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 liquid water, but by the material it carries, from logs to cars. or urban furniture. How decisive was the DANA that wreaked destruction a week ago?
The engineer and flood expert who has best studied the channel in the Poyo Rambla, Félix Francés, calculates that the dragging of materials increased the volume of the flood by between 20 and 40%, which is equivalent to a 50% increase. of the density of water and its buoyancy. “This mixture of water and sediment increases the destructive capacity of the flood and its dragging capacity,” explains the professor of hydraulic engineering at 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 drags and the denser the mixture that goes down the ravines, multiplying its destructive capacity. “In the most extreme cases, the water looks like chocolate and is capable of knocking over houses, because it has much more density and exerts a greater thrust,” he summarizes.
In the most extreme cases, the water looks like chocolate and is capable of knocking over houses, because it has much more density and exerts a greater thrust.
Felix French
— Professor of hydraulic engineering at the UPV
This is not the only effect associated with this increase in material, indicates Félix Francés, who together with his team has developed a mathematical model called TETIS which allows you to simulate the behavior of these mixtures in floods like last Tuesday. “By increasing the density of the mixture, the ease with which cars float increases,” he adds. “And the accumulation of sediment increases the cost of subsequent cleaning, which is one of the most important.”
The pending adaptation
Tools such as TETIS, developed by Félix Francés’ team, are a good example of incorporating the role of sediments in this type of studies, as it allows us to simulate how it is incorporated into the channels and how it moves in different scenarios. This can also serve to design adaptation and protection strategies, although sometimes experts are not consulted.
In 2007, this engineer participated together with the Júcar Confederation in a flood risk management plan for the Rambla del Poyo that was not applied: “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 now it is not taken into account except for exceptions.” In 2007, this engineer participated together 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 we must try to ensure that the water carries less sediment at source,” he says. “If you don’t stop the erosion there, there is little to do further down,” he concludes.
Visible from space
Francisco J. Tapiador, professor of Earth Physics at the University of Castilla-La Mancha (UCLM), remembers that the rivers of the Mediterranean have the peculiarity that they flow through what ancient geography books called ‘clayey Spain’, and that causes them to carry large amounts of mud when they overflow. “The fertility of the Valencian orchard is, in fact, the result of the secular overflows that have been flooding those lands with each cold drop (DANA), leaving a very fertile soil,” he points out.
This sedimentation is what is seen 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 have been affected by flooding, while more than 60,000 people, 15.2 km of railway tracks and 531.6 km of roads have been damaged. seen potentially affected.
“One of the problems is that the areas where there were previously only old orchard huts and barracks, which were easily rebuilt after floods, have now been transformed into blocks of apartments and semi-detached houses, also multiplying the population,” says Tapiador. “Building in what were always orchards, or on the edge of streams and waterways, is always a bad idea.”
Understudied sediments
Proof that sediment drift is poorly studied was presented in a recent review by a team of researchers from the Higher Scientific Research Council (CSIC). In it, scientists warn that although the mud deposited by the flood is associated with the greatest material damage (since it renders household appliances useless and deteriorates furniture), most of the scientific studies of flood danger are carried out with models in which simulate that what circulates through the channels and banks is clean water, devoid of mud.
Of around 4,000 studies published in scientific journals that we compiled, only 30 or 40 took sediments into consideration.
Daniel Vazquez Tarrio
— MCU
“Of around 4,000 studies published in scientific journals that we compiled, only 30 or 40 took sediments into consideration, a very small percentage,” explains Daniel Vázquez Tarrío, co-author of the study. Although we can already have an idea of what happened, he warns, the density of the fluid that was produced in this destructive flood can be known precisely. a posterioriwhen the remains and structural damage are analyzed in more detail.
“Depending on the density of the fluid, the processes and deposits that are generated change,” he says. “With that 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 material 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 comes to blocking sections of the channel.
“The sediment transport processes introduce uncertainty about the magnitude of the flood and this is not adequately taken into account when these processes are studied,” emphasizes 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 sediments are 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 phenomena
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 this cannot be good, when many times that is not the case and you have to look at it case by case,” says Díez Herrero. If the water ran loose and clean, the velocity would be much higher and would cause greater damage.
“We must think of vegetation and woody material as a fundamental part of the river systems that must be taken into account, because they are part of the river,” says 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 off and they would have sunk, because the energy invested in transporting the sediment has not been subtracted from that current,” says Díez Herrero.
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