Morphology of Building Development as an Element of Urban Ventialtion System

Warsaw, the largest Polish city, is located in the central part of the country, on the Middle Mazovian lowland, on the Vistula River. According to the Köppen-Geiger classification, the city is situated in a humid continental climate zone. It covers an area of 517 km² and has a population of 1.8 million. Due to natural and historical conditions, Warsaw’s building development forms a compact central system cut by the Vistula valley. Like most large cities, Warsaw is affected by the urban heat island phenomenon, which is sure to increase as a significant increase in building development is planned there over the next 50 years [10, 11]. Smog is another progressing climate problem. According to the current IQAir rankings, among Polish and European cities, Warsaw ranks relatively high in this respect [13]. The system of ventilation corridors, created in 1916 by Tadeusz Tołwiński as part of the Sketch of the Warsaw Regulatory Plan, is of fundamental importance for the ventilation of Warsaw. The system emerged during a crucial period when loosely built-up suburbs were incorporated into the city’s territory, whereas its area tripled (from 33 to 121 km²). The system consists of 9 greenery-covered, undeveloped strips of land that stretch in a radial pattern from the outskirts to the city center (Fig. 1). They provide the route of air inflow from the outer city areas to the downtown. The airflow is caused by local winds or city breezes on windless days (the period of weak winds and atmospheric silence in Warsaw amounts to 1/3 of the year) [11]. In the 1990s, the aeration corridors were freed from the administrative protection of agricultural land. Since then, they have become the most attractive locations for housing investments, whereas continuous attempts are being made to develop the areas gradually. The Vistula valley remains the only continued uninterrupted corridor in the central part of Warsaw.

This system, although already depleted, remains important for air exchange in Warsaw and is thereby protected by The Study of the conditions and directions of spatial development in Warsaw [14]. However, no coherent pro-ventilation policy exists regarding buildings developed in the areas outside the corridors. The existing planning regulations and procedures with which the intensity parameters and the building development shape issues are regulated fail to account for the ventilation criterion. Thus, spatial characteristics such as the orientation, compactness, and porosity listed in Table 1 are not applied in the aerodynamic aspect. This observation is disturbing, as a large part of the development is shaped in a quarterly form, closed to the free wind flow. Thus, the intensive building development densification process in Warsaw, which has been underway since the 1990s and is forecast for the next 30 years, may significantly inhibit air exchange in the city.

Figure 1.

Krakow is located on the Vistula River in the southern part of Poland, at the junction of the Cracow-Częstochowa Upland, the West Beskidian Foothills, and the Sandomierz Basin. It is located in the same climate zone as Warsaw. The city area equals 327 sq. km, whereas its population equals 0.8 million. Althugh Cracow is smaller and less populated than Warsaw, it is more clearly affected by smog. In fact, it takes the lead as a large Polish city most exposed to this phenomenon. This is due to the geographically unfavorable shape of the city, whose center is located in a valley. Thus, Crakow is not susceptible to prompt air exchange. The smog situation has recently improved thanks to administrative measures to reduce the number of coal-heated households and limit transport pollution. However, the complete elimination of smog requires multidirectional actions, including air exchange intensification. Air exchange is also important with regard to the urban heat island phenomenon. Although less intense in the case of Cracow as compared to Warsaw, the problem also occurs and requires intervention. Cracow’s building development is formed as a central system, slightly more stretched along the east-west axis, along the Vistula riverbed.

Unlike in Warsaw, no clear layout of corridors free of building development is present in Cracow’s urban structure. Naturally-formed terrains in the form of river and stream valleys and terrain depressions act as corridors. In The Study of the conditions and directions of spatial development in Cracow, seven such corridors and four areas (called aerosanitary corridors) were distinguished, with recommendations for their protection [15]. However, these areas can hardly be called a continuous, comprehensive system that could operate on the scale of the city as a whole. As in the case of Warsaw, such a system had been initiated during the partitions as part of The Regulatory Plan for Great Cracow, when the city’s territory was significantly enlarged by new peripheral areas [16].

However, the system failed to be recognized as important for city ventilation and was insufficiently protected. Over 100 years later, only certain areas have remained excluded from development for natural reasons. These, however, influence the local climate rather than the mesoclimate of the entire city. Despite the efforts made by the authorities to extend the protection of these areas (for example, the western corridor was enlarged), the situation can only be improved to a limited degree. The building development spatial features, such as orientation, compactness, and porosity, fail to be accounted for in the aerodynamics context. Thus, similar to other large Polish cities subject to the process of development densification, Cracow faces the risk of inhibited city ventilation due to intensified development and continuous increase in the paved area ratio.

In recent years, research has been undertaken on the so-called dynamic ventilation of Cracow generated by a system of ventilation towers and chimneys (Fig. 2) [17]. Such possibilities are tested at The Wind Engineering Laboratory, Cracow University of Technology by Andrzej Flaga’s team. The concept is to use ventilation towers (in the form of masts with a fan system) to stimulate horizontal air movement along the existing aerosanitary corridors. The air directed in this manner would reach a system of central ventilation chimneys, where it would be discharged upwards above the temperature inversion zone (Fig. 3). The size of such structures would be considerable. The towers’ height is estimated at 80 meters, whereas the chimneys would stand at 400 meters. Work to assess the feasibility of implementing this solutionis is underway. The research results indicate measurable effects in terms of air exchange. Nevertheless, such implementation is rather costly and generates several obvious problems (e.g., overwhelming scale, aesthetic consequences for the city image), as well as issues that might be difficult to predict. Advanced research into the effectiveness of the system and the possibilities of its implementation is underway. However, it seems unlikely that it will be implemented.

Figure 2.

Figure 3.

The capital of India, New Delhi, is the world’s third-largest agglomeration. The city is located on the Yamuna River in the northern part of India in the Hindustan lowlands.

Delhi is located in the subtropical humid climate zone. It covers an area of 1483 km², whereas its population stands at 26 million. The city has a central layout, with the eastern part of the very center situated on the river. In India, as in other cultures based in hot climates, solutions to protect against the sun and intensify natural air exchange are rooted in the traditions of building development shaping and urban layouts. However, the city’s size, the density of its population, the transport concentration, and the industry make New Dehli one of the world’s most air-polluted cities (alongside some Chinese cities and other cities in India) [12]. This situation results from significant emissions of pollutants related to high population density, energy based mainly on fossil fuels, and economic activities. Agricultural activities, for example, consisting of burning fields surrounding the city (a practice rooted in the water protection law, which shortens the time to clear the fields following harvests) are another reason. The situation is further aggravated by the city’s poor ventilation, especially in cooler, windless autumn and winter seasons, when the air becomes dry. The area’s physiography is also unfavorable, as it is sheltered from the wind by a mountain range.

India is not rich or developed enough to introduce effective, systemic political and economic strategies for pollution source reduction in cities. New Dehli covers a vast area and is marked with a complex structure of buildings created and transformed since Antiquity. Neglected, chaotic, and overcrowded districts occupy large areas of the city. In this situation, creating a coherent aeration corridor system seems unfeasible, especially given that air pollution is also present in the city outskirts. It is also impracticable to use the morphological features of building development, listed in Table 1, for airflow regulation at a scale big enough to reduce the problem significantly. Traditional Indian architecture was adapted to the local climate by using solutions which increase ventilation. Yet, these methods were not applied in the contemporary architecture and urban planning of New Dehli. Hence, the plans to build ventilation towers, known as smog towers, began to be considered.

The first tower of this type was implemented in a compact, post-colonial development on one of the commercial streets of New Delhi. The tower stands at the height of 24 m. In the part of the ground floor, which forms a kind of a wide pedestal, fans were introduced. Hepa-filtered air is discharged through a centrally-located chimney (Fig. 4). The data published in commercial materials concern the possibility to purify air within the area and a radius of 1 km of the tower to harmless values. [18]. Although the city authorities are proud of this investment and have announced the construction of further towers, strong criticism of this solution is being raised. The implementation lacks system assumptions, unlike in the case of the solution considered for Krakow; scientists question the tower’s effectiveness [19]. In addition, the tower is to be powered by grid-supplied electricity, with over 70% of India’s electricity generated by burning coal. Therefore, the solution may be seen as a temporary provisional measure, rather than part of a well-considered ventilation concept.

Figure 4.

Masdar City is a new eco-city erected in the United Arab Emirates in 2006. It belongs to the Emirate of Abu Dhabi (in the northern part of the Emirates, along the Persian Gulf) and is located several kilometers from its borders. The city lies in the desert climate zone, and its area equals six sq. km. Currently, the number of permanent residents amounts only to around 300, but ultimately the area should be populated by about 50,000 inhabitants. Masdar City is an experimental development implemented as a testing ground for new pro-ecological technologies. It may be seen as a display of possibilities, technological advancement, and wealth of the United Arab Emirates; thus, it faces a certain amount of criticism from communities promoting sustainable development. However, it should not be overlooked that the city boasts numerous well-thought-out passive urban solutions. New concepts are combined with traditional solutions, characteristic to hot climates, that support ventilation inside buildings and around them [3]. The possibility to design the building development completely from scratch allowed for creating a system of smaller and larger spaces where airflow is properly directed. The main part of the city is arranged in the shape of a square intersected by two diagonal, wide corridors covered with greenery. Hot, northwestern winds fall into the corridor spaces. After being cooled by green areas, the air is introduced into built-up structures through the lateral exchange. Colder winds from the east also penetrate the main corridors and enter deeper into the building development. The street grid is shaped in such a way as to avoid elongated corridors, which allows the free flow of hot north-west winds. The cool breezes from the east are caught by evenly spaced wind towers modeled on traditional Arabian wind catchers (bagdirs) (Fig. 5). Thanks to them, cool air reaches the lower parts of the streets, contributing to a favorable microclimate at the pedestrian level. The city uses model solutions that combine various environmental and utility goals, with no need for advanced and costly technologies but rather in a way close to local tradition and culture.

Figure 5.

Jätkäsaari is a new residential area in Helsinki. Currently, the area is under construction, and its completion is scheduled for 2030. The district is located in the southwestern part of the city, on a peninsula in the Finnish Baltic Bay. Helsinki lies in a humid continental climate. The area of the district covers 0.68 km², whereas its projected population will amount to around 18,000. As in the case of Masdar City, planning the development for a relatively large area from scratch allowed for the implementation of a well-thought-out ventilation strategy based solely on designing the development properly [3]. The district’s structure is based on a grid of rectangular closed quarters. Both the targeting and the size of the quarters have been adapted to climatic conditions. Building development quarters pass each other, thus creating a discontinuous street pattern in the direction of cold period winds. The quarters guided in this direction have narrower sides, which prevents the penetration of cold air inside the courtyard sand the streets are winding, discontinuous. Warm summer winds, in turn, hit the streets continuously. Such an arrangement constitutes natural channels with which the penetration of warmer air into the city at the pedestrian level is enabled. The longer dimensions of the building development quarters’ sides towards this direction provide the courtyards with ventilation in the summer. Additionally, air exchange is favored by differences in the heights of building development quarters (Fig. 6). Jätkäsaari is an example of an urban structure in which, despite the lack of large open areas, prompt ventilation has been achieved solely through the morphological features of the building development.

Figure 6.