Case study:Restoring the river continuity of the Bresle River by returning it to its original bed in Sénarpont
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Project overview
| 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 |
Project summary
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
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
Image gallery
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Catchment and subcatchment
Site
Project background
Cost for project phases
Reasons for river restoration
Measures
MonitoringHydromorphological quality elements
Biological quality elements
Physico-chemical quality elements
Any other monitoring, e.g. social, economic
Monitoring documents
Additional documents and videos
Additional links and references
Supplementary InformationEdit Supplementary Information
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