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 | User:eptb de la Bresle |
| 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.
Monitoring surveys and results
■ Monitoring
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.
■ Outcome of the project and outlook
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.
Returning a watercourse
to its original bed
The new channel in the talweg in 2014.
The diverse, intermediate zone in 2014, following the works.
Pierre-Marie Michel EPTB Bresle Pierre-Marie Michel EPTB Bresle
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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