Flooding: What consequences does frost have on the dikes?

The flood situation in Lower Saxony remains tense. But after the constant rain, a change in the weather now brings freezing cold. What consequences do snow and frost have on the dikes?

The flood situation remains tense in several regions of Lower Saxony. Thousands of emergency services, such as those from the Technical Relief Agency (THW), fire departments and the police, are still on duty because of the flood, said a spokesman for the Ministry of the Interior in Hanover.

The situation in Oldenburg is so explosive that the authorities are developing procedures for a targeted opening of the dike on the Hunte in an emergency – if necessary by blowing it up. There are therefore two scenarios: In the first, an excavator that is operational around the clock is supposed to dig an emergency opening in the dike so that the water can drain into the Osternburg Canal. However, if the dike is too wet from the water and can no longer be accessed by the excavator, targeted blasting could be carried out in the area.

Water will recede

But there is relaxation in sight. The water levels on rivers in Lower Saxony could fall in the coming days. A tendency towards falling water levels is expected, said Anne Rickmeyer, director of the state agency for water management, coastal and nature conservation. However, it could still take several days or even a week until the water levels are below the critical level.

In Bremen, where there is a warning of major flooding in the Wümme area, the situation remained static on Saturday night, according to the fire department. There were no corresponding operations. Nevertheless, one cannot speak of a relaxed situation, said a spokesman.

After continuous rain now frost

The falling temperatures could now mean an important change: after the continuous rain, a change in the weather brings freezing cold to large parts of Germany. According to the DWD, there will still be rain showers in the center of the country and in the southwest today, while snow will fall in some areas in the rest. There’s a lot of noise in Lower Saxony today NDR Maximum temperatures around freezing point expected during the day. There will be moderate frost at minus four to minus seven degrees tomorrow night. For the following days, meteorologists expect light permanent frost, mostly during the day.

But what does that mean for flood areas? Does the frost also bring hope that the dikes will become more stable and the danger will therefore be reduced? And what do the sub-zero temperatures mean for buildings or agriculture in the affected regions?

“Just a building block in the Overall rating”

In Saxony-Anhalt, the announced permanent frost could be helpful in fending off flooding: “That will play into our hands,” said a spokeswoman for the Mansfeld-Südharz district disaster team. The dikes would solidify in the frost.

Other experts are more critical with regard to the dikes. It cannot be said in general to what extent frost has a positive or negative effect on dike stability, said the director of the Lower Saxony State Office for Water Management, Coastal and Nature Conservation (NLWKN), Anne Rickmeyer, to the dpa news agency. “In order to maintain flood protection, weather conditions must always be taken into account and are only one component in the overall assessment of the situation.”

Positive for tightness

If a dike is exposed to frost without any water effect, the soil will solidify at a certain depth – depending on the temperatures and the duration. “This can have a positive effect on stability and tightness if water accumulates.”

However, the situation is currently different in many places: water levels are still high in some places and masses of water are pushing against dikes. “If there is relatively warm water on the dike before the frost period, the area below the water level will not freeze and no positive effects will occur there.” Above the water, however, the earth’s surface of the dike will freeze and prevent further precipitation from penetrating. “That would then be seen as positive.”

Water is “fixed” by frost

Other experts, such as aquatic ecologist Christian Wolter, also expect another positive effect: If snow falls instead of rain or water freezes, it will be “fixed” and will not lead to a further increase in flooding. “The frost will ensure that less water flows in. So the flood situation should ease,” said the expert in an interview tagesschau.de.

Concerns for homeowners

Homeowners, on the other hand, are very worried about the combination of flooding and frost. The masonry can be damaged if it is damp. “When water freezes, it expands by ten percent,” says Norbert Gebbeken, an expert in structural engineering at the Bundeswehr University in Munich. “And this expansion can create such a high pressure that material or components are really destroyed” – especially if parts of the building are already soaked with moisture.

However: “Moderate and not long-lasting frost will not cause standing water in the basement to freeze,” says Christine Buddenbohm from the Central Association of the German Building Industry. The depth to which frost penetrates is deeper in the mountainous south than in the north. In the middle of Germany the depth is 80 centimeters. Homeowners should still pump out full basements if possible, but at the same time take other risks into account.

“Soils need frost”

Agricultural experts have mixed feelings about the current situation. “Frost is usually a blessing for our soils,” says Christoph Tebbe from the Thünen Institute for Biodiversity in Braunschweig. The normal winter frost is good because it loosens the ground. “Our soils need frost.”

But things look different when the arable land is completely submerged in water at very low temperatures, explains Tebbe. “The soil then doesn’t regenerate over the winter and it is too dense.” It would then no longer be able to absorb as much water in the spring. Ultimately, this could affect harvests.