Feasibility study for Urrutiénéa dam removal, France

Introduction

The Nivelle River is a 45 km long cross-border coastal river. The first 12 km flows into Spain, while the remaining 33 km are in France, including the river’s mouth into the Atlantic Ocean. This river supports a variety of migratory fish, including two species of shad, sea lamprey, European eel, Atlantic salmon, and sea trout. The number of salmon that swim up the river each year has declined from an average of over 200 fish between 1990 and 2000 to 100 fish (and in some years fewer than 50) over the last 20 years. This species is in sharp decline and is particularly threatened in this basin. The causes of the decline are varied, as in other rivers (habitat quality, water quality, dams, commercial fishing, etc.).

The river is also home to the last population of freshwater pearl mussels in the Pyrenees, which live in symbiosis with salmon and trout. This population is also in serious danger of extinction, and significant efforts have been made in recent years to save it. The upstream part of the river is home to other rare and endangered species, such as the Pyrenean Desman (a small aquatic mammal endemic to the Pyrenees) and the White-clawed Crayfish. Currently, the upstream section of the river has the best conservation status and serves as a spawning ground for salmon and trout. However, this part of the river is inaccessible due to the Urrutienea Dam.

Project context and opportunity

Several fish passes have been built on the two main dams in the downstream part of the Nivelle, but the Urrutienea Dam was never equipped, despite the high stakes for biodiversity. The owner consistently opposed the construction of a fish pass. For this reason, the project team purchased the dam, intending to remove it once they became the owners.

This project also had the support of the Government of Navarra in Spain, as it would enable the return of salmon to the Spanish section of the Nivelle. In the context of climate change, the movement of fish (and all aquatic organisms in general) is becoming an increasingly important issue.

This project was of significant local importance, requiring detailed preliminary technical studies.

Project aims

The project team conducted topographic surveys, estimated the volume of sediment that would be remobilised, and produced technical design documents. During the preparatory phase, the team defined measures to limit the impacts during removal and prepared the specifications, selecting the company that offered the best skills at the most competitive cost. They also developed the permission procedure during this phase.

Project achievements

Thanks to the preparatory work conducted, the project’s technical feasibility, the primary goal of the study, has been confirmed. The sediment quality is deemed satisfactory, allowing for natural sediment transit without the need for removal from the river. The dam’s area of influence has been accurately identified, and the topography and required resources for the upcoming works are well-defined.

Facing challenges, the association successfully negotiated with the dam owner, eventually acquiring the dam and the associated former fish farm independently of the Open Rivers Programme grant on 1 August 2022. Another hurdle involved opposition from a neighbouring owner, a local authority with ownership of an old mill downstream. Continuous mediation efforts led to a favourable agreement, publicly announced at the last Natura 2000 meeting on 11 September 2022.

A thorough survey of the dam’s concrete quality revealed a combination of reinforced concrete (with embedded iron) and stone components, which helped to size the demolition efforts. Additionally, an analysis of the potential impacts from the dam removal, considering both positive and negative effects on the environment, ecosystem functioning, and fish populations, was conducted.

This project was completed in 2022, and in 2023, the dam was removed through a follow on grant from the programme.