Case study:Removal of the Kernansquillec dam over the River Léguer

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Location: 48° 34' 9.61" N, 3° 24' 37.56" W

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Project overview

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Status	Complete
Project web site
Themes	Hydropower
Country	France
Main contact forename	Catherine
Main contact surname	Moret
Main contact user ID
Contact organisation	Association de la Vallée du Léguer
Contact organisation web site	http://www.riviere-du-leguer.com/
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 Léguer is a first-category watercourse, classified as a “river with migratory fish” is one of the region’s most renowned salmon rivers. Between 1920 and 1922, a dam was built across the Léguer in order to supply the Vallée paper mill - one of the biggest factories in the area - with electricity. The dam, approximately 15 metres tall, created a reservoir with a length of approximately 1.5 kilometres. Despite the presence of a fish pass, it was difficult for salmon to cross and this formed an impassable obstacle for eels. In 1965, the paper mill went out of business. The dam then supplied electricity to EDF (French electricity supply ompany). Due to its poor condition, the former licence-holder decided not to renew its application. The dam went into the public domain in 1994.

In addition to the interruption of river continuity, there was significant silting up of the reservoir whose eutrophication was clearly visible. The risk of the dam failing became a growing concern and, due to the lack of maintenance, the Kernansquillec dam started cracking and became a direct threat to homes situated downstream. Following significant flooding in the winter of 1995, the government announced plans to dismantle the dam on the grounds of public safety.

In 1996 the dismantling began with the gradual draining of the reservoir; this drainage process would take five months. 90,000 m³ of sediments were extracted from the orginal river bed. Three 50-centimetre weirs were constructed in order to retain any sediments that might have escaped the suction dredging and reduce the risks of regressive erosion. The dam was then demolished.

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Image gallery

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Kernansquillec dam and its reservoir in the last century.

The Kernansquillec dam in 1996 during the pumping out of mud from the reservoir prior to drainage.

Site of the former dam in October 2009: footbridge and successive weirs.

The Léguer valley after the removal of the Kernansquillec dam.

The Léguer within the former reservoir area in October 2009: variations in the stream flow and particle size distribution.

Another view of the Léguer within the former reservoir area in October 2009: variations in the stream flow and particle size distribution.

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 Léguer Subcatchment River name Léguer Area category 100 - 1000 km² Area (km2) 280 km²28,000 ha <br /> Maximum altitude category Maximum altitude (m) Dominant geology Siliceous Ecoregion Western Plains Dominant land cover Extensive agriculture Waterbody ID Site Edit site Name Kernansquillec - Plounévez-Moëdec WFD water body codes FRDR0046 WFD (national) typology WFD water body name Pre-project morphology Impounded Reference morphology Single channel, Sinuous Desired post project morphology Heavily modified water body Yes National/international site designation Local/regional site designations Protected species present No Invasive species present No Species of interest Dominant hydrology Artificially regulated Dominant substrate River corridor land use Extensive agriculture, Intensive agriculture, Grassland Average bankfull channel width category 5 - 10 m Average bankfull channel width (m) 1010 m <br />0.01 km <br />1,000 cm <br /> Average bankfull channel depth category Average bankfull channel depth (m) Mean discharge category Mean annual discharge (m3/s) 66 m³/s <br />6,000 l/s <br /> Average channel gradient category Average channel gradient Average unit stream power (W/m2) Project background Edit project background Reach length directly affected (m) 2000 m2 km <br />200,000 cm <br /> Project started 1996/09/01 Works started Works completed 2001/01/01 Project completed Total cost category 1000 - 5000 k€ Total cost (k€) 1400 k€1,400,000 € <br /> Benefit to cost ratio Funding sources French Ministry of Industry, Water Agency, French Ministry of Envinronment, European Community Cost for project phases Phase cost category cost exact (k€) Lead organisation Contact forename Contact surname Investigation and design 10 - 50 k€ 15 k€15,000 € <br /> Stakeholder engagement and communication Works and works supervision 1000 - 5000 k€ 1400 k€1,400,000 € <br /> Post-project management and maintenance Monitoring Reasons for river restoration Edit reasons for restoration Mitigation of a pressure Hydropower, Reservoir impoundment Hydromorphology Continuity for organisms, Continuity of sediment transport, Quantity & dynamics of flow Biology Fish: Species composition, Fish: Abundance Physico-chemical Other reasons for the project Flood risk management Measures Edit Measures Structural measures Bank/bed modifications Dam removal Floodplain / River corridor Planform / Channel pattern Removal of impoundments Other Non-structural measures Management interventions Social measures (incl. engagement) Rehabilitation of site by local associations, Creation of a centre to show visitors the historical heritage, Creation of a twelve kms foothpath to visit the area, Video made by Eau et Rivières de Bretagne, Brochure available in a website Other Information provision Monitoring Hydromorphological quality elements Edit Hydromorphological quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Continuity for organisms Yes Yes Yes Improvement 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 Yes Yes Inconclusive 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 [[]] Yes Yes Yes Improvement 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 Case study description by ONEMA (French National Agency for Water and Aquatic Environments) Additional links and references Edit links and references Link Description http://www.riviere-du-leguer.com/ Supplementary Information Edit Supplementary Information