like this latest DANA occur, in order to face these risks. “The damage could have been minimized with an appropriate policy,” he says.
