Why the level deviated, the experts can currently only speculate. After the GA reported that restaurant operator Michael Opgenorth from the Bundehäuschen in Oberkassel had noticed a higher water level than usual at the Bonn gauge, the WSA checked the facility at the height of the Bonn Opera. "Apparently, the defect occurred over the weekend," Krekel says. Next to the Rhine is a vertical shaft that is directly connected to the river via a pipe. Based on the principle of communicating tubes, the water in the shaft rises in the same proportion as the Rhine water. Two sensors - one as a backup - then report the level digitally to the control center. "Presumably, the pipe became clogged with sand, which prevented the water in the shaft from rising," Krekel said. The incident can only be investigated in more detail once the level has dropped again, he added. The Bonn fire department reported that flotsam had blocked the measuring device.

The city of Bonn had not even noticed the glitch until it was corrected; it had relied on the wrong figures. Until Tuesday noon, the wrong level of 7.40 meters was still displayed. The WSA and the Rhineland-Palatinate Flood Control Center had manually raised their level by 40 centimeters, so that it then stood at 7.80 meters. Although the water level seemed strange to some city employees, it was not checked further - there is also an old-fashioned black-and-yellow staff gauge on the banks of the Rhine that indicates the correct level. "The incident has changed the way we look at the water level," says fire department spokesman Frank Frenser. Normally, one can trust the official reports. The situation is not dramatic, he says, because the fire department and the city work with a good half-meter buffer to avoid being surprised by the water levels. "The measures we have taken so far do not change because of the level correction."

By Thursday, however, it will be different. The delay now results in a different flood forecast for Bonn. Accordingly, it is no longer at 7.75 meters, but at about 8.30 meters. This in turn has an impact on the protective measures. From eight meters, flood mark II applies, then shipping traffic is stopped. In addition, the Rhine overflows its banks in Beuel. A dam is built in Wolfsgasse, sparing the Rheinaustraße from flooding up to a level of 9.50 meters. In Bonn, Charles-de-Gaulle-Straße is closed downstream to Hermann-Ehlers-Straße.

In any case, Michael Opgenorth is glad that he did not rely solely on the water level, but also on his feeling. "Sometimes experiences are better than measured values," he says. If he hadn't taken the precaution of clearing everything out of the restaurant, he would have incurred damage of well over 150,000 euros.

(Original text: Nicolas Ottersbach, Translation: Mareike Graepel)