2023-08-20 05:13:54
What can you do “organisationally” to counteract a flood, the Mühlviertel communities asked themselves after the flood disaster of 2002. At that time, the calm Aist swelled into a torrent and caused enormous damage. According to Gutau Mayor Josef Lindner (SPÖ), who has been in office since 2003, each municipality will not be able to do anything on its own. 27 municipalities joined together to form the Aist flood protection association. Lindner is the chairman.
In the catchment area of 640 square kilometers, 84,000 people are potentially affected by flooding from the Aist with Feld- and Waldaist and their tributaries. The region is concerned with protective measures for more global precipitation events as well as for local severe weather. In the case of small-scale events, it is difficult to create an overall solution because “you don’t know what will happen when and where,” says Lindner. The aim is that less water runs off the surface. The mayor, who is also active in the fire brigade, can confirm that the local heavy rain events are becoming more and more frequent and massive. Likewise, drought, which blocks the absorption capacity of the soil, is an issue in the Mühlviertel.
The association consults with the torrent and avalanche control as well as the water district Linz as technical support. The funding track is the Hydraulic Engineering Funding Act. The highest level of protection is expected from retention basins, which are supposed to “curb the source effects of the floods and absorb flood peaks,” says Lindner. A need for three large reservoirs designed for HQ100 (100-year flood) was identified for said region.
But these large pools are controversial, especially the local “Initiative for water management, climate and nature conservation” pursues a different philosophy. She wants to keep the water in the region through small retention landscapes with domestic ponds, infiltration troughs, cisterns and green roofs. “Two-thirds of the farmers here have stopped keeping livestock, there are large manure deposits everywhere that could be used for roof water,” says chairman Fritz Robeischl as an example. So it’s not just about “briefly holding back flood water in large retention basins” but also about measures to counteract the impending water shortage. Instead of building concrete walls “across the Aist,” one had to “hold back the water to the roots of the flood, to the streams”. Water bodies in the region would cool and bring “water vapor upwards, which promotes new precipitation,” Robeischl continues. Because, he is convinced: “We will still ask and pray for every cubic meter of water.”
In the course of the discussion about the “right protection” – the first large retention basin Poneggenbach near Schwertberg was completed in 2015 – the association commissioned a study from the Grieskirchner civil engineers Thürriedl & Mayr for the north of Freistadt in 2016, which should clarify what it would mean building several small pools instead of large ones. “That would only reach 15 percent of the catchment area. That means only 15 percent of the water would be held back, 85 percent would continue to flow. That’s not effective,” says Lindner, reflecting the result.
Two retention measures are currently planned north of Freistadt and between Kefermarkt and Freistadt. The planning for the basin in Rainbach (Passberger Steg) on the Feldaist with a capacity of 250,000 cubic meters is well advanced, and the majority of the landowners have given their approval. With the other Grünbach project, the association is still in the middle of negotiations for the landowners’ declarations of consent.
At the same time, the statute of the association states that the sealing in the places should be restricted. So it is obligatory for the member communities to create small catchment basins when building settlements. Furthermore, “places with a view of the surface water must be designed accordingly, in order to bring as little water as possible into the drain,” says Lindner. Open spaces along the streams are also “respected more”.
The Gutauerbach, a tributary of the Waldaist, lies in its village of Gutau in masonry from the 1970s. The regulation cannot be reversed, but a rake was installed above the village, which stops the material being carried, but still allows the water to pass. In the case of the Waldaist, on the other hand, because of the very sparse population, it is sufficient to protect a few properties directly on the river line, i.e. “local property protection” is sufficient. According to the chairman, between 2009 and 2014 almost six million euros were invested in various small building measures in the region of the flood protection association.
The association has also drawn up a forest concept with the aim of switching from spruce to deciduous forests in order to increase seepage in the forest again. Spruces have shallow roots, but they can quickly fall over on banks. This leads to blockages along the streams and rivers. And bridges were also built over the “structurally restricted” streams, Lindner describes the situation. After the heavy rain comes the water that carries the spruce trunks with it. Blockages develop, the water accumulates, at some point the masonry breaks and the wave comes. In 2020, for example, the Stampfenbach caused massive damage in the Gutau and St. Leonhard areas when 114 liters of rain per square meter fell within a very short time. According to Lindner, a project to improve protection for this area is being implemented.
Long-term protective measures also include land management measures. So that the soil again has “greater chances of absorbing more water”. Due to the compaction of the soil, not least due to the cultivation of the fields with heavy equipment, there is less seepage and increased runoff into the streams and rivers during heavy rain, Lindner wants to briefly address a sensitive topic.
The “debris management” of solids lying on the ground is also important for flood protection. In the municipality of Schwertberg, a sand trap was built in 2023 together with the water district of Linz, which makes it easier to remove the debris and offers more protection for the communities downstream.
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