As far as rain is concerned, Hamburg Kummer is used to it. But not as in May 2018, when a so-called urban flash flood devastated several plots of land in Bergedorf-Lohbrügge. Some researchers believe that such heavy rain events could increase in the future. City hydrologist Andreas Kuchenbecker has commissioned Hamburg: Water to develop a heavy rain index - an instrument that could help protect against sky flooding.

ZEIT ONLINE: If it rains, then you just need to look at the puddles, it's already clear if the rain is strong. Why an index?

Andreas Kuchenbecker: You can see that the rain is strong. But not how strong - and how dangerous. The question is, for example, if the puddle you see can be the same. Or if she may continue to build up, because it will continue to rain all day long and the sewer system will reach its limits. With our index, we divide heavy rain into hazard levels, which works much like the wind force scale.

TIME ONLINE: And then?

Kuchenbecker: Then anyone on the Internet can see where it is raining and how big the risk is.

Andreas Kuchenbecker © private

ZEIT ONLINE: How do you know how strong it is raining somewhere in Hamburg?

Kuchenbecker: We combine the data that our city-wide measuring points collect with the records of the rain radar operated by the German Weather Service. The radar measures continuously how close the rain falls. The collecting pots spark through every five minutes, which is just arriving on the ground.

ZEIT ONLINE: And when do I go home and ever get the bucket out?

Kuchenbecker: Well ... That depends on how your property is ordered. Did you take care of the backflow flaps in the drainage system? Are the gutters cleaned regularly? Is the basement protected against the ingress of water? Once the area on our map is no longer blue or green, there is cause for concern. This happens quite often: In the last ten years, we have measured 181 heavy rainfalls in the city of Hamburg, each eighth so strong that basements or underpasses could run full. If you convert them into the new index, then eight of them were even index 6 and stronger. You even have to expect a disaster situation.

ZEIT ONLINE: The highest value of the scale is 12. How often has it been reached?

Kuchenbecker: Fortunately, the upper end of the scale has never been measured in Hamburg. But that's happened in Münster: In 2014, there were enormous rainfall of 292 liters per square meter in seven hours, index beyond 12! Münster was largely flooded, a person drowned in his basement. That was the maximum water charge that clouds can theoretically absorb.

ZEIT ONLINE: Starting today, the index is officially online. Internally measured but since May. Have there been any records?

Kuchenbecker: Oh yes, and what kind of one. That was the day when I went on vacation with my family. The children behind were using the smartphone, the app with the index. We were already quite far away, when this storm developed, just above our house. Back they were really fun, I was getting restless. "I think it's going to turn yellow." "What?" "No, orange." "Honestly?" "Stop, now it's red!" The value went up to the really unlikely 10, all right over our neighborhood, and I already saw our stuff swimming. But the family stopped me from going back. Luckily nothing happened.

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Data protection

ZEIT ONLINE: So you have already taken care?

Kuchenbecker: Well, of course my job influences my priorities. That is also the main reason why we bother with the online index: make people aware that they need to arm themselves. So far, rain intensities were indeed specified in return times - mathematically correct, but somewhat difficult to understand. In heavy rain, the scale went off once a year and reached "once in more than 100 years". If someone says that a devastating heavy rain statistically occurs only once in 100 years or even less, then it sounds to lay people relatively reassuring, as if you would not have to expect in his lifetime. That is not so.

ZEIT ONLINE: You mean a "rain of the century"? Was not there just a few around Hamburg?

Kuchenbecker: Yes, exactly. Five in two years. The one in Niendorf, one in the harbor area, two in the surrounding area and then in 2018 the torrent in Bergedorf-Lohbrügge.

ZEIT ONLINE: Well, just what's changing in the climate. Could not it be?

Kuchenbecker: Could. In a long-term review. But first of all there is a much simpler explanation: Although the five-hundred-year rainfall is geographically located all in the Hamburg region, but strictly speaking, it was never the same place. The areas in which heavy rainfall occurs are often very limited locally, especially in Hamburg with its different landscapes and microclimate conditions. The estimation that such a rain will happen once in a hundred years is calculated for an area as small as the rain gauge. Even if a district sank in a heavy rain of the century class last week, it can happen to another part of the city the next day.

ZEIT ONLINE: Do you have the impression that we are already in the middle of change? Were heavy rain events more?

Kuchenbecker: We have been measuring and documenting the rain of Hamburg for a hundred years. So far our data show no such increase. However, we only evaluate the measuring stations every ten years after climatic changes. It will be time again next year. Then we'll know more. It is true that floods have already increased in places. It's about rainfall, which the city used to digest quite well in the past. That's the re-compaction. When houses come, the drainage areas often disappear.

ZEIT ONLINE: How can you counteract this?

Kuchenbecker: We want to develop Hamburg into a sponge city, so that rainwater is first collected and later slowly released. Green roofs, hollows and ponds but also emergency storage under sports fields, in parks or as a rain playground help. In addition, the city nature can help with areas where water seeps away. If you do not plaster your garden, but plan many overgrown areas with thick layers of humus and large trees that delay the rainfall, which also reduces the risk for yourself - and all of us.

ZEIT ONLINE: So foliage in the fall not wegrechen and dispose of, but as a humus-based on the beds?

Kuchenbecker: As long as you remove it on the street neat, so that the outflows do not clog there - yes!

From Monday, the heavy rain index will be live streamed by Hamburg Wasser:

That's what the numbers mean:
SRI 1-2: The water should be able to drain through the sewer system without impairment.
SRI 3-5: Not all water can be drained. In unfavorable locations, the rainwater can accumulate on the surface and basements or underground shafts flood
From SRI 6: The rain becomes a disaster prevention case: Larger areas could be flooded.