Gironde Estuary

With an area of 635 km², the Gironde estuary is the largest estuary in Western Europe. The alluvium carried by the Garonne and Dordogne rivers contributes to the formation of sandbanks (vasards) in the Gironde estuary: each year, between 1.5 and 3 million tons of suspended particles settle, forming banks that can develop into islands. The Mortagne-sur-Gironde polder and Île Nouvelle, formerly cereal-growing areas, were purchased by the Conservatoire du littoral after Storm Martin in 1999, when they were partly overtaken by the sea. The Conservatoire decided not to block the breaches on these lands but to let the tides act freely and to monitor the evolution of these environments.

Mortagne-sur-Gironde polders

On the north bank of the Gironde estuary, polders were created between 1960 and 1970 in the commune of Mortagne-sur-Gironde with the aim of establishing intensive agricultural production on estuarine salt meadows that were previously underutilized and regularly covered by brackish waters.

Storm Martin in 1999 created breaches in the embankment system of a polder located downstream of the port of Mortagne-sur-Gironde, and water flooded this area. To protect the homes, a new dike was then built further inland, closer to the former shoreline. The Conservatoire du Littoral acquired the parcels located within the former embanked zone, and since then, the Regional Conservatory of Natural Spaces (CREN) Poitou-Charentes has been managing these lands.

At this site, the sedimentation rates observed are remarkable: within ten years, the original topographic level was regained, and a gradual recolonization by vegetation typical of estuarine marshes began. Beyond the intrinsic qualities of these habitats , the reed beds act as a buffer by absorbing part of the energy of the incoming swell, which reduces its impact on the coastline.

Nouvelle island

L’Île Nouvelle first appears on maps in 1825 as two islands, Île Bouchaud and Île Sans-Pain, shaped by the meeting of sediments from the Garonne and Dordogne rivers and marine sands. The island appears as a single entity after 1866.

From the mid-19th century to the 1960s, Île Nouvelle was farmed for its lands enriched by the estuary’s silt. Initially a vineyard area, it escaped the phylloxera plague that devastated French vineyards at the end of the 19th century thanks to regular flooding of the river, which drowned the aphid responsible for the disease. Viticulture gave way to cereal farming in the 1960s. The island’s last farmer ceased activity in 1997.
It was for its remarkable ecological potential that the Conservatoire du Littoral purchased the island in 1991. Management was entrusted to the Gironde Department.

Two major climatic events influenced the Conservatoire’s and Department’s decisions on projects to undertake.
During Storm Martin in 1999, a breach opened in the dike of Île Sans Pain, causing significant flooding of that part of the island and the village, raising awareness about the nature of projects needed. From then on, a renaturation program involving the deliberate opening of a breach seemed possible. The manager and owner drew on Dutch experiences with marine reconnection to initiate this renaturation program. The island was twinned under the EUROSITE2 program with the Dutch island of Tiengemeten.

Storm Xynthia in 2010 created a new breach in the northeast dike of Île Bouchaud and accelerated the fully embraced objective from the beginning: “to let the island flood with river waters.” This northern part, directly connected to the river, is evolving rapidly: the island is losing its forest cover, a mudflat is forming and is gradually being replaced by a rich and welcoming reedbed.

In 2006, the Conservatoire launched a management plan to define the site’s long-term objectives. These can be summarized as renaturing this agricultural island through a “flexible” management of the coastline on the northern part (Bouchaud) and controlled water level management on the southern part of the island (Sans Pain). This management approach, started over 10 years ago, is restoring the original state of the estuarine marshes that existed before the “viticulture and agriculture period.” Flora and fauna quickly recolonize the areas, biodiversity returns.

Alongside the island’s renaturation, the Conservatoire and the Gironde Department wanted to preserve the memory of the people who lived and worked on the island by conserving and highlighting its built heritage. A large restoration program of the buildings on Île Sans Pain (the winery, the manager’s house, the school, and agricultural workers’ housing) and hydraulic structures was completed in 2016. To welcome the public (a key aspect of the project), a trail was created in the southern part with wooden decking and a hide.

The northern zone has continued to evolve freely since the 2010 storm, with a buffer enclosure protecting this “free evolution” area from the southern enclosure that receives visitors.
Île Nouvelle is a Protected Natural Area (Espace Naturel Sensible) managed by the Gironde Department, a true “open-air laboratory” for renaturation. It offers visitors a chance to discover estuarine marshes, their species, and the identity landscapes of the Gironde estuary. It preserves the memory of the past life of the “îlouts,” its former inhabitants. Finally, it serves as a showcase of the flexible coastal management approach engaged for nearly 10 years, symbolizing adaptation to climate change and a magnificent demonstration of the benefits nature offers us.

The northern part of the island (the former Île de Bouchaud) is closed to the public to ensure a peaceful space for the many bird species that frequent it.

Project actions and outcomes

The Gironde estuary is a valuable site for the Adapto project because this area benefits from comprehensive monitoring organized by the managers; the evolution of these territories will serve as a reference for other sites that are currently at earlier stages in the process. The Gironde estuary can thus contribute to the Adapto network of sites facing similar challenges.

The presence of the two experimental sites will also enable reflections on the future of other polders in the Gironde estuary under similar conditions, allowing local stakeholders to conduct prospective studies exploring multiple possible development scenarios.

The Adapto project will primarily implement detailed monitoring actions: birdlife, fish populations, crustaceans, habitat mapping, sedimentation analysis and modeling, social acceptance, ecosystem services, and landscape evolution.

This comprehensive data set will allow local stakeholders to understand and appreciate the ecosystem benefits provided by these natural spaces.

Nouvelle insland

The breach at high tide – July 2018

The breach at low tide – July 2018

The projects undertaken and the work carried out over the past ten years provide many answers about the services offered by marine reconnection: changes in vegetation cover, diversity of bird species, refuge and nursery areas for fish, etc.

The Adapto project will implement, in particular, two actions:

• The implementation of the IRSTEA research agreement (Institute for Research in Environmental Sciences and Agriculture) related to ecosystem services in order to establish an inventory of the benefits linked to estuarine reconnection (2018–2020).
• The implementation of the “ecological quality habitat indicators” method with the MNHN (National Museum of Natural History).
• Monitoring of the bathymetry of the breach and the channel by BRGM (Geological and Mining Research Bureau).
• A social perception study of the site’s stakeholders and users.

This set of data will enable local stakeholders to understand the ecosystem services provided by nature and the feasibility of flexible management on an estuarine island.

Chronology

Mortagne-sur-Gironde polders

• September 2018: Renewal of the photo campaign for the landscape observatory of the Mortagne-sur-Gironde polders – CREN + aerial photo campaign.

Ile Nouvelle

• March 26, 2018: Presentation of Adapto to the management committee bringing together elected officials of Gironde, the manager (Cd33), and the Conservatoire du littoral.
• May 30, 2018: The Adapto project is presented to DREAL Aquitaine.
• Summer 2018: 47 hectares of Public River Domain are assigned to the Conservatoire du littoral by the Grand Port Maritime de Bordeaux.
• Summer 2018: Screening of the film “Et demain, on fait quoi?” (“So tomorrow, what do we do?”) at a school in Pauillac. Organized by the Aquitaine Delegation in partnership with CPIE Médoc (Permanent Center for Environmental Initiatives), already very involved in its educational program on climate change and flexible management issues.
• September 13, 2018: Presentation of the Adapto project to the local steering committee (Copil) which includes managers, partners, State services, scientists, and users.
• November 2018: Signing of a partnership agreement with IRSTEA (2018-2020). Implementation of a research and development program to evaluate the services of estuarine reconnection by studying the trophic functioning of aquatic communities and the associated ecosystem services approach (economic/ecological component).
• January 17, 2019: Site management committee meeting including Cdl/Cd33 with elected officials from the department, presentation of scientific monitoring programs under the Adapto project.
• February 7, 2019: Conference in Sadirac (33), presentation of Adapto by the Aquitaine delegation on flexible coastal management actions for pilot sites in Aquitaine (Leyre Delta – Ile Nouvelle). Conference organized by the Terre et Océan Association. 40 participants.