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City of Bonn

Construction of the rainwater retention basin on Hauweg on schedule

In recent years, Bonn has experienced frequent heavy rainfall events. In order to better protect the district of Kessenich from flooding in the future, the City of Bonn is building a rainwater retention basin on the Venusberg slope.

View into the 8.5 meter deep excavation pit of the machine that will be used to drill a 90 meter long relief channel underground to the new rainwater retention basin.

The structure is necessary because the existing retention basins on Bergstrasse have already overflowed several times during rain events in recent years, causing damage to infrastructure and buildings on Bergstrasse and Rosenburgweg. The new underground basin on Hauweg, which is designed for a 20-year rainfall event, will further increase flood protection.

As things stand, the City of Bonn's civil engineering department is investing around 2.2 million euros in the complex construction project - less than initially calculated. The rainwater retention basin is being built on the site of the former sports field on Hauweg in the landscape conservation area. The project is on schedule.

Preparatory work began in winter 2023

The preparatory work began in winter 2023 with the felling of subordinate tree growth. With the preparatory work in spring 2024, rootstocks were cleared, a construction site access road was created and the backfilled surface of the sports field, which was contaminated with heavy metals, was excavated to a depth of half a meter. The contaminated excavated soil was taken to a landfill site in accordance with legal regulations. A total of around 2,400 cubic meters of soil was disposed of professionally.

In May and June 2024, archaeological investigations were carried out into a suspected 6,000-year-old Neolithic rampart in the triangle between Robert-Koch-Straße, Bodelschwinghweg and Hauweg. However, no archaeological relics were found.

Drilling a relief channel

The actual work for the rainwater retention basin, which will be built as an open earth basin in a near-natural construction method, started in December 2024. A 90-metre-long relief channel with a diameter of 1.20 meters is currently being drilled underground from an approximately 8.5-metre-deep excavation pit to the new rainwater retention basin. When the storm sewer in Hauweg gradually fills up and backs up during heavy rainfall, the wastewater automatically flows into the rainwater retention basin - and out again when the water level drops.

Work on the pipe jacking has been running around the clock in three shifts since March 24, 2025. From a technical point of view, it is beneficial if the pipe jacking runs continuously and does not come to a standstill. The Office for the Environment and Urban Greenery has approved the necessary night work. The city informed local residents of this before the work began. The drilling work should be completed within a week, by March 27.

View of the relief channel being created, which is being drilled at the bottom of the excavation pit using pipe jacking. In the foreground, earth and rock being transported out of the channel on a conveyor belt.
Project manager Wolfgang Frömbgen (right) and head of department Sebastian Richter from the municipal civil engineering office next to one of the concrete pipes with a diameter of 1.20 meters that will be used for the construction of the relief sewer.

From April, 24,000 cubic meters of soil will be excavated

From April, around 24,000 cubic meters of soil will be excavated and removed for the rainwater retention basin. The future rainwater retention basin has a storage volume of around 6,400 cubic meters. As diluted wastewater will be dammed up, the basin will be sealed with a plastic sealing membrane covered with eco-paving. The embankments will be covered with topsoil and sown.

The earth basin is scheduled for completion at the end of 2025.