In the Haute-Savoie department, located in French Rhône-Alpes region, the aquatic environment management and flood prevention expertise to restore the Dranse river delta

For the last few years, the area surrounding the Dranse delta has been facing the impact of rampant urbanisation caused by the development of industry, tourism, and new residential areas. The Lower Dranse, between Pont de la Douceur and the Dranse delta, formed a wide area where the river deviated.

The flood in 2015 served as a warning of the risk of flooding in this downstream section of the Dranse and highlighted the importance of restoring the area in an ecomorphological way.

There are many factors at play in this part of the delta:

• A reduction in the space available for the river to flow properly, caused by lateral constraints: river bank protection, crossing structures, etc.
• Sedimentary imbalance caused by the extraction, now prohibited, of large volumes of material from the river bed and from the mouth of the Lower Dranse
• A reduction in sediment additions from upstream of the catchment basin, caused by stabilisation of the slopes and the influence of hydraulic structures, causing hydrological impacts and sedimentary blockages

Teams at SUEZ Consulting are helping Chablais planning association restore the river's ecological functions.

In order to meet its many challenges, the project is being co-developed by all its stakeholders and financing bodies, namely the Dranse River Delta's nature reserve, the Direction Départementale des Territoires (regional planning and development department), the municipalities, the Communauté de Communes (federation of municipalities), the RM&C water agency, private stakeholders, the French Fishing Federation, the French planning and housing authority DREAL (risk and environment), and Chablais planning association.

To optimise and try to reduce the project's greenhouse gas emissions, our teams worked with the Ecole Polytechnique Fédérale de Lausanne to record and analyse the site's greenhouse gas emissions prior to the start of the project.

The entire project focuses on the ecological challenges and susceptibilities of the natural environments:

Wherever possible, we reuse materials from the site and adopt nature-based reinforcement solutions. The technical innovations applied are based on nature-based solutions, such as soil bioengineering and reusing local products, plants and aggregates.

The longitudinal and transverse profiles of the river will be modified to improve flood control whilst restoring upstream aquatic habitats.

The use of local varieties of plants from the natural habitat is in line with restoration of the ecological functions of natural environments. Their use for plantation, redevelopment or reinforcement enhances the resilience of ecosystems.

These plants, certified by the brand "Végétal local", are suitable for any operation to preserve or restore biodiversity, complete with local collection and production channels.

Helped by specialised ecologists, teams at SUEZ Consulting adapted the project to minimise its environmental impact, in order to identify and apply the following avoidance and reduction measures:

• Consideration of the habitat of water shrews and preventive demarcation for beaver reproduction
• Upkeep of sectors with steep banks and installation of a lining to encourage nesting of kingfishers and wild martins
• Creation of hibernacula
• Installation of barriers to protect fauna during the construction phase - particularly for amphibians
• Modifications to the project to meet piscicultural issues and protect the spawning grounds of the lake trout, this section of the Dranse river being the lake trout's largest reproduction zone
• Creation of ponds for amphibians
• Replanting of levelled areas by sowing and planting “Végétal local” certified plants
• Measures to tackle Invasive Alien Species and treat Asian knotweed
• Work in keeping with the Dranse delta regional nature reserve management plan