Removal of drainage ditches on wet meadow on Coat Carriou

National Id
The project involves the removal of drainage from a wet grassland at the head of a small Breton coastal river watershed in an area subject to nitrogen pressure. Located at the bottom of an hydromorphic valley, the Coat Carriou meadow has been drained by two ditches to allow the plot to be mowed. Collecting runoff from the hillside, the two ditches caused a short circuit in the meadow, leading the nitrate-laden runoff directly to the river stream. The two drainage ditches were filled in with the aim of eliminating the short circuit, allowing all water flowing to spread over the meadow. The aim was to improve the purifying effect of the grassland on the waters from the hillside, while maintaining agricultural activity. The short circuit has effectively been eliminated, allowing all the water on the slope to pass through the wetland where the purifying capacities make it possible to remove all the mineral nitrogen present in the water. As the purification capacities were already optimal before the work, they were not improved, but all water can now be purified. The reduced bearing capacity allow to mow the meadow only in dry years. It has been included in the grazing path of a herd of brittany pie-noir cows, able to graze wet grassland.
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The in-depth description of the case study
Location description
The Coat Carriou site is located at the head of a small coastal river in South Finistère. The site occupies the right bank of the stream, in a relatively steep valley. The plot is occupied by a meadow. The other surrounding plots are meadows and crops, and a wooded hillside on the other side. The plot was acquired shortly before the project by an organic farmer who wanted to integrate it into his grazing land.
RBD code
FRG-La Loire, les cours d'eau côtiers vendéens et bretons
NWRM(s) implemented in the case study
Climate zone
cool temperate moist
Mean annual rainfall
900 - 1200 mm
Light or indepth?
Monitoring parameters
The follow-ups implemented were carried out from 2014 (before the works) to 2016.
Piezometric monitoring
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.
Physico-chemical monitoring
The evolution of nitrogen concentrations in the water table across the wetland was measured by taking water from the piezometers.
Ecological monitoring
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.
Control site
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.
Biophysical impact assessement method
Comparison with a control site
Project scale
Installation date
Area (ha)
0.3 ha
Factors constraints
The plot is located in a very hydromorphic valley bottom
Favourable preconditions
With the objective of improving water quality in a watershed under nitrogen pressure, it was necessary to restore the purification function of the wet grassland by removing drainage ditches to restore water flow through the wetland and raise the water table.
Design contractual arrangement
Arrangement type Responsibility Role Comments Name
Design consultation activity
Activity stage Key issues Name Comments
Design land use change
Land use change type
Design authority
Authority type Role Responsibility Name Comments
Initiation of the measure
Communauté de communes du pays Fouesnantais (CCPF) (federation of municipalities of Pays Fouesnantais)
Initiation of the measure
Réseau expériemental de rehabilitation de zones humides (RERZH) (wetland rehabilitation experimental network)
Key lessons
It would have been interesting to measure the flow rates and nitrogen contents in the ditch water to quantify the gain provided by the removal of these short circuits.
Success factor(s)
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.

Financing type Comments
Barrier type Barrier role Comments
secondary barrier
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
main driver
The subsidies helped convincing the local community to invest in the project.
main driver
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
Financing share type Share Comments
Policy description
The plot is located in a very hydromorphic valley bottom, whose groundwater level is influenced by both flows from the hillside and the watercourse. It drains to a marine area sensitive to algal blooms and the concentration of nitrogen in the water is high.
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.
Policy target
Target purpose
Policy pressure
Pressure directive Relevant pressure
Policy area
Policy area type Policy area focus Name Comments
Policy impact
Impact directive Relevant impact
Policy wider plan
Wider plan type Wider plan focus Name Comments
Policy requirement directive
Requirement directive Specification
Information on Ecosystem improved biodiversity
Vegetation evolved between 2014 and 2016 in some areas of the plot, with the appearance of more hygrophilic species in the old ditches and downstream of the plot. In addition, the recovery of some species such as the Sharp-flowered Rush has increased significantly over the entire profile studied. There is also an increase in the average number of species per quadrat, indicating a diversification of the existing grassland. Many species of acidophilic lowland and paratourous grassland have been identified, representing a gain in terms of heritage species.
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.
Information on Increased groundwater level
Visual observations on site show a greater saturation of soil water on the southern part of the plot. This remained saturated throughout 2016, making mowing impossible, whereas it was before the ditches were filled.
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.
Information on Soil quality overall soil improvements
The reduction of dissolved mineral and organic nitrogen after the work is maintained. The same trend can be observed at the control site, and the difference between the two sites is not statistically significant. There is no observed effect of the work on orthophosphates.
These results show that the wetland was already functional before restoration in terms of nitrogen purification.


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