Isère Amont, a major development project sponsored by Symbhi (Combined Isère Water Basins Authority) is continuing in the Grésivaudan valley, north of Grenoble. While work is almost finished immediately upriver of Grenoble, further conversion work is planned until 2021 in the very north of the valley. The challenge is to control the risk of flooding in all built-up areas and restore value to the region.
- Crédits : © Egis - Nathalie Moralès
From the river bed to its banks, construction work is progressing at pace: vegetation clearing, levelling, consolidation and construction of watertight walls, protection of dwellings, environmental and landscaping work, etc. Leading these multiple projects, Egis is deploying all means to reduce the risk of flooding in the Isère plain - a very real risk due to frequent and regular rises in river level and the risk of flood wall breaches.
Following the first works tranche which began in the downstream section of the Grésivaudan valley in 2012, the 19 municipalities located upriver of Saint-Ismier are now the focus of the new project in tranches 2 and 3.
The approach turns out to be quite innovative: “Rather than only work on flood walls, 16 controlled flood washlands with total capacity of 35 million m³ are being created to allow a rising River Isère to flood farmland and natural zones and therefore protect residential areas,” explains Olivier Manin, Isère Amont project manager at Symbhi. This system is made to work effectively by means of several structures which direct water flow towards or from these fields, or in certain cases which prevent the Isere from flooding back up tributaries. All of these works help to regulate the river flow rate and thereby prevent flood walls from being breached.
The new projects firstly relate to the north of the Grésivaudan Valley (Pontcharra, Le Cheylas, Barraux…) and its middle part (Crolles, Lumbin). “We work primarily in built-up zones with high potential social and economic impact, and in areas subject to high flood frequency,” adds Arnaud Le Peillet, project manager at Egis. “But sooner or later, all the municipalities will see trucks and diggers at work in their area. Because, step by step, we are building a protective system for the entire valley.”
Works tranches 2 and 3 are being conducted concurrently and aim to meet three major challenges: reduce peak flooding on the Isère, support flood walls and restore value to the natural environment. “As has been done before, we must ensure that the work performed does not make matters worse downriver or upstream of sites where work is in progress in the event of higher river levels,” notes Jean-Philippe Pinchart, works director at Egis. “To do so, the experience that we acquired during the construction of tranche 1 taught us a lot in terms of organisation and logistics on this type of work.”
The works also provide for the environmental restoration of the river, connecting it with natural environments (300 ha of alluvial forest) by revitalising natural areas (restoration of seven former gravel pits and three backwaters) and by optimising the valley’s biological corridor which comprises 20 ha of plantations, 35 km of hedges and seven fish ladders. With the construction of paths, green stops and access ramps, the inhabitants will also benefit from better access to the banks of the Isère.
“Beyond flood protection, Isère Amont is a true regional planning project which takes into consideration all the environmental, agricultural and landscape aspects and leisure activities in connection with the river, thereby capitalising on local river heritage. It is an exciting project which matters a lot to the valley’s residents. This is why we put a fine point on factoring in all the aspects of the project in each of our tasks,” concludes Arnaud Le Peillet.
- Crédits : © SYMBHI.PNG
On completion of the work planned in the valley, all the developed zones will benefit from adequate flood protection to withstand the 200 year flood of 1859. Works director Jean Philippe Pinchart adds: “In addition to reinforcing flood walls and creating the washlands which constitute the virtually invisible flood protection architecture, the environmental and landscaping developments in the project contribute to the heritage of the Isere valley.”
The Isère Amont project involves 29 municipalities and 300,000 inhabitants.
All the projects planned aim to satisfy several objectives: