Heavy rain and flooding surprised Saarland and Baden-Württemberg this weekend. And new storms have been announced. Can sponge cities be part of a solution?
Sunken cars in the city, people saving themselves from the floods on the roofs of houses, rescue divers on duty – that’s what it looked like in some cities and towns at Pentecost. Saarland and Baden-Württemberg were particularly affected. In the next few days the rain will also move across Bavaria.
As early as 2017, the Bavarian state government commissioned the “HiOS” project – short for: “Surface runoff and flash flood information maps”. Scientists from the Technical University of Munich and the Ludwig Maximilian University (LMU), among others, should investigate how great the risk of flash floods is in the various regions of Bavaria.
The result: A good third of Bavaria is at high or very high risk from flash floods and flooding, especially in the Alpine region and in the southeast of Bavaria. A good quarter of Bavarian cities are considered to be at risk, and 16 percent are considered to be at very high risk.
“Sponge region”. flood plain
In order to reduce this risk, the Bavarian Ministry of Agriculture has launched the “Sponge Regions” campaign. The aim is to support municipalities in making settlement areas water-sensitive in order to make villages and cities climate-proof.
With the help of new drainage and rainwater retention basins, new floodplains or the unsealing of areas, more water will seep into the ground. Since many towns and communities are flooded after heavy rain events, there is an urgent need for action. The Free State wants to contribute up to 90 percent of the planning costs.
Redirection via pipe system
Aubstadt in Lower Franconia was flooded three times in 2018. The community in the Rhön-Grabfeld district lies in a valley surrounded by fields. During heavy rain, the water can flow into the town unhindered. Not only water and mud are washed into the area, but also fertilizers and pesticides from the fields, which then end up in the sewage system.
As part of the local redevelopment, the sewer network was initially separated. The wastewater from the houses and rainwater from the roofs and streets now run in separate systems. This relieves pressure on the sewerage system during heavy rain.
Since the renovation in 2018, the rainwater that used to run into the town has also been collected on the mountain and directed around the town via a pipe system. There the water is stored in rain retention basins. Here it can seep away slowly, or is delayed and passed on at a slow flow rate.
This has two big advantages. On the one hand, the groundwater can be replenished in this way, and on the other hand, less water arrives in the villages and towns downstream, which reduces the risk of rapidly rising water levels.
Aubstadt in Lower Franconia was flooded three times in 2018. Now the city has, among other things, built rain retention basins in an attempt to better cope with heavy rain events.
Climate resilient Villages and landscapes
Municipalities are preparing for heavy rain events. However, this requires cooperation between municipalities. The mayor of Aubstadt’s neighboring municipality of Herbstadt, Georg Rath, puts it in a nutshell: “If it’s not just us who do this, but many municipalities, then the water will arrive in Bad Neustadt or Bad Kissingen much more slowly and then the spa garden there may remain dry .”
A new system for Tree watering in the test
The “Sponge Region” campaign also aims to store excess rainwater to enable continuous irrigation during hot periods. The Bavarian State Institute for Viticulture and Horticulture (LWG) in Veitshöchheim in the Würzburg district has been researching the topic of “sponge city” since the 1970s.
It started with testing different plants and their water absorption capacity. Numerous green roofs were also examined. During heavy rain, these release the excess water with a delay, thus relieving the strain on the sewer network.
Now there could be a solution for trees in inner cities. The test series has been running for a few months. This is an underground reservoir in which water is collected during heavy rain. A kind of “sponge” is integrated into this reservoir as a wick material, which draws the water up to the plant. In this way, the vegetation on the surface could be supplied with water during dry phases. With the materials tested so far, a “water riser height” of 70 centimeters has already been achieved.
Underground storage facilities are intended to help ensure balanced irrigation for trees, for example in inner cities.
Waiting for summer heat and drought
Claus Prinz from the LWG is excited about the summer. “I’m really hoping for a hot, dry summer and as much evaporation as possible in my system so that I can then see how resilient this wick is even under extreme conditions because I want to measure its maximum performance.” The results, which the researcher expects in about two years, will show whether the system is suitable for practical use.