Study the feasibility and technical details of restoring ecological continuity at 3 obstacles on the Gabarn river.
The Gabarn is a small river 2.6 km long. It is home to a very large population of white-clawed crayfish, a native species in serious danger of extinction. This small river is part of a larger group of small tributaries of the Gave d’Ossau in this area. This population represents a very high conservation priority for this species. It forms a metapopulation that is still functional.
However, around 1 km from the confluence, the river is channelled into an impassable concrete culvert for around 100 m. Two other, shorter culverts (3 m) are also present along a 200 m stretch upstream of this obstruction. In all, there are 3 consecutive culverts.
| Project status | Completed |
|---|---|
| Project type | Dam removal: pre-demolition |
| Country | France |
| River | Gabarn |
| Focal species | White-clawed crayfish, Brown trout |
| Organisation | Fédération de Pêche des Pyrénées-Atlantiques |
| Website | Fédération de Pêche des Pyrénées-Atlantiques |
Upstream of the first culvert, there are no crayfish to be found in the creek, even though the habitats are suitable for them. This is explained by the lack of connectivity between the upstream and downstream sections of the creek. In the past, intense events (flooding, severe low-water levels, etc.) may have prevented crayfish from recolonising upstream. The project involves removing the 3 nozzles entirely. The project’s team already have an agreement in principle from the owner, who is in favour of the project.
The aim of the project is to restore these 3 culverts in order to restore connectivity. The project team will carry out investigations and technical studies to complete the project:
1 – Complete topographic surveys of the 3 culverts,
2 – Investigation of site sensitivities and study of the hydraulic functioning of nearby wetlands,
3 – Technical dimensioning of the project on the new stretch of river,
4 – Study of the hydromorphological consequences of the dam removal. Impacts on the hydraulic capacities of downstream bridges. This is an essential point, as the longest culvert opens onto an old, partially dilapidated bridge.
5 – Definition of accompanying measures to limit the impacts during the construction phase,
6 – Drafting of authorisation requests,
7 – Drafting of specifications for the recruitment of companies.
Technically, removing culverts #2 and #3 presents no major difficulties, aside from creating access for the excavator.
The project for culvert #1 is more complex, as it involves excavating a new channel over 100 metres long, at a significant depth. The dimensions of the new channel are based on observations of unaffected sections downstream. This work will necessarily require the removal of some of the material. Topographic measurements (during spring and summer 2025) and geotechnical surveys (through boreholes in November 2025) allowed the project team to define the constraints and technical difficulties in order to find solutions. Materials are mostly of good quality, but the team is looking for sites as close as possible to minimise the project’s carbon footprint. The initial preliminary sketches presented to the landowner, and the refined project drawings in December 2025, were well received by the site owner.
Before starting work, the team wants to address two final points: identifying the source of the local crayfish contamination and obtaining approval from the road authority downstream of the project. These issues would not completely halt the work if left unresolved, but based on experience, the team prefers to err on the side of caution to ensure the project’s success.
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