In 2007, the River Sorne burst its banks, causing damage worth millions of Swiss francs in Delémont. Since then, the town has taken numerous steps to prevent flooding. However, the Delémont Marée Basse project is not just about flood protection – it is a community project.
Text: Cornélia de Preux
The summer of floods in 2007 is one that the people of Delémont (JU) will not forget in a hurry. Extremely heavy rainfall on 8 and 9 August caused the River Sorne to rise rapidly and burst its banks. The entire Morépont district flooded, submerging basements, underground car parks and other underground areas, and causing damage costing 10 million Swiss francs.
Today, there is no longer any visible trace of the disaster in Morépont. And thanks to the Delémont Marée Basse (DMB) project, many things have changed. The River Sorne winds its way peacefully between trees and shrubs and islands of shingle and greenery. It is now twice the width it used to be: 40 metres wide instead of 20. The revitalisation work has created a natural environment that offers habitats and nesting sites for numerous animal and bird species. The area is close to the town centre and attracts large numbers of visitors, including schools, which use it as an open-air science lab. The new town park has play areas, recreational zones and space for various events.
Recognising the opportunities
“In the Delémont Marée Basse project we are ensuring safety, while taking account of the need for revitalisation and creating a bonus for the local people: direct access to the river”, says Cédric Neukomm, a local authority engineer in Delémont. “The floods of 2007 were a shock. But instead of focusing exclusively on flood protection, we saw opportunities to improve people’s quality of life.”
David Siffert, who works in the FOEN’s Flood Protection Section, was a local authority engineer in Delémont from 2009 to 2018. He recalls that the most urgent problems had to be tackled immediately and the authorities built two embankments at the most at-risk sites, some distance from the river. For the DMB project work, the town was divided into three sectors: the En Dozière natural habitats sector, the Morépont district and the town centre. Work on the construction site, which covered more than three kilometres, began in 2010.
More space for the river
The first environmental measures were implemented in the En Dozière sector, where more space was created for the river. This also increased the flow, which was particularly beneficial to the fish. Open spaces connected to the river and dry meadows were created. The campsite acquired a beach and the footpaths were upgraded. These measures were drawn up in a participative process and paved the way for similar initiatives.
The DMB project is impressive, not only because of its environmental and socio-economic components, but also because of its structural measures. These include a structure not far from the town centre that can discharge water onto the railway line that runs between Delémont and Basel if the volume of water exceeds the capacities of the flood defences.
The town gets a facelift
The centre of Delémont is heavily built-up and the River Sorne has only a limited amount of space here. Work is currently being carried out to deepen the river bed, lower the foundation walls and reinforce their embankment function. These measures are designed to increase the flow capacity by 30 percent. There are also plans to add plants to the side walls and create a footpath. Municipal allotment gardens will be created on the corner of Rue Pré-Guillaume. As in the Morépont district, the new structures in the town centre have been designed with great care. Architectural competitions were organised to choose designs for all the river crossings, such as the new Collège and Haut Fourneau footbridges. The idea, however, is not only to make the town centre more attractive, but also safer. In order to reduce the residual risk of flooding and damage to new buildings, all ground floors will be built higher in future and buildings will include protective measures. As local authority engineer Cédric Neukomm explains, the DMB project is holistic and sustainable: “It is holistic because we looked at the main protection measures from every angle to ensure we achieved the greatest benefit for the town for the sum invested. And it is sustainable because the structural measures were agreed in consultation with the population, which made it possible to secure them for the long term.”
The project entailed major challenges: first the town had to assess the hazards and risks posed by flooding. The analyses showed that the town could expect damage costing up to 120 million Swiss francs. The project will cost around 15 million Swiss francs. The town’s inhabitants were involved in the process at an early stage so that their wishes and needs could be taken into account. And when the project team was set up, they made sure it included people with a range of expertise – in underground engineering, hydraulic engineering, applied ecology, landscaping and public space
Extra benefits for the community
The project is creating plenty of extra benefits for the local population: natural spaces are being enhanced and the River Sorne integrated into the socio-economic life of the town. It has also led to other projects which, according to David Siffert at the FOEN, “would never have been realised without the DMB”. For instance, the town park in Morépont has been extended and the conditions necessary to develop the Gros-Seuc eco-district, where more than 350 homes are due to be built, have been created. In addition to the main bulk of funds to revitalise the river and the surrounding area, other public loans were also approved and used to finance the discharge channel by the railway line, the footbridges, the town park and other projects.
A further plus of the DMB project is an area of around 15,000 square metres in Morépont that has been reserved for the river and adjoining town park. In the En Dozière sector, a zone previously approved for construction has even been set aside for nature instead. The works in the town centre are not yet finished. There are still some finishing touches to be made: connecting the built-up areas along the Sorne and improving access to the narrow stretch of the river. These works are due to be completed by 2024 at the latest.
A whole town on board
Around 70 percent of the 15 million Swiss francs needed for the work on the Delémont Marée Basse project is being provided by the federal government and the canton. The remaining costs must be borne by the town. Around a quarter will be met by the insurance company Die Mobiliar and by the national railway company SBB, which will both benefit from the flood protection measures. A comprehensive participation process with the local population played a major part in ensuring that the financing of the revitalisation of the River Sorne met with widespread approval across the whole commune. In 2009, 83 percent of those entitled to vote in Delémont voted in favour of the loan. Nearly 50 representatives of the canton and commune, political parties, the insurance industry, environmental organisations and agriculture were involved in the planning process, along with property owners and the general public.
Last modification 03.06.2020