Vertical limit

Skyscrapers are popular in Mumbai and many other Indian cities are queuing up to get them. However, Subhankar Mitrasays that many hidden problems come with the heights

July 25, 2014 05:50 pm | Updated 05:50 pm IST - Chennai

The Mumbai city skyline

The Mumbai city skyline

Internationally, a building that touches or exceeds 150 metres in height is considered a skyscraper. Until recently, Mumbai was the only Indian city with high-rise buildings, but others are now catching up, with New Delhi, Kolkata, Ahmedabad, Hyderabad and Bangalore looking at projects.

Chennai is not on this list, and that might be a very good thing. Here are some huge demerits to high-rises.

Urban wind tunnel Rise in the elevation of a building increases the distance of the wind shadow and minimises the air flow at the street level behind the building. Near high-rise buildings, the local wind speed is high even in summer. In addition, high-rise buildings tend to create a turbulent flow of the gradient wind as a result of increasing the roughness of the boundary layer surface.

Increased air pollution In summers, local wind speeds near skyscrapers are very high and troublesome. The ventilation conditions in urban spaces and major streets with high vehicular traffic have significant impact on the concentration of air pollutants at the street level. The high velocity and turbulent wind at the street level results in the mixing of the highly polluted low-level air with cleaner air flowing above the urban canopy.

Effect on urban radiation High-rise buildings absorb direct and reflected solar radiation of the surrounding low-rise buildings and convert it into heat via convection of long wave radiation. However, when buildings are of different heights, the walls of the higher buildings absorb part of the reflected and emitted radiation and block a portion of the sky, resulting in reduced solar exposure and long-wave emission from the roofs of the lower buildings.

Increased urban temperature The size and density of built-up areas affect urban temperatures. In the congested centres of large cities, temperature levels are generally higher than in the suburbs. The peaks in temperature occur during clear and still-air nights, also called Urban Heat Island. Excessive opacity of high-rise buildings in city centres results in concentrated heat generation by high-density land use (traffic, lighting, heat exhaust) and contributes to the creation of urban heat islands.

Effect on night-time cooling Nocturnal radiation is a major climatic factor that reduces atmospheric heat in urban areas located in hot, dry regions. Nocturnal radiation decreases as the density and height of built-up urban masses increases. High-rise buildings store solar energy during the day and release it slowly into low-speed local wind, especially at night. The vertical distance between cool winds above building roofs and the ground surface is long, and this results in decreased radiant cooling during the nights.

Low-rise buildings that match tree heights of 12-15 metres, on the other hand, penetrate night-time ventilated cooling at the ground level and also store cool radiation through built-up urban areas.

Cost factor Tall buildings are colder in winter and hotter in summer than regular buildings and, therefore, require more heating and more cooling. This is particularly true of modern glass towers.

Thus, a lot of energy is required to keep these high rises functioning. Exterior cleaning and maintenance of high-rise buildings can be very costly and dangerous.

High-rise buildings are costlier and take longer to build, and impose a heavy load on civic infrastructure. The average construction cost per square foot is 20-25 per cent higher if the building has more than 12 floors.

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