Case study:Removal of the Ver mill weir on the Sienne River

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Location: 49° 6' 53", -1° 18' 42"
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Project overview

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Status Complete
Project web site
Themes Environmental flows and water resources, Habitat and biodiversity, Hydropower, Hydromorphology, Social benefits
Country France
Main contact forename Romuald
Main contact surname Genoël
Main contact user ID
Contact organisation Intermunicipal board for the development and maintenance of the Sienne (SIAES)
Contact organisation web site
Partner organisations Sienne AAPPMA, National agency for water and aquatic environments (Onema), Intermunicipal board for the development and maintenance of the Sienne (SIAES), Seine-Normandie Water agency, Manche departmental territorial and maritime directorate, University of Caen
Parent multi-site project
This is a parent project
encompassing the following
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Project summary

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The Sienne is mentioned in Lists 1 and 2 of Article L. 214-17 of the Environmental code for migratory fish, including Atlantic salmon, sea lampreys, river lampreys, brown trout, sea trout and eels.

Approximately 40 transversal structures exist along the river, representing one structure every two kilometres on average. Most of these structures serve no economic purpose and half are in ruins.

The Ver mill, built centuries ago, had not served an economic purpose for decades. The mill weir, 1.7 metres high, was in very poor condition.

In 2009, the mill was purchased by a new owner. The State services and the Water agency explained to the new owner the situation with the structure and the issues at hand in the sector. The owner accepted the removal of the structure given the safety issues (the dilapidated condition) as well as the regulatory and economic context (the publication of a new river classification in 2012, taking into account new species in the design of fish passes (Art. L. 214-17 of the Environmental code). The owner relinquished his water rights in November 2010.

The Ver mill weir was completely removed. The rubble of the stone weir was used on the right bank to create groynes to centre flood waters. The project included maintenance of the riparian vegetation upstream. A nearby pond was restored to replace the mill pond that had been used by the local fire department. Finally, fences and watering points were set up on neighbouring lands.

Monitoring surveys and results

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In 2010, before the demolition, the Seine-Normandie Water agency established a monitoring point (physical-chemical quality, macro-invertebrates, diatoms) 200 metres upstream of the weir. This monitoring point is checked every two months for the physical-chemical data and every two years on average for the hydrobiological data.

Post-works monitoring was carried out one month, six months and one year after the demolition, directly by Onema and the Water agency. This monitoring dealt with the spawning redds of sea lampreys and with the hydromorphological compartment (topography, flow patterns, substrate measurements, etc.).

The Departmental federation for fishing and the protection of aquatic environments for the Manche department monitors each year salmon populations, including an index on the abundance of juvenile salmon.

In 2012, the geomorphology (erosion dynamics, stream power, etc.) was monitored by an intern from the University of Caen. The Intermunicipal board for the development and maintenance of the Sienne (SIAES) undertook photographic monitoring of the changes in the riverbed and the banks for the first three years following the project.

The removal of the Ver mill weir made possible a return to a natural riverbed in the formerly impounded reach, over a distance of 1.7 kilometres.

The elimination of the reservoir resulted in an array of diverse flow patterns of running water. For example, seven riffles reappeared, alternating with rapids and more lentic flow patterns. The return of the riffles has made available a surface area of 2 400 square metres suitable as spawning habitats for fish species.

The removal of the weir also significantly improved the upstream migration conditions (wounds, delays, etc.) for salmonids, eels and sea lampreys between the English Channel and the Airou tributary. In 2012, 13 spawning redds for sea lampreys were observed on the site of the former impoundment, thus confirming the suitability of the habitats made available following the elimination of the structure.

The discovery of riffles in the sector entailed the creation of a monitoring point for the abundance index of salmon juveniles (IAS index). In 2011, the index stood at 49 at the monitoring point (high status). That represents significant progress for the monitoring point because the index was zero (no riffles) prior to the removal of the weir. These results indicate effective restoration of continuity for fish. The areas upstream of the Ver mill are now available and the suitability of the reproduction habitats in the former impoundment is also manifest.

An increase in fishing activities has also been observed in the sector, due notably to the increase in the salmon population.

However, the improvement in the passage of migratory fish at the Ver mill is limited by the existence of other structures blocking movement in the river. There remain two or three structures that make passage difficult or impossible for fish and block their access to spawning redds upstream and in the tributaries.

The physical-chemical monitoring revealed an improvement in the oxygenation of the river waters and the river effectively achieved good physical-chemical status in 2011. Samples of benthic macro-invertebrates taken in the years following the removal of the weir indicate very high habitat quality.

Geomorphologically speaking, the Sienne would appear to have returned to a more natural hydraulic gradient. Post works, major quantities of coarse sediment reappeared on the site of the former impoundment. This sediment has been and continues to be transported by two-year winter floods. The river has also recommenced natural adjustment via lateral erosion in the concavity sections of meanders.

In 2010, the Seine-Normandie Water agency launched a study to obtain a comprehensive and shared assessment of the hydraulic structures in the Sienne basin. The study contained proposals for work on structures to restore river continuity, taking into account local environmental, economic and social issues, while also assessing the value of maintaining the structures. The overall objective of the study was to look at a strategy to restore river continuity in the Sienne basin, taking into account the heritage value of the structures.

Lessons learnt

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Catchment and subcatchment


Name Sienne River
WFD water body codes HR336B
WFD (national) typology
WFD water body name
Pre-project morphology
Reference morphology
Desired post project morphology
Heavily modified water body No
National/international site designation
Local/regional site designations
Protected species present No
Invasive species present No
Species of interest
Dominant hydrology
Dominant substrate
River corridor land use
Average bankfull channel width category 10 - 50 m
Average bankfull channel width (m) 18
18 m
0.018 km
1,800 cm
Average bankfull channel depth category
Average bankfull channel depth (m)
Mean discharge category 10 - 100 m³/s
Mean annual discharge (m3/s) 8
8 m³/s
8,000 l/s
Average channel gradient category 0.001 - 0.01
Average channel gradient 0.001
Average unit stream power (W/m2) 4.3586666666666
4.359 W/m²

Project background

Reach length directly affected (m) 1700
1,700 m
1.7 km
170,000 cm
Project started 2010/10/01
Works started
Works completed
Project completed 2010/10/31
Total cost category 50 - 100 k€
Total cost (k€) 52
52 k€
52,000 €
Benefit to cost ratio
Funding sources Seine-Normandie Water agency (90%), Basse-Normandie Regional council (5%), Sienne AAPPMA (certified association for fishing and protection of aquatic environments) (5%)

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
Post-project management and maintenance

Reasons for river restoration

Mitigation of a pressure Impoundments (not hydropower)
Hydromorphology Continuity for organisms, Continuity of sediment transport, Flow velocities
Biology Fish: Abundance, Fish: Species composition
Other reasons for the project


Structural measures
Bank/bed modifications Weir removal, Creation of groynes
Floodplain / River corridor Maintenance of riparian vegetation, Restoration of pond
Planform / Channel pattern
Non-structural measures
Management interventions
Social measures (incl. engagement)


Hydromorphological quality elements

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

Biological quality elements

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

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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All information on this page is copied from the the AFB, The French National Agency for Biodiversity.