THE RESILIENCE OF CITYSCAPES

 

The resilience of cityscapes against climate change is predominantly determined by
the properties of their surfaces and the spatial arrangement of the buildings. These
factors induce the occurrence of urban heat islands or flooding. When global radiation
reaches a surface it may be reflected (Albedo) or transformed to sensible or latent
heat flux. Whereas plants are able to transform the sun energy into biomass, oxygen
and air humidity, regular building surfaces (e.g. plaster) emit sensible heat flux. Plants
regulate the urban microclimate while conventional surfaces lead to microclimatic
extremes and reduce the thermal comfort within cities. Aside from the positive
microclimatic effects plants are also able to store water in contrast to sealed regular
urban surfaces.
A research group investigated the multitude of positive effects of green urban
infrastructure like green roofs, living walls and greened permeable pavements. The
impact of green infrastructure on an urban fabric has been visualized by computer
modeling tools. The computer model results showed that all tested green technologies
provide benefits to the urban microclimate and water storage capacity.
They make clear that green infrastructure is an answer to increase the resilience of
cityscapes worldwide.


The challenge

Cities are subjected to permanent transition in a multitude of aspects. There is a clear
trend to urbanization and more than fifty percent of the world’s population now live in
cities. Two effects of this influx can be observed: the occupied city area grows and
density increases. At the same time citizens request more infrastructure from cities
such as public transport, recreation and sewage systems. City planners are
challenged to combine the pressure of growth and integration of satisfactory
infrastructure.
Within this context the global climate is changing. Cities are affected proportionately
more by the rise of temperature and extreme weather conditions as described in
London’s Urban Heat Island report or by Formeyer et. al [2009]. As a consequence,
the quality of life suffers and the attractiveness and competitiveness of cities as
liveable places are diminished.

Spatial arrangement of buildings, design and properties of city surfaces and usage of
plants are frequently mentioned as potential solutions to the improvement of the
resilience of cityscapes towards climate change [Katzschner, 2011].
Global radiation reaches city surfaces and can be reflected or transformed to sensible
or latent heat flux. The arrangement of buildings in consideration to the sun has a
significant effect on shading and cooling of buildings as shown in figure 1 and figure 2.

Figure 1: Commercial area in Vienna (Auhof Center). Left: areal image; right: mean mean radiant temperature near surface [Trimmel,

Figure 2: Residential area in the 9th district of Vienna (Canisiusgasse). Left: areal image; right: mean radiant temperature near surface [Trimmel, 2013, unpublished].

In contrast to regular surfaces (plaster, tin or brick) plants convert sun energy through
photosynthesis and transform it into biomass (carbon fixation), oxygen and air
humidity. Consequently it is assumed that plants ameliorate the urban microclimate
(by adding humidity and reducing radiation and wind speed) while regular surfaces
reduce the thermal comfort of cities. Aside from the positive microclimatic effects
plants are also able to store water.

In this paper the multitude of positive effects of plants used as green infrastructure
(green roofs, living walls and greened permeable pavements) are discussed in detail
to illustrate the potential of plants to contribute to the improved resilience of cities
against climate change.


Methodology

In the course of this research the microclimatic effects, building physical properties
and water retention potential of 14 green roofs, 5 living walls and 9 surface
consolidation methods have been monitored using sensor technology. The following
parameters have been measured continuously: albedo, air temperature profile, air
humidity profile, substrate and plaster temperature, heat flux, substrate humidity, water
balance. Additionally thermal photographies were done to measure the radiant
temperature. Figure 3 illustrates the sensor based measurement principle by means of
a green roof. On basis of a two minutes measurement interval for all parameters the
ten minutes average has been calculated and safed by a data logger.

 

Figure 3: Measurement principle for acquisition of microclimatic and building physical properties of green roofs.

 

To project the measured effects of green infrastructure on the city scale the microscale
modelling software ENVI-met [Bruse, 2012] has been used. Three representative
urban typologies of the city of Vienna have been chosen to simulate the effects of
virtually applied green infrastructure on the microclimate in comparison to the their
present state. The chosen typologies have been subjected to scenarios of climatic
framework conditions from 1980-2010 and 2050-2080 (as projected by Formayer,
2011 and ZAMG, 2012) and different levels of integration of green infrastructure. The
following parameters have been simulated: mean radiant temperature, potential
temperature, PMV (predicted mean vote).


Results field measurement campaigns

With an average albedo of 20%, the albedo of green infrastructure is comparable to
brick and many other typical urban surfaces. The ability of green infrastructure to
evapotranspirate – in contrast to standard urban surfaces – plays a key role in
influencing the microclimate. In addition the shading effect of plants is decisive to
improve the PMV.
Figure 4 correlates the relative air humidity with the air temperature. The
evapotranspiration effect of a living wall on the relative air humidity can be seen
clearly.

Correlation of surface near relative air humidity (rH) and air temperature:

 

Figure 4: Correlation of air humidity and air temperature: green graph shows the correlation of the living wall; blue graph shows the correlation
of the plaster facade.

Figure 5: Influence of evapotranspiration on the radiant temperature. Left: photo of 
living wall; right: thermal photo of a living wall; temperature differences indicated in Grad Celsius

 

The following figures 6 and 7 provide information on the temperature profile of a green
roof and a tin roofing. The air temperature was measured 0.4 m above ground and 5
cm above ground. For the green roof the substrate temperatures have been measured
at the surface and in the middle of the construction. For the green roof and the tin
roofing the building envelope temperature has been measured (see figure 3). This
temperature also represents the surface temperature of the tin roofing.

 

                                                     Temperature profile - Material C                    

Figure 6: Temperature profile of an extensive green roof (12 cm total construction height).

 

Figure 7: Temperature profile of light grey tin roofing.

 

Results microclimate modeling

The results of the field measurement campaigns have been used as basis for
microclimatic simulations. To illustrate the effects of green infrastructure on the
microclimate the following virtual scenarios have been chosen for a residential area (a
historical perimeter block development area Canisiusgasse) in the 9th district of
Vienna. The major purpose of the developed scenarios is to quantify the potential
microclimatic effects of virtually applied green infrastructure on a representative urban
typology of Vienna.


Climate scenarios

The cooling potential of green infrastructure is the focus of the performed simulations.
This potential can be seen best on extremely hot days. The 99% percentile of diurnal
air temperature maxima has been calculated using climate prognosis for the actual
climate (1980-2010) and future climate (2050 – 2080) [Formayer, 2011]. The 99%
percentile is the temperature value that is not exceeded on 99% of the days of the
respective period. Hence, it represents the 110 hottest days in a 30 year period.


Greening scenarios

Two different intensities of applied green infrastructure have been chosen to be able to
estimate the ratio of necessary green infrastructure to adapt the urban fabric to climate
change.


The minimum greening scenario

This scenario focuses on urban surfaces, with the highest impact on the urban
microclimate (radiation, size) when greened: Unsealing of all private owned parking
lots, pavements and inner courtyards, extensive green flat roofs with a gradient less
than 5 %* and greened area on all south exposed facades.


The maximum greening scenario

The maximum greening scenario focuses on technically seen maximum applicable
integration of green infrastructure on microclimatic relevant surfaces:
all parking lots, pavements and inner court yards are unsealed, all flat roofs with a
gradient less than 5 %* are greened intensively, roofs with a gradient between 5 and
20%* are greened extensively, all South, West and East facades are greened.
[*Gründachpotentialkataster 2012   http://www.wien.gv.at/umweltschutz/raum/gruendachpotenzial.html]


Climate change effect on potential air temperature

The temperature increase in the area of the Canisiusgasse predominantly affects the
south, west and east exposed facades. Protected areas, as inner court yards are less
affected. The open spaces warm up homogeneously by 2.25°C above the actual 
average temperature (see figure 8).

Figure 8: Change of potential air temperature from today to projected climate in 2050 [Trimmel, 2013, unpublished]..

 

Surface near mean radiant temperature:

Figure 9: Surface near mean radiant temperature based on climate period data
1980-2010 of Canisiusgasse. Left: status quo, middle: minimum greening scenario,
right: maximum greening scenario [Trimmel, 2013, unpublished].

 

The minimum greening scenario reduces the mean radiant temperature next to the
south exposed facade by 20°C (see green line on south facade).
The maximum greening scenario reaches a mean radiant temperature reduction of
25°C near the facades and as a result of unsealing the parking areas an average
reduction of mean radiant temperature of 5°C.


PMV - predicted mean vote

PMV describes the human thermal wellbeing. The air temperature, air humidity, mean
radiant temperature and wind speed are the relevant input variables of the predicted
mean vote. Additionally human activity and clothing affect the PMV. The following
figures 10 and 11 illustrate the effect of the different types of green infrastructure:
green roofs, facade greening, unsealed surfaces in relation to the status quo of the
selected residential area.

Figure 10: Influence of green roofs in maximum greening scenario based on projected
climate period data of 2050-2080. Left: surface near PMV, right: PMV at roof level
[Trimmel, 2013, unpublished].

 

Green roofs show the least influence on the near-surface PMV. On roof top level a
significant reduction from 3 to 1 was simulated.

 

Figure 11: Left: influence of living walls in maximum greening scenario Right:
influence of unsealing in maximum greening scenario (both based on projected
climate period data of 2050-2080 [Trimmel, 2013, unpublished]).

 

Living walls affect their immediate surroundings. At south exposed facades a PMV
reduction from 4.5 to 3 has been simulated. Permeable surfaces constitute minor
effects on the PMV than green roofs and living walls. The reduction calculated for
sunny areas is from 4.5 to 4 and shady areas from 2.5 to 2.

The combination of all types of green infrastructure showed the best results in terms of
melioration of the PMV. The PMV of south exposed facades could be reduced from
4.5 to 2 and the PMV of the pavements from 2.5 to 1.5 (see figure 12).
This simulation result makes clear that it is necessary to apply a mixture of green
infrastructure types to achieve maximum effects.

Figure 12: influence of all types of green infrastructure in maximum greening scenario based on projected climate period data of 2050-2080
[Trimmel, 2013, unpublished].

 

Summary

In a changing climate the status quo of our cities leads to a reduction of thermal
comfort during heat episodes. Green infrastructure can act as a buffer for climatic
extremes. Measures have to be taken to at least keep the level of today e.g. in terms
of PMV.
As the presented research shows, green infrastructure is able to significantly increase
air humidity by evapotransipration. The effect of shading, cooling and protecting
building surfaces could also be demonstrated by the comparison of a tin roof and an
extensive green roof (figures 6 and 7). This fact is relevant to the building itself (and
especially the durability of the roof’s waterproofing) and the urban microclimate since
the radiation of sensible heat correlates with the surface temperature
(Stefan-Boltzmann law).
By means of computer simulation the measurments at test sites have been transferred
to representative urban typologies of the City of Vienna. To find out, which
microclimatic effect could be achieved by green infrastructure two greening scenarios
have been applied on urban typologies and subjected to todays and future climatic
framework conditions. The simulations make clear, that the urban microclimate can be
ameliorated by integration of green infrastructure.
Therefore green infrastructure is justifiably as one appealing solution to improve the
resilience of our cities against climate change. Apart from the microclimatic effects and
the positive influence on thermal comfort, green infrastructure provides a broad range
of added values: water retention, health promotion and psychological effects (stress
reduction), habitat and habitat connection for fauna and flora, biodiversity and urban
farming.
But it must be pointed out that a single green roof or living wall are no more than a
drop in the bucket. To achieve reasonable improvement of the resilience of cities or to
strongly affect neighborhoods a combination of different types of green infrastructure
and a network of green infrastructure throughout the city is necessary [Yu and Hien,
2005].
Although the benefits of green infrastructure and their potential to adapt cityscapes to
climate change are clear, the implementation at a degree used for the simulations may
be difficult to achieve. Most flat roofs could be greened with existing technical
solutions as shown in the simulations. But facades are often protected or of high
architectural value. There are also many regulations that need to be overcome as e.g.
fire protection regulations in Vienna.
Another constraint is the fact that most buildings are privately owned. Incentives have
to be developed, as e.g. private-public-partnerships.
Landscape architects play a significant role in the application of green infrastructure,
being the profession that designs urban open spaces. They shall be the ambassadors
of green infrastructure within cities to ensure a more livable and comfortable urban
environment.

 

References

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Gruendachpotentialkataster (2013).
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Formayer, H., Clementschitsch, L., Hofstätter, M., Kromp-Kolb, H. (2009). Vor Sicht
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Katzschner, L. (2011). Urban Climate Strategies Against Future Heat Stress
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ZAMG (2012). Zentralanstalt für Meteorologie und Geodynamik. A-1190 Wien. Hohe
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