A rare sight: The total emptying of the Schlegeis Reservoir

On 2nd March 2026, VERBUND completely drained the Schlegeis Reservoir, belonging to the Zillertal Valley power plant group, to inspect all submerged plant components and carry out any necessary maintenance works in accordance with a government regulation.

The Schlegeis Reservoir in the Zillertal Valley, which belongs to the VERBUND plant group, is one of Austria’s largest reservoirs and has a storage volume of 126.5 million m³ of water with a maximum depth of around 116 metres. To ensure safe operation, it must be drained at defined multiannual intervals as stipulated by the water authority. The last complete emptying took place in 2013. This is usually done in the winter months, when there is little natural inflow. This makes it possible to inspect underwater plant components and carry out maintenance works as needed.

Total reservoir drainage requires conscientious and long-term planning. The water level was slowly lowered weeks in advance, with the water being lowered via the turbines as planned. The amount of residual water that could no longer be processed through the turbines was drained via the bottom drain. On the morning of 2nd March, it was gradually opened up to ensure that the impact on flora and fauna was minimised. With a flow rate of about 12 cubic metres per second, emptying was not complete until the afternoon. In the next few weeks, all structures that are constantly under water during normal operations, such as inlet structures and the headrace channel, will be cleaned, inspected and maintenance work will be carried out. The hydraulic steel components will also be closely inspected.

Once the maintenance works have been completed, the bottom drain will be closed again at the end of March 2026 and rebuilding will start. As a result, the components will disappear below the surface of the water again for years to come. Depending on rainfall and snowmelt, it will take a few weeks for electricity generation to restart. By the late summer, the Schlegeis Reservoir will be full again, in keeping with the season.

The dam wall: 725 metres long, 131 metres high, and well monitored

Work on the reservoir starts immediately after draining. "We have to remove the deposits in the space in front of the bottom drains," explains VERBUND Plant Group Manager Marco Fiegl. The more than 700-metre-long and 131-metre-high dam wall can now "relax" for a few weeks. The slope of the wall changes depending on how much water presses against the concrete arch. This is shown in the measurements inside the wall. This is continuously monitored with sensors and measuring points. "Dams are one of the best monitored structures in Austria. The dam wall has around 1,000 measuring devices. The most substantial of these are the geotechnical pendulum systems," says Fiegl. The data from the Schlegeis dam wall is evaluated around the clock in the central control room in Mayrhofen. There, employees can respond immediately if something unusual happens.

Ecological monitoring of the draining process

The project is supported by an environmental building inspection body and supervised by comprehensive monitoring measures. Ecological monitoring started months before emptying and will only be completed around a year after draining.

During the draining process, no sediments are removed from the reservoir, and instead only the sediments that are mobilised by the water’s natural dragging power are transported onward.