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", -0° 8' 7"
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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
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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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Image gallery


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)
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Catchment and 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

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) 3.5
3.5 m
0.0035 km
350 cm
Average bankfull channel depth category
Average bankfull channel depth (m)
Mean discharge category
Mean annual discharge (m3/s) 0.929
0.929 m³/s
929 l/s
Average channel gradient category
Average channel gradient 0.0018
Average unit stream power (W/m2) 4.6855044
4.686 W/m²


Project background

Reach length directly affected (m) 26686 m
26.686 km
2,668,600 cm
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 €
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 €
Water Agency
Post-project management and maintenance
Monitoring



Reasons for river 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

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

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

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

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

Any other monitoring, e.g. social, economic

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


Monitoring documents



Additional documents and videos


Additional links and references

Link Description

Supplementary Information

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