The flood relief comes into effect when the water level of a barrier structure exceeds the target - mostly due to increased, out of sequence water inputs. It protects the dam or the dam wall from extraordinary loads and is intended to prevent the water from overflowing over the top of the structure, since in the worst case both could lead to the failure of the dam . The flood relief must be designed for the greatest possible flood ( design flood ).
The flood relief consists of an inlet, a forwarding (also transport) and an energy conversion structure. For forwarding an often spillway chute used for energy conversion usually the stilling basin of the main outlet of the barrage.
In the event of an attack , the water is drained off in a controlled manner via the dam system or to the side after it has reached the destination. In the case of dams, the overflow is usually designed as a side drain next to the barrage and founded on natural (i.e. undisturbed) soil. This is mainly because an overflow over the dam has to be constructed more difficult because of the expected settlement of the embankment, since it is mobile. After the overflow, the excess water is drained through a channel (often designed as a cascade ) on the side of the dam into the drain of the dam. Once at the bottom of the valley, the water usually flows through a stilling basin , where the kinetic energy of the water is largely converted: the water is deprived of its high speed before it enters the flowing water of the underwater .
In the case of dam walls, the flood is drained more often than in the case of dams via the barrage. With this type of flood relief, the excess water flows over the top of the wall at predefined points, or openings are provided in the wall. The water is then led down on the air side of the wall and reaches the stilling basin in the area of the wall base.
Flood relief tower
Another type of construction is the flood relief tower. This is located in the storage space of the storage system and often has an overflow in the form of a hold-up funnel . For this reason it is also known as a chalice, tulip or trumpet. If the water level of the dam exceeds the destination, the excess water flows into the overflow. The water is fed under the dam through the tower and a connecting pipe or tunnel system into the drainage of the dam.
Active flood relief
In contrast to the attack and the flood relief tower , in which the flood can only be drained with the help of the flood relief when the storage target has been exceeded, there are also systems that can be described as "active" flood relief. In the case of active flood relief, the inlet structure is below the waterline of the dam target and is normally closed by a gate valve.
This form of flood relief is sometimes used for dam walls, with the inlet structure often being integrated into the barrage. If there is a threat of flooding, the lock is opened and the water flows through pipes in the dam to the air side, where it is directed to an adjoining jump via a chute. The jump throws the water in a high arc into the stilling basin , from which it flows into the natural water.
The intake structure can, however, also be built into one of the two valley flanks surrounding the dam. In this case, after the gate valve has been opened, the water flows through tunnels to the other side of the dam, where it is also fed into the natural waters via a chute and a subsequent jump.