Urbanisation and Concretisation: What a City Loses When It Is Paved

In one line

Rain that used to soak into the ground now runs off it, and everything that follows, the floods, the falling water table and the heat, follows from that one change.

In the wording a student can write in an exam: urbanisation is the increasing concentration of population in towns and cities, and concretisation is the replacement of permeable ground by impervious surfaces; its consequences are the loss of infiltration and therefore of ground water recharge, an increase in the volume and speed of run-off and so in urban flooding, the loss of the water bodies and flood plains that stored the excess, the urban heat island, and the loss of the ecological functions of the soil and the tree cover; and the legal instruments that reach it are the development control regulations, the reservation of open space in a development plan, rainwater harvesting mandates, the Wetlands Rules, 2017, and the public trust doctrine.

The physical mechanism, which is the whole answer

A vegetated soil takes rain in. Some evaporates, some is used by plants, and much of it percolates to the water table. Very little runs off, and what does runs off slowly.

A paved surface takes none in. All of it runs off, and it runs off fast.

The consequences follow arithmetically.

The peak flow rises. The same rain arrives at the drain in a shorter time, so the peak is higher. A drain sized for the old catchment overflows.

The volume rises, because none is lost to infiltration.

Recharge falls, so the water table under the city falls even as the flooding on its surface increases. A city can flood and run out of water in the same year, and Indian cities regularly do.

And the temperature rises. Concrete and asphalt absorb and re-radiate heat, there is no evaporative cooling from vegetation, and the built form traps the air, which is the urban heat island.

What is destroyed on the way

The water bodies. A tank at the edge of a town is flat, dry for part of the year and recorded as waste. It is the first thing built on, and it was the thing that stored the flood.

The flood plain, which is by definition the land the river uses when it is high, and which is developed because it is flat and near.

The natural drains. A city's nallahs are its storm drainage, and they are progressively narrowed, built over and turned into sewers.

The tree cover and the soil, and with them the shade, the evaporative cooling and the infiltration.

The legal instruments, and why each is weak

Development control regulations prescribe ground coverage, setbacks and open space, so that a proportion of each plot remains unpaved. The weakness is that the required open space is very often paved, and few regulations require it to be permeable.