Case study:Removing a weir and renaturalising the Iton River on the Plis industrial site in Hondouville
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- 1 Project overview
- 2 Image gallery
- 3 Catchment and subcatchment
- 4 Site
- 5 Project background
- 6 Reasons for river restoration
- 7 Measures
- 8 Monitoring
- 9 Additional documents and videos
- 10 Additional links and references
- 11 Supplementary Information
|Project web site|
|Themes||Environmental flows and water resources, Flood risk management, Habitat and biodiversity, Hydromorphology, Water quality|
|Main contact forename||Corine|
|Main contact surname||Seigneurbieux|
|Main contact user ID|
|Contact organisation web site||http://www.sca.com/fr/|
|Partner organisations||Saviton, Seine-Normandie water agency, Eure departmental territorial and maritime agency, Onema, Haute-Normandie regional environmental directorate, Egis Eau consulting firm, SETHY company|
|Parent multi-site project|
| This is a parent project
encompassing the following
The industrial site in Hondouville was constructed in the beginning of the 1900s as a textile factory. A weir was built across the Iton with a bypass leat leading to a mill, called the Plis mill. The outdated Plis weir measured 1.1 metres high with the gates closed and 0.8 metre with the gates open. It caused numerous malfunctions in the river, including a break in sediment transport, an obstacle to the passage of trout and eels, degraded water quality, a more uniform flow pattern and siltation of habitats (an impounded reach extending 600 metres).
In addition, the Courtieux stream flowed through the industrial site, itself located in a floodable zone, before reaching the Iton. In the lower section, the stream runs through a culvert, but due to its poor positioning, the stream is clogged with sediment over more than 350 metres, with as a result a break in continuity between the stream and the river.
Finally, there was a risk of accidental pollution due to the position of the Courtieux stream and the factory leat in the middle of the industrial installations and no means to control the water entering or exiting them. The position of the factory wastewatertreatment plant, on an island in a floodable zone, was also a problem.
Saviton informed SCA Tissue France on the problems caused by the Plis weir, on the possibility of a public subsidy to remove the structure and proposed providing assistance to the industrial company during the operation. SCA Tissue France had no further use for the weir and wanted to avoid incurring costs for its maintenance, consequently, it accepted the idea of removing it.
The fact that the company owned the meadow opposite the site was a chance to restore the flood storage zone and to reduce the flood risk for the factory. In addition to removing the Plis weir, the final project included filling in the factory leat to improve access to the wastewater-treatment plant, bypassing the Courtieux stream to avoid the industrial site and the creation of a wetland in the meadow opposite the site. SCA Tissue France decided in favour of the project given that it was economically beneficial for site operation and corresponded to its environmental values.
The work took place in three phases:
The gates, concrete glacis and spillway were removed. The factory leat and part of the low-water channel upstream of the weir were dredged. The island downstream of the factory channel was eliminated to enable a distribution of flows similar to that prior to the works.
The future riverbed was dug out, the banks were graded to a slight slope and replanted. The former bed was filled in with landfill from the new bed. The bunds along the banks were removed to create a wetland and the section of the lot along the Iton was cleared.
The factory channel was filled in with the landfill from the meadow. The banks along the industrial site were reinforced with sheet piles to protect the buildings. The outlet of the wastewater-treatment plant, originally in the factory channel, was extended to the Iton.
Monitoring surveys and results
The pre works monitoring done in 2012 dealt with the biological (benthic macroinvertebrates) and morphological (topographic surveys of the long and cross profiles, sedimentation levels) compartments. The first survey after the works in 2015 (n+1) dealt with the morphological compartment (topographic surveys of the long and cross profiles, water depths, substrates and current velocities) at four points along the riverbed and the flood plain, in a zone stretching 400 metres upstream and downstream of the former weir. The monitoring programme is scheduled to continue for two more years (n+2 and n+3).
This project opened and restored 800 metres of the Iton (though there remain five structures downstream and 200 upstream) and restored 600 metres of favourable habitats for category-1 species thanks to the elimination of the impounded reach. Sediment transport was previously blocked by the weir, but has now been restored. The restoration of the meadow provides the river with a flood expansion zone and would appear to be a potential habitat for the yellow-bellied toad, whose presence has been observed near the site. One year after the works, the hydromorphological studies on the Iton at the site of the former weir revealed a modification in the long profile with a drop of 25 centimetres in the water level upstream and an increase of 25 to 30 cm downstream. In economic and flood-security terms, the project is beneficial for SCA Tissue France in that the firm succeeded in protecting its installations from the river and avoided maintenance costs. The risk of accidental pollution was reduced by returning the Courtieux stream to the talweg and by filling in the factory channel. However, due to the small amount of available land, the new bed of the Courtieux stream is straight and the banks could not be planted as planned. The drop in the water level indirectly damaged the foundations of the industrial installations along the river by exposing them to freezing temperatures. That being said, SCA Tissue France and its partners consider the project a success because it met both the economic and ecological objectives, thanks to the technical support from Saviton and the implication of the industrial company.
Catchment and subcatchment
Cost for project phases
Reasons for river restoration
Hydromorphological quality elements
Biological quality elements
Physico-chemical quality elements
Additional documents and videos