Unsealing the inner city - green life instead of gray asphalt

Around 40,000 m2 of asphalt in the city center are to be unsealed and planted - with planting of a total of 1,000 new trees in the city . The unsealed areas can be co-watered by the Frankfurt bridges' ring main. The cistern system of the bridges can be supplemented in more remote areas by local cisterns that can store rainwater on site from the building roofs around the unsealed area.

Content: Valuable green spaces can be created by unsealing areas within Frankfurt's urban area

Thanks to the ring main, these areas can be well irrigated. In addition to the water source "ring main", rainwater from the surrounding buildings can also be collected in cisterns for the unsealed areas.

Where water cannot be supplied to more remote unsealed areas during dry periods by direct connection to the Frankfurt Bridge irrigation system, tankers can tap water from the ring main and transport it to the green spaces.

The unsealing of cities should be further promoted

In the meantime, there are also hot summer days in Germany with maximum temperatures of over 40 degrees. It gets particularly hot and stuffy during such heat waves in cities like Frankfurt: The air comes to a standstill in the urban canyons and heated buildings, walls and streets can literally turn city centers into baking ovens.

The reason for this is the materials from which cities have been built since the post-war period: Asphalt, concrete and especially glass heat up much more than wood, clay or most natural stones.

On hot days, the difference between downtown Frankfurt and the surrounding area can be as much as ten degrees.

In addition, in recent years there is the other extreme: heavy rain and flooding.

Unsealing is the solution: Just as every square meter of asphalt and concrete heats up the city, every little green space cools it down. At the same time, every unsealed infiltration area absorbs floodwater from the streets.

Inner cities as green hotspots - where there was gray before, lush greenery will provide fresh air and cooling in the future

The ring of bridges around the city center serves as a water supplier.

This is what rises in cities usually look like today....

..and this is how they can look in the future with rainwater harvesting

Downtown not just a gray open space....

..but also as a lively recreational area

Flowers instead of asphalt

Everyone knows them: the squares and pedestrian areas concreted over in the sixties, seventies.

The Hauptwache in Frankfurt is just one example - large areas, almost without greenery. Squares like the one in front of St. Catherine's Church can become green oases where everything except the walking areas is unsealed - making them more attractive for people: a living space and not just a functional area.

The permanent preservation of the unsealed areas is planned in cooperation with the bridge company

Unsealed green spaces need water and care - both can be synergetically supported by the bridges: The ring main of the bridges represents the basic structure of a watering network that can be used by the Parks Department for Frankfurt's urban green spaces.

It also makes sense to cooperate with the bridges' gardening crews in the maintenance of the new high-quality areas in order to realize synergies.

For each area to be unsealed, it is necessary to check the subsoil and the light conditions

Basic plan Hauptwache: Almost all beds are planted in tubs, as the B-level is located below.

Exemplary irrigation of a large unsealed inner city area: In addition to the water source "ring main", rainwater from the surrounding buildings can also be used for unsealed areas

For 2000 m2 of unsealed and planted new beds at the Hauptwache, for example, so much water is needed that, in addition to the tank runs from the ring main, cisterns should be sunk into the ground to collect and store rainwater from the surrounding roofs. It should be noted, however, that coordination with surrounding building owners for use of their roof runoff could be difficult, as most are still likely to combine rainwater and wastewater close to or within buildings using conventional piping structures.

Why should only the prominent spots of a city actually be unsealed?

With good planning and pragmatic irrigation solutions, entire neighborhoods can be unsealed and greened.

Not only prominent places should be greened: Wherever possible in the city, the soil should be unsealed and new green spaces created: on pedestrian islands, at the roadside, on squares and in pedestrian zones.

Even if this cannot be implemented from one day to the next, it should be conceived as infrastructure planning and implemented successively.

As part of the planning of the Frankfurt bridges, an unsealing plan for the city center has been drawn up as an example, because with the help of the bridges there is an extensive network of standpipes for the irrigation of the unsealed and greened areas: No matter where new greenery is to be created

- no drinking water may be used for watering, but a connection to a source of irrigation water or rainwater must be ensured.

In Frankfurt, a total of around 40,000 m2 can be unsealed and greened. And there is also room for around 1,000 trees in the city center.

The unsealing program for the city center works with different modules

Not everywhere the ground can simply be torn up and trees and flowers placed: The unsealing plan therefore provides suitable modules for different locations.

Trees: Within the bridge ring 1000 trees can be planted, along the outer arms another 2000.

Bucket: Along the roads, in many places there are rows of posts facing the roadway, which are replaced by planters without a bottom: This way, the plants in them are protected and can root in the soil.

Area: Usually, smaller dead corners or even larger traffic islands are lending themselves to unsealing.

Plat Arches: In particularly narrow, but also particularly bleak places next to the roadway, where no wider tubs fit, slender planting arches can provide a more beautiful streetscape.

When planting urban trees, tree species must be selected according to their location: Above B-levels or underground car parks, for example, only trees that are suitable for buckets can be planted

Green stops benefit not only nature

Green stops already exist in Frankfurt. This concept can be extended to hundreds of stops in Frankfurt with the help of the Frankfurt Bridges irrigation system. To ensure long-term greening, ground-based planting is recommended.

Lighting on the planting arches ensures well-being in such streets even after dusk. Heavily trafficked streets with narrow sidewalks are also to be upgraded by planting arches - especially those with residential buildings

Google Maps Stiftung Altes Neuland Frankfurt

Experts on the Topic

Hochschule Geisenheim - Institut für Landschaftsbau und Vegetationstechnik — Technische Universität München - Zentrum Stadtnatur und Klimaanpassung (ZSK) — Stadt der Zukunft - Stadtbegrünungstechnologien — Technische Universität München - Professur für Green Technologies in Landscape Architecture

However, the areas to be unsealed are not directly adjacent to the bridges with their ring mains. Their irrigation is therefore a challenge - The solution: clay planters with integrated water reservoir

For planters without a bottom, the rim of the bucket can be filled with water as a reserve for dry periods

These special tubs have no bottom - the tub substrate has direct connection to the soil. This allows the roots of the potted plants to spread in the soil and supply the plants with water.

The water storage system is integrated into the bucket wall: This is double-walled (outer wall glazed, inner wall without glaze), which means: hollow inside. The water can be poured in through an opening at the edge. It is held there temporarily and only trickles down successively. Thus, unlike conventional tubs, the root spaces do not simply dry out, because they have access to the reserves in the soil - and quite a while "wall storage" surrounding them.

The solution for lower plants in the middle of the asphalt: the invisible water reservoir

There are also areas where tubs cannot be placed (e.g., because an important viewing angle for motorists would be blocked), but where (lower) plants would still be an asset. There you have to switch to planter boxes that can be sunk into the ground.

Also, since only low planting is possible in these places, you do not need as much water as when a bush or shrub is to grow in a planter for years and decades. So these planter boxes normally have a closed bottom, which facilitates water retention.

Such a planter box of, for example, a size of one meter by one meter contains 40 centimeters of substrate in the upper part, while in the lower part there is a retention space. This holds up to 600 liters of water, which can be filled via an inlet pipe. This allows small low plants to get by for several dry weeks.

Plants on traffic islands can survive dry spells by using a potted water reservoir.

In the course of unsealing, new beds will be created. There, infiltration from the surrounding road area is a suitable source of irrigation.

The bed is designed like a trough so that surrounding water can drain into the bed and seep away. The pollutants that are brought in from the sidewalk or the street should be decomposed and bound by an intact soil life, among other things, so that negative effects on the groundwater do not occur.

If urban trees are newly planted, they can be equipped with "survival aids" from the very beginning - and wherever possible, be connected to the rain gutters of the adjacent roofs

City trees over B levels or Subway tunnels

If you want to unseal and plant trees in areas where there is no soil below the surface, but only tunnels, you have to limit yourself to trees that can be planted in tubs- they need special water reservoirs, as there is no corresponding layer of humus as a reservoir.

Even if the bucket is designed as a reservoir - these trees need to be watered more often by tanker truck.

Conclusion: With the help of the Frankfurt bridges, the water supply can be ensured for unsealed areas of the city center.

Unsealed surfaces in the city have positive effects on the urban climate, biodiversity and the well-being of people.

Suitable planting and irrigation modules have been developed for a wide variety of versified areas such as pedestrian islands, narrow strips on sidewalks or former parking lots.

The unsealing plan for Frankfurt's city center is an integral part of the bridge planning. It also includes the planning of an irrigation system for the unsealed areas that, like the irrigation on the bridges, functions with little plastic and with the least possible loss of irrigation water through evaporation.

Greenery on the bridges

Plantlife along the bridges

Nature conservation and biodiversity

Vitalization of the urban greenery

The green metropolis of the future

The goal: to arm the city against drought and floods

Water distribution with the bridge-ring-line

Frankfurt: Current urban climate