Case study:Restoring the river continuity of the Bresle River by returning it to its original bed in Sénarpont

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Location: 49° 53' 20", 1° 42' 59"
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Project overview

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Status Complete
Project web site
Themes Environmental flows and water resources, Habitat and biodiversity, Hydromorphology
Country France
Main contact forename Pierre-Marie
Main contact surname Michel
Main contact user ID
Contact organisation EPTB de la Bresle
Contact organisation web site http://www.eptb-bresle.com
Partner organisations
Parent multi-site project
This is a parent project
encompassing the following
projects
No
Summary of the projec (According to EPTB Bresle)

Project summary

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The works, done at the end of 2013, consisted of creating multiple channels along the original bed, using some of the former bypass channels of the abandoned mill. Some of the earthwork was done in the standard manner with machines designed for wetlands. The rest was done by the river itself which carved out its bed in the former bypass channels. This hydraulic earthwork was facilitated by the presence of gates to adjust discharges and to create a bankfull, morphogenetic discharge. Left to its own devices for over a month, but watched over by the earthworks company, the river redrew a perfectly natural bed. This method, thanks to the natural erosion, redistributed the coarse sediment of the banks and thus avoided the high cost of trucking in material. This method also avoided any compaction or damage to the alder and ash stand, which maintained its functions.

The result was a more natural river bed. The leat, was filled in with a soil and gravel mix brought in from outside the area to stabilise the terrain, in compliance with the commitment made by the project manager to the land owners. Additional works, such as putting up fences and creating drinking points for livestock, was done in 2015 to maintain the farming activities while protecting water quality.

Monitoring surveys and results

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Biological monitoring of this project is based on fish populations. The pre-works situation was assessed in 2013 on the basis of an electrofishing campaign in the leat. The assessment was carried out with redd countst on the project site in 2013 and 2014, and in 2015 with a count in the upstream 8 kilometres of river made accessible for fish by the works (the count was carried out in a partnership with the Onema salmonid centre). Post-works monitoring was done from 2013 to 2015. Two inventories were carried out in 2015 by the Seinormigr association, using the IAT (trout abundance index) protocol. The initial and post-works assessments did not implement identical methods, i.e. the first was a fish rescue in the former leat and the second implemented the IAT protocol. The results are nonetheless useful on the basis of individual density calculations.

The bypass of the Sénarpont hydraulic structure made possible to restore the continuity of the Bresle River a further eight kilometres upstream. The works also improved the functioning of the alder and ash wet woodland by enhancing the supply of water. The operation also restored 650 metres of river by reducing the impounded reach to 400 metres and reinjecting water into a number of side channels (250 metres). The connection between the riverbed and the side channels in the project sector created new expansion zones (in non-critical areas) for flooding. These lateral connections improve water quality through enhanced self-cleansing and the creation of greater habitat diversity, a positive factor for biodiversity. The former impounded reach was replaced by a series of diversified flows in multiple channels in the valley bottom. Fine sediment was removed to reveal the coarse substrate that is now renewed naturally thanks to the restoration of the morphodynamic process.

Monitoring of fish revealed that prior to the works in 2013, trout density was 0.5 fish per 100 m². In July 2015, after the works, the measured density was 5.6 trout per 100 m². The trout population gained in numbers by a factor of 11 with a significant increase in the percentage of the juvenile population (30% of the total in 2013, 85% in 2015). The site has thus become highly favourable for spawners and the growth of juveniles. At the end of 2013, just after the diversion of the river to its original bed, several sea trout were observed spawning in the restored channel. Five redds for migratory salmonids were observed on the site. One year later, a dozen redds were noted. During the inventory, bullheads and eels were also caught. The strong point of this project is the restoration of the overall functioning of the river and of its side channels for a relatively small amount of money. The Bresle EPTB succeeded in defending its restoration objective and in negotiating over a long period to convince all the land owners and the town council. Today, the results are positive. Local residents have easier access to the nature and take pleasure in observing a dynamic river with a diversified ecology. This project was all the more beneficial that a number of mill owners who were previously hesitant to work on their installations are now ready to launch operations to restore river continuity.

Lessons learnt

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Image gallery


The Sénarpont weir, a total barrier for fish, in 2012 prior to the works (Stéphane Forgeois, Onema).
The diverse, intermediate zone in 2014, following the works (Pierre-Marie Michel EPTB Bresle)
The new channel in the talweg in 2014 (Pierre-Marie Michel EPTB Bresle)
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Catchment and subcatchment



Site

Name
WFD water body codes FRHRSAV07
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 sea trout, Atlantic salmon, sea and river lampreys, European eels brown trout, brook lamprey
Dominant hydrology
Dominant substrate
River corridor land use
Average bankfull channel width category 5 - 10 m
Average bankfull channel width (m) 7
7 m
0.007 km
700 cm
Average bankfull channel depth category
Average bankfull channel depth (m)
Mean discharge category 1 - 10 m³/s
Mean annual discharge (m3/s) 1.07
1.07 m³/s
1,070 l/s
Average channel gradient category 0.01 - 0.1
Average channel gradient 0.002
Average unit stream power (W/m2) 2.99814
2.998 W/m²


Project background

Reach length directly affected (m) 660 m
0.66 km
66,000 cm
Project started 2013/11/01
Works started 2013/11/01
Works completed 2015/05/01
Project completed 2015/05/01
Total cost category 100 - 500 k€
Total cost (k€) 150
150 k€
150,000 €
Benefit to cost ratio
Funding sources Preliminary study: AESN 80%, NORIAP 20%. Works: AESN 100%.

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

Mitigation of a pressure Barriers to fish migration
Hydromorphology Continuity for organisms
Biology Fish: Abundance, Fish: Species composition
Physico-chemical
Other reasons for the project Restore the hydromorphological characteristics and Consolidate and revitalise the EU-listed alluvial wetlands.


Measures

Structural measures
Bank/bed modifications New riverbed along old channel
Floodplain / River corridor
Planform / Channel pattern
Other Putting up fences and drinking points for livestock
Non-structural measures
Management interventions Adjusting discharges and creating a monophogenetic discharge, Management regarding letting the river draw a natural river bed
Social measures (incl. engagement)
Other


Monitoring

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
http://www.onema.fr/sites/default/files/pdf/afb rex r8 bresle gb v3.pdf All information on this page is copied from the AFB, The French Agency for biodiversity

Supplementary Information

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