Flood protection What happens in Bonn when the Rhine reaches a certain level?
Bonn · During flash floods or when the snow melts, the water level in the river Rhine quickly rises. Inundations of water can cause devastating damage. We explain the water levels in Bonn and what to do when it becomes critical.
Persistent precipitation, heavy rainfall and melting snow all cause rising water levels in the Rhine and the streams around Bonn. In extreme cases, the water can burst the banks and pose a danger for nearby residents. The last floods in Bonn to exceed the level of 10 metres were in 1993 and 1995. However, the flood alert plan considers water levels of five metres or more as ‘minor high water’, and levels of six metres or more are ‘high water mark I’ - even at these levels things can become critical in the Bonn area.
The Rhine level (called ‘Pegelstand’ in German) is measured around the clock by the City of Bonn in cooperation with the Federal Waterways and Shipping Administration and can be accessed online.
Vulnerable areas in Bonn
Beuel is the district most affected by flooding of the Rhine. The riverbank promenade in Beuel floods from a level of seven metres. The highest water level in Beuel was measured in 1993 at 10.13 metres.
In the Bonn district, the Rathenauufer between Erster and Zweiter Fährgasse is the first to be affected. Here the situation becomes critical for the riverbank promenade and roadway from a water level of seven metres. The riverside promenade at Alter Zoll will be flooded from a level of 7.50 meters. The district of Graurheindorf is also particularly affected by floods. The water from the Rheindorfer Bach floods the Estermannstraße from a level of about 8.70 metres.
In Mehlem in the district of Bad Godesberg, it becomes critical at a water level of about 8.90 metres. This is when the water runs into Rüdigerstraße by Am Glückshaus.
When the water level is high, the cycle paths in Bonn can also be partially flooded. On the left bank of the Rhine, the cycle paths are completely under water from a level of 6.50 metres. On the right bank of the Rhine, this happens at a water level of 7.50 metres. Diversions will be provided in the city area via signposted cycle routes. The diversion leads away from the Rhine onto a cycle route that runs parallel to the path along the Rhine and leads to the southern city boundary.
Maps detailing flood danger and risks are drawn up for all bodies of water posing a potential risk. The maps can be accessed online via the Bonn city map (see box below). The affected areas in the city are marked on the map according to different flood scenarios: a frequent flood (every 10-20 years), a medium flood (every 100 years) and an extremely rare flood (less than every 100 years). The maps show the corresponding water depths and flow velocities in addition to the extent of flooding.
What happens at which water level?
According to the flood alert plan, the following water levels in Bonn are important gauges for the City of Bonn and the Water and Shipping Authority:
- 5 metres: Minor high water (Kleines Hochwasser)
- 6 metres: High water mark I (Hochwassermarke I): ships are only allowed to sail slowly and in the middle of the river.
- 6.50- 6.60 meters: The Graurheindorf-Mondorf ferry stops operating.
- 7 metres: In Beuel, access roads to the Rhine are closed. The public will be informed by loudspeaker announcements if the level is forecast to rise to 9.50 metres.
- 7.15 metres: The stops Bad Godesberg Fähre and Bad Godesberg Rheinufer are closed.
- 7.20 metres: In Bonn, the access roads to the Rhine will be closed from Legionsweg to Schaumburg-Lippe-Straße.
- 7.40 metres: Medium high water (Mittleres Hochwasser): The fire brigade sets up an emergency task force. The 66 tram line in the area of Königswinter at the Sealife-Center floods. The stop will then no longer be served.
- 7.50 metres: The Beuel Technical Operations Centre is set up and the first sandbag walls are built. Recycling containers are relocated.
- 7.60 metres: The Mehlem-Königswinter ferry service is suspended.
- 7.90 metres: The Dollendorf-Bad Godesberg ferry service is suspended.
- 8 metres: High water mark II (Hochwassermarke II): Shipping traffic is suspended. At just over eight metres, the Rhine overflows its banks in Beuel. North of the Kennedy Bridge, the Rheinaustraße is spared flooding up to a level of 9.50 metres by the dam at Wolfsgasse. In Bonn, Charles-de-Gaulle-Straße will be closed downstream to Hermann-Ehlers-Straße.
- 8.20 metres: Major high water (Großes Hochwasser): Technical emergency lines are set up in Bad Godesberg and Bonn. Footbridges are built in the Rheinstraße.
- 8.50 metres: A command post is set up in Graurheindorf. Water runs into Mertensgasse. In Bonn's city centre, the riverside promenade at Alter Zoll is flooded.
- 8.60 metres: In Mehlem it becomes critical. This is when the water runs into Rüdigerstraße by Am Glückshaus.
- 9.50 metres: In Mondorf, the Rheinallee is flooded, cellars fill up. Around 100 residents are affected. In addition, the new flood protection in Beuel fails at this level. The first streets to be flooded are Rheinaustraße south of Kennedybrücke, Steinerstraße and Johannesstraße. Footbridges must be constructed.
Flood risk for Bonn's streams and creek systems
Not only the Rhine is affected by heavy rainfall. The Cologne district government has designated a floodplain for a once in 100-year flood for Bonn's streams and creek systems. The floodplain designation was created for areas in which a flood event is statistically to be expected once in 100 years (HQ100). Flooding can occur during heavy rainfall at the Mehlemer Bach and the Godesberger Bach in particular. The Hardtbach system and the Katzenlochbach valley are also classified as flood risk areas.
Bonn City’s Civil Engineering Office has introduced an automatic warning system that permanently monitors the water levels of the streams and, in extreme cases, sends messages to the fire brigade's operations control centre and to the on-call services of the Bonn Civil Engineering Office. If a flash flood is imminent, the fire brigade triggers the siren alarm, posts the information on the Nina warning app and initiates announcements on local radio stations.
When the water level rises - City of Bonn recommends precautionary measures
When the water rises, residents in vulnerable areas should take precautions to prevent major damage. The City of Bonn recommends the following:
- Evacuate cellars, garages and other parts of the building affected by high water levels
- Remove vehicles from the garage, pavement or street and park them in safe places outside the flood zone
- Prepare waste containers and bulky waste for special collection - in good time before the flood wave arrives, all waste containers are emptied in special collection
- Check the building’s drainage system and backwater flaps in the basement
- Protect the heating systems
When the water recedes
As soon as the flood water has receded, cellars and other rooms should be pumped out quickly. Otherwise, there is an increased risk of mould, and moisture can penetrate deeper into the floor and masonry. The fire brigade in Bonn does not provide pumps to drain water during periods of flooding. Only when the water has receded is it possible, in extreme cases, to get help from the fire brigade. The fire brigade warns against pumping out the water too early. Differing pressure conditions under the basement floors and on the outer walls can cause damage to the building.
At higher water levels, the groundwater can also endanger buildings located in areas further away from the Rhine. Cellars and other rooms should therefore be checked regularly and appropriate measures taken.
(Original text: GA, Translation: Caroline Kusch)
