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 objected to permanent transition in a multitude of aspects. There is a clear trend to urbanization. Meanwhile more than fifty percent of the world population lives in cities. Hence two effects can be observed: the occupied city area grows as well as the density. At the same time citizens request more infrastructures from cities as public transport, recreation, sewage system etc. City planners are challenged to combine the pressure of growth and integration of satisfactory infrastructure.

 

On top of that the global climate is changing. Understanding actual publications and predictions of meteorologists, cities are affected over proportional by the raise of temperature and extreme weather conditions. As consequence, the quality of life suffers, attractiveness and competitiveness of cities 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.

 

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.

commercial area Vienna

      
Figure 1: Commercial area in Vienna (Auhof Center). Left: areal image; right: mean radiant temperature near surface [Trimmel, 2013, unpublished].

 

commercial area Vienna2

 

 

  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 consume in course of the photosynthesis process sun energy and transform it into biomass (carbon fixation), oxygen and air humidity. Consequently it is assumed that plants meliorate the urban microclimate 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 urban infrastructure like green roofs, living walls and greened permeable pavements are discussed in detail to illustrate the potential of plants as contribution to the improvement of the resilience of cities towards climate change.

 

Methodology

 

In the course of different research projects the microclimatic and building physical properties as well as the water retention potential of 14 green roofs, 5 living walls and 9 surface consolidation methods have been monitored. 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.

Measurement principle

 

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 green infrastructure on the microclimate in comparison to the status quo. The chosen typologies have been subjected to scenarios of climatic framework conditions (1980-2010 and 2050-2080) and different levels of integration of green infrastructure [Formayer, 2011 and ZAMG, 2012]. The following parameters have been analyzed: 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 air humidity and air temperatureFigure 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.

Influence of evapotranspiration on the radiant temperature

      
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 [°C].

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. This temperature also represents the surface temperature of the tin roofing.

Temperature profile of an extensive green roof

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

Temperature profile of light grey tin roofing

 

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 scenarios have been chosen for a residential area a historical perimeter block development area Canisiusgasse in the 9th district 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 includes: 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 includes: 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 green 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).

Change of potential air temperature from today to projected climate in 2050

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

Surface near mean radiant temperature

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 thanks to 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.

Influence of green roofs

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.

influence of living walls

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.

influence of all types of green infrastructure

 
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 and 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. Green infrastructure is justifiably one appealing solution to improve the resilience of our cities against climate change. Apart from there microclimatic effects and the undoubtedly positive influence on our 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, 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 reasonably improvement of the resilience of cities or strongly affected neighborhoods a combination of different types of green infrastructure and a network of green infrastructure is necessary. The application of green infrastructure needs professional planning by landscape architects.

References

Bruse (2012). http://www.envi-met.info/dwiki/doku.php last access 6.3.2013

Gruendachpotentialkataster (2013). http://www.wien.gv.at/umweltschutz/raum/gruendachpotenzial.html

Formayer (2011). Aufbereitung von Klimaszenarien für die Klimafolgenforschung. Oesterreichische Gesellschaft für Meteorologie, Tagungsband des 4. Oesterreichischen MeteorologInnentag.

ZAMG (2012). Zentralanstalt für Meteorologie und Geodynamik. A-1190 Wien. Hohe Warte 38.