Mr. Böttcher, some cellars in Bremen and the suburb of Lilienthal were already full, even though it didn’t feel like it was raining yet. Where did all the water come from? We saw that some houses near the dike were under water, even though the streets in front of them were dry. It is possible that the water can also come through the sewer system. As a rule, it rained heavily beforehand. In Lilienthal, 30 to 40 liters per square meter of rain fell across the area within 24 hours. That alone is enough to flood some basements. However, the water usually only comes from below when it seeps into the soil over a long period of time.
What is the point of dike protection if the groundwater on the other side endangers residential areas? Such a situation occurs when it rains a lot in the affected region, but the water levels of the river have not risen to the point where the polders on the dike are flooded . But dike protection has nothing to do with it.
But? That depends very much on the soil. For example, water cannot seep down as easily through layers of clay. It sometimes collects in this layer a few meters or even centimeters below the surface of the earth. During prolonged rainfall, the ground above is quickly saturated with water.
How long does it take for groundwater to reach the surface? This cannot be said in general terms. In some places it takes three hours, in other places it takes ten days, in other locations it never happens.
How can you protect yourself from rising groundwater? Protection actually occurs when the house is built. In areas threatened by groundwater, houses are built in tubs or on a terp. Of course, there are also houses that are 50 years old and come from a time when no one took this problem into account or there was little knowledge about the nature of the soil. These owners then have to think about how to renovate the floor slab and masonry of their basement. Of course it’s not that easy. There may be cracks in older buildings. Pumping systems that collect and extract the water from the outside of the house can sometimes help. But it’s all very complicated.
Can it be predicted when the floods will recede? This depends on the flow speed and gradient in a region, but also on further rain. The slower the water moves in a river or stream, the longer the flood lasts. A completely different level of protection is required in the Ahr Valley than in Lower Saxony on Aller, Leine or Hunte. Within 36 hours, 300 liters of rain per square meter fell on steep terrain with high flow rates. The flood drained away again after a short time, but left enormous damage behind.
Flood protection systems and dikes in Lower Saxony, on the other hand, must be equipped so that they can keep the water out for one to two weeks. It becomes dangerous if there are burrowing animals at work or trees growing in the dike. The more holes in the dike, the worse it is at keeping out flooding.
The fire brigade in Bremen-Borgfeld said on the second day of the flood that the river levels were slowly falling, but the overall flood level was stagnating. How can this be explained? The ground has become so saturated with water that the groundwater can no longer escape into deeper soil layers. In order to reduce the flood level, it should no longer rain at such times. The water then has to flow away via rivers.
And how likely is that? We are now expecting a period of high pressure. The very heavy rainfall is now over. The levels may then fall in the next five to ten days. But then the winter weather comes upon us…
… and is that good or bad? Where the helpers have previously been active in double-digit plus temperatures, the temperatures will drop to minus ten degrees at night. The water that has not drained by then will freeze over in the next few days. Buildings that are standing in water and whose masonry has become waterlogged then suffer from the frost. This could cause further damage.
And if the ice melts, will the floods of the New Year be repeated? No. The water freezes from top to bottom. There is still water under the ice cover and it is slowly draining away. We can see this in puddles. After the first frost they look very dark. This is due to the water under the ice layer. Once the water has seeped into the ground, the ice layer has a milky glow and becomes thin and crunchy. There is a cavity under the ice puddle. With large areas of ice, this cavity does not form; instead, the area sinks with the water.
Can residents in flood areas still take out their skates? You should definitely not do this because the areas are unstable. Ice surfaces in flood plains are constantly working. The water underneath drains away, creating cracks in the ice that can break open unexpectedly. There is a danger to life in these areas.
People always say that climate change is threatening groundwater and now it is rising. What is true now? What we are currently experiencing in Lower Saxony, Thuringia and Saxony-Anhalt was already indicated by the climate scenarios 30 years ago. We have known for decades that climate change is causing drought periods to increase, as well as long periods of rain with higher amounts of precipitation and floods. And depending on whether it rains more or not at all, the groundwater level also rises and falls. We can also predict quite reliably that the situation will get worse in the future.
Germany must adapt to this and, for example, practice water management. This is also the reason why there will be the first German Climate Management Conference in Hamburg in September.
How exactly can we imagine this management? During periods of heavy rain, we have to store water in order to replenish the groundwater and moisten the soil. There will be very different solutions for this from region to region. When the groundwater has reached the surface, the soil is of course saturated. But letting the water drain away is also part of it. But the water is not the biggest problem.
But?The human being. We have to seriously consider whether and how we want to build in these floodplains. Whether insurance companies still pay for water damage there and whether it is still affordable. But the most interesting question is: Are we prepared to financially support those who knowingly want to live in risk areas and are not insurable?
What is your answer? In my view, withdrawal from these areas will be an important part of the adaptation strategy. The water stakes its claim on the areas and we cannot escape this forever with insurance and construction measures. We can’t afford that.
So there is no master plan like “How do I protect my house”? Of course you can build a dike around your property, build a terp, install pumps. But do you want and can you afford such expensive measures? In the end, there could no longer be any elementary insurance that is willing to insure high-risk locations and there may then be no more money from the state at these locations. But we will also see municipalities, communities and cities designating new flood risk areas and taking people to court because the value of their real estate and land is falling and they feel dispossessed. So there is still a lot to be discussed.
They also live in flood plains. Have you already decided what to do next for you? Our house is on a higher part that is not at risk. The flood is therefore not a problem for us. But I wouldn’t want to build in the valley either. When you build a house, you have to be aware of what to expect in certain areas. In some areas, floods are neither surprising nor unexpected.
