Extension of the Main dyke begins
16.02.2023 – The extensive construction work to extend the Main winter dyke begins in the Rumpenheim section with the ground-breaking ceremony. Representatives from the city of Offenbach, the state of Hesse and residents from Rumpenheim came together for this symbolic act on Tuesday afternoon.
This means that a project that has been planned for more than a decade and an important measure for flood protection in Offenbach is now being implemented. Necessary preparatory work has been carried out in recent weeks. With the extension of the Main dyke, the entire section in Rumpenheim will receive a modern dyke system that will also protect the district against a 200-year flood.
The section in Rumpenheim is scheduled for completion in the fall of this year. The expansion of the Main dyke in the city center, between the Carl Ulrich Bridge and the former paint factory site, now the Innovation Campus, is planned for 2024 and 2025.
Head of Planning and Construction Paul-Gerhard Weiß: "Today is a good day for protecting the people of Offenbach from the dangers of flooding, which we will have to expect more frequently in future due to climate change. By renovating and raising the dyke in Rumpenheim and then in the inner city area, we are improving protection against extremely high floods, which statistically occur every 200 years." Weiß also spoke about the planning process in the past: "The road to the first excavator bite was long, because the Main dyke, as the construction of the century, characterizes our city in terms of urban development and it is absolutely understandable that many design issues were taken into account in the planning process and that rescheduling was necessary. The consultations between the numerous parties involved were very complex and yet always goal-oriented. Those involved also included residents from Rumpenheim, whom I would like to thank today for their excellent cooperation." As some of the construction work is also being carried out on private land and in gardens, residents are directly affected and are concerned about their property. "We take these concerns seriously and have taken various measures to protect local residents. These include, for example, groundwater monitoring, the assessment of the properties in order to record the current condition with legal certainty and flood protection measures during the construction period," explained the Head of Planning and Construction, who also thanked District President Brigitte Lindscheid for the high level of financial support from the state of Hesse.
The renovation will ensure the stability of the dyke. Even in places where mobile flood protection elements are used. In the event of flooding, the conditions for dyke defense as well as for dyke maintenance will be considerably improved. This includes the creation of dyke protection strips and the construction of continuous access roads and paths.
The Darmstadt Regional Council (RP) advises local authorities on the technical solution and on the application for funding for the measure. In the event of a dyke defense, the RP acts in an advisory capacity for the municipal emergency services. "You can see how important joint action is, especially in this area. The next extreme flood will come. We have to be prepared for it," emphasizes District President Brigitte Lindscheid. "Together with the local authorities, the state of Hesse is making great efforts to protect people. I would like to thank everyone who is facing up to this elementary challenge."
Sheet piling takes over the function of the dyke structure
In future, the actual flood protection will no longer be provided by the masonry dyke itself. Instead, sheet pile walls will be installed up to eight meters deep in the ground directly in front of the dyke (on the river side). Their function will be to prevent the water from entering the dyke and the adjacent streets and residential areas. At the same time, renovation work is being carried out on the dyke structure.
The necessary explosive ordnance detection is currently still being carried out. As the sheet pile walls extend up to eight meters deep into the ground, the investigations must also be carried out to this depth. Superficial detection would not be sufficient. It is therefore necessary to drill to a depth of six meters along the sheet pile axis. This is followed by a ground radar measurement using a special measuring device. The borehole is then filled with swellable clay pellets. If no abnormalities are detected during the measurements, the actual construction work can begin.
As already became clear during the planning process, not all trees can be preserved for the measures. The necessary tree felling below the castle park and in the field on the border with Mühlheim took place at the beginning of 2022. At this point, it was already apparent that the row of trees along the cycle path between Neugasse and Kleines Gäßchen had developed so much over the course of the entire planning and approval process that the construction of the sheet pile wall would have a significant impact on the crown and root area. As a result, the trees were examined by an expert in tree statics in order to assess the necessary intervention in terms of stability. It was found that most of the trees had advanced fungal infestation and, as a result, decay in the trunk and root area. Based on these findings and taking into account other factors, such as the need for crown and root pruning, nine of the fourteen trees were removed. The five poplars in the west will be retained. All felling has been agreed with the higher nature conservation authority. Compensation will be made as part of the overall measure.
Construction process
Due to the constant risk of flooding, the construction work is only being carried out in sections. The dyke upgrade, which is around 550 meters long in total within the Rumpenheim district, is therefore being carried out in five sections. A further construction section is located in a field on the outskirts of Mühlheim.
The construction work is being carried out from west to east. Work will begin in the area where the "Kleines Gäßchen" road joins. First, the sheet pile walls are hydraulically pressed into the ground. Then the sheet pile wall is clad with sandstone. Finally, a sandstone cover plate is placed on top of the sheet pile wall, onto which the mobile flood protection system can be placed in the event of flooding.
The Deichtor piers, including the foundations, will be dismantled and replaced with reinforced concrete. The piers will also be clad with sandstone.
In the area of the former "Schiffchen" restaurant, the natural stone wall will be retained for reasons of monument protection. To ensure that this wall is sufficiently stable and still retains its historical character, it was agreed in consultation with the monument protection authority to remove the upper stones and place a new reinforced concrete beam on top. The new reinforced concrete beam will then be clad with the existing quarry stone material. This will preserve the historical character of the wall. In addition, the wall will be stabilized with vertical and horizontal tie rods.
The area of the pumping station and the underground car park is already well protected against flooding. Additional protection with mobile dam beams is only required in the event of exceptionally severe flooding. For this purpose, new concrete foundations will be placed on the ceiling of the pumping station and the underground car park. In the first floor garden, the concrete foundations will be laid so that they are concealed under the gravel of the path surface and only need to be uncovered in the event of flooding.
The historic ice deflector at the eastern end of the castle garden cannot be retained for structural reasons. In consultation with the monument protection authority, it was agreed to erect a new, round pillar structure a few meters east of the old location. This will also be clad with sandstone.
In the field area towards the Mühlheim district, the existing field path will be raised by up to 30 centimetres and at the same time upgraded with concrete paving as a dyke defense path.
Impairments and protection during the construction period
During the construction work, the area in front of the Main in Rumpenheim will only be accessible to a limited extent and traffic will be diverted in sections. The important ferry connection to and from Maintal will be maintained during the entire construction period, but there may be temporary obstructions. The heavily frequented Main cycle path has been shifted towards the Main and thus leads around the construction site. During the construction work in the area below the Schlosspark, the cycle path is closed in this area. A signposted detour for cyclists then leads around the castle park via Schloßgartenstraße. The streets leading to the Main - Schmiedegasse, Fischergasse, Untergasse, Neugasse and Kleines Gäßchen - will temporarily become dead ends depending on the progress of the construction work. Access to the individual properties for residents and the emergency services is guaranteed at all times.
The parking lot in the Main forecourt will be closed during the construction period as it will be used as a construction site installation area. During the construction work in the field area on the border with Mühlheim, the farm road there is also inaccessible.
The City of Offenbach is endeavoring to keep the disruption caused by the construction work to a minimum. All work has been approved by the relevant authorities. Nevertheless, disruptions such as truck traffic, noise pollution and soiling cannot be ruled out. The City of Offenbach and the construction company ask all residents and those affected for their understanding for the inconvenience.
The protection of property and the safety of residents who live in the immediate vicinity of the construction site and are also affected have the highest priority. Groundwater monitoring has been carried out for a year now and groundwater levels are being closely monitored. This is intended to rule out structural damage caused by changes in groundwater flows. In addition, all properties have been "evidence-protected" by an expert. This means that not only the city but also the residents have legal certainty regarding the condition of their buildings during the construction work.
Flood protection will also be ensured in cooperation with the ESO Offenbacher Dienstleistungsgesellschaft mbH water authority. The construction company also has sufficient fill and scaffolding material in stock so that it can be installed at an early stage in the event of imminent flooding.
The entire project is being funded by the state of Hesse. The city of Offenbach receives around 85 percent of the eligible flood protection costs. In December 2021, Environment Minister Priska Hinz presented the city of Offenbach with a funding decision amounting to 6,225,000 euros. The funding was increased to 12,928,510 euros in June last year. The funding is being processed by the Wirtschafts- und Infrastrukturbank Hessen.