Dam and dyke maintenance measures on the river Inn as an improvement to flood protection

The Inn was and is known as a mountain river with rapid and extremely stormy floodwater. In connection with the use of renewable energy from hydropower, the construction of reservoir stages and the associated dams and dykes also establishes floodwater protection. In order to best guarantee the protective function and security of this dam and dyke construction, on the one hand, VERBUND employees regularly conduct maintenance and service measures. However, on the other, this also has the effect that the dam areas continue to remain popular recreation and leisure areas for the local residents, characterised by species-rich flora and fauna. Extensive ecological dam and dyke maintenance concepts are currently being developed in close consultation with the nature conservation authorities in order to continue to ecologically enhance the dams and dykes.

"Our employees regularly control the dams and dykes on our Inn reservoir stages," said Klaus Schöler, group manager of the 13 VERBUND Inn power plants. "They observe the possible flow abrasion caused by the river, the dyke vegetation, the dyke crest and the bank slope, analyse effects of activities of burrowing animals and maintain the associated seepage trenches and passages. Thus, the security of the dams and dykes is not only guaranteed during normal operations, but especially during time of flooding."

A precious area of alkaline grassland

The maintenance measures primarily comprise the removal and thinning out of wooded areas in the winter months, which is carried out with consideration for bird life outside the bird breeding season. VERBUND employees give special attention to the precious plant habitats. The regular mowing, but also the initial mowing that takes place very late in the year, leads in the long-term to species-rich areas of alkaline grassland, which are meanwhile ecological important.

With the maintenance measures carried out to date, sites have already been created for orchids from the originally bush-covered areas, such as e.g. the Miliary Orchid, the Marsh Helleborine or the Heath Spotted Orchid, but there has also been the creation of places in our cultivated landscape for rare bushes, grasses and other flowering plants.

In order to combat non-native plants, which crowd domestic plant species out, such as the Himalayan Balsam, for example, it may be necessary to carry out mowing more intensively during several periods of growth. "However, this is only limited to subareas," says Schöler and expands upon this by saying: "If possible, we are striving to establish alkaline grassland here. This is nutrient-poor and suppresses the growth of woody plants. In addition, they represent replacement sites for plants specific to wetland areas, namely, the stinging nettles, which are no longer encountered so frequently in the wetlands."

Vegetation of dams through shrubs and plants

Since, in the case of flooding, the growth of woody plants can be responsible for damage to the dam body, this is not permitted on the waterside of the dam. It is for this reason that there are no woody groves to be found on dams and dykes. They would put too much strain on the building structure and thus endanger its stability. Tree roots can generate preferred seepage paths and damage the impermeable core and blanket of the dyke. That’s why tree populations are only permitted far removed from the foot of the dyke e.g. at the drainage channels.

However, in order to provide relief in the landscape and to create the possibility of places of refuge and breeding areas, VERBUND has deliberately left several bushes in the upper region of the dyke, on the landward face. Recreational athletes and those seeking relaxation thus enjoy this diverse natural landscape along the course of the river.

Comprehensive ditch systems for stability and drainage
So-called seepage ditches play an important role in dam stability. Schöler explains: "These are ditches along the foot of the dam, which safely channel off the seepage water that can leak from the dams and dykes as a result of flooding, and thus ensure a constantly safe dam body."

It’s not only the dams, but also the landside areas behind the dams and dykes that need to be safely drained so as to prevent damage to the buildings and properties of local residents. Extensive ditch systems have thus been built, leading the surface water to pump stations, from where it is then pumped back into the river once more via the dams and dykes.

As a basic principle, the discharge capacity of the entire ditch system must not be impaired. It is for this reason that it is regularly maintained by VERBUND employees and cleansed of mud and deposits. In the process it is also necessary to remove larger foreign bodies in order to prevent it from lying across the drainage cross-section. "It can also be the case that barriers erected by beavers have to be removed at times in order to ensure secure drainage," says Schöler. "However, this only takes place in close consultation with the regional nature conservation authority."