Flood Defence in Mainz
Each of the properties of Zollhafen Mainz should be able to withstand extreme flood events during the construction period and later during normal daily operations. The buildings must not suffer any damage even if the Rhine rises significantly above its banks. Expressed in figures, this means that the protection target for the Zollhafen Mainz must be achieved for a water level of 86.53 meters above sea level (flood 200) plus 50 cm freeboard as a safety buffer. This corresponds to a flood that statistically occurs every 200 years. This protection target also applies to the adjacent neighborhoods of Mainz Neustadt.
In addition, it was important to the owner Zollhafen Mainz GmbH & Co. KG – a company of CA Immo Deutschland GmbH and Mainzer Stadtwerke AG – to incorporate the wishes of the city of Mainz and the citizens of Mainz into the planning right from the start and to reconcile them with the requirements of building law. These consisted above all in the desire for open spaces and recreational areas that would once again make the banks of the Rhine and its special landscape area on the water accessible to all.
At the start of the project, Mainzer Stadtwerke had already set the decisive thrust in motion. Among other things, as a partner in the EU project “FloodResilienCity”. This project aims to link the increasing demand for more real estate and other construction projects with the growing need for more and better flood protection measures in northwestern European cities along rivers. omniCon brought its experience with sustainable neighborhood developments near riverbanks and innovative, flood-adapted planning and construction to the project at an early stage. This has resulted in a project with a nationwide role model character for preventive flood protection and safe and worthwhile living and working on the waterfront.
“Due to the expertise and experience of our team, we can assess the conditions in Mainz very well. We can judge whether the plans of our project partners are suitable or whether they need to be readjusted. In this way, we can support the project teams with our consulting services and lead the project to success in the interests of the developers,” says Dr. Patrick Kaurisch, head of the land preparation and development department at omniCon Gesellschaft für innovatives Bauen mbH (omniCon), describing his department’s role in this project. omniCon is a subsidiary of CA Immo and is responsible for the construction management of the Zollhafen Mainz project.
From the industrial port back to the green meadow
Geologically, the area of the Zollhafen Mainz is considered to be one of the lower terraces of the Rhine and is thus a natural floodplain. In order to reclaim the industrial and harbor areas, the former Rhine floodplain was filled up according to the planning of the then city architect Eduard Kreyßig.
The banks of the harbor were initially enclosed by quay walls and in the 1960s secured against undercutting with steel sheet pile walls. The surfaces were sealed. While this meets the requirements of industrial use, it also no longer provides a habitat for native, water-borne species. The floodplain was lost as an ecosystem and recreational area.
“Not everything in Mainz Zollhafen is being overplanned with buildings, but recreational and compensatory areas are also being created,” summarizes Franz Motzko-Lisy, project director of omniCon and Zollhafen Mainz GmbH.
When preparing the land and developing the new Zollhafen Mainz quarter, the degree of sealing is not simply reduced, but the green area is quadrupled. Of the total 1.6 hectares of green space, 1.2 hectares alone will be created along the promenade of the north shore, the so-called north pier. These green areas will be arranged in terraces. The preliminary work, such as quay removal and bank lowering, will be completed by the end of 2021. The “city balcony”, the “sports bay” and the “park meadow” will offer the residents of the neighborhood a local recreation area right on their doorstep. The design comes from the landscape architecture firm SINAI. For flood protection and the reclamation of the riparian area as a habitat for native and endangered species, however, one part of the lower park meadow will be of particular interest: the “Wild Urban Floodplain”.
Floodplain is the term used to describe areas of the landscape that are regularly inundated by floodwaters. Floodplains can absorb considerable amounts of water and thus help to mitigate flood waves. Floodplains are characterized by flat terrain and vegetation consisting of meadows and scattered shrubs or so-called floodplain forests. They provide valuable habitat for numerous animal and plant species and support the preservation of biodiversity along the Rhine.
“We plant here what comes closest to a natural growth of vegetation on the banks of the Rhine,” Motzko-Lisy reports. “You give nature a push so that the natural vegetation grows quickly and healthily. Then you allow nature to unfold in peace and a natural biotope to emerge. What is innovative here is that we consciously include natural occurrences in our planning instead of fighting them as we did in the past. Here, a social rethinking is taking place that we owe to our young generation.”
The probability of flooding increases – a test for the customs port of Mainz in February 2021
Although the extreme flood events for which the Zollhafen Mainz is designed are statistically very rare, flood protection was taken into account with the utmost care in the planning.
This is because global climate change will lead to extreme weather events becoming more frequent. According to the Federal Environment Agency, annual precipitation will increase in northwestern Europe over the next 80 years, particularly in the winter months. The frequency of heavy precipitation is also expected to increase. This may lead to increased flood events, including along major rivers.
Therefore, in addition to the green floodplain, further retention space is gained in the Zollhafen Mainz by lowering and exposing the historic bank reinforcement at the harbor entrance and the terraced banks. Flood waters above a level of 86.2 m above sea level are channelled in a controlled manner through the canals of the port islands.
At the beginning of this year, the customs port experienced a flood event. Persistent precipitation and thawing weather caused the Mainz water level to exceed 6 meters. However, this flood did not damage the buildings of Zollhafen Mainz that were completed or under construction, according to Wolfram Albert-Göppinger, Construction Manager at omniCon. “The flood protection measures are, after all, already taken into account during construction. The flood in February was not nearly high enough to pose a serious threat to the building fabric. But it was enough to check whether all the cellars were watertight. And as far as I’m aware, all is well.”
Zollhafen Mainz is scheduled for completion by 2025. More and more developers and investors are active on the approximately 30 construction sites, while the first tenants are already moving into their apartments or offices. Due to its proximity to the water, but above all due to the people who will live, work or spend their leisure time here, Zollhafen Mainz will become an urban, lively and livable neighborhood in the coming years.