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.

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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)

Catchment and subcatchment Select a catchment/subcatchment Edit the catchment and subcatchment details (affects all case studies in this subcatchment) 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 Edit site 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 Edit project background 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 Edit reasons for restoration 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 Edit Measures 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 Edit Hydromorphological 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 Edit biological 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 Edit Physico-chemical 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 Edit Other responses Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Monitoring documents Upload monitoring documents Additional documents and videos Upload additional documents The French National Agency for Water and Aquatic Environments (ONEMA) case study Additional links and references Edit links and references Link Description Supplementary Information Edit Supplementary Information