Elevated crossing over a ravine

A rural road crosses the ravine at grade, on a low embankment. During floods this ford behaves like a dam: it holds back the water, is overtopped and is cut off, just when it is most needed as an emergency route and a link with the neighbouring towns. Rebuilding it as it was would bring back the same geometry and the same weak point.

The starting decision is not to rebuild the ford but to replace it with an elevated structure that leaves the channel section clear and keeps the road open during a flood as well. The crossing no longer obstructs the flow and no longer acts as a single point of failure for the road network.

The outline solution proposes a deck of around 50 m in length and 10 m in width, some 500 m², with two lanes and a segregated cycle and pedestrian path for people on foot and by bike, who currently use verges and tracks. It is designed as a skewed slab on deep foundations, piles or micropiles with pile caps, because the structure bears on the channel and has to withstand scour. These dimensions are indicative: the design will set them.

The final sizing is determined by the hydraulic study. The design calculates the 25-, 100- and 500-year return-period flows, calibrates them against the October 2024 DANA floods and uses a two-dimensional model to check the freeboard and clearance under the deck, and that the new structure does not raise water levels upstream. The river basin authority’s permit is sought on the basis of that study.