#13 - Urban planning in cities with hot and dry climates: the case of Cairo

 Dossier Hot climates  hot climate  urban planning  development  territory  eco-district
Published by La rédaction C21

Despite the awareness of the need to focus on sustainable urban development in the 1980s, Egypt faces a lack of convergence between the sustainable urban design model and post-revolution urban modernism. In 1952, it focused on social housing plans and the theoretical values of post-modernist urban planning in a warm country like Egypt (see: Ali, 2003; Dona, 1999; Fahmi & Sutton, 2008; Stewart et al., 2004; Sutton & Fahmi, 2001).

 

The Egyptian climate: hot and dry

 The importance of climate study is focused on its effect on the landscape at two levels: the first level concerning the behavior of individuals according to their activity and their circulation in an open space. The second levels is the impact of climate on other factors in space such as soil quality, plant species, and existing facilities and determine the factors and components and their proportions, and visual perception, which depends on the quantity of lighting, its orientation and choice of materials. " (El-Gamal, 1995).

 

Climate can be defined as the set of characteristics specific to each region in terms of temperature, wind, humidity, lighting and other factors.

 

There are several factors that affect Egypt's climate, such as its position between latitudes 22°C and 31.36°C north latitude, with the Tropic of Cancer passing at the southern limit of the country, this position shows that Egypt is within the hot and dry tropical zone, where only the dry tropical region (desert) located between latitudes 180°C, 30°C north and south is known.

 

This site offers the Egyptian territory a significant amount of solar radiation. The sun sends almost vertical rays, on the territory of Egypt in summer, has a more moderate strength during spring and autumn and weakens during the winter months. Because of this orbital position, the duration of the day increases in summer to 14 hours, which increases the amount of solar radiation reaching the earth during this period. The flat surface dominates Egyptian territory, especially in the valley and delta, and in most parts of Western Sahara.

 

Adapted to natural conditions

"The orientation of urban space and its H/L ratio (height of the building / width of the roadway) are the most influential factors on climate performance in urban spaces. However, there are design measures that can be taken to have better control to reduce climate problems and thus improve the quality of life in existing urban spaces, where the orientation or H/L ratio can no longer be changed. These measures include wind control, solar radiation, humidity, temperature, and air pollution.

The lack of awareness about improving the thermal performance of spaces is a major problem in Egypt. There are many reasons for this problem. One of these reasons is that the client and the architect are more often concerned about individual construction than urban design. There has been a trend in recent decades to build spectacular isolated buildings rather than considering uniform urban design.

The lack of awareness about improving the thermal performance of spaces is a major problem in Egypt. There are many reasons for this problem. One of these reasons is that the client and the architect are more often concerned about individual construction than urban design. There has been a trend in recent decades to build spectacular isolated buildings rather than considering uniform urban design.

 

This is why one of the most important reasons for the success of the thermal performance of public spaces lies on the designer's ability to respect these factors, his ability also to take into account the needs of the users of the place (for example, such spaces are usually used during the day only). Therefore, the design must be optimised to provide favourable conditions during the day.

 

Shade and humidity: two key factors for good urban planning

 

Shading is particularly effective in the summer climate of hot and dry regions, as solar radiation is dominated by the direct component. In order to obtain sufficient shadows, we must know the best proportion of spaces in public places. It should be noted that in urban areas, it is difficult to control the environmental elements on a large scale, due to exposure to direct solar radiation and the difficulties of creating a comfortable climate. On the other hand, there are more shaded areas in medium and small spaces where exposure to direct sunlight is lower.

 

Plant elements also play an important and essential role in the environmental balance. They are very important because of their functional diversity and because of their different qualities and characteristics; they absorb solar radiation, increase humidity, filter sand and dust winds, etc. It can even be said that in areas where the ecological balance is disturbed - or in other words the environment is polluted - the concentration on the plant elements is necessary to eliminate this pollution.

 

The phenomenon of perspiration is an important element in the water cycle because the rise of water vapour contributes to the process of cloud formation. The phenomenon of perspiration that is carried out by plants is therefore a favourable element in the natural water cycle.

 

However, some problems encountered in residential areas should not be ignored, such as putting new plants near existing buildings with shallow foundations, or obstructing light with branches. These problems can be avoided by choosing trees that consume little water, and by keeping a sufficient distance between the tree and the window, either by planting it at a sufficient distance or by pruning the offending branches.

 

Cities generally suffer from increasing urbanization, industrialization rates and urban climate pollution, leading to high temperatures. In addition, there are vapours from motor vehicles, neglected street dust, industrial vapours, smoke and soot. It is therefore necessary to study environmental pollution in cities and try to limit its spread in order to control it by various means in order to obtain appropriate environmental conditions in the city, write new health laws and ensure their enforcement.

 

General recommendations for urban areas in Cairo

 

Urban and architectural treatment

Studies have shown that streets with high H/L ratios (building height/ lane width), where buildings provide shade, are an advantage in summer conditions. On the other hand, in winter, a form dispersed in urban areas, with streets with a low H/L ratio, is preferable. However, since the summer season is longer than winter and it is more difficult to adapt behaviour and clothing in warm conditions, most streets should be designed to be comfortable in summer. This would require new urban areas that are much more compact than today's to improve micro-climatic conditions during the warm season (Johansson, 2006).

 

The increase in shading in urban areas

It is important to increase shade to reduce the effect of solar radiation in summer. For areas that have shown higher percentages of dissatisfaction such as seats and aisles, facilities that respect the local climate and the thermal needs of visitors should be provided.

 

Shading by trees

Based on the requirement of a comfortable climate, a suitable type of tree can be selected, such as deciduous trees such as mulberry trees. In summer, they provide shade. On the other hand, deciduous trees lose their leaves in winter and allow solar radiation to heat buildings.

 

Shading by built structures

Shade can be created by installing lightweight wooden lattices, or by using an overhanging roof with removable panels, so that they can be tilted if necessary, to protect against the summer sun and leave the sun in winter. Horizontal shading devices above sidewalks or colonnades below the upper floors can also be installed.

 

The increase in green spaces

Trees can be used as windbreaks to protect buildings and outdoor areas (such as lawns and terraces) from hot and cold winds. The reduction in wind speed behind the windbreak depends on their size, density, cross-sectional shape, width and length, the first two being the most important factors. When the wind is not blowing perpendicular to the windbreak, the protected area is increased. This is why it is more important to design wind breaks to reduce maximum wind speed in extreme climates (Brown & DeKay, 2001).

 

Vegetation also absorbs solar radiation, which in turn maintains the temperature of the ground and eliminates much of the heat and light reflected in adjacent spaces (Gregory 1989). This is why it is necessary to reduce flat surfaces that do not contain any plant elements, using paving systems combined with plant elements, and vegetation as an alternative to physical elements in the landscape such as walls and walls (Fig.21).

 

hot climates urbanism Cairo

 

A system of linear greenways or boulevards that converge towards the city centre would help maintain the movement of fresh air. Provided the soil is sufficiently moist, a single isolated tree can sweat up to 400 litres of water per day. This perspiration, with the protection of the sun's rays, creates a cooler environment around the tree. On a hot summer day, temperatures can drop significantly under trees due to cool breezes from convective currents and shade from the direct sun. Seeded areas can be up to 5 to 8°C colder than built-up areas, due to a combination of evapotranspiration, reflection, shading, and cold storage. (Brown & DeKay 2001) To optimize existing green spaces, several actions are possible:

- The identification of priority green spaces to preserve them.

- Conservation and development operations for the enhancement.

- The use of existing green spaces to improve the climate by using favourable climatic elements, such as redesigning lanes so that they pass under trees to exploit their shadows.

- Exploiting current green space opportunities in future plans and linking all green spaces to avoid making isolated islands into green spaces that do not work together on climate improvement.

- Protecting the growth of urban vegetation that surrounds the neighborhood and maintaining it.

- The planting of palm trees that adapt the climatic conditions of the areas to the arid climate.

- Reduction of irrigation needs through multi-use design of green spaces.

 - The selection of trees with a larger shade area, and requiring little maintenance and water, such as Albizia julibrissin, Eucalyptus, Pinus pinea, Mulberry, Acer rubrum, and Quercus phellos.

- Consideration of vegetation before construction.

- Develop new green spaces in areas that are destroyed due to negligence or vandalism.

- Encouraging people to plant their roofs and private open spaces.

- The study of plant growth to select those that best adapt to the given location, without too much care, and that will have enough space to reach their adult size. Plants are intended to be appreciated by city dwellers and should not become a maintenance problem, due to poor location, disease and/or necessary pruning (Eckbo 1978).

 

Wind control

It is desirable to increase the movement of fresh air and to block the hot wind or the one that carries dust in open spaces by several types of actions:

- Increase green areas in and around urban blocks.

- Make a belt of large deciduous trees to filter the wind and resist the phenomenon of dust storms.

- Develop the landscape using desert shrubs and trees that reduce the effect of sandstorms, such as Pinus elderica, Fraxinus pennsylvanica and Eucalyptus.

- Arrange the plants to control the wind movement.

The increase in humidity (Fig. 22).

 

hot climates urbanism Cairo

 

- Develop the landscape using desert shrubs and trees that reduce the effect of sandstorms, such as Pinus elderica, Fraxinus pennsylvanica and Eucalyptus.

- Try to guide wind movement in urban areas by opening the lanes to cool winds that have passed over water bodies before reaching the urban mass.

- Plant deflection can be used in conjunction with landforms and architectural materials to modify the airflow over the landscape, and around or through buildings.

- Install windbreaks perpendicular to the direction of the winds to be controlled.

- Use windbreaks dense enough to form a solid barrier.

- Provide openings between plants to allow air to move from the high pressure area to the low pressure area to create a positive circulation through these spaces.

- Arrange plants to control wind movement

The increase in humidity                                                                                                                                        

The presence of a body of water on or near a site helps to reduce the microclimate of the site. An industrial fountain or lake acts as a heat reservoir, gradually warming during the spring and remaining at a relatively constant temperature throughout the hot season. When the air temperature is very high, even the slightest breeze on the water will produce evaporative cooling and make the climate more bearable (Todd, 1985). (Fig.25).                                                                                                                                                      

Plants provide surfaces where winds can attract humidity by evaporation, which cools the site. They also provide moisture through the natural process of perspiration (outdoor evaporative cooling mechanisms), which provides indoor and outdoor comfort by lowering the temperature of

 

 An article by Ahmed El Hozayen, Assistant in the Architecture and Urban Design Programme at the German University of Cairo

Moderated by : Alice Dupuy

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