Flood and level measurements When the water was where, at which point and why
Region · During the heavy rain, the floods devastated the region in many places, some level measuring points in the flood area were broken or overwhelmed. Experts explain when the water was at which point.
Many gauge measuring points in the region's flood zone stopped working on the day of heavy rain (Wednesday, July 14). They were not designed for such extreme values. Example: Steinbach. There, it rained 178 liters per square meter during the course of the day. As a result, the dam overflowed, clogged the emergency outlet and was damaged on the outside of the dam. From 11 a.m., the level of the Steinbach near Arloff rose rapidly and continuously, from 38 centimeters to over 1.80 meters. This last measured value was reached at 8 pm. Then the water level gauge failed.
The Steinbach flows into the Orbach and the water then flows via Swist and Erft into the Rhine. The next gauge in the Swist near Morenhoven recorded the strong rise from about 1 pm. If one assumes a flow velocity of eight to ten kilometers per hour, this can be explained, because it is 16 kilometers from Arloff to Morenhoven as the crow flies. In the village of Swisttal, the crest of the wave was reached around Thursday, between 6 and 8 o'clock in the morning, although it is not entirely clear whether the actual height did not exceed the measured values, because the gauge was simply not able to document it. The water level did not level off again until noon. 18 kilometers away, in Bliesheim, the crest was reached at 10 a.m. on Thursday. According to a rough estimate by the experts, in the event of a dam break at the Steinbach Dam, the flood wave would have reached the town of Rheinbach after one hour, Heimerzheim after two hours and Erftstadt after four hours. However, the forecast has many unknowns, so these are only approximate values. The fact that the area of heavy rain was stuck over the region for hours also meant that a lot of water came down in many places at the same time, so that the inflow(ux) of surface water filled the rivers from all sides at the same time.
The gauge measuring the Ahr River in Altenahr also stopped working around 8 p.m. Wednesday. The adjusted curve shows a rise from about 2 p.m. from a level just below one meter to the crest of about six meters in the early morning hours. Data computer scientist Thomas Strub has analyzed the flood situation on the Ahr in detail and made cautious initial assessments. According to this, the current flood is likely to have exceeded the marks from the previous high of June 13, 1910. He speaks of a plausible maximum water level in Altenahr of 5.74 meters, while the estimates of the state consider a value close to seven meters possible. Strub has calculated a water discharge of 550 cubic meters per second for 2021, while the 1910 flood is said to have been 496 cubic meters per second. He estimates that the 1804 flood was another 2.5 times as high.
Original text: Jörg Manhold
Translation: Mareike Graepel
