View source for Case study:Elimination of a pond along the Erve River and conservation of the historical heritage in Sainte-Suzanne-et-Chammes

{{Case study status
|Approval status=Draft
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{{Location
|Location=48.096239, -0.3573440000000119
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{{Project overview
|Status=Complete
|Themes=Environmental flows and water resources, Habitat and biodiversity, Hydromorphology, Social benefits, Water quality
|Country=France
|Main contact forename=Xavier
|Main contact surname=Seigneuret
|Contact organisation=Erve River board
|Multi-site=No
|Project summary=The existence of numerous weirs in the Erve is the main disturbance in that they separate fish populations, modify aquatic habitats and impact sediment transport. 

The reservoir in Sainte-Suzanne had become filled with up to two metres of mud and the stagnant water made the site less attractive for anglers and the public.

In 2009, the Contract for the restoration and maintenance of the Erve proposed the elimination of the two gates and the resulting reservoir. However, the proposal did not make it to the actual project stage.

In 2010, the towns of Sainte-Suzanne and Chammes (that became a single town in 2016) and the Coëvrons intermunicipal board decided to restore the Grand-Moulin by creating a leat in order to conserve the town’s historical heritage and use the site as a showcase and an educational facility.

The project was accepted by the Departmental territorial agency on two conditions:

• that the water-intake structures be passable by aquatic fauna and comply with regulations concerning the minimum biological flow;

• that the two flap gates and the reservoir be eliminated as compensatory measures, particularly given the fact that the excessively high water level would have hindered operation of the Grand-Moulin (overrunning of the wheel).

The project started in 2010 with the emptying of the reservoir, very progressively in order to limit the outflow of the fine sediment. The flap gate and the concrete spillway of the reservoir were then removed. Thanks to these two operations, the mud in the former reservoir stabilised and the river reformed its bed very naturally.

In 2012, the second flap gate located upstream and an obsolete, wooden dam located next to the pumping station were removed.

The two flap gates were replaced by two distribution structures, i.e. rock chutes with macro-roughness surfaces evenly distributed. These structures maintain the minimum discharge in the natural riverbed. In parallel, side leats were created to supply the flume of each mill via buried conduits.

In the former reservoir, the river was left to its own devices in order to produce a natural riverbed in the sediment. In 2013, part of the mud was nonetheless spread on a slight slope at the outlet of the Grand-Moulin tailrace in order to improve the stability of the zone. The banks were planted.

This project was a chance to reconcile ecological restoration and the conservation of the historical heritage (a “showcase” site). The local population has rediscovered the Erve River. A set of walking trails has been created and fly fishing is now practised in the lotic sections. The Grand-Moulin has been restored and now serves to welcome the public to an exhibition on the various uses of water as a driving force (grinding flour, paper production, hydroelectric generation).
|Monitoring surveys and results=The natural evolution of the river is monitored by the River board which may decide to intervene in certain areas, if necessary.

The pre-works monitoring was carried out in 2010 outside the zone of influence of the former reservoir and dealt with the benthic macrofauna (WFD- IBG biological index). No hydromorphological monitoring was done during the project.

Since the elimination of the flap gates in 2011, annual monitoring of the fish community (IPR index), the benthic macrofauna (WFD- IBG biological index) and diatoms (IBD index) has been carried out. This monitoring work was undertaken inside the zone of influence of the former reservoir.

Visually speaking, the project is a success and the river rapidly carved out its bed in the area previously occupied by the reservoir. Vegetation colonised the banks, thus creating alternating zones of light and shade along the river. The flow patterns became more diversified and the previously uniform substrate now varies between sand, stones and large rocks. A general improvement in the fish and invertebrate compartments is due to the diversification of habitats in the former impounded reach (1.3 km).

The results of the macro-invertebrate and diatom indices are positive, including a significant improvement in the WFD- IBG biological index from 2012 to 2014. The level of quality for the index was quite high in 2014, with greater taxonomic richness and the presence of taxa more sensitive to pollutants.

For the IBD diatom index, the situation has remained stable since 2011, with a varied and balanced diatom community and high physical-chemical quality of the water, notably in terms of nutrients (nitrogen, phosphorous, suspended matter, etc.).

Monitoring of the fish community revealed a reduction in the number of species adapted to lentic environments (roach, bream) or those less sensitive to the quality of the environment (stone loach), and an increase in species adapted to lotic environments (bullheads, minnows). No juvenile trout were noted in the sector, an indication that the necessary reproductive conditions may not be available for the species. In 2014, no eels or brook lampreys had yet been observed on the site. Their absence may be due to the presence, downstream, of transversal structures that block the movement of the species.

This project opened up 2.6 km of river in the upper section of the Erve basin. River continuity nonetheless remains a serious problem because there are still 22 weirs downstream that block the movement of fish and the nearest is only 400 metres away.
|Project title=Elimination of a pond along the Erve River and conservation of the historical heritage in Sainte-Suzanne-et-Chammes
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{{Case study subcatchment}}
{{Site
|Name=Erve River
|WFD water body code=FRGR0486
|Heavily modified water body=No
|Protected species present=No
|Invasive species present=No
|Average bankfull channel width category=5 - 10 m
|Avrg bankfull channel width=6.5
|Mean discharge category=0.1 - 1.0 m³/s
|Mn discharge=0.9
|Average channel gradient category=Less than 0.001
|Avrg channel gradient=0.00051
}}
{{Project background
|Reach length directly affected=400
|Project started=2010/05/01
|Project completed=2010/08/31
|Total cost category=100 - 500 k€
|Total1 cost=227
|Funding sources=Loire-Bretagne Water agency: 50%; Departmental council: 20%; Regional council: 10%; Erve River board: 20%
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{{Motivations
|Specific mitigation=Reservoir impoundment,
|Hydromorphological quality elements=Continuity for organisms, Continuity of sediment transport,
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{{Measures
|Bank and bed modifications measure=Emptying reservoir, Removal of flap gate, Removal of spillway, Dam removal, Creation of side leats, Implementation of distribution structures
|Floodplain / River corridor=Planting of banks
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{{Supplementary Information
|Information=All information on this page is copied from ONEMA, The French National Agency for Water and Aquatic Environments.
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