Floods simulated in the laboratory to better understand them

First the deafening noise of a permanent flow. Then a slight smell of chlorine. However, we are neither at the edge of a river nor at the swimming pool, but in a laboratory of the National Research Institute for Agriculture, Food and the Environment (Inrae) in Lyon. This laboratory is dedicated to the simulation and study of floods and their often dramatic consequences. “Each year, in France, floods cause 650 to 800 million euros in damage and continue to kill,” reminds Inrae. It is the first natural risk in France, to which a third of the municipalities are exposed.

In a channel 18 meters long by 3 meters wide, two water inlets model the confluence of two rivers with two different flow rates, 85 l/s and 35 l/s. By varying the inflows, the researchers are studying the formation of eddies, their speeds and their consequences in terms of alluvial diffusion. Inrae's hydraulics and hydromorphology laboratory (HHLab), in Lyon, on January 9, 2023. — In a channel 18 meters long by 3 meters wide, two water inlets model the confluence of two rivers with two different flow rates, 85 l/s and 35 l/s. By varying the inflows, the researchers are studying the formation of eddies, their speeds and their consequences in terms of alluvial diffusion. Inrae’s hydraulics and hydromorphology laboratory (HHLab), in Lyon, on January 9, 2023.

In the vast hall of 300 square meters of this hydraulics and hydromorphology laboratory, the facilities are impressive. A first channel is 18 meters long by 1 meter wide, and its inclination can vary from 0% to 5%, to represent, as desired, a mountain stream or a plain river. Researchers can also modulate the speed of flow, with a maximum of 300 liters per second. It is mainly used for the study of sediment deposits, integrating other variable parameters, such as the grain size (from 10 to 30 microns) or the concentration of sediments at the entrance to the canal, explains Céline Berni, research officer in hydromorphology rivers.

Alongside it, taking up even more space, is another channel, 18 meters long and 3 meters wide, with a depth of 13 centimeters. With an inclination of 0.1%, it is mainly used to study river overflows, allowing a better understanding of the effects of extreme floods. These instruments are surrounded by glass walls for better observation.

Pollution

To model the “major beds” – contrary to what the expression suggests, these are the beds created by the river when it overflows in the event of high flow, the “minor bed” being the one where it flows permanently – , glass benches are installed in the canal, with synthetic grass to represent a meadow, for example, or wooden sticks for the forest. This allows to study the “transitions of land cover, from grassland alone to grassland plus forest”.

Doctoral student Clément Fagour studies the physical phenomena of flow on a 1/50 scale model of a network of streets and buildings. It injects blue-tinted water to study the diffusion of pollutants. Inrae's HHLab, in Lyon, on January 9, 2023. — Doctoral student Clément Fagour studies the physical phenomena of flow on a 1/50 scale model of a network of streets and buildings. It injects blue-tinted water to study the diffusion of pollutants. Inrae’s HHLab, in Lyon, on January 9, 2023.

In the Inrae Lyon laboratory, on January 9, 2023, Clément Fagour works on floods and pollution in urban areas. On a 5.4 mx 3.8 m model of a network of streets, materializing intersections, openings in buildings, he studies, using 90 sensors, the diffusion of pollutants by varying their origins and the flow rates.

These facilities have existed since 2013, and most of the work has been funded by the National Research Agency (ANR). On each piece of equipment are fixed a large number of lasers, ultrasonic sensors to measure the height of water, electromagnetic flowmeters, turbidimeters to calculate the concentration of sediments, or probes and acoustic Doppler velocimeters. Everything here is calibrated and measured according to the subject of study. Most of the projects, lasting three to four years, are funded by the ANR and respond to calls for tenders from socio-economic actors who participate in their funding, such as the Compagnie Nationale du Rhône , water agencies, etc. This equipment is also used by doctoral students who come to work on their thesis, for a period of three years.

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