Down by the Inn, that’s where the corn grows highest. Three meters and in some cases even higher, says one of the farmers in the fire station in Vogtareuth, a small community on the eastern bank of the Inn, about halfway between Rosenheim and Wasserburg. How high will the mud and sand in the fields be down there? What other substances will the water bring with it? And what will it mean for your forest if around a tenth of the entire community area is flooded?
These are the questions that many landowners and forest owners in Vogtareuth are asking themselves and which they are now addressing here in the fire department hall to all the officials and scientists from Munich, Augsburg, Rosenheim and Kassel. That evening they will present to them the “Inn Study”, which the Free State and its State Office for the Environment (LfU) ordered from the Technical University of Munich ten years ago and published for the first time this year. Vogtareuth plays a leading role in it. At some point it could be decided here whether Wasserburg and Passau and many places in between will sink into the floods during the next big flood or not.
A flood polder at the Feldkirchen barrage in the Vogtareuther Au could hold more than 17 million cubic meters of water, around a tenth of the Sylvenstein reservoir on the upper Isar. If the floodplain were to be deliberately flooded over a length of six kilometers during an extreme flood, this could take away the peak of the wave and reduce the water level a few kilometers further in Wasserburg by 55 centimeters. Even for Passau, where the Inn flows into the Danube, that would still be 25 centimeters. This is what the scientists from the Technical University of Munich and their colleagues from Kassel and Vienna calculated in their models. “Feldkirchen is perhaps the most burning project,” says the responsible department head from the LfU.
The researchers considered many possible locations along the 210 kilometers of the Bavarian Inn from Oberaudorf to Passau and along the 70 kilometers of the Salzach from Freilassing to Burghausen and examined ten of them in more detail. Vogtareuth remained their favorite, followed by a polder near Inzing with a capacity of almost 63 million cubic meters, which, however, was just before Passau and would therefore be of no use to all the other places above.
The LfU and the Ministry of the Environment emphasize that the new Inn study has not yet been planned. However, the result does not surprise anyone in Vogtareuth. Mayor Rudolf Leitmannstetter says he was presented with practically exactly the same thing in 2014, the year after the most recent major flood on the Inn and Danube in June 2013. The damage in Bavaria was estimated at 1.3 billion euros at the time. Years ago there was a rough cost estimate of around 50 million euros for a flood polder in Vogtareuth.
How often such a facility would be used is unclear. The benchmark for common state flood protection measures is the so-called hundred-year flood, i.e. a flood that statistically occurs once every hundred years. This means that all places on the rivers should be able to cope without polders. But experts have long been expecting more frequent extreme events. The polders are intended to defuse floods that, according to current statistics, are considered to occur every 300 or even a thousand years.
Before such a polder is actually built near Vogtareuth, Inzing or elsewhere on the Inn and Salzach, the Free State wants to try to better use the 15 barrages on the Bavarian Inn operated by the Austrian electricity company Verbund for flood protection. If water is released in a controlled manner from the weirs in good time before the wave, this can also bring a few crucial centimeters downstream – even significantly more than on the Danube, where the gradient is smaller. And all these weirs already exist. “The infrastructure is there,” says a representative of the Ministry of the Environment in Vogtareuth. On the other hand, there are many measures that are ready for construction, but not all of them have the necessary money. What will happen next on the border rivers Inn and Salzach will have to be discussed with the Austrians and the power plant operator Verbund in the coming months and years.