Cities are about sharing. In a city we share schools and colleges and specialized training institutions. We share medical facilities and specialized hospitals. We share school and college playgrounds, pocket parks, larger public gardens and football or cricket stadia. We share theatres and concert halls and places for public gatherings. We share museums, we share a zoo.
We also share the hardware of a city: its water supply, its sewage treatment and disposal system, its transport network. All these shared physical and social amenities require land, which for each amenity cannot fall below a certain minimum area if there is to be no loss in quality of service.
The larger the city the more the things we can share. We also have a larger choice of jobs. That is why our cities have been getting larger and larger. The larger the city the more compact it needs to be if it is to continue functioning well, without loss of the quality of life that brought people to it in the first place.
To many of us high-rise development seems undesirable for a variety of reasons. Chiefly of course it is the loss of human scale that is discomfiting. Small buildings seem cosier and more friendly than large impersonal and imposing ones. Also, with two- or three-storeyed buildings placed directly on a street, without compound walls, there is an interaction between the open space of the street and the enclosed space of the house. Houses can look out on the street, mothers can keep an eye on children playing, and people at home can either participate in what is going on in the street or withdraw, as they prefer. At street level, cafés can spill out onto wide footpaths and patrons enjoy the open space while enjoying the meal.
But low-rise living comes at a cost. The settlement overall must stretch over a larger area. If the city is compact, transit systems are lower in capital cost, lower in operating and maintenance cost, and lower in consumption of resources. We do not want urban sprawl, not only to conserve land but also because we want to minimize people’s travel time.
Densities should be amenity driven
Densities are fundamental and should be the consideration that drives all urban planning. Density means the number of people in a given area. Once we decide per capita areas for amenities and open spaces, we know how much land we need to reserve for these for any given density — that is, the number of people within any given area.
The National Building Code 2016 spells out how much land is needed for schools, hospitals, parks and so on for any particular size of population. Converting this to provision of a per capita land area we arrive at a need for 14 sqm/ capita at city level for open spaces and all public amenities.
We divide our total land area three ways: land for roads and public transit; land for amenities; and land for plots on which buildings can be built. Roads and transit take up about 30% of land area. That leaves us with 70% to be divided between amenities and buildable plots.
In Figure 1, we have four different scenarios, with different ratios of buildable area to amenity area (also called public area). All calculations are for 14 sqm/ ca for amenities.
The somewhat startling and counter-intuitive finding this diagram reveals is that the more area you devote to common amenities and open spaces, as a proportion of the total, the higher the population that the total land area can support with full scale local amenities. We see that if planned public areas are 14% (one-fifth) of the developable area (that is, the ‘buildable plot’ : ‘public area’ ratio is 4:1) the city-wide density will be 10,000 p/ sq km. But if public areas are 35% (half the developable area with a buildable : public ratio of 1:1) city-wide densities could be as high as 25,000 p/ sq km.
So the higher the proportion of public areas to total area, the higher the overall city density can be in terms of persons per overall hectare without loss of access to adequate amenities. Clearly, if we want a more compact city we should make our public areas as high as possible in relation to our buildable plot areas.
Reducing the buildable : amenity ratio will of course increase individual plot densities on the buildable private plots. For overall densities ranging from 10,000 to 25,000 persons/ sq km, buildable plot densities will range from 179 to 714 persons/ ha.
For a better understanding of what densities from 10,000 p/ sq km to 25,000 p/ sq km imply in physical terms we convert these into the average number of storeys of construction. The number of storeys for any given density depends on two parameters: (i) the average size of built-up space per person, and (ii) the share of the plot that is taken up by the plinth of the building, what we call the footprint, expressed as a percentage of the plot area.
To arrive at the number of storeys we have a two-step process. We need to introduce an intermediate parameter, and that is FSI (Floor Space Index), the ratio of the built-up area on a plot to the area of the plot.
TABLE 1
| Overall city density p/ sq km → | 10,000 | 15,000 | 20,000 | 25,000 |
|---|---|---|---|---|
| Buildable: Amenity Plot Ratio → | 4 | 3 | 2 | 1 |
| Buildable Plot Density p/ ha → | 179 | 286 | 429 | 714 |
| Apartment Size sqm (Family of 4) ↓ | Cell values below are FSI | |||
| 30 | 0.2 | 0.4 | 0.7 | 1.5 |
| 60 | 0.5 | 0.8 | 1.4 | 3.1 |
| 90 | 0.7 | 1.2 | 2.1 | 4.6 |
| 120 | 1.0 | 1.6 | 2.8 | 6.1 |
| 150 | 1.2 | 2.0 | 3.4 | 7.7 |
| 180 | 1.4 | 2.4 | 4.1 | 9.2 |
| 210 | 1.7 | 2.9 | 4.8 | 10.7 |
TABLE 2
| Footprint → | 20% | 40% | 60% | 80% |
|---|---|---|---|---|
| FSI ↓ | Cell values below are number of storeys | |||
| 0.5 | 3 | 2 | 1 | 1 |
| 1 | 5 | 3 | 2 | 2 |
| 2 | 10 | 5 | 4 | 3 |
| 3 | 15 | 8 | 5 | 4 |
| 4 | 20 | 10 | 7 | 5 |
| 7 | 35 | 18 | 12 | 9 |
| 10 | 50 | 25 | 17 | 13 |
Reading the two tables together, we see that for low income people occupying 30 sqm apartments we can have them in one-, two- or three-storeyed buildings and still achieve our desired overall density of 25,000 persons/ sq km. Those with 210 sqm apartments, to achieve the same density, would need to be in buildings over 50 storeys high, on small footprints to allow for good air circulation and daylight.
What prevails in Mumbai
Mumbai has about 4 sqm/ ca (as against our desired 14), and whatever it has is also unevenly distributed. The central difficulty is that the private : public land area in Mumbai is 6:1 as against our desired 1:1. Our effort towards correction should therefore be to look for opportunities where lands held by government or government agencies is turned over to truly public use. Government land is public land, owned by the public, and therefore must be put to the use that is in the highest public interest. This is not necessarily monetization. The first call must be that of inadequate amenities, and it is to expanding these that any change of use should be directed.
Let us work with a specific example, the reconstruction of BDD Chawls that are on a 22 hectare site at Worli in Mumbai. How can living conditions there be improved?
If we follow our suggestion of equal land areas for amenities and for buildable plots after setting aside 30% for roads (including parking), we are left with 7.7 ha for building plots and 7.7 ha amenities.
The really challenging problem is the 7.7 ha that are to be built upon must accommodate the 9,600 currently resident families, with the further requirement that each apartment must be 45 sqm (which is what residents have been currently promised by government).
Stacked streets
A possibility emerges from our observation that the cityscapes we like best are those in which streets are not too wide and have two- or three-storeyed buildings on either side. If the street is pedestrian-friendly, or better still pedestrian only, that makes it even more desirable. So why not take this configuration and stack it vertically?
Imagine therefore an indoor street 9m wide, on either side of which are three-storeyed buildings (ground + 2 upper), covered by a repetition of the same configuration. At each indoor street level we have a street 9m wide and 9m high that runs through the building from end to end, with both ends open. Off each “street” (no vehicles), on either side are three floor levels. Each floor is let us say 9m deep opening to the outdoors on the outer face, and opening to the indoor street on the inner face. With windows on the outside as well as the indoor street we should expect good ventilation.
We can accommodate 20 buildings on the site, plus a cricket ground and plenty of space for other amenities. Each building is 7 stacked streets, each 9m high, for a building height of 63m (21 floors). Lifts serve only the streets, not each individual floor. So lifts have 7 stops, not 21, a feature that should significantly reduce waiting times for lifts.
What the present exercise shows is that reconstruction of old sites can both yield improved local and city amenities, with larger apartments in localities where densities are already among the highest in the world in conditions that are at present really dismal.
Conclusion
We started with the argument that in planning cities densities matter. We may not be able to control them directly, but they can be controlled, indeed should be controlled indirectly through the use of building control regulations.
If we want cities to be good places to live in what is of central importance is that the population be well served with common amenities like schools, hospitals and parks. Working with standards of how much land is required for such amenities and open spaces per capita, we assume there must be no compromise on this. If that is so, the population any given locality can support will be determined by the amenities provided.
Working with the specific example of BDD Chawls at Worli, we see how redevelopment can take place in a way that both improves indoor space for residents (from 16 sqm to 45 sqm per family) and simultaneously improves amenities, both local and city-wide.
Shirish B Patel is one of the three authors of the idea of New Bombay, and was the first Director of Planning and Works for Navi Mumbai.
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