Leaky dams, explained in one storm

Not a dam, a delay

A leaky dam is a few logs fixed across a small stream, with a gap underneath. In normal weather, the water passes under it and nothing changes. In a storm, the stream rises above the gap, and the dam holds part of the flood for a short time before letting it go.

One dam does almost nothing. In the Haut-Lône headwaters we built 146 of them in 2024, spread across the small streams of the plateau, with wet woodland planted along the banks. Each one adds a little delay, and the delays add up.

One storm, two gauges

We read the effect with gauges above and below the dams and compare each storm with the record. Since the dams went in, the flood peak reaches the bottom of the headwaters 3 h 40 min later than before.

For the valley downstream, that is time to close flood gates, move cars and warn residents before the river arrives. Leaky dams do not stop a flood. They move it, which is often what a town needs most.