Facts: Heavy rain and extreme weather
During a downpour, a lot of rain falls in a short time. The rain is experienced as fierce and heavy.
SMHI’s definition of downpour is at least 50 millimeters in one hour or at least 1 millimeter in one minute. Almost all downpours occur in summer.
Extreme precipitation refers to amounts that significantly exceed normal, for example, during a month or in a day or an hour.
Source: SMHI
In the future, Sweden is expected to experience more floods caused by torrential rain, high flows in waterways and high sea levels.
Already today, extreme rains are more frequent than before, and in the year 2100 they may be 20-40 percent more common than today, calculations from SMHI show.
In order to adapt Sweden to more and more intense extreme rains, we need to take major measures, and the preventive work will be central, according to Malin Mobjörk, PhD in environmental change and deputy head of department at Formas, a government research council for sustainable development.
An important part is about urban planning.
— In many cities we have seen a great densification and we have valued building close to the beach. It leads to increased vulnerabilities, she says.
Anders Wörman, professor of watercourse engineering at KTH. Green cities absorb water
Among other things, cities need to plan to divert bodies of water to places where the water can be collected. Taking care to preserve and create green areas in a city can be crucial in dealing not only with heat, but also with heavy rain.
— In a city with very hard surfaces, it becomes more difficult for large amounts of rain to get anywhere. But a city that has a lot of vegetation – the green city – can both moderate the heat but also absorb water, says Mobjörk.
In Norway, parts of a dam at the power plant in Braskereidfoss, northeast of Oslo, collapsed during storm Hans.
An important part of preparing for the extreme weather of the future involves sizing hydropower dams so that they do not collapse, according to Anders Wörman, professor of watercourse engineering at KTH. Here, climate change is already taken into account when building.
“When you have determined a design flow at hydropower plants, you can add a certain safety margin to try to take into account the expected change in these extreme flows,” he says.
Malin Mobjörk, doctor of philosophy in environmental change and deputy head of department at Formas, a government research council for sustainable development. Two different wallets
According to Wörman, a difficulty when carrying out the climate adaptations is deciding what safety margin to have.
Even the division of responsibility – and who is to pay – is not always obvious. An example concerns embankments, something that is used in, for example, Kristianstad and Karlstad.
Here, a difficulty is weighing the cost of implementing an embankment against the risk of a flood occurring and the costs that arise then. According to Wörman, the fact that it is a question of two different wallets can cause it.
— If the flood exceeds, it is the insurance company, and by extension those who pay the insurance premium, who pay. When the embankment is made, it may be the municipality that takes the measures. Then it goes on the tax bill. It can make it difficult to make economically rational decisions about such a measure, he says.
Reduced risk at Slussen
But even if much remains to be done, much is already being done today to reduce the risk of flooding. Anders Wörman takes the rebuilding of Slussen in Stockholm as a current example.
Here, they have also created a wider flow path for the water, which makes it possible to drain larger amounts of water from Mälaren to the Baltic Sea. Something that reduces the risk of flooding in the entire Mälardalen.
“The common man probably doesn’t think about the fact that there has become a wider, free flow path under the bridge. But it has been a very important part of the climate adaptation for the city of Stockholm, explains Anders Wörman.