Opinion Indian cities have a drainage problem

Another important fact is that traditionally our cities had a large area which was capable of absorbing a considerable part of the runoff. But over the years, every city has, by design or illegally, allowed more area to be paved, concretised, or consolidated. This has resulted in the generation of more stormwater than what was factored in while designing drains — this strains the capacity of drains and results in flooding.

Other than the estimated magnitude of the rain, a drain’s capacity is decided according to its catchment area — the land from which the runoff flows into it. The bigger the catchment area, the greater the volume of the stormwater and the bigger the size of the drain that is required to pass this water safely to the next level drain. As water moves from multiple small drains of the network to join the next level drain, its volume and the flow rate keeps on increasing. A bigger cross-section is, thus, required for this water to pass, without overflowing.

Usually, stormwater flows from a higher elevation to a lower one. However, at times, conditions are not conducive for storm water to flow under gravity — this is especially so in underpasses or some low-lying areas. This leaves no option but to deploy pumps to evacuate this water. This requires scientific handling. Storm drains are thus designed according to the size and population of a city and its longitudinal slope. This implies that no part of the city should be flooded if the rainfall is less than or equal to what was factored in while designing the drainage systems.

Over time, all of these conditions are violated — the city expands, its population increases, land use changes. This is what lies at the core of the flooding problem. Drains tend to get clogged when silt, moving with the stormwater, settles and when solid waste is dumped into them. This ends up reducing the carrying capacity of the drains to evacuate stormwater.

Another major issue is that stormwater drains invariably carry sewage in a majority of our cities, including Delhi. This is largely due to poorly managed sewerage systems, and also because some parts of the city are outside the sewerage networks. For a drainage system to work well, at least for the estimated rainfall, the drains must be clean. The ground reality is that in almost every city, desilting is not done properly. Moreover, large stretches are permanently covered and, therefore, virtually impossible to desilt. It is important that access should be created in such drains to ensure effective desilting.

Over the past two decades, the effects of climate change have added to the flooding woes. One of the main implications of climate change in India has been the increase in magnitude and intensity of rainstorms. Rainstorms today are of much greater intensity and have rendered the existing drainage infrastructure completely ineffective. This was factored in the framing of the Manual on “Stormwater Drainage Systems” for the Ministry of Housing and Urban Affairs in 2015. It was suggested that cities should consider one in five years or a higher return period for designing or remodelling their storm drainage network to deal with the vagaries of climate change. But the storms that are being experienced recently are of much higher magnitude and intensity. They end up overwhelming the infrastructure, and every city experiences an increased magnitude of flooding.

This problem needs dedicated and continuous attention and use of scientific tools — the simulation framework on a GIS platform, for instance. It simulates the movement of the run-off and tries to attune drainage systems to changes in population and land use. Such a framework was provided to the Government of Delhi and is capable of providing solutions to Delhi’s flooding.

It is equally important to understand that it is not possible to wish away flooding because storms today are bigger than what was factored in while designing the drainage system. However, it is possible to reduce the severity of floods by taking a series of actions. One simple strategy that can work well is to reduce the magnitude of stormwater by taking multiple measures such as enhancing groundwater recharge, creating underground storage to retain/detain water, rejuvenating waterbodies and stopping their encroachment. There are examples of cities creating rooftop gardens to retain part of the rainwater. Some developed countries are also installing big stormwater tunnels to carry the stormwater from the city and release it into a nearby river or water body. However, such solutions are very expensive. Anyway, they require basic street-level drainage infrastructure.

The writer is former professor, IIT Delhi, and founder director, INRM Consultants, an IIT Delhi incubate company