General
National Id
France_10
Summary
Due to major developments over the past two centuries, the Eau Morte Valley (a tributary of Annecy Lake) has a hydrosedimentary dysfunction with incised areas and areas with sediment excesses and an alluvial marsh disconnected from flooding. To reduce the impacts of the river's floods on uses, and to prevent the risk of urban areas flooding, a functional restoration programme for the valley has been developed by the federation of municipalities of Annecy lake sources. It was based on the renaturation of the Eau Morte bed, the functional restoration of the Giez marsh and the construction of crossing structures. This work has given the marsh a role as a flood expansion area. This project was also carried out by involving local residents in a process of territorial dialogue so that they themselves would be actors in the area and would participate in its management.
The in-depth description of the case study
Location description
The Eau Morte is the main tributary of the Annecy lake. Like its tributaries, the St Ruph and the Glière, it has its sources in the Bauges and Bornes mountains. The Giez marsh is located downstream of the valley, and the Eau Morte flows through it before flowing into Annecy Lake.
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 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.
NUTS Code
Rhône-Alpes
RBD code
FRD-Le Rhône et les cours d'eau côtiers méditerranéens
Transboundary
0
Source(s)
NWRM(s) implemented in the case study
Longitude
6.2615
Latitude
45.7610
Site information
Light or indepth?
In-depth
Monitoring maintenance
Monitoring impacts effects
1
Monitoring parameters
Floral monitoring
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.
Piezometric monitoring
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.
Hydraulic modelling
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.
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.
Piezometric monitoring
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.
Hydraulic modelling
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.
Performance
Biophysical impact assessement method
Floral monitoring
Piezometric monitoring
Hydraulic modeling
Piezometric monitoring
Hydraulic modeling
Design & implementations
Project scale
Large
Application scale
River Basin
Installation date
2012-2017
Area (ha)
166 ha
Favourable preconditions
The choice of these NWRMs followed the diagnosis of hydrosedimentary dysfunctions in the Eau Morte Valley, which had highlighted the need to restore the hydraulic functioning of the Giez marsh, and to restore sediment transit in the valley. Their interest lay in the global approach applied throughout the valley.
Design contractual arrangement
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Design consultation activity
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Design land use change
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Lessons, risks, implications...
Key lessons
As concerns floodrisk, the restored marsh perfectly played its water retention role in 2018. The flood expansion took place in the marsh, avoiding upstream as well as downstrem impacts.
Success factor(s)
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Attitude of the public
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main factor
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Financing
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Driver
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Policy, general governance and design targets
Policy description
The project covers the Giez marsh, as well as two other areas: the Mercier plain, which is 2 km upstream of the marsh, and the village of Faverges, which is located directly upstream of the plain.
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.
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.
Policy target
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Policy pressure
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Policy area
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Policy impact
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Policy wider plan
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Policy requirement directive
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Biophysical impacts
Information on Reducing flood risks, quantity
Two successive floods of similar intensity occurred in 2015 and 2018. In 2015, the golf course and agricultural land upstream of the marsh were flooded, while in 2018 they were not affected, and it was the marsh that filled with water. The diversion channel operated in the desired format: instead of having a direct supply from the Eau Morte, the diversion channel was supplied by overflows from the watercourse on the left bank.
Photo gallery
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.