The hydraulic lab, located at Irstea’s Lyon-Villeurbanne center, has exceptional equipment to simulate flooding and improve our understanding of these natural events. Flooding in the laboratory!
In order to fully understand the utility of this equipment, we have to look at the event itself: the high waters that lead to flooding. When rivers overflow, they leave their regular (minor) bed to run through their major bed which includes grasslands, forests and habitats. The way in which the land is used influences the way the water flows, which raises a question: how will developments to the major bed impact the next flood?
Although scientists have access to digital models to study rivers and water flows, these are sometimes not robust enough to deal with the specificities of various catchment areas (topography, developments, etc.), and their results need to be validated by data. It can, however, be dangerous to collect data directly during major floods, and data for extreme events is sometimes completely missing. How can we predict these extreme floods that are, by definition, rare?
Through laboratory experiments, of course! The hydraulic lab makes it possible to simulate floods and acquire specialized knowledge of flows and the effect of ground use. It's an ideal tool in flood risk prevention.
A laboratory river
The hydraulic lab is an extraordinary platform measuring 300m2. Built in 2013, it comprises 2 glass channels.
It is unique in Europe due to:
- the size of these glass channels: 18m long and 3m wide for the first channel and 18m long and 1m wide for the second (to study sediment transport)
- the ability to take optical measurements across the entire flow (due to the glass panels)
- the potential to work with both coarse and fine sediments.
How does it work? Laboratory experiments make it possible to study a simplified reality: the main channel (large channel) represents a small scale model of a river (1/100 scale). Glass benches represent the major beds. Grasslands are represented by synthetic grass, while wooden sticks are used for forests.
Using this facility, researchers can isolate physical phenomena for several configurations (grasslands only or grasslands and forests) and in controlled conditions. A series of sensors are used to collect precise data relating to outputs, water levels and flow speeds, making it possible to predict overflows in flood plains.
Improving predictions for extreme floods
In particular, the lab is used to simulate extreme/rare floods, in order to prevent the risk of flooding and risks linked to land use. Using field data from specific floods (for example, the hundred year flood of the Rhone in 2003), “traditional” digital models have been developed and are used for extreme floods. Are these data valid and reliable?
These extreme floods are, by definition, rare; researchers therefore lack data collected from the field to qualify them. Laboratory experiments make it possible to obtain data and carry out simulations. “Improved” models can then be developed and compared to “traditional” models during experiments: what's the difference between field data and laboratory data? Researchers quantify uncertainties in levels and speeds, two parameters that characterize how dangerous floods are for people and property. At the same time, they are meeting the requirements of the European Flood Directive in terms of understanding extreme events.
These simulations go beyond basic research and are relevant for nuclear power stations. A simulation guide for extreme floods targeting managers is being developed.