The Lyon metropolitan area planted its first "rain trees" in November 2021. This pretty name, invented locally, covers an equally interesting process that promotes the infiltration of rainwater.
Creating a "rain tree" involves expanding the space at the base of existing urban trees in a way that facilitates the infiltration of stormwater into the ground. This involves not only enlarging the area dedicated to the tree, but also removing anything that may interfere with root development and replenishing richer soil. Curb cuts are also opened to create infiltration trenches for rainwater to irrigate the tree pit. The benefits are many.
Stormwater Management
The first motivation is stormwater management : water that infiltrates no longer runs off the road. The city uses trees instead of pipes to manage stormwater.
Each tree can infiltrate water and recharge the water table. This reduces their watering during the summer. Another benefit is the fight against heat islands. These trees will have more water available to play their role of natural air conditioner by evapotranspiration. The metropolis has also previously identified the red heat island areas to position the projects. These larger spaces at the foot of the trees also make it possible to plant different species and thus promote biodiversity. Residents are invited to take care of these new green spaces through a partnership with FNE .
In total, one hundred streets will be landscaped with rain trees by 2026. For the moment, the cost is quite high because the companies are not specialized in these works. However, it should quickly decrease due to the mass effect. Indeed, this way of planting with pits will now be generalized with each new tree plantation.
Waterproofing
The rain tree is one of the nature-based solutions that can help adapt to climate change. It is part of a larger scheme, carried out as part of the European Life Artisan project (ofb.gouv.fr) Le projet Life intégré ARTISAN (ofb.gouv.fr), which aims to make the city permeable, i.e. to remove the asphalt and replace it with soil that allows water to infiltrate.
This is a way of building the city differently that requires thinking about water policies upstream of urban development and road projects and working together and across the various departments of the community.
For more information on this scheme, please find attached the technical booklet "Rain trees" elaborated by the GRANDLYON metropolis.