Flood peak attenuation
Some dams are designed to attenuate flood peaks, that is to say reduce the maximum flow of the river during a flood. Normally empty at the arrival of the flood, they fill up to prevent the water from continuing its course. The volume thus temporarily stored is returned to the river after the flood has passed.
These developments often go hand in hand with embankment systems to directly protect inhabited areas located further downstream.
There are two types of such developments:
- Reservoir on the river course. The dam is equipped with an open sluice located at the base of the dam which limits the incoming flow while the reservoir fills up. The Villerest dam on the Loire thus makes it possible to attenuate major floods of the river.
- Reservoir fed by a canal diverting water from the river. The territorial basin public establishment "Seine Grands Lacs" manages three large reservoirs on the Seine, Aube and Marne rivers which contribute to the system protecting in particular the Île-de-France region and Paris against major floods of the 1910 flood type.
These structures are designed for a flood level known as the protection flood. For all floods below the protection flood, the downstream areas experience only limited flooding.
Warning: if the actual flood is greater (in flow, in volume) than the protection flood, the reservoir fills up and the flood flow passes through the surface spillway: the flood is no longer attenuated, with the flow in the river also increasing very rapidly.