The Effects of Urban Topography

The topography of a region can modify the speed and direction of the wind, and also have some influence on temperature, mainly through combined effects of air drainage and radiation.  In addition, the problems of air pollution in some cities are worsened due to the trapping effect of streets surrounded by high buildings, and consequently the vertical mixing of the atmospheric layers does not take place (OCDE, 1975).

The main variable in the analysis of pollution in urban spaces is ventilation, which implies a vertical exchange. This exchange can be achieved by three processes: the first is diffusion, a slow process that exists even with no wind; the second is the vertical movement caused by the friction of the wind, and the third is convection, resulting from the vertical differences in temperature. The last two produce turbulence, i.e. the changes in speed overlapping the average speed of the fluid (CPU, 1983).

The exchange of air between the urban spaces and the higher levels of the atmosphere allow their ventilation.

Figure 1A

 indicates this exchange; the ventilation of the urban space depends on the movement of air through the hypothetical plane that defines the top of that space. The exchange of air and of pollution through the plane is achieved by diffusion or by air movement resulting from the action of the wind (CPU, 1983). In hours with no movement of air the products of emissions of vehicles leave the urban space by diffusion. This is the case during thermal inversions. The concentration of the emissions in this case depends on the relationship between the volume of the urban space and the surface of the plane. The volume/surface ratio is an indicator of the difficulty of diffusion. For days with air movement the degree of street ventilation also depends on its proportions .

Figure 1B to 1D

In both cases, the proportions of urban spaces with smaller widths than heights reduce ventilation and the speed in eliminating the products of emissions from the vehicles in these spaces.

References

CONSEJO DE PLANIFICACION URBANA; 1983. Esquema Director de Ordenamiento Urbano. Vol. I (Estudios de transporte); p. 52-123 (Parte II: Impacto ambiental); Municipalidad de la Ciudad de Buenos Aires, Argentina.

OCDE; 1975. Roads and Urban Environment. Paris.