Tackling urban flooding

River flooding has been a common phenomenon in India for centuries. However, urban flooding is a more recent issue, emerging over the past few decades. The problem seems to worsen each year. In cities like Chennai, Bengaluru, Mumbai, and Delhi-Gurugram, hardly a monsoon passes without life being severely disrupted for several days. Water enters low-lying buildings and apartment basements, transportation is severely affected, and daily life is thrown into disarray.

Unfortunately, many of the measures taken by the authorities have failed to provide long-term solutions and can be seen as knee-jerk reactions. This article aims to analyse the possible causes of urban flooding and suggest strategies to mitigate this growing problem.

The rapid population growth in Indian cities over the last five-six decades has necessitated extensive investment in infrastructure, including buildings, roads, and rail networks. The consequence of such development has been the replacement of permeable surfaces (which permit seepage into the ground) by an impermeable surface such as concrete, leading to increased surface runoff.

As a result, storm water drains, originally designed to carry a specific amount of water, are now required to carry two to three times their original capacity. Climate change further exacerbates this problem, as it increases the frequency and intensity of extreme weather events. Going by current trends the world over, climate change can no longer be ignored. Large-scale permanent construction in the vicinity of the stormwater drains does not permit widening of the drains to accommodate increased water flow, leading to overflowing and subsequent flooding. While rainwater harvesting in apartment complexes, a mandatory requirement in many cities, can reduce surface runoff, paved surfaces such as roads will continue to generate significant runoff.

In one instance in Bengaluru, urban flooding was ingenuously addressed by constructing a wide drain beneath a road and supporting the road with a concrete slab. But this solution requires road closures for several months, making it feasible only for short, highly distressed stretches.

Lakes play a crucial role in mitigating floods due to their capacity to absorb excess water. Unfortunately, in many cities, lakes have been encroached upon for commercial gains, reducing their capacity to store water and increasing the risk of flooding. The problem worsens when escape channels from lakes, known as ‘Raja kaluves,’ are blocked by new buildings. These channels, which interconnect lakes, are essential for transferring water from one lake to another.

When blocked, they exacerbate flooding. However, in some areas of Bengaluru, stormwater has been successfully diverted to nearby lakes. This can only be done when the topography is favourable and the required gradients are available. An additional benefit of such a scheme is the rise in groundwater levels in the vicinity of lakes. 

Silt accumulation in stormwater drains also reduces their discharge capacity, further contributing to flooding. Periodic clearing of silt can help reduce flooding but is unlikely to solve the problem entirely due to the factors mentioned above. 

Urban flooding is now a global phenomenon. The International Association for Hydro-Environment Engineering and Research recognises this fact and has devoted an entire issue of its magazine Hydrolink to ‘Drop Structures’, which are essential to prevent flooding in several large cities. Such structures have been successfully deployed in London, Abu Dhabi, Hong Kong, Singapore, and several cities in the United States.

Drop structures operate by diverting water from a stormwater drain when the level of water rises to what is considered an unsafe level from the point of view of flooding to a huge underground tunnel. These tunnels are, in some instances, as much as 100m below the ground. Such a deep location apparently ensures that the foundations of existing structures are not affected.

The water collected in the tunnel can be pumped back to the ground level at a later time and used for purposes other than drinking, such as construction, gardening, and street cleaning. In effect, the strategy not only avoids flooding but is effective in rainwater harvesting. The technique is yet to be used in India to contain urban flooding. We should adopt the practice before we incur further economic loss by way of loss of property, fall in production, and lack of a healthy environment for living. 

(The writer is a retired professor of civil engineering, IIT, Roorkee)