Sixty metres becomes ninety. Along 14 km of the Rhône between Bex and Yvorne, the cantons of Vaud and Valais are redrawing the river’s cross-section as a designed object. The bed will average 90 m instead of 60 m and open to as much as 300 m in places. Reinforced dikes and new ones hold it on both sides. The public file for this Chablais priority measure (MP Chablais) of the third Rhône correction opens today, 3 October, and runs until 2 November 2026. The État de Vaud calls the programme «le plus grand projet de protection contre les crues de Suisse», the largest flood-protection project in Switzerland. Chablais is its biggest single piece.

Width is the design variable

I carry loads for a living, so I notice when engineers change the variable they design with. The first two corrections, in 1860–1890 and 1930–1960, narrowed and straightened the Rhône to win farmland. After that, the only answer to a bigger flood was a taller wall. Rhône 3 reverses the logic: give the water room sideways and the same discharge passes at a lower level. The dikes stay serious. RTS reports that Vaud’s Conseiller d’État Vassilis Venizelos and his Valais counterpart Franz Ruppen describe the works as «dimensionnés pour protéger durablement la plaine contre une crue extrême», sized to protect the plain for the long term against an extreme flood.

The measure costs CHF 368 million excl. VAT. Ruppen told RTS that this figure has to be weighed against «2,2 milliards de francs de dégâts potentiels» from extreme floods in the region. The trade-off is just as clear: every hectare of riverbed is a hectare no longer farmed, built on or zoned for anything else.

One strip, many users

That land question was worked through, not avoided. Valais decided in May 2024 to revise its part of the programme, and the take of arable land has since dropped to about 20 ha, roughly 40% less than the first design. Blick romand reports three new footbridges for walking and cycling and seventeen public stops along the banks. The strip also carries habitat. Sébastien Beuchat, Vaud’s director of natural resources and heritage, told RTS that «le Rhône pourra davantage divaguer et créer des milieux alluviaux riches en biodiversité»: the river will be freer to wander and form alluvial habitats. The same section serves as flood defence, promenade and floodplain.

Three levels, eight communes, one river

The cost split is as carefully designed as the cross-section. The Confederation pays 47–57%, Vaud and Valais 20–25% each, and the communes 2–3%. Eight communes share the stretch: Bex, Ollon, Aigle and Yvorne in Vaud, and Massongex, Monthey, Collombey-Muraz and Vouvry in Valais. Baublatt sets out the timeline: at least five years of appeals, permits and preparation, then about ten years of works, finishing between 2040 and 2045. The whole correction runs 162 km from Gletsch to Lake Geneva. Its works began at Visp in 2009.

The credit goes to people whose names rarely reach a headline: the Rhône 3 team (ECF-R3) and the water-resources division DGE-EAU in Vaud, and the DMTE in Valais. These hydraulic engineers, surveyors and land negotiators took a first 2 km widening at Yvorne/Vouvry (CHF 100 million) to public inquiry in December 2022. Vaud states openly that the existing dikes «présentent aujourd’hui des signes de fragilité». The teams’ answer is not a higher wall but a second path for the water. Looking back from late in this century, the networks that coped were the ones that built in spare capacity before they needed it.

←TODAY: CHF 368 million to widen 14 km of the Rhône from 60 m to 90 m. The file is open from 3 October to 2 November 2026.
→3012: In Zurich-3012, the river corridor is the plain’s oldest public building, and nobody remembers it as a flood work.
Fulcrum: Width bought now is spare capacity that a century of farmers, walkers and floods can all share.

Atelier: Landscape architects, civil engineers and planners working on the Chablais plain will feel this cross-section in their site levels, access routes and zoning for the next fifteen years. If an AI assistant summarises the French dossier for you, check every level and width it quotes against the drawing. Monday move: have one person load the inquiry sections for your nearest commune into the office model as a locked reference layer.

Hack: Compare how much water a 60 m bed and a 90 m bed carry at the same depth, using Manning’s equation for a trapezoidal channel. The inputs below are illustrative. Replace them with the levels, slope and roughness given in the inquiry file.

n, S, h, m = 0.035, 0.001, 5.0, 2.0   # roughness, slope, depth m, bank slope (illustrative)
for b in (60, 90):
    A = (b + m*h)*h; P = b + 2*h*(1 + m**2)**0.5
    print(b, "m bed:", round((1/n)*A*(A/P)**(2/3)*S**0.5), "m3/s")

With these illustrative inputs, not the project’s design values, capacity goes from about 830 to about 1,220 m³/s. That is nearly 50% more before a dike has to rise by a centimetre. In Grasshopper, the same formula can drive a section that you send to Archicad through the PAZ Grasshopper↔Archicad Library.

There is an information session in Aigle on 6 October at 18:00, but the file stays open long after that evening. Open it at vd.ch/rhone3 before 2 November and redraw one section at the scale of your own model.

Sources & Further Reading

• Primary: État de Vaud — Mise à l’enquête publique de la Mesure prioritaire du Chablais (Rhône 3)
• Reinforcing: RTS — Le volet chablaisien de la troisième correction du Rhône mis à l’enquête