When a flood occurs like the one that killed more than 200 people last Tuesday in the DANA that hit Valencia, a significant part of the damage is not caused by the liquid water, but by the material it carries, from logs to cars or street furniture. . How decisive was the DANA that sowed destruction a week ago?
The engineer and flood expert who best studied the Rambla de Poyo canal, Félix Francés, calculates that the entrainment of materials increased the volume of the flood between 20 and 40%, which is equivalent to an increase 50% of the density. water and its buoyancy. “This mixture of water and sediment increases the destructive capacity of the flood and its driving capacity,” explains the professor of hydraulic engineering from the Polytechnic University of Valencia (UPV) to elDiario.es.
In this situation, a paradoxical result occurs, according to the specialist: the more liquid water falls from the sky, the more material it carries and the denser the mixture that descends into the ravines, thus multiplying its destructive power. “In the most extreme cases, the water looks like chocolate and is capable of overturning houses, because it has much more density and exerts greater thrust,” he summarizes.
In the most extreme cases, water looks like chocolate and is capable of overturning houses, because it has much more density and exerts greater thrust.
French Felix
— Professor of hydraulic engineering at UPV
This is not the only effect associated with this increase in material, indicates Félix Francés, who with his team developed a mathematical model called TÉTIS which makes it possible to simulate the behavior of these mixtures during floods like last Tuesday. “By increasing the density of the mixture, the ease with which cars float increases,” he adds. “And sediment buildup increases the cost of subsequent cleanup, which is one of the largest.”
Adaptation on hold
Tools such as TETIS, developed by Félix Francés’ team, are a good example of integrating the role of sediments in this type of studies, because they allow their incorporation into channels and their movement in different scenarios to be simulated. It can also be used to design adaptation and protection strategies, even if sometimes experts are not consulted.
In 2007, this engineer participated with the Júcar Confederation in a flood risk management plan for the Rambla del Poyo which was not implemented: “One of the measures we considered was reforestation, or the creation of microdams.
“Although it is mentioned [la sedimentación] “As a factor to be taken into account in the work associated with the 2007 European Framework Directive on flood risk management”, emphasizes Francés, “the reality is that for the moment it is not taken into account account, with some exceptions. In 2007, this engineer participated with the Júcar Confederation in a flood risk management plan for the Rambla del Poyo that included this factor, but it was not applied. “One of the measures we considered was reforestation, or the creation of microdams, because you have to try to ensure that the water carries less sediment to the source,” he says. “If we don’t stop the erosion there, there’s not much we can do further down,” he concludes.
Visible from space
Francisco J. Tapiador, professor of Earth physics at the University of Castile-La Mancha (UCLM), recalls that the rivers of the Mediterranean have the particularity of crossing what old geography books called “clay Spain”. , causing them to carry large amounts of mud when they overflow. “The fertility of the Valencian orchard is in fact the result of centuries-old overflows that flooded these lands with each drop of cold (DANA), leaving very fertile soil,” he emphasizes.
This sedimentation is what we see from the satellite in the images obtained after DANA, where the entire landscape appears covered in mud. Data collected by the Copernicus monitoring system reveals that more than 4,100 hectares of land and 3,906 buildings were affected by the floods, while more than 60,000 people, 15.2 km of railway lines and 531.6 km of roads were potentially damaged.
“One of the problems is that areas where previously there were only old orchard huts and barracks, easy to rebuild after floods, have been transformed into blocks of apartments and semi-detached houses, also multiplying the population,” explains Tapiador. “Building in what have always been orchards, or along creeks and waterways, is always a bad idea. »
Little-studied sediments
Evidence that sediment drift is understudied was presented in a recent study carried out by a team of researchers from the Superior Council for Scientific Research (CSIC). In it, scientists warn that although mud deposited by flooding is associated with the greatest property damage (since it renders appliances useless and damages furniture), most scientific studies on flood risk are carried out with models in which they simulate what clean water, devoid of mud, circulates in the canals and banks.
Of the approximately 4,000 studies published in scientific journals that we compiled, only 30 or 40 took sediment into account.
Daniel Vazquez Tarrio
— MCU
“Out of about 4,000 studies published in scientific journals that we compiled, only 30 or 40 took sediment into account, a very small percentage,” explains Daniel Vázquez Tarrío, co-author of the study. Even if we can already have an idea of what happened, he warns, the density of the fluid produced during this destructive flood can be known precisely. a posterioriwhen the remains and structural damage will be analyzed in more detail.
“Depending on the density of the fluid, the processes and deposits generated change,” he explains. “With this, you can get an idea of its mechanical properties, from records of past events.” And it is not only important to take into account the materials carried by the water, but also to carefully study the different effects that vegetation, cars or fragments of infrastructure uprooted by the flood can have when it occurs. acts to block sections of the canal.
“Sediment transport processes introduce uncertainty into the extent of the flooding and this is not sufficiently taken into account when studying these processes,” underlines Andrés Díez Herrero, co-author of the ‘work. “We ask that they be taken into account when estimating the danger of flooding, because there are no simple answers to this type of phenomenon; “More sediment is not always more destructive.”
We ask that sediments be taken into account when estimating the danger of flooding, there are no simple answers in this type of phenomenon
Andrés Díez Herrero,
— IGME-CSIC
In the case of vegetation, he cites as an example, it also produces a slowing effect on the speed of the current. “People see the bridges full of logs and the houses full of mud and think that it can’t be good, when often that’s not the case and you have to look at things on a case by case basis,” explains Díez Herrero . If the water flowed freely and cleanly, the velocity would be much higher and cause greater damage.
“We must consider vegetation and woody materials as a fundamental element of river systems that must be taken into account, because they are part of the river,” explains Ana Lucía, co-author of the study. “You need to know it and study it more.” “Without sediment, the water could have eroded the foundations of the bridge pillars, some would have come loose and they would have sunk, because the energy invested in transporting the sediment was not subtracted from this current,” explains Díez Herrero .
