14 January 2020

Let it rain

From issue to resource, how to make the most of rainwater



An Olympic goal


The French swimmer Émile Paulus won the first ‘Traversée de Paris’, Paris crossing, after a swim of 11.6km in the river Seine against eight other swimmers. It was 1905. Today, the average Parisian has a difficult time imagining themselves dipping their feet into the river. Excepting the Villette basin, the Seine is unsuitable for bathing. 


This might soon change, since Paris has big plans to clean the river for the 2024 Olympics, when crowds will be able to watch swimming competitions sitting along the riverbanks. And once the games are done, the river will be open to all along the whole Greater Paris area for a swim or just a quick plunge.


The journey of a raindrop


There are several elements that contributed to the current pollution levels of the Seine, a significant one being that the equivalent of 1,000 Olympic swimming pools of polluted waters ends up in the Seine every year due to rainfall.


In a natural setting, where earth and sky face each other without barriers, the life of most raindrops is simple enough: 50% infiltrates in the earth, 40% goes back into the air, and only 10% runs off. In Paris, where 66% of surfaces are impermeable, rain has a harder time to find its way back into the ground. Instead, most of it runs off, joins the sewers, which, sometimes, can’t manage the extra water, overflows and ends up in the Seine, bringing with it all the impurities collected along the way.


A projection study done by the city estimated that the amount of polluted water ending up in the Seine, if current conditions remain unchanged, would increase by 76% in the next twenty years. Paris’s city council response was to unanimously vote for the ParisPluie plan in March 2018. The plan aims to create more permeable and green spaces in the city.


The ParisPluie plan


The principle of the plan is simple: making the most of rain as close as possible to where it falls. Because the vast majority of annual precipitation in Paris, up to 80%, is considered ‘soft/current’, compared to 20% of storm rain, the plan addresses only the impact from more frequent rainfall.


From green roofs with rainwater collectors, like the one on top of a gymnasium in the Belleville area, to permeable cycling paths like the one in the Avenue de la Porte d’Ivry, ParisPluie proposes a multitude of solutions to attain its goal of converting 570 hectares in the city into green spaces, and of reducing water runoff into the Seine by 21%.


The plan favours solutions that will have multiple positive effects on the city and its people. For example, the green area created by the public authority in Clichy Batignolles will positively affect biodiversity, it will create an îlot de fraicheur, ‘coolness island’, with a temperature that will be between two to three degrees fresher than neighbouring areas with impermeable surfaces, not to mention it will make the city more pleasant and create a space for social interaction. 

“We can capture the water and then turn it into a resource rather than a waste.”


“The green roof is usually the first surface that collects rainwater,” notes Lionel Zint, technical manager at the construction company Soprema.  “This opportunity allows us to manage it more easily, to valorise it and not only to see it flowing into the gutters and the sewers, but to capture it and then turn it into a resource rather than a waste.”


Other ways to capitalise on rainwater are water storage facilities, like underground containers or cisterns outside of apartment buildings. These can collect potentially bigger quantities of rainwater and can be combined with green areas and gardens for watering purposes. The water collected could also be reused by housing complexes for cleaning and washing. 


Some solutions involve making the most of every available square centimetre, like letting vegetation grow between the paving stones of the courtyards of condominiums in the 4th Arrondissement.


Everyone involved


The ParisPluie plan was put into local urban regulation in September 2018, therefore involving everyone in the project of making Paris a permeable city. In practice, this means that any new construction or refurbishment plan has to take into account rainwater management and should put into place a solution to achieve a minimum of water absorption. 


The calculation of this minimum is based on a study that considers the sewage network absorption capacity and the composition of the soil in a set area of Paris. In fact, in some areas of Paris, the composition of the soil is such that it can’t absorb too much water or the stability of the ground will be compromised. 


This means, for example, that a building built on a plot of 850 square metres in a zone considered as having a normal capacity of infiltration needs to be able to absorb 10.2 cubic metres of water every rainy day.


To help understand the new regulation and to discuss solutions and opportunities, the city has organised a series of meetings with relevant stakeholders called ‘Les Rencontres du ParisPluie’. 


“We are moving towards very simple solutions, but this is great innovation.”

In these meetings the solutions that are discussed are not very high tech. “We are moving towards very simple solutions, but this is great innovation,” explains Élodie Brelot, an expert in urban development and hydrology, while participating in one of the meetings.


The city also published an extensive guide collecting different solutions and highlighting each solution’s performance in terms of water absorption, the advantages on the environment, the cost, the maintenance commitment, and the disadvantages.


Schools as green oases  


Falling under the ParisPluie umbrella are several schools and colleges in Paris, which collectively occupy more than 70 hectares of urban space. This extensive surface, now mostly made of asphalt and other impermeable materials, has a big potential to be transformed into green spaces and to help increase the city permeability.


The project will involve around 30 schools every year, building schoolyards that are more plant-based, that use more natural materials, and more permeable soils.  The schools will replace the asphalt with colourful and innovative porous materials, they will add green roofs or urban gardens, plant more trees to create more shade, and build water fountains. 


The project will involve children and adults in co-designing their perfect solution, so that the new layout of their schoolyards will not only be sustainable, but also adapt to different uses, like playing, developing motor skills, and sharing space.


In the spirit of sharing the new schoolyards will be open on weekends and holidays with high temperature forecast, so that people whose health could be at risk because of the hot weather, such as homeless or elderly people, may have access to a fresher space and have some respite from the heat wave. At least until 2024 when a swim in the river Seine will be another option to keep cool.  

The ParisPluie plan explained

EcoQuartier Clichy-Batignolles (17th Arrondissement) ©Agence Parisienne du Climat

Example of outdoors water reservoir

Kids play in their new permeable schoolyard

Working together at a 'Rencontre du ParisPluie'


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