|In addition to the fact that vegetation-based techniques allow banks to be rebuilt in an environmentally-friendly way, by recreating as closely as possible the natural functions of a bank, it has also been demonstrated that these techniques also promote the return of fish fauna and in particular young fry.
Within the frame of this programme to build and renovate such installations, the VNF Inter-regional North-East Department (NE Dept.) has for several years been conducting bank renovation initiatives using vegetation-based techniques.
To evaluate the ecological impact of these installations and to obtain feedback allowing us to improve the techniques used, ecological monitoring has been implemented since 2007.
This monitoring focuses on water quality adjacent to the banks, the fish, in particular young fish (fry) and aquatic invertebrates.
For fish, the results obtained since the start of the study show that banks renovated using vegetation-based techniques are almost as attractive for fish as natural banks.
In effect, fish appear to find in these installations the habitat and feeding conditions needed to support reproduction and growth.
It is all the more true that it is also the case that when the conditions are suited to this, the VNF Inter-regional North-East Department installs hydraulic systems (with the aim of creating nurseries) close to the banks reconstructed by soil bioengineering, that are of natural appearance but which are entirely man-made.
However, banks with sheet piling or block arrays do not provide such favourable conditions for fish.
For invertebrates, the results are less conclusive.
Even if the results seem to indicate a slight improvement using vegetation-based techniques over sheet piling, the methodology does not allow us to draw a clear conclusion. This point will be addressed more specifically in a forthcoming study.
As for water quality, the measurements show good quality for all of the sites studied but do not allow us to show any difference between the different types of installation.
In addition to these results, the studies also allowed us to define the technical criteria for maintaining bank functionality that is practically equivalent to natural banks, such as for example favouring plant immersion at the bank head, constructing flat sections at least one metre wide, creating protected coves at the front of the installation. In 2010, year of biodiversity, VNF plans to continue this monitoring to extend its data and improve its techniques to be able to recreate habitats that are attractive for the aquatic fauna and flora.
For further information, contact the Water-Environment section of the NE Dept. on +33 (0)3 83 35 36 80 or by email at firstname.lastname@example.org
| Methodology information:
For the fish fauna study, the research section conducts electric fishing to determine the number of species present, the number of individuals per species and their state of development.
For macroinvertebrates, samples are taken at different parts of the bank and in the channel. Counting the number of individuals and species allows the definition of a score reflecting the overall quality of the environment in which they live.
Over the different sites distributed across the North-East rivers and canals, comparisons are made between natural eroded banks and banks that have been renovated using vegetation-based techniques on the one hand, and between banks protected using conventional techniques (sheet piling or block arrays) and banks that have been renovated using vegetation-based techniques on the other hand.
- Macroinvertebrates: Small animals that live on the bottom of waterways and canals, or spend part of their lifecycle there, from the bank to the centre of the channel, on the surface of the sediments or in the sediments (molluscs, worms, insect larvae, small crustaceans). They are sensitive to environmental conditions and can be useful for defining the quality of the aquatic environment.
- Sheet piling: Bank defence system, usually made from metal. Sheet piles are driven into the ground and interlinked to form a cohesive wall, called a sheet piling curtain, and hold back the bank’s earth and ensure water-tightness.
- Electric fishing: Electric fishing consists of passing a low current in the waterway using a conducting rod with a ring at its extremity, which is supplied by a battery. The fish are thus slightly shocked and float at the surface where they can be easily collected using a net. This fishing technique may be used only by persons specially trained and authorised. It is used solely for conducting studies or to protect fish during work.Once the collected fish have fully regained consciousness, they are released back into their natural habitat.