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LIFE
ON THE CANALS
Canal
banks : a return to "green" solutions !
Yachts and cruisers leave the lowest
chamber of the triple lock at Trollhättan. The fourth
lock is in the background, behind the basin excavated in
the rock.
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The four experimental sites for consolidation of banks of the
Canal des Deux Mers in Midi-Pyrenees region have all been developed
and planted, and a works reception visit took place in late June.
A
delegation of 15 elected representatives and technicians from
Midi-Pyrenees Regional Council visited the four VEV experimental
sites: Deyme and Castanet on the Canal du Midi, Lacourtensourt
and Montech on the Canal latéral à la Garonne, to
assess and discuss the different aspects of the works just completed.
The picture taken during the visit shows part of the bank treat-ed
at the Deyme site, 4km upstream of Vic lock on the Canal du Midi.
D.R
Site de Deyme, en cours de travaux
Behind the plane trees is the cycle path laid out by the département
of Haute-Garonne. The site presents an open landscape, in a rural
environment, but on the immediate outskirts of the Toulouse metropolitan
area. It was thought preferable here to preserve visibility, avoiding
the more imposing shrub beds that were considered appropriate
on other sites.
At the bottom of the slope, the former towpath had almost disappeared
over a few kilometres as a result of bank erosion. The first task
was to restore the original bank cross-section by making a berm,
at the edge of which two rows of willow piles, up to 2m long,
were driven in at 50cm intervals. These piles effectively pin
onto a small platform small willow branches, over which are laid,
linearly, fascines of conifers, completed by a compacted earthy
material. This bank protection should effectively dissipate the
wave energy caused by the boats' wash, thus protecting the young
helophytes planted between the piles and the slope. The helophytes
planted on the berm are scirpus, reeds, creeping-jenny, sedge
and Deschampsia.
It should be noted that the narrowing of the canal surface by
1 to 2m does not affect the navigable conditions, since the final
configuration corresponds to the former dimensions.
Evaluation of the four projects will provide valuable information
and comparisons, especially as the four sites present widely varying
conditions.
At Lacourtensourt, for example, the landscape contractors and
VNF (project manager of behalf of Midi-Pyrenees) have already
reported the theft of a significant number of newly-planted shrubs,
as well as the malevolence of water rats which have literally
devoured the alder saplings. This rodent, an unfortunate (and
accidental) import from Northern America, had already been responsible
for making the Montech Canal embankments permeable, forcing VNF
to close the canal around 1990.
The life of a waterway engineer is thus full of surprises, often
unpleasant, hence the importance of the programmes for restoration
and raising awareness of riparians and users, to minimise the
disorders and facilitate future maintenance.
Midi-Pyrenees Regional Council and VNF in any event hope that
it will be possible to benefit from these experiments to develop
a large-scale restoration programme, which would render the canal's
original aspect and cross- section, thanks to the reintroduction
of vegetal engineering techniques.
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