Case study:Removing a weir and renaturalising the Iton River on the Plis industrial site in Hondouville

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Location: 49° 8' 24.24" N, 1° 7' 9.58" E

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Project overview

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Status	Complete
Project web site
Themes	Environmental flows and water resources, Flood risk management, Habitat and biodiversity, Hydromorphology, Water quality
Country	France
Main contact forename	Corine
Main contact surname	Seigneurbieux
Main contact user ID
Contact organisation	SCA
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 projects	No

Overview of the project (Egis Eau, 2013)

Project summary

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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 The outdated Plis weir measured 1.1 metres high with the gates closed and 0.8 metre with the gates open.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

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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.

Lessons learnt

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Image gallery

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The meadow after restoration of the wetland (2014) (SCA Tissue France).

Filling in the factory leat following the removal of the Plis weir (2014) (SCA Tissue France).

The Plis weir in 2014 (SCA Tissue France).

Catchment and subcatchment Select a catchment/subcatchment Site Edit site Name WFD water body codes FRHR259 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 Yes Invasive species present No Species of interest brown trout, grayling, bullheads, stone loach, eels and brook lamprey Dominant hydrology Dominant substrate River corridor land use Average bankfull channel width category 10 - 50 m Average bankfull channel width (m) 1717 m <br />0.017 km <br />1,700 cm <br /> Average bankfull channel depth category Average bankfull channel depth (m) Mean discharge category 1 - 10 m³/s Mean annual discharge (m3/s) 3.73.7 m³/s <br />3,700 l/s <br /> Average channel gradient category Less than 0.001 Average channel gradient 0.0002 Average unit stream power (W/m2) 0.426892941176470.427 W/m² <br /> Project background Edit project background Reach length directly affected (m) 200200 m <br />0.2 km <br />20,000 cm <br /> Project started 2014/01/01 Works started Works completed Project completed 2014/12/31 Total cost category 100 - 500 k€ Total cost (k€) 359359 k€ <br />359,000 € <br /> Benefit to cost ratio Funding sources Seine-Normandie water agency (80%), SCA Tissue France (20%) 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 Monitoring Reasons for river restoration Edit reasons for restoration Mitigation of a pressure Pollution incident, Barriers to fish migration Hydromorphology Continuity for organisms, Continuity of sediment transport, Quantity & dynamics of flow Biology Fish: Abundance, Fish: Species composition Physico-chemical Other reasons for the project Reduce the risks of accidental water pollution and of flooding, Renaturalise the Courtieux stream Measures Edit Measures Structural measures Bank/bed modifications Dam removal, Dredging of channel, Removal of island, Riverbed dug out Floodplain / River corridor Replanting of banks, removal of bunds, Reinforcing of banks Planform / Channel pattern Other Non-structural measures Management interventions extension of water treatment outlet 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 Biological quality elements Edit biological quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative 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 Additional links and references Edit links and references Link Description http://www.onema.fr/sites/default/files/pdf/afb rex r1 itonscatissue gb v3.pdf All information on this page is copied from the AFB, The French Agency for biodiversity. Supplementary Information Edit Supplementary Information