Case study:Removal of the Brosses weir on the Soanan River

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Location: 47° 32' 12.19" N, 3° 41' 29.40" E

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Project overview

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Status	Complete
Project web site
Themes	Habitat and biodiversity, Hydromorphology, Urban
Country	France
Main contact forename	Pierre
Main contact surname	Gadiolet
Main contact user ID
Contact organisation	Development board for the Chères and Azergues plain
Contact organisation web site
Partner organisations	Rhône local office of Onema (National agency for water and aquatic environments), SMRPCA, Departmental federation for fishing and the protection of aquatic environments for the Rhône department (FDPPMA 69)
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 Brosses weir was completely destroyed and the rubble was removed, except for a few large rocks left in the bed, where the weir once stood. During the works, the river was temporarily diverted to avoid the fine sediment being washed away. The dissipation pool of the weir served as a sedimentation pool. A rescue fishing campaign was undertaken prior to the works to limit the impact on the fish population.

Monitoring surveys and results

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Pre-works monitoring was carried out on several compartments of the hydrosystem. The characteristics of the morphological compartment (long and cross profiles, substrate measurements, flow patterns) were identified prior to the works. The fish and riparian-vegetation compartments were also assessed by the FDPPMA 69 and the SMRPCA respectively. Finally, pre-works monitoring of the spawning redds was done by the Rhône local office of Onema. Post-works monitoring was done on the same compartments in 2011.

The removal of the Brosses weir opened 2.5 km of river and potential spawning redds for brown trout. The free movement of fish species and sediment was restored between this section of the Soanan and the Azergues River.

On the site of the former weir, the Onema local office noted, post works, more diversified flow patterns and a shift from lentic to lotic facies. These changes in facies would seem to have stimulated the populations of bullheads and juvenile brown trout, whose respective densities were multiplied by factors of ten and three one year after the works.

In 2017, following numerous floods including a particularly morphogeneous flood in 2016, it was decided to pursue monitoring on the Soanan. The flood in question reduced some of the irregularities in the long and cross profiles of the river. The slope is now similar to the steady slope observed in the rest of the river. However, regressive erosion upstream of the former weir has been observed.

Analysis of the monitoring data revealed reactivation of sediment over a distance of 120 metres upstream of the former weir and, downstream, sediment progression to the confluence with the Azergues River. The fine sediment (sand and silt) disappeared from the upstream zone, revealing gravel beds. In just one year, the natural sediment dynamics were restored in this section of the river.

Prior to the removal of the weir in 2009, no spawning activity by brown trout had been observed over a distance of 200 metres below the weir. Two spawning redds were subsequently observed in 2010, i.e. during the winter immediately following the removal, and twelve spawning redds were observed in December 2011. On the basis of the monitoring results for fish, the species commonly found in calm waters and ponds/lakes had disappeared following the removal of the weir. The number of brook lampreys fell virtually to zero due to the disappearance of the sand and silt substrates. The studied river section had returned to its initial fish-population context comprising essentially salmonids. In 2017, the trout population had returned to a certain equilibrium in that all age classes were represented. However, juveniles were under-represented due to two very hot and dry summers the previous two years.

An action programme for flood prevention (PAPI) and a river-basin contract are currently in effect in the Azergues basin. These two instruments aim to enhance the coordination of management work to control floods and restore aquatic environments by developing contacts and project development in the area.

Lessons learnt

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Catchment and subcatchment Select a catchment/subcatchment Site Edit site Name Soanan River WFD water body codes FRDR571 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 5 - 10 m Average bankfull channel width (m) 88 m <br />0.008 km <br />800 cm <br /> Average bankfull channel depth category Average bankfull channel depth (m) Mean discharge category 0.1 - 1.0 m³/s Mean annual discharge (m3/s) 0.80.8 m³/s <br />800 l/s <br /> Average channel gradient category 0.01 - 0.1 Average channel gradient 0.01 Average unit stream power (W/m2) 9.8079.807 W/m² <br /> Project background Edit project background Reach length directly affected (m) 300300 m <br />0.3 km <br />30,000 cm <br /> Project started 2010/09/01 Works started Works completed Project completed 2010/09/30 Total cost category 10 - 50 k€ Total cost (k€) 2727 k€ <br />27,000 € <br /> Benefit to cost ratio Funding sources Rhône-Méditerranée-Corse water agency (60%), Rhône-Alpes region (20%), Development board for the Chères and Azergues plain (SMRPCA) (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 Reservoir impoundment Hydromorphology Continuity for organisms, Continuity of sediment transport Biology Fish: Abundance, Fish: Species composition Physico-chemical Other reasons for the project Measures Edit Measures Structural measures Bank/bed modifications Weir removal Floodplain / River corridor Planform / Channel pattern Other Non-structural measures Management interventions Rescue fishing campaign, temporary diversion of river 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 All information on this page is copied from the AFB, The French Agency for Biodiversity.