Case study:Reactivating river dynamics in the Vieux-Rhône River (non-navigable section) at Cornas, Roubion and Petite Île

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Location: 44° 57' 49.44" N, 4° 50' 50.61" E

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Project overview

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Status	Complete
Project web site
Themes	Habitat and biodiversity, Hydromorphology, Monitoring
Country	France
Main contact forename	William
Main contact surname	Brasier
Main contact user ID
Contact organisation	Compagnie nationale du Rhône (CNR)
Contact organisation web site	http://www.cnr.tm.fr/en/
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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In order to improve the functioning of the river system and facilitate the passage of flood waters, it was decided to directly address the cause of habitat degradation and lateral disconnection by removing the structures created during the 1800s. The experimental work undertaken was intended to reverse the trend toward aggradation of the banks and the river margins, still in progress even today, in order to restore the hydraulic and ecological dynamics of the river. The study of historic aerial photos in June 2005, pertaining particularly to the morphological evolution of the Rhône around the city of Montélimar, revealed that certain sites: • had a number of favourable characteristics for work to reactivate the river margins; • did not appear to have any major vulnerabilities concerning their ecological value or any heritage issues. The mobilisation of sediment (self-dredging) by the current appeared to be sufficient. Consequently, three sites (Cornas, Roubion and Petite Île) were selected for removal of the lateral dikes.

Monitoring surveys and results

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A brief ecological characterisation study was carried out prior to the work. Following the work, a complete ecological monitoring programme (vegetation, birds, amphibians, odonata, fish) was initiated as part of the overall authorisation procedure. The year 2012, the year following the end of the work, was selected as a reference point. In September 2012, the Compagnie nationale du Rhône initiated monitoring of the fish compartment in the three pools on the Cornas site. The monitoring will continue until 2016. Birds, odonata, amphibians as well as the terrestrial and aquatic vegetation are monitored by a consulting firm. The annual campaigns will also be pursued from 2012 to 2016.

For the time being, only preliminary results have been published because the available data concerns only the first year of monitoring. The general idea behind the work to reactivate river dynamics is to initiate a hydraulic and geomorphological process using the energy of the river and its floods. The desired ecological changes and improvements will become visible following floods and over a fairly long time span. The data from the geomorphological monitoring are still being analysed, however some changes are already visible. For example, signs of river dynamics have been observed on two sites (Cornas and Petite Île) during annual floods.

The fish community in the pools is the product of the fish trapped by the floods in May and June 2012 and that survived over the summer. The most common are smaller fish (carp, bitterling, etc.) and juvenile bleak, roach and bream.

In terms of terrestrial and aquatic vegetation, a strip of helophytes has developed on the exposed banks, where a number of indigenous species (Ranunculus sceleratus, Najas marina, etc.) have appeared.

At Cornas, wetland vegetation has developed in conjunction with pioneer species and undesirable species (Japanese knotweed) along the channel.

Studies on odonata revealed the presence of a dozen species, including Gomphus simillimus and Coenagrion mercuriale, the latter being protected nationwide. There are indications of amphibian reproduction and various species (natterjack toads, common toads and midwife toads) have been observed.

Lessons learnt

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Image gallery

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Old Girardon dike along the Rhône where flood debris accumulated, before the dike was opened. September 2011 (Compagnie nationale du Rhône).

Connection of flood storage zones in Cornas, during flooding. December 2012 (Juan Robert).

Opening of the Girardon dike at Cornas. October 2011 (Juan Robert).

The fallow Petite Île site prior to the work. October 2011 (Compagnie nationale du Rhône).

Vertical earthwork on the Petite Île site. The stratification (gravel layers) is clearly visible (Compagnie nationale du Rhône).

Catchment and subcatchment Select a catchment/subcatchment Site Edit site Name WFD water body codes WFD (national) typology WFD water body name Pre-project morphology Reference morphology Desired post project morphology Heavily modified water body National/international site designation Local/regional site designations Protected species present Invasive species present Species of interest Dominant hydrology Dominant substrate River corridor land use Average bankfull channel width category Average bankfull channel width (m) Average bankfull channel depth category Average bankfull channel depth (m) Mean discharge category Mean annual discharge (m3/s) Average channel gradient category Average channel gradient Average unit stream power (W/m2) Project background Edit project background Reach length directly affected (m) Project started Works started Works completed Project completed Total cost category Total cost (k€) Benefit to cost ratio Funding sources 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 Hydromorphology Biology Physico-chemical Other reasons for the project Measures Edit Measures Structural measures Bank/bed modifications Floodplain / River corridor Planform / Channel pattern Other Non-structural measures Management interventions 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 Supplementary Information Edit Supplementary Information