Construction and operation of dams

There are 74 gravity dams in Germany, but only one arch dam (Ofenwaldsperre, 27.50 metres high), one arch gravity dam (Okertalsperre, 75 metres high), one arch row dam (Linachtalsperre, 34 metres high) and one pillar dam (Oleftalsperre, 59 metres high). The main reason for this is that arch dams, for example, require special terrain conditions (steep slopes with a high load-bearing capacity), which are very common in the Alpine region but are rarely found in the German low mountain ranges.

A typical gravity dam is the 40 metre high dam wall of the Möhne dam. Its cross-section is almost triangular so that the horizontal force of the water can be optimally transferred into the bedrock using the dam's own weight. The arched ground plan of the wall also compensates for temperature-related expansions caused by movement.

In contrast to dam walls, dams are dams made of rock fragments or earthworks, which usually consist of the supporting parts and a sealing part. This seal is placed either on the water-side embankment or inside the dam. Dams made of a single material that seals and supports at the same time are generally only built for small dam heights or dykes.

The Ruhrverband's Sorpe dam, which went into operation in 1935, was one of the first dams with a concrete inner seal. With a dam height of 69 metres, it was the highest dam in Germany at the time. One of the first dams with a surface seal is the 58 metre high dam of the Henne Dam, which went into operation in 1955. This construction method was used at 50 dams in Germany.

Bottom outlets & service outlets

The water stored in reservoirs is used for various purposes - for example, to ensure a certain water flow in the underlying body of water or to generate energy. For this reason, dams usually have several closable outlets. The so-called bottom outlets, which lead directly into the lower reaches of the dam, are essential for the operation of a dam. This lower course, i.e. the body of water that continues to flow below the dam, must never be allowed to dry out completely, as otherwise animals and plants would not be able to survive there. In addition, the dam can be completely or partially emptied via the bottom outlets if repairs need to be carried out, for example.

The bottom outlets of large dams have two closures. One of these, the so-called regulating device, is used to control the discharge. The amount of water that is to flow through the pipe can be precisely set here. The second valve provides additional safety. It can be closed if the control element needs to be repaired or if problems occur with it. A typical control device is a ring valve, typical shut-off devices are wedge gate valves, butterfly valves and ball valves.

In addition to the basic outlets, dams can also have so-called operating outlets, which serve the further functions of a dam. These include, for example, pipes that transport the dam water to a hydroelectric power station for power generation or drinking water pipes from the dam to the drinking water works.

Flood relief

The main function of a dam is to store water. During floods, however, the reservoir space may not be sufficient and more water may flow into the dam than it can hold. However, dams in particular must not be flooded under any circumstances, as the water flowing over the dam would wash out the dam and destroy it within a short time. But dams should also not be flooded in an unplanned manner. For this reason, every dam must have a spillway to safely discharge excess water.

In the case of dams, openings are provided in the wall through which the water can run off. The wall surface is reinforced so that it can withstand prolonged overflow. At the foot of the dam wall, a water basin collects the outflowing flood water and drains it into the lower course without causing any damage. In order to protect dams from overflow under all circumstances, the spillway is usually located on the side of the slope. An inlet channel collects the excess water and guides it through a discharge channel to the foot of the dam. There is also a water basin here, which absorbs the water's energy and drains it away.

The Biggetal dam has a special feature: A 50 metre spillway tower rises out of the water in front of the dam. If necessary, it channels the excess water into an underground tunnel and from there into the lower reaches of the dam.

Dams are perceived by the public as technical structures in nature. They are often seen as recreational and leisure facilities and their technical purpose fades into the background. The fact that dams store large amounts of energy and therefore pose potential dangers is often not recognised by the public. Dams and reservoirs are considered safe. It is important that the structural and operational safety of the dam is guaranteed at all times. An essential aspect of the safety of dams, reservoirs, barrages and sedimentation basins is dam monitoring.

Due to the considerable risk potential, dams must be constructed, maintained and operated in accordance with the relevant laws and regulations and the generally recognised rules of technology. As the operator of eight dams with a total volume of 463 million cubic metres, five barrages and a number of sedimentation basins, the Ruhrverband is therefore also obliged to carry out regular dam monitoring.

Depending on the class, dams are usually monitored using sensors that measure various parameters such as water level, pressure, temperature and movement. These sensors are usually installed in the impoundment and send continuously recorded data to a database system, which can be used to assess the size of the recorded data and therefore the safety of the system, taking various variables into account. At particularly safety-relevant measuring points, an alarm is sent to the on-call service at any time of day or night if a specified value is exceeded. All automatically collected measured values are compared with the respective static or dynamic alarm values when they are imported into the database. If the corresponding alarm value is exceeded or not reached, an e-mail or text message is sent to the operations centre management. If necessary, appropriate measures can be initiated to ensure the safety of the system.