The increasing concentration of people in cities and suburban areas is a global phenomenon. The resulting urban sprawl has an impact on the water cycle: soil sealing intensifies surface run-off while the development of sewage and rainwater disposal systems literally short-circuits the natural rainwater circulation process. These phenomena are suspected of increasing flood risks, degrading water quality (due to vehicle pollution) and deteriorating the ecological status of the aquatic environment (linked to river levels).
Monitoring the Yzeron river basin for over 20 years
Since the end of the 1990s, Irstea scientists have sought to define the precise impact of urbanization on the hydrological regime of an entire river basin. Their case study was the Yzeron river basin, located on the western edge of greater Lyon and characterized by its large size (150 km²) and suburban features (patchwork of urban and rural areas). The scientists therefore installed experimental measuring systems for rain and river flows across the whole river basin. Their aim was to understand the processes involved (infiltration, run-off, evapotranspiration, etc.) and their consequences on river behavior at the river basin scale in order to provide local authorities and engineering firms with precious knowledge.
The equipment allowed Irstea scientists and associates at the Experimental Observatory for Urban Hydrology (OTHU) to continuously monitor the area, supplying data to a database created 20 years ago. Ultimately, they were able to create predictive hydrological models capable of taking into account soil sealing and development and management methods used for sewage and rainwater systems.
Instrumenting the Yzeron river basin
Six rain gauges and 7 hydrometric stations (to measure water levels) belonging to Irstea complete the operational measurement network (rain gauges provided by Lyon and hydrometric stations provided by the Rhône-Alpes Regional Department of the Environment, Planning and Housing (DREAL)). The data is available and can be consulted online.
The way in which the network is linked makes the instrumentation unique, allowing it to study the river basin's upstream and downstream response and to sample various types of land use (urban, predominantly rural or predominantly forest).
An original tool yields new results
The Irstea team recently developed a new hydrological model of the river basin , incorporating land use and rainwater management parameters. Using this simulation tool, they tested various future urbanization scenarios (extending urbanization at current rates, intensification, etc.) with various rainwater management configurations to determine their influence on river flows.
The tool is unique in its field and led them to several conclusions:
- In a river basin such as the Yzeron, the rainwater management method has more impact on the hydrological regime than the urban sprawl itself.
- Of the various rainwater management methods, collecting rainwater at the source - an alternative method that involves infiltrating the water as near as possible to where it falls - best moderates water levels and therefore has the least impact.
- Finally, a result that confirms previous studies: while urbanization has a confirmed influence on low water levels (more extreme) during summer and on low intensity flooding (increased impact), it has no impact on high intensity flooding. In the Yzeron, this can be explained by the fact that the highest floods correspond to a response by the upstream portion of the basin, which is primarily rural.
Recognition for this river basin research
In 2016, the Yzeron river basin joined the river basin network, a network of experimental hydrological observatories that bring together several French research organizations and fifteen river basins. The Yzeron acts as a pilot site due to its characteristics: the density of its observation network, its urban and suburban configuration, its large size and unrivaled amount of historical data.
For Flora Branger, a hydrologist at Irstea Lyon-Villeurbanne in charge monitoring the Yzeron basin, "joining this network is a seal of approval that recognizes our work. The network will enable us to implement new partnerships, make the most of our expertise and attract new skills, such as geophysics or isotopic geochemistry, to continue studying the river basin."
For more information
- Feature. Flood risk: research raises the bar
- Consult the web pages of the Hydrology and Hydraulics (HHLY) team and the Irstea Lyon-Villeurbanne Center
 Work carried out as part of the dissertation by Mériem Labbas through the ANR Assessing the Vulnerability of Peri-Urban Rivers project (AVuPUR) and the EC2CO project - The role of land use in relation to energy and water flow modeling in urban and suburban environments (Rosenhy).