Case study:Removal of twenty small structures and diversification of the Main Channel of the River Couasnon

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Location: 47° 30' 53.35" N, 0° 8' 6.77" W

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Project overview

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Status	Complete
Project web site
Themes
Country	France
Main contact forename	Sylvain
Main contact surname	Royet
Main contact user ID
Contact organisation	Syndicat Intercommunal pour l'amènagement of Couasnon
Contact organisation web site
Partner organisations
Parent multi-site project
This is a parent project encompassing the following projects	No

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Project summary

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The river Couasnon was resized and straightened during the 1970s and 1980s. Numerous hydraulic structures line the river and affect its hydraulic operations. Approximately two-thirds of the Couasnon’s flows are diverted into the mill structures and reaches. Hydraulic structures and cleaning and straightening operations caused the water quality to drop and damaged habitats. The structures prevented the free movement of fish, caused the siltation of the river, led to the proliferation of aquatic vegetation and increased the water temperature. They also led to the disappearance of the brown trout in the upstream section of the river. The pumping of water from the Couasnon for irrigation is now prohibited, thus rendering many of the structures useless. In 2004 was signed a Contrat restauration entretien (CRE- Restoration and Maintenance Contract), whose purpose is to establish free-flowing water and diversify habitats. Between 2004 and 2008, the flaps of the 22 structures were lowered. In the section classified as a fishery in the first category, 550 tonnes of blocks were used to diversify the bed and create over 110 deflectors. 1,500 tonnes of gravel was deposited in the river bed in order to reconstitute riffles.

Monitoring surveys and results

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Lessons learnt

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Catchment and subcatchment

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Catchment

River basin district	Loire-Bretagne
River basin	Pays-de-la-Loire

Subcatchment

River name	Couasnon
Area category	more than 10000 km²
Area (km2)
Maximum altitude category	500 - 1000 m
Maximum altitude (m)
Dominant geology	Siliceous
Ecoregion	Western Plains
Dominant land cover	Extensive agriculture, Woodland, Urban
Waterbody ID

Site

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Name	Maine-et-Loire
WFD water body codes	FRGR0453
WFD (national) typology
WFD water body name
Pre-project morphology	two arms, straightened, risized
Reference morphology
Desired post project morphology
Heavily modified water body	true
National/international site designation	EU - Special Protected Area
Local/regional site designations
Protected species present
Invasive species present
Species of interest
Dominant hydrology	Artificially regulated
Dominant substrate
River corridor land use	Intensive agriculture
Average bankfull channel width category	2 - 5 m
Average bankfull channel width (m)	3.53.5 m <br />0.0035 km <br />350 cm <br />
Average bankfull channel depth category
Average bankfull channel depth (m)
Mean discharge category
Mean annual discharge (m3/s)	0.9290.929 m³/s <br />929 l/s <br />
Average channel gradient category
Average channel gradient	0.0018
Average unit stream power (W/m2)	4.68550444.686 W/m² <br />

Project background

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Reach length directly affected (m)	26686 m26.686 km <br />2,668,600 cm <br />
Project started
Works started	2006/06/01
Works completed	2009/08/01
Project completed
Total cost category	100 - 500 k€
Total cost (k€)	207 k€207,000 € <br />
Benefit to cost ratio
Funding sources	Water Agency

Cost for project phases

Phase	cost category	cost exact (k€)	Lead organisation	Contact forename	Contact surname
Investigation and design
Stakeholder engagement and communication
Works and works supervision	100 - 500 k€	207 k€207,000 € <br />	Water Agency
Post-project management and maintenance
Monitoring

Reasons for river restoration

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Mitigation of a pressure	Abstraction
Hydromorphology	Quantity & dynamics of flow, Continuity for organisms
Biology
Physico-chemical	Temperature
Other reasons for the project	Flood risk management

Measures

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Structural measures
Bank/bed modifications	Removal of 22 flaps, Introducing sediment
Floodplain / River corridor	Creation of a flood expansion area
Planform / Channel pattern
Other	Prohibition of pumping of water
Non-structural measures
Management interventions
Social measures (incl. engagement)
Other

Monitoring

Hydromorphological quality elements

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quality elements

Element	When monitored		Type of monitoring		Control site used	Result
	Before measures	After measures	Qualitative	Quantitative
Quantity & dynamics of flow	Yes
Channel pattern/planform	Yes

Biological quality elements

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quality elements

Element	When monitored		Type of monitoring		Control site used	Result
	Before measures	After measures	Qualitative	Quantitative
Fish: Species composition	Yes
Invertebrates: Taxonomic composition	Yes
Invertebrates: Abundance

Physico-chemical quality elements

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quality elements

Element	When monitored		Type of monitoring		Control site used	Result
	Before measures	After measures	Qualitative	Quantitative

Any other monitoring, e.g. social, economic

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Element	When monitored		Type of monitoring		Control site used	Result
	Before measures	After measures	Qualitative	Quantitative

Monitoring documents

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Image gallery

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A flap on the Couasnon at Chavaignes, prior to its opening.

A flap on the Couasnon at Chavaignes, afterwards (2008).

A flap weir on the Couasnon at Baugé, prior to its removal.

A flap weir on the Couasnon at Baugé, afterwards (2008)

Additional documents and videos

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Additional links and references

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Link	Description

Supplementary Information

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