Heavy rain and flooding

Science Teams Activities

Preparation of the excursion in class

Heavy rainfall event in Cologne on 14.07.2021

Isohyetics of the precipitation totals

Precipitation totals in the Flehbach catchment area range from 87.43 to 108.78 mm. This means that the value increases from east to west. This is also approximately the direction of flow of the Flehbach.

Assessment: As the Flehbach flows through forest for long stretches, surface runoff is low, as forest has a high storage capacity. In the further course, there are repeatedly meadows and unsealed areas along the river, which also have a good water storage capacity.

The heavy rainfall on 14 July is an event that occurs less frequently than every 100 years.

• Evaluation of the water level of the Flehbach

20210714_Flehbach_Straßenbrücke Am Gräfenhof – Marktplatz Brück

• • The discharge behaviour of the Flehbach from 10 to 22 July is mixed. On 10 July the Flehbach had a water level of 0.5 m. Around 12 noon on 11 July, a water level of 0.75 m was reached due to increased precipitation. In the morning of 13 July at around 3 a.m., the level sank to a minimum of 0.48 m. This is followed by heavy rainfall on 14 and 15 July, which causes the Flehbach to reach its highest level of 1.21 m at 7 a.m. on 15 July. Subsequently, the level drops to 1.02 m within one day, and from here it drops rapidly to 0.9 m. From noon on 16 July until 22 July, the level slowly drops to 0.8 m and slowly returns to normal. Normal conditions are nevertheless not reached

• The Flehbach (Aleyna & Jasmeen)

  • The Flehbach rises in the area of the Tütberg in the Königsforst in Bergisch Gladbach. It is a 17 kilometre long river and flows through the forest area, the Königsforst and then through the districts of Brück and Merheim. In Merheim it joins the Bruchbach and thus flows into the Faulbach. Through the Faulbach it flows into the Rhine and then flows into the North Sea.

On 14.07.2021, a total of 144.8 mm of precipitation fell on the HHG.

The annual average is 88 mm in Cologne-Wahn. In comparison, twice as much fell on 14.07. as normally falls in the whole month of July. In addition, approx. 2 mm normally falls every day in July, which again illustrates how extreme the precipitation was on 14.07.2021. The amount of precipitation in the Flehbach catchment area ranges from 87.43 to 108.78. This means that the value increases from east to west. This is also approximately the direction of flow of the Flehbach.

Since the Flehbach flows through forested areas for long stretches, surface runoff is low because forests have a high storage capacity. Further along the river, there are always meadows and unsealed areas that can also store water well. The heavy rainfall on 14 July is an event that occurs less frequently than every 100 years.

Dr. Dulgoß from the University of Cologne already told us during her lecture that something like this happens every 400 years.

Introduction to the excursion 

• Dear students,
  • On Thursday, 02.12.2021, we, the EF geography course of Mrs. Busch, went on an excursion to the Flehbach as part of our PULCHRA project. Since a heavy rain event occurred this summer (14.07.2021) and 144mm of precipitation were measured at our HHG weather station, we dealt with the question: What effects does a heavy rain event have on the Flehbach in Cologne-Brück and what possibilities are there to avoid flooding?
  • We left at 9:45 am for the bus stop “Ostheim”. We took the bus to the “Frankfurter Straße” stop and from there we took the train to the Flehbachstraße station. There we met Mr. Vedder from the Stadtentwässerungsbetriebe Köln, who accompanied us the whole time and showed us around the Flehbach.

1. At the first station, Mr. Vedder told us what renaturation measures had already been undertaken. The Stadtentwässerungsbetriebe want to keep the Flehbach as natural as possible and at the same time protect Brück from floods of the Flehbach. Then we went to the marketplace of Brück, where the Flehbach flows underground. Measures are also planned for the canal, especially to protect the animals in the water.
2. The second station is at the end of the canal, where the Flehbach continues to flow above ground. This is the measuring point for the water level calculation. Mr. Vedder explained to us that there is a big rubbish problem there and that the Flehbach must therefore be cleaned regularly. They also want to make the stream wider and more natural with the help of wood.
3. Then we went to the third station. There, a sand trap was built in the Flehbach floodplain. This is a kind of large lake to filter out sand and sediments and to protect against flooding. There and at the 14 other places along the Flehbach, the University of Cologne put up QR codes with information about the Flehbach. This is the so-called Flehbach LEAP.

• • After this station, our excursion was over. Mr Vedder had planned to send some of our students into the water, but the water level was too high – to our good fortune.

The Locations

• Location I: Flehbachstraße – Existing measure near the KVB stop

  • The existing measures on the Flehbach in Cologne-Brück are two years old. Until then, the Flehbach ran in a very straight line and was then widened and diverted by the renaturation measures. Now it has a longer course again and more meanders (bends). This has a great impact on flood protection, as the flow is slowed down and there are alternative areas in the parks where the water masses can flow to and from during floods. In addition, renaturation increases the habitat for animals and plants. Renaturation involves building a structure out of wood, plants, and stones. During an older clean-up campaign with the scouts, they made sure that poisonous and non-native plants were removed.

• Location II: south of the Brücker Marktplatz (Malin, Alexander, Helena)

  • Hidden in a stainless-steel box, a laser measures the water level and flow velocity of the Flehbach before the tunnelling begins.
  • A big problem is that people constantly throw rubbish into the Flehbach. For example, electric scooters, shopping trolleys, mannequins and dead animals are regularly found, which can block the opening of the tunnel. This prevents fish and other animals living there from swimming through the tunnel to the other side and can lead to a waterlog. The rubbish is held up by a coarse grid and later taken out and transported away by a mobile task force of the STEB.
  • Another problem is that concrete shells are still attached to the bottom of the riverbed and must be removed, as concrete does not provide any structure and thus no habitat for animals living there, such as fish, snails, small crayfish, etc. The river is also trying to be restored to the Flehbach. In addition, attempts are being made to give the Flehbach back its natural course and to create a natural environment, i.e., to renaturalise the entire stream. This can be achieved by using tree trunks as flow deflectors. This takes a few years, but the stream can develop a natural shape by itself. Excavators must be used to remove the concrete elements, but these may only be used during breaks in vegetation, i.e. from November to February.

• Location III: In the Flehbachaue

• • „Die Krebssperre“
  • The crayfish barrier stops the migration of crayfish. Non-native crayfish species often carry diseases to which native crayfish are not immune. The crayfish barrier thus serves to protect native species. The crayfish barrier works by constructing small “waterfalls” in the direction of flow of the stream, which the crayfish cannot overcome.

• • „Der Sandfang“
  • A sand trap is a basin- or pond-like widening and deepening in a watercourse that is constantly flowed through. Because the water moves more slowly here, heavier solid particles can settle. The sand trap in the Brücker Flehbachaue is an almost natural pond. The basin is located in the middle reaches of the Flehbach. Because the stream water constantly flows through the sand trap and cannot be diverted, it can only be emptied during a dry period in summer. During sand trapping, about 600 sqm of sand and silt are removed, and the total volume can reach up to 900 sqm.

Conclusion on the excursion

• Everyone enjoyed the excursion to the Flehbach because it was very informative. I also found it even better because of Mr. Vedder’s explanations, as he made you really listen to him. Above all, I liked that he used comparisons so that you could visualise things well. It was also good that he involved us and asked us questions, for example, to make us think.

1. At the first station, it is important that a few measures already show a clear effect after a few years.
2. At the Brücker Marktplatz, Mr. Vedder showed us that the tunnel is a barrier for animals, as some of them do not swim through it.
3. The southern section is to be renaturalised in the next few years. We will keep a close eye on this.

Development Of The City Challenges

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