Ecologically stealth construction works » for the reinstatement of the corridor’s functionality through an existing highway

The 100kms section of A63 highway in Aquitaine, South-West France  is supporting a heavy traffic between Spain and Northern Europe.

The bidirectional double lane widening works (undertaken between 2011 and 2013) was entrusted to concessionaire Atlandes with the requirement to improve environmental integration of the highway, particularly regarding  ecological connectivity of wildlife corridors at a regional scale and quality of water resource.

One of the project’s main challenges was to reconnect the ecological corridors from either side of a busy highway.  In its initial state, the highway was almost impassable for major species such as the European Otter, the European Pond turtle and migrating fishes such as Lamprey and Eels.

The most efficient and standard ecological way to reestablish the connectivity is to install concrete ledges etc. via the existing bridges and culverts.  However, these are reducing the hydraulic capacity of existing structures, which were already considered as under-sized for centennial storm events.  Some structures had also waterfalls, making them particularly difficult to be crossed by aquatic fauna.  In addition, the construction of concrete ledges along the existing culverts would generate severe issues since it requires the drying out and temporary diversion of the stream.

The challenge was then to reinstate the ecological connectivity for small and semi-aquatic animals without negatively affecting the hydraulic capacity of the existing structure and limit impacts induced by the construction works to the minimum .

Egis developed the “ecologically stealth construction work” concept for stream/river with the collaboration of a construction company (Colas), by :

1 - Installation of wooden ledges, designed for use by small and semi-aquatic animals (otter, mink…), adapted to the biological needs of the species : stepped ledges, made of wood, are fixed to the structure with non-rusting metal brackets.  This set-up has been confirmed as useful in terms of :

  • ecology ; no interruption to the flow of water, no need to pour concrete and no suspended material diffusion…,
  • hydraulic ; flow capacity roughly preserved,
  • construction;  faster and economic since the lightweight materials was easy to move and quick to install in a few days by boat or on foot without significant disturbance within the stream bed.

2 - Installation of water flow control systems at the impassable waterfalls to stimulate the ascent and descent of migrating Lamprey and Eels with strong involvement of the National Rivers Authority in France (ONEMA).

3 - Protection fence and signage to preserve the protected water-born plant communities during works.

The post-construction monitoring showed that for 3 years the culverts have been regularly used by fauna, noticeably by semi-aquatic mammals.  The wooden ledges have been resistant despite many heavy floods.

The “ecologically stealth construction work” resulted in minimal perturbation to the stream or waterway and to wildlife during the construction period, a period which is normally the most sensitive. The concept produced satisfying results regarding the upgrade of the ecological corridor functionality, truly documented by photo traps monitoring.

Upgrading existing infrastructure, ecological connectivity, sensitive construction phase