The headwaters of the Eau Morte Basin are very productive in terms of sedimentary materials, due in particular to the presence of suspended moraines. Its valley is an alluvial plain resulting from the progressive filling of an ancient lake by the progressive addition of sediments.
The conservatory of natural spaces of Haute-Savoie carried out a monitoring of plant communities in the alkaline lowland sector of the Giez marsh.
In addition, monitoring according to the Rhomeo flore protocol was carried out in June 2014 and June 2019 in the Giez marsh, in order to calculate the floristic engorgement index, the floristic soil fertility index and the floristic quality index.
A piezometric monitoring was carried out by the conservatory of natural spaces of Haute-Savoie thanks to 4 piezometers.
A second monitoring will be set up by the federation of municipalities of Annecy lake sources, based on the 9 piezometers used in 2006 for the diagnosis of the valley, and still present on the site.
A hydraulic model of the marsh was developed by the project leader. It is used to assess the hydrological behaviour of the valley for a given flood.
Design & implementations
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Lessons, risks, implications...
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Attitude of the public
The 2015 flood then proved to be a lever for the conduct of the territorial dialogue and the involvement of local residents in the project. Today, the transversal approach between elected officials facilitates the implementation of flood prevention within a framework of upstream-downstream solidarity.
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Attitude of relevant stakeholders
The main obstacles were the time needed for elected officials to get involved in the issue, and to get shoreline property owners involved. The federation of municipalities of Annecy lake sources has taken on the role of mediator.
The 2015 flood and its impacts initially acted as a brake, as several residents wanted to carry out emergency work.
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Policy, general governance and design targets
The Giez marsh is part of a complex of wetlands annexed to Annecy Lake, called the Cluse du lac d'Annecy. It is crossed by the Eau Morte, with which it is in piezometric equilibrium. It is made up of a mosaic of environments: molinia grasslands, terrestrial reed beds, alkaline low bogs, alluvial white alder forests and rivers. It has a rich biodiversity (400 plant species and 354 listed animal species), including protected species. The marsh belongs to the Natura 2000 site of the Cluse du lac d'Annecy, benefits from a biotope protection decree since 1990.
The marsh development work has led to its gradual disconnection from the watercourse that crosses it. An accumulation of ice jams at the entrance to the marsh caused an accelerated deposition of materials in the Eau Morte bed. It reached the level of the upper bank, causing overflows upstream of the marsh, and limiting sedimentary transit through it.
The Mercier plain occupies the alluvial valley of the Glière, and is covered by an alluvial forest. The watercourse is strongly incised, and is therefore disconnected from its major bed.
The village of Faverges is crossed by the Glière river. The bed of the river includes several crossing structures that cause breaks in ecological continuity and a risk of flooding during 100-year floods.
Objectives of the NWRM: Reduce the risk of flooding in the village of Faverges, directly upstream of the Giez marsh (golf and agricultural uses) and at the Sollier-Verthier level;
Reconnect the Giez marsh with the Eau Morte floods;
Restore sediment transit in the valley;
Restore ecological continuity between a spawning area favourable for trout and Annecy Lake.
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