2.1. Playing with urban geometry and morphology
2.1. Playing with urban geometry and morphology
Embedded Files
LASER/F urban microclimate simulation software, official website
Urban geometry and morphology. Ratti et al. , 2003.
As Oke demonstrates (Figure 16a), heat island is proportional to street aspect ratio (Height/Width). As a consequence priority should be given to urban geometry, which offers the smallest H/W ratio, ie. wide streets and low-rise buildings. This is because small H/W ratio allow infrared radiation that is emitted from urban surfaces to escape to the atmosphere, rather than getting trapped in canyon streets.
Figure 16a. Relationship between street geometry (H/W ratio) and maximum heat island intensity (Oke 1987).
This kind of geometry also promotes street ventilation (Figure 16b) and therefore results in better heat evacuation (and also pollution) . We know that when the wind speed in the streets increases, the heat island automatically decreases in intensity.
Figure 16b. The role of urban geometry (H / W) on street ventilation. Case (a) good ventilation, case (b) compromised ventilation, case (c) poor ventilation with street insulation.
To avoid densely built-up areas (Figure 17a) and restore air exchanges between town and its environment, it is necessary to decrease the global volume occupied by buildings. This can be done: 1) in plan by an increase of the distances between buildings in all horizontal directions; 2) in height with small infrastructures. This measure also makes it possible to reduce the overall roughness of the city, and therefore reduces friction and the height of zero-plane displacement (Figure 17b).
Figure 17b. Reduction of mean building height (ZH) also reduces the level of zero-plane displacement (Zd). Bradford, 2015.
Improving the ventilation of the city to avoid the stagnation of hot air in the streets during the night, allows the night cold breezes to penetrate to the heart of the city and transfer the heat out of town. To do this, it is necessary to construct low rise buildings, avoid the compactness of the city center and fragmentation must be encouraged (Figure 18). This can be done with green or blue corridors to separate the neighborhoods, avoid adjoining buildings that stop the wind, use the right orientations in relation to the direction of the wind, promote the nocturnal catabatic slope breezes...
Figure 18. Urban morphology: avoid urban compactness, introduce green corridors, promote nocturnal catabatic slope breezes...
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