In order to improve the urban climate, urban sprawl must be avoided and unsealing and shading of formerly hot asphalted streets must be promoted.

The concept of the Frankfurt bridges represents an opportunity for major cities worldwide to defuse the conflict of interests in modern urban planning: If, in the past, people were forced to sprawl into the surrounding countryside at the expense of the city's climate as the population grew, or if already built-up areas in the city becamde denser, now a space that previously had a rather negative effect in the city is being put to positive use: Above dark asphalted streets, Frankfurt's bridges stretch through the city like a green lung with 1 million square meters of greenery. With their water ring, they contribute to further unsealing and greening in the entire inner city area: a completely new and urban climate-friendly form of redensification.

Chapter content: The chapter provides an overview of the most important factors influencing the urban climate and the role played by bridges in this context

First of all, the result of the analyses on possible consequences of the Frankfurt bridges on the supply of cold and fresh air from the surrounding area is presented. In connection with this, the indirect positive effects of the bridges are described: they help to create living space without urban sprawl and excessive densification in the city.

It is then described how Frankfurt's bridges, both as a building and as a network structure, have an effect on various aspects that positively influence the urban climate: The bridges provide a transport landscape without pollutant emissions above conventional traffic with combustion engines; they create unsealed, irrigated areas and increase cooling shading in the city through the bridge's own corpus as well as indirectly through the irrigation of existing and additional urban trees.

Cold and fresh air supply from the surrounding area are important factors for the urban climate

Good urban planning takes these so-called "cold air corridors" into account in any infrastructure project: after all, grassland and wooded areas around cities are capable of producing cool, clean air and should be preserved wherever possible.

The better this cool and fresh air flows into a city along so-called "air corridors" (usually green corridors or rivers), the more pleasant the urban climate becomes. These cold and fresh air corridors should not be built on under any circumstances

Good urban planning therefore avoids urban sprawl in the surrounding countryside

Urban sprawl must therefore be avoided in good urban planning - at least as far as possible.

With the Frankfurt Bridges, a concept has been developed that creates new living space without building on the surrounding area, which is valuable in terms of the city's climate: "cold air formation areas" in the surrounding area can thus be preserved, from which fresh and cool air can flow into the city.

Built-up bridges over traffic roads is a good concept for urban planning in terms of urban climate - worldwide!

Shirin Kriklava - Stiftung Altes Neuland Frankfurt

The Frankfurt bridges avoid urban sprawl and are a new, more compatible form of redensification: they thus help to protect the city's cold and fresh air corridors

Successive development of the surrounding area is currently the only solution to growing housing demand in most cities and municipalities - including Frankfurt. This is to change with the bridges: Here, redensification does not take place in already built-up areas, but over dark gray asphalted traffic roads, which in any case have no positive influence on the urban climate. If unsealed and greened bridge areas rise above them, these stretches will then even become "green lungs" for the city.

In particular, Frankfurt's green belt must be protected from urban sprawl with regard to the supply of cold and fresh air

With population growth, every city is usually faced with the dilemma of whether it is better to develop the surrounding areas or to increase the density in the city centre in order to create more living space

More than half of the world's population lives in cities - and by 2050 this figure is expected to rise to two-thirds. To accommodate them all, the only options available so far are either (1) to "redensify" cities, i.e. to build on the last remaining open spaces and, if necessary, to build upwards, or (2) to build on the surrounding countryside, thus sacrificing valuable areas for the formation of cold air. Neither is exactly conducive to the urban climate.

Redensification in already built-up areas occurs sporadically in Europe and, in the case of individual buildings, has no perceptible impact on the urban climate. Applied on a large scale, however, it very quickly becomes problematic for the urban climate, as many high-density cities in Asia or South America show.

The Frankfurt bridges offer a solution to this dilemma, which, according to the analysis, does not noticeably impair the cold air and fresh air corridors of Frankfurt

After examining the ventilation and cold air pathways, it can be assumed that the Frankfurt bridges will not have any large-scale influence on the urban climate, as they are too small in terms of area in relation to the total city area and, with an average thickness of the bridge corpus of only two metres, they also do not represent any significant restrictions for the air pathways.

Accordingly, the bridges do represent a form of "densification", but since this densification is accompanied by comparatively low building density and takes place far away from settlement climatic compensation areas - namely above the traffic roads - it is a form of densification that does not affect the urban climate on a large scale.

Experts on the Topic

Universität Duisburg Essen - Lehrstuhl für Angewandte Klimatologie und Landschaftsökologie — RWTH Aachen - Geographisches Institut - Lehr- und Forschungsgebiet Physische Geographie und Klimatologie — HU Berlin - Geographisches Institut - Klimageographie — TU Dresden - Institut für Hydrologie und Meteorologie - AG Stadtklima — Universität Freiburg - Institut für Geo- und Umweltnaturwissenschaften - Professur für Umweltmeteorologie — TU Kaiserslautern - Professur für Physische Geographie und Fachdidaktik - Angewandte Stadtklimatologie

The effect of the urban climate on humans is complex

In cities, human well-being is shaped by various influences. Of particular importance are not only the thermal conditions, which affect the air temperature But also the air-hygienic situation, which concerns the accumulation of air pollutants, and the characteristics of the wind field are also significantly involved in the perception of the urban climate.

It is therefore not surprising that the ideal of a healthy urban climate is characterised above all by moderate temperatures and pleasant wind conditions, while the air quality should be as good as possible.

Shirin Kriklava - Stiftung Altes Neuland Frankfurt GNU

A small-scale analysis of the urban climate focuses on the following three aspects: (1) Immission load, (2) Thermal load and (3) Gustiness

(1) Immission pollution occurs when air pollutants accumulate in the urban atmosphere and do not dissipate. (2) Thermal pollution occurs especially in summer due to strongly heated and stagnant air masses. (3) Unpleasant air currents in the form of gusts often form at the edges of buildings and in street canyons.

(1) Dense building stock can lead locally to increased immission loads

Buildings are an important factor for the urban climate: if buildings are very close together in certain places, air pollutants can collect there to a greater extent.

simonkr - istockphoto.com / Hert Niks - unsplash.com

(2) Between buildings, the air can hardly circulate and heats up more easily, which can lead to thermal stress locally

At the same time, more and higher buildings also mean increased shadows - which in turn can have a cooling effect. However, shade is usually only really pleasing outdoors, as it is perceived as an impairment of the light situation in built-up areas - at least in Central and Northern Europe.

(3) The local flow situation at a particular location in the city can be significantly altered by new development structures under certain circumstances

On the one hand, a structure can reduce the wind speed close to the ground because the flow path is obstructed. This has a negative effect not only on air hygiene but also on thermal comfort: Air pollutants that are released close to the ground (e.g. exhaust gases from vehicles) can be transported away more poorly. Experts refer to this as "aggravation of the immission situation in the road space".

The local climate also changes when there is no more wind on the ground: heat can no longer be dissipated as well - resulting in more intense heat islands in hot weather.

Lucas Lindau - Stiftung Altes Neuland Frankfurt GNU

Depending on the situation, however, new development can not only obstruct the wind: on the contrary, it can also ensure that the wind is channelled, catches somewhere and even causes turbulence.

This is positive for air hygiene, because the air pollutants are diluted; the thermal load can also be reduced - especially on hot days - by the increased gustiness: The locality is better ventilated, so to speak. However, strong gusts can also drastically reduce the local comfort factor - despite better air quality and moderate temperatures - at the latest when your sun hat flies off.

The shape and layout of buildings are important factors for the urban climate, especially when it comes to the issue of gustiness

Common example: Dense development in particular can lead to turbulence

With dense development, the wind near the ground is disconnected from the overlying flow. This reduces the "wind speed near the ground" and the street space is less well ventilated.

In order to demonstrate the extent to which wind movement in the street canyons will be reduced, flow and climate models can be used to simulate the conditions before the start of the construction phase. In this way, it is possible to estimate in advance of the construction measures how the air currents will behave on a small scale at the individual locations in the city.

However, turbulence does not only occur with dense development, but also with the exact opposite: the solitary buildings

If individual buildings protrude above the average building height, as is the case with some high-rise buildings in Frankfurt, very turbulent flows can occur in the vicinity of these solitary buildings close to the ground due to the downward deflection of the upper flow into the street space: Complex turbulences then occur when flowing around the edges of the building and downwind of the structure. This leads to a strong increase in local gustiness. Anyone walking through the skyscraper canyons in Frankfurt can sometimes feel this very rudely.

Although the Frankfurt bridges are not solitary structures, similar effects can also occur as a result of plateaus, such as the planned bridge structures. Therefore, the complex flow conditions caused by the bridge structures must also be thoroughly investigated and simulated in the course of the preliminary planning.

And sometimes wind and turbulence fields occur below the "mean roof level" - i.e., on the façade below the edge of the roof

Experts refer to this as channeling and jet effects: The wind whistles through the road as through a channel. The narrower the channel becomes - or even if two channels (i.e., roads) merge into one channel - the flow velocity increases and the so-called "Venturi effect" occurs.

In any case, this effect will be investigated for the bridges during the preliminary planning by means of flow simulations. Although road traffic under the bridges remains largely unaffected, such flows could become very unpleasant for pedestrians.

Experts on the Topic

Universität Augsburg - Institut für Geographie - Stadt- und Geländeklimatologie — FU Berlin - Institut für Meteorologie - AG Stadtklima und Stadtmessnetz — Ruhr-Universität Bochum - Geographisches Institut – Klimatologie — Universität Freiburg - Institut für Geo- und Umweltnaturwissenschaften - Professur für Umweltmeteorologie — Universität Kassel - Institut für Landschaftsarchitektur und -planung - Fachgebiet Umweltmeteorologie — Universität Mainz - Geographisches Institut - Klimatologie (Stadtklima) — Universität Münster - Institut für Landschaftsökologie - AG Klimatologie

All three aspects that affect the urban climate locally can be significantly mitigated by a high proportion of vegetation area in the city

Vegetated areas have several advantages: in a city park, for example, there are no cars and there are no other potential sources of pollution.

Unsealed and greened areas do not heat up as much and therefore represent so-called relief areas and counteract urban overheating.

Parks have trees and bushes instead of buildings, so that there is no gustiness due to canyon shapes and the like.

Accordingly, every city can positively influence its urban climate by increasing the proportion of inner-city vegetation areas.

The Frankfurt bridges, with their 1 million square metres of green surface, represent an extension of the urban vegetation areas

Overall, the bridges do not have a negative impact on the urban climate: neither do they increase the accumulation of pollutants, nor do they lead to heat islands, nor do they favour gusts - on the contrary: some feel-good factors, such as cool and "fresh" (i.e., low-pollutant) air , can be favoured by the bridges.

The shadows cast by a building keep it cool: and Frankfurt's bridges shade the streets below them, which would otherwise heat up in the blazing sun.

Unsealed surfaces that can absorb rain and irrigation water also contribute to a pleasant local climate with evaporative cooling: And the surfaces of the bridges are unsealed wherever there is not a vehicle driving or a house standing.

Google Earth / Stiftung Altes Neuland Frankfurt GNU

The bridges also offer many advantages due to their net character

The Frankfurt bridges are not only positive for the city climate as a shaded, greened and irrigated structure in themselves: they also open completely new possibilities as a special unique infrastructure system for the city by (1) providing a platform for emission-free traffic, (2) irrigating unsealed areas in the city, (3) thereby enabling more shady tree cover in the city and (4) also transporting water to a variety of fountains and newly created water bodies.

(1) Zero-emission bridge traffic helps reduce the use of internal combustion engines in the city

In most cities, motor vehicle traffic is the main emitter of nitrogen oxides and is also largely responsible for fine and coarse particulate pollution. In the Rhine-Main region, too, traffic is the largest polluter in the ground level.

With their zero-emission hydrogen and electric vehicles, the Frankfurt bridges not only provide climate-friendly local transport for the bridge district, but also relieve the strain on the road traffic that flows beneath them with around 30 million passenger journeys a year - and thus help to reduce pollution in the city.

The solar energy generated on the bridges is used, among other things, for hydrogen production and thus also enables numerous hydrogen filling stations for traffic on the ground. For e-cars, there are charging options at hundreds of bridge piers, which also promotes the move away from the internal combustion engine.

(2) The bridges have a positive impact on the areas along their course

In addition, the bridges as a structure along their course make a positive contribution to the urban climate: by irrigating the green areas to the right and left of the bridge as well as many unsealed areas in the city, their positive effect extends beyond their own bridge corpus.

(3) The bridge as an irrigation infrastructure helps in reducing thermal load in the city

Good urban planning provides for unsealing and planting of road surfaces, because soil does not store heat as strongly as asphalt. This has a cooling effect, especially at night. If the soil is also wetted or soaked by irrigation water or rain, then it serves as a particularly intensive cooling surface.

The most effective way of providing thermal relief is to plant large-crowned trees, because trees cast shade, and this cools streets noticeably.

However, urban planning must be carefully considered here, because not all trees are the same: some trees emit trace gases from which ozone is formed in sunny weather; others have such dense crowns that - especially when planted in rows as avenues or at bottlenecks - they act as flow obstacles and keep out cooling winds. Tree selection must therefore be carried out with professional support.

The bridges help the city to have more trees

The bridges are crisscrossed by a water ring main that allows them to bring water to the city over a wide area.

They enable the irrigation of 40,000 square metres of unsealed area and the planting and irrigation of around 1,000 additional trees in the city centre - thus making a significant contribution to increased shade in the city.

(4) With the help of a water supply structure such as the bridges, urban planning can also create more fountains, misting systems or even water areas to cool down on hot days

The bridges' ring mains can supply water for hundreds of fountains or fogging systems. It also feeds the new bathing lake on the Nidda.

But all these water-cooling sources must be carefully planned, because under certain weather conditions they can also have the opposite effect: Because evaporation can also lead to muggy air instead of the cool breeze:

Conclusion: Frankfurt's bridges do not have a negative overall impact on the city's climate

The Frankfurt bridges do not affect Frankfurt's cold and fresh air corridors to any perceptible extent - quite the opposite: as they represent a concept to avoid urban sprawl in the surrounding area and to relieve the city during redensification, they have a positive long-term effect on the supply of cold and fresh air. They also offer more advantages than disadvantages in terms of microclimate: they hardly increase the accumulation of pollutants, but at the same time promote coolness through shade cast by their own corpus as well as newly planted irrigated trees; likewise, they create a pleasant urban climate through unsealed, greened and irrigated areas.

Frankfurt: Stadtklima aktuell

Auswirkungen der Brücken

Maßnahmen neben den Brücken

Die CO2-Bilanz der Brücken

Stadtklima – Weltklima

Das Ziel: Die Stadt gegen Trockenheit und Flut wappnen

Die grüne Zukunftsmetropole

Vitalisierung des Stadtgrüns