how are they implemented to combat flooding?

While techniques that rely on the services and resources provided by ecosystems develop and demonstrate their relevance, Cepri has just published a guide that presents solutions to prevent the risk of flooding: benefits, types of actions, conditions for implementation … Decryption.


Faced with general overconsumption and ever-increasing energy needs, many voices are being raised to call for sobriety. A key word for environmentalists for several years, it has also added territories and technical services, as we saw during the last energy transition meetings. How do you reduce the redundant and optimize the available resources? That’s what we’re seeing this summer via a web series to show how sobriety also applies to waste management, water and sanitation, energy and mobility. This week’s theme is water and sanitation.

For decades, the development and equipping of the territory was carried out using exclusively civil engineering techniques called “gray solutions”. Without questioning their interest in particular contexts (such as dykes in urban centers), they have shown their limits and even their disadvantages. In addition to the ecological impoverishment of the environment, they can amplify the risks: this is the case of dikes that accelerate the flow, which can promote flooding downstream. Faced with this observation, a fundamental movement has arisen since the 2000s, namely to resort to “Solutions based on Nature (SfN)” (also called NBS “Nature-based solutions”).

The IUCN (International Union for Conservation of Nature) World Congress in Hawaii then adopted a resolution in 2016 defining them for the first time. These are “actions aimed at protecting, sustainably managing and restoring natural or modified ecosystems to directly address society’s global challenges such as the fight against climate change or the management of natural risks”.

A holistic ecosystem approach

It is in this context that Cepri has just published a guide for local authorities about NBS to prevent the risk of floods (1). This document is part of the Life ARTISAN project coordinated over a period of 8 years (2020-2027) by the French Office of Biodiversity (OFB) and means “Increasing the resilience of territories to climate change by encouraging nature-based adaptation solutions (SafN)”. It is now a matter of relying on the services and resources that ecosystems provide. Thus, following the definition of NbS, these SafNs aim both to adapt to the effects of climate change while enabling “a net gain for biodiversity and co-benefits for society”.

The guidance primarily recalls this concept and the associated definitions, while clearly specifying that it is a matter of implementing one or more concrete actions for the restoration, management or protection of environments “in the context of “a holistic ecosystem approach”. Such an approach requires it to include “ecological, societal, political, economic and cultural issues at all levels, from the individual to the collective, from the local to the national, from the public or private sphere”. A local authority may, in particular, take into account the protection of biodiversity in its efforts to reduce the risk of flooding. However, it often does not include adaptation to climate change in the project, which nevertheless constitutes one of the societal challenges for IUCN. Through several examples, the guidance presents, firstly, the advantage of using these solutions, which contribute to reducing the risk of flooding in a territory, by limiting the danger (the case of the dune ridge) with an impact on the height and speed of the water, but also on the duration of the ​​the flood and on the effects of “small” floods… They also make it possible to adapt the area to the presence of the risk of floods and reduce its vulnerability by giving more space to water, by raising buildings or by adapting networks.. .

Many SafNs to reduce all types of flooding

A number of (non-exhaustive) measures are then developed based on the functioning of natural ecosystems to reduce the risk of runoff, river overflows and harbor subsidence. These are actions that allow you to:

  • reducing runoff by promoting better infiltration into the water table (such as draining land with a net gain in biodiversity, revegetation in urban areas, creating vegetated pathways for water such as ditches or valleys in cool islands, etc.), lowering flows or storing water;
  • reduce the risks of flooding from stream overflows by improving the natural upstream-downstream flow of streams (restriction of levees and their cover in urban areas, stream embankment, restoration of the minor bed, creation of vegetated benches in beds), providing more room for the stream (creation or restoration of flood extension zones and/or a wetland, etc.);
  • reduce sea flooding by limiting it (using “soft” methods to maintain the dune belt, limiting erosion and increasing the roughness of the foreshores – intertidal areas – and the approaches to the coasts, securing and replanting erosion-limiting seagrass beds such as Posidonia), by providing space for the sea, as for streams, which can lead to moving constructions from the coast inland and to renature these spaces…

A commitment: elected and convinced technicians

But choosing these solutions and implementing them is not a simple method. This especially requires a commitment from the public figure with elected officials and convinced technicians who argue and explain the reasons for the choice. This choice may indeed satisfy some local actors (environmental protection associations, public bodies involved in nature conservation, etc.), but it may also give rise to misunderstandings, reluctance or even resistance (permanent landowners opposed to the project, e.g. example). To do this, it is necessary to associate the actors affected by the project as far upstream as possible. If necessary, we can be accompanied by a mediator in case of conflict rather than risk a lawsuit.

This requires building a flood risk prevention strategy that relies on multiple techniques (not just infrastructure) to adapt to the effects of climate change while preserving biodiversity. This new strategy must be incorporated at all levels: in the territorial project, in planning at different scales (Sraddet, SDGE, PCAET, Scot and PLU(i)), at the level of the hydrographic basins (Sdage and PGRI), in development operations. ..

A common and transversal approach

It should also be noted that the SafNs have gained importance in the context of taking the Gemapi skill. After a first phase where we were mainly interested in flood protection, the trend is now towards a combined approach with the management of water environments. There is no longer a dichotomous vision because the nature-based solution combining IP with GEMA at the watershed scale is much more effective. But let’s not fool ourselves: moving from an IP approach to Gemapis is not easy. This requires more skills for design offices, but also for community services, in hydroecology, hydrobiology, hydraulics, infrastructure or ecology… with the need for a common and cross-cutting approach between all services. It is therefore important that local authorities can be advised by actors specializing in risks, but also biodiversity (ecologists, specialized design offices, associations, etc.).

Communicate and evaluate

The guide also confirms the importance of being an educator, showing examples of comparable results in other territories, but also of evaluating the SafN project, of showing its concrete results for the territory. It is often this lack of evaluation and quantitative results “that is currently lacking in the SafN projects studied and that can sometimes call into question the credibility of the project”.

Ultimately, it has been shown that the main advantage of nature-based adaptation measures is that they generate multiple benefits beyond flood prevention while optimizing certain costs. On this topic, the guide presents examples of funding that can be mobilized according to action types and contracting parties.

However, he concludes that “it is in the search for this complementarity between ‘grey’ and ‘green’ solutions, of a middle way that combines these two types of solutions, that local authorities can find answers to the challenges that lie ahead. on map term in terms of flood risk management”.

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