3 transects of 5 piezometers each placed in the direction of the steepest slope of the site, with a water level reading every 15 days.
The evolution of nitrogen concentrations in the water table across the wetland was measured by taking water from the piezometers.
Two transects were carried out on the plot to monitor the evolution of the flora. A mapping of the vegetation of the plot is also carried out. Qualitative inventories of amphibians, lepidoptera, orthoptera and odonates have been carried out.
Monitoring is also carried out on a similar control site in a good state of conservation. The control plot is located upstream on the same watercourse.
Design & implementations
|Activity stage||Key issues||Name||Comments|
|Land use change type|
Lessons, risks, implications...
|Success factor type||Success factor role||Comments|
The main interest of the restoration work was to eliminate the short-circuit effect of the ditches. After the works, all the water from the hillside flows through the wetland and benefits from its purifying effect. The main opportunity was the call for projects sent by the network, which convinced project leaders who were still hesitant to launch an action and carry out their project successfully. Financial leverage is important, as the work is part of territorial contracts and is financed by the Water Agency. As monitoring and evaluation are carried out by RERZH, the project leader can benefit from monitoring without additional cost to the structure. The technical support and assistance on regulatory dossiers provided by RERZH was also an important lever.
|Barrier type||Barrier role||Comments|
The major constraint is related to the particular hydrological functioning of the wetland, which, despite the drainage ditches, had still a very wet character. This reduced the demonstrative aspect of the work sought under the RERZH.
|Driver type||Driver role||Comments|
Availability of subsidies
The subsidies helped convincing the local community to invest in the project.
The land owner was willing to include the meadow in the grazing path of his herd and was therefore interested in improving the fodder quality by reducing the Juncus effusus and not too sensitive to a loss of bearing capacity of the meadow.
|Financing share type||Share||Comments|
Policy, general governance and design targets
Originally a light peat bog (thin layer of peat), the plot was drained to reduce hydromorphy and thus allow the entry of machinery to exploit the meadow by mowing. Drainage is provided by two ditches 80 to 120 cm deep that collect water from a spring and drain the groundwater from the hillside. The groundwater level is relatively high despite this drainage, due to the double supply from the hillside and from the accompanying groundwater of the river.
As a result, the prairie vegetation is hydrophilic. Botanical inventories carried out in 2014 by the National Botanical Conservatory of Brest made it possible to obtain a mapping of habitats and a list of 72 taxa present. The Soft Rush (Juncus effusus) is present in large quantities.
Faunal inventories carried out by Bretagne Vivante and the Breton Mammological Group have revealed the presence of Palmate Newt (Lissotriton helveticus), Common Toad (Bufo spinosus) and Grass Frog (Rana temporaria). Five odonata species have been observed, including the Golden-ringed Dragonfly (Cordulegaster boltonii), a key species at the departmental level. Eleven species of orthopterans have been observed on the site, as well as the Mole Cricket (Gryllotalpa gryllotalpa), rare in Bretagne. Two protected semi-aquatic mammals are using the site: the Southern Water Vole and the Eurasian Water Shrew.
The watershed is subject to nitrogen pressure. The water reaching the meadow by resurgence at the foot of the hillside can reach concentrations of 44 mg/l of nitrates upstream of the plot. These decrease significantly as they flow into the plot, indicating that nitrogen has been reduced in the wetland. As a result of prairie drainage, some of the water enters the stream without passing through the wet meadow and its purification capacity is therefore partially short-circuited.
The objective of the NWRM is to restore hydraulic operation of the meadow to allow:
- optimize nitrogen abatement processes by denitrification by raising the groundwater level;
- to improve forage quality by controlling the diffuse rush with appropriate management;
- to promote a fauna and flora typical of wetlands;
- to demonstrate to the stakeholders of the territory the interest of this type of restoration for the improvement of water quality.
|Pressure directive||Relevant pressure|
|Policy area type||Policy area focus||Name||Comments|
|Impact directive||Relevant impact|
|Wider plan type||Wider plan focus||Name||Comments|
The work begun in the summer of 2014 seems to have been favourable to the Grass Frog, which immediately exploited the new temporary puddles available. The second year of prospecting confirms this colonization of the site by the species.
Orthopteran processions linked to wetlands (Conocephalus dorsalis, Gryllotalpa gryllotalpa, Chorthippus albomarginatus, Stethophyma grossum, Tetrix ceperoi...) are well preserved with little evolution between before and after work because the vegetation already had a very marked wet grassland character before work.
The Southern Water Vole is still present in high density on the site in 2015. By 2016, the occupation of the northern ditch had decreased significantly, especially in the area between the former central ditch and the watercourse, where the ragondine has become well established. A ragondin control campaign launched by the CCPF in 2016 should make it possible to avoid the continuation of the replacement of the Vole by this species.
There is no detected impact on the Eurasian Water Shrew.
Piezometric levels at low water level after work are slightly higher (10 centimetres on average) in the piezometers closest to the filled ditch, but this observation is not statistically significant. In addition, the same phenomenon can be observed at the control site, showing a variation more related to meteorology. However, on the restored site, there is an increase in the restoration date and a decrease in the discharge period, showing an increase in the saturation time of the wetland soil.
In the absence of a piezometer in the area where a groundwater rise was observed visually, it could not be quantified.
These results show that the wetland was already functional before restoration in terms of nitrogen purification.