The bridges will make Frankfurt an (even) greener city: 1 million m2 of green space will be brought into being on the bridges, 100,000 square meters of vitalized and 40,000 square meters of unsealed green space will be created in Frankfurt's inner city area, while at the same time the aim is to plant over 1,000 new trees. All of this needs to be irrigated. The Frankfurt Bridges provide the necessary irrigation water for this.
Approximately 600,000 to 800,000 m³ of irrigation water will be required for the green spaces created in connection with the bridge project.
This will not be taken from Frankfurt‘s – problematic – drinking water supply; rather, the bridges will collect rainwater and construction site water to bring it to storage locations, from where it will also be taken and distributed again for watering the plants.
More than 1,000,000 square meters of green space will be created by the flower beds and meadows on the bridges alone. In addition, an unsealing plan provides for the creation of new green spaces on 40,000 square meters in the city: Where asphalt previously covered Frankfurt's ground, flower beds and meadows are to be created. In addition, around 1,000 new trees are to be planted in the city center. The challenge here is that all of this needs to be watered.
Approximately 600,000 - 800,000 cubic meters of irrigation water are estimated to be needed for this purpose, depending on the dryness of a year. Groundwater and rainwater should therefore be kept in the city so that it is available for watering the plants.
For this purpose, a pipe system for irrigation water separate from the city‘s sewage system must be created. Rainwater as well as groundwater from excavation pits, which is currently pumped into the Main River, should be collected in it or transferred.
In addition, all of this must be stored for the drier periods of the year when it is needed.
Frankfurt needs about 65 million cubic meters of drinking water per year: about 20 % of this is obtained in Frankfurt, the remaining share is imported from the Vogelsberg, the Kinzigtal, the Main-Spessart and the Hessian Ried, which increasingly leads to groundwater depletion there.
The wastewater from the buildings, together with rainwater from the street sewers, is treated in Frankfurt's wastewater treatment plants and then discharged back into the Main River. In order to be able to use it as irrigation water instead, it would have to be treated even further. Therefore, it cannot currently be used to irrigate the new green spaces to be created.
Where would you get 800,000 cubic meters of water? And even if you had it: Where do you want to store this volume? And could you store these rough volumes of water - how can the water from those reservoirs be re-distributed to the green spaces?
Unfortunately, the water does not always arrive at the places where it is needed and usually not at the times when it is urgently needed.
The Frankfurt Bridges offer the solution: water is "collected" when it occurs, i.e. when it rains or the river water level is higher, or when groundwater is being pumped out at major construction sites to enable civil engineering work: The bridges pass by everywhere and can collect up to 1.6 million cubic meters of rainwater and up to 2 million cubic meters of construction site groundwater as pipeline carriers.
The next step is to store the water until it is needed during dry periods. The bridges reach possible storage locations in all directions - this solves the problem of storing huge amounts of water, which would be difficult in the middle of the city.
And last but not least, the irrigation water must then be delivered to where the plants need it. Here, too, the bridges with their ring structure and arms offer an extensive network of lines and taps.
A look back at the past reveals a clear picture for Frankfurt: Compared with the period from 1981 to 2010, rainfall has fallen by around 12 percent over the past 10 years. In the course of climate change, the average amount of rain will probably continue to decrease. In addition, gradually increasing temperatures and sunshine intensity lead to longer vegetation phases and more evaporation. In addition to the amount, the distribution of precipitation has also changed - heavy rainfall events and dry periods have tended to increase.
When rain falls on the bridge surface, it is collected by the plants, the substrate and the retention layer. In this way, up to 250,000 m3 of water can be released with a time delay to the cisterns in the roadway floor under the bridges and - again with a time delay - to the sewage system.
The cisterns will be laid under the roadway area in the course of bridge construction. With a storage capacity of approx. 90,000 m³, you can temporarily store a very heavy rain event over all connected areas.
The Frankfurt Bridges will create a network throughout the city. In this way, the various water sources and storage facilities can be connected with each other: the new swimming lake to be created in Niddapark, a wide variety of infiltration areas, rainwater from roofs and parking lot canopies, and groundwater from construction sites - they will all be linked by the bridges.
One often reads the demand that modern cities should become "sponge cities". That sounds like wet basements and dry rot. Another term for the same goal is much nicer: the "water-sensitive city. What is really meant by this?
The "water-sensitive city" braces itself against longer periods of drought and, at the same time, more frequent and extreme rainfall events, by no longer draining accumulating water out of the city as quickly as possible, but instead keeping it in the city's water balance and reusing it.
In the medium term, this could be collected rainwater and water from construction pits. In the long term, however, German cities will also have to treat their wastewater to a greater extent than in the past and at least use it as irrigation or service water. In Frankfurt, the prerequisites are currently being created by the introduction of the fourth treatment stage in the city's wastewater treatment plants.
Frankfurt must continue to work on using less of the surrounding area for its water needs.
The Frankfurt Bridges can collect the required 600,000 to 800,000 cubic meters of water from various sources as part of the bridge project, transport it to storage locations, and make it available again for irrigation when needed.
They also represent an urban infrastructure that can add millions more cubic meters of water to Frankfurt's water balance in the distant future: by also being able to transport even larger volumes of groundwater and rainwater, as well as further treated wastewater, to storage locations.