Carried out by Angers Loire Métropole, the project consists of a study to evaluate the role of the embankments on the Authion river, in order to improve the protection of people in the event of a major flood of the Loire and/or the Authion in the Pays de la Loire region.
- 30 May 2020
The project provided an understanding of a complex hydraulic system located in the Loire Valley. Clarifying the role of the facilities leads to improved safety of property and people, as well as a better consideration of the flood risk in this sector.
This study evaluates the consequences of a flood by looking at several scenarios (banks bursting and flooding over, pumping failure) and the return of water to the Loire. It mainly concerns the districts of Sorges, the Moulin Marcille, La Guillebotte, La Chesnaie, Saint-Aubin, Les Ponts-de-Cé and the Vernusson industrial estate in Angers.
The project aims to identify the flood risks in the study area and to clarify the role played—or not played—by embankments. It also includes taking better account of the flood risk in land use planning in a very hydraulically complex sector on the banks of the Loire.
Protecting the populations located on the banks of the Authion River
In its downstream sector, the Authion Valley is heavily urbanised, particularly in the communes of Trélazé and Ponts-de-Cé. This sector is particularly complex from a hydraulic and water management point of view: the area protected by the great Loire levée is itself broken down into hydraulic 'traps' by secondary embankments. This system is crossed by the Authion canal, which drains the entire Authion valley and is the only outlet in the commune of Les Ponts-de-Cé.
The hazard assessment study carried out in 2014 by the DREAL Centre does not address the issue of secondary embankments, while recommending specific hydraulic studies to understand their role.
Following these conclusions and in anticipation of GEMAPI's assumption of jurisdiction in the region, Angers-Loire-Métropole wanted to launch a hydraulic study of the downstream section of the Authion Valley. The purpose of this study is to better understand the role of the Authion embankment. The idea is to find out how the water would spread through the town and how it would drain away. The ultimate aim is to improve protection for residents behind this structure in the event of a major flood of the Loire and/or Authion rivers.
Logically linking together local actors and the improvement of knowledge, it was first necessary to understand and model the functioning of the rivers and the present assets at stake (houses, companies, etc.). The second step was to better understand the role played by embankments through their physical description and behaviour in different flood scenarios in these areas.
Classification of works
A more accurate hydraulic model of this complex area was produced, as well as the choices available for each embankment, and whether or not it should be classified as a protective structure. The latter is important to ensure the safety of property and people. Indeed, a categorised structure would need regulated and rigorous monitoring to ensure it fulfilled its protective role.
The project made it possible to clarify the situation of this sector with regard to the flood risk and to confirm the safety of property and people. The elected representatives were able to commit to the possible opportunity to classify the works as a system of dykes. The classification of it being a containment system was finally decided, while the principle of transparency of the other works studied is upheld.
Total investment and European funding
Total investment for the 'Flood risks - Flood study of the Ponts-de-Cé, Trélazé and Sainte-Gemmes-sur-Loire sectors' project is EUR 70 000; the contribution of the European Regional Development Fund amounts to EUR 35 000 under the 'ERDF Interregional Operational Programme for the Loire' cooperation programme for the 2014-2020 programming period. The investment falls under the ‘Energy and Climate Union’ priority.