Why does Delhi flood? The answer lies in our urban stormwater management

Every year, we witness flooding in our large metropolises and most state capitals. Normally, Delhi is in the news for flooding due to the overflowing of Yamuna river when large volumes of water are released from the Tajewala dam upstream. However, on June 28, 2024, we witnessed flooding from a high rainfall episode in south Delhi. What explains the frequent in-situ urban flooding in Delhi? This happened in Bengaluru and Lucknow in 2023 as well. High rainfall and climate change are undoubtedly a reality. But is it just that, or something more? 

To understand the recent flooding in Delhi, let us first understand why 15 minutes of rainfall causes flooding on the Mehrauli-Badarpur road (MB road) in south Delhi. What does this have to do with understanding Delhi’s urban stormwater management challenges and those of other cities? 

Our cities, particularly our large metropolises of Bengaluru, Delhi and Lucknow, among others, are rapidly growing, resulting in very large built-up urban landscapes with dense habitation and unplanned and informal settlements all around their periphery. As our cities’ built-up areas expand, we are witnessing unprecedented in-situ flooding from rain, as seen in our major cities.

Bengaluru has expanded from 100 square kilometres (sqkm) to 800 sqkm in the last few decades. Delhi is bursting at the seams into neighbouring cities in Uttar Pradesh and Haryana. With a plethora of construction activity, paved roads and underground construction works like basement parking and underground metro lines killing valuable aquifers, where do we expect the rainwater to go?

Each city has its own distinct watersheds and drainage areas. Delhi has three and so does Bangalore. Determining which areas will flood should be straightforward; these will primarily be at the catchment’s lower end. 

There is a pattern of urban flooding, in which rainwater flows from one part of the city to another, because no stormwater conservation measures are in place where it rains. The lack of open spaces, parks, natural lakes and water bodies within the dense unplanned settlements adds to the difficulty. 

As a result, even short rain episode in south Delhi’s Sangam Vihar colony causes the majority of the rainwater to flow down the MB road. Stormwater then floods the dense settlement of Dakshinpuri colony and its residential areas. It then flows into the Dakshinpuri stormwater drain, meandering through posh and affluent areas of south Delhi before entering the Barapulla main drain and eventually the Yamuna.

The extremely heavy rainfall of 230 mm on June 28 caused massive amounts of stormwater from Sangam Vihar and subsequent catchments to flow down towards their natural drainage lines. With massive new construction projects in and around Safdarjung and south Delhi, including underground metro lines and large buildings with multi-tier basements, consuming valuable aquifer recharge potential, where could rainwater be absorbed?

There was not enough time for the natural drainage system to channel all the stormwater to drain out of upscale south Delhi localities.

If we are to address our cities’ urban flooding challenges, we must first understand this aspect of in-situ urban flooding, which includes stormwater flooding from both large built-up and unplanned residential areas.

What needs to be done?

The Delhi Drainage Master Plan of 2018 identified flooding hotspots of Delhi. These were all along the drainage lines and already well known. But what needs to be done to address this is not explained.

Unless rainwater conservation is first implemented in and around the farthest catchments of our city watersheds, in colonies such as Sangam Vihar, to prevent their fifteen-minute rainfall deluge from flowing where it flows, flooding downstream cannot be controlled, nor can valuable rainwater conserved. Given the twin challenges of extreme rainfall caused by climate change and large built-up areas that have resulted in in-situ urban flooding, we must consider both water conservation and stormwater drainage in our cities.

A recent study by Delhi-based think tank Centre for Science and Environment (CSE) of Delhi’s water supply, sanitation systems and stormwater challenges (using the case study of Sangam Vihar) highlighted this aspect of stormwater management as part of our Global South water-sensitive cities framework.

Delhi has a total of 426.55 km of natural drainage lines and approximately 3,311.54 km of engineered stormwater drains, which are managed by 11 different agencies.  Yet these seem to not work for addressing urban flooding.

The Delhi Master Plan 2041 does lip service to addressing stormwater management and conservation, stating, “New development should be made in such a way that overall runoff from the area does not increase with the proposed urbanisation. For that, necessary measures in the form of ponds, parks, porous pavements, green belts, artificial lakes / or other rainwater harvesting and storage structures, etc may be adopted.”

It does not address how this will happen, given the densification of our urban population and increasing floor area ratio ceilings within Delhi. What infrastructure and what planning are required to address urban flooding in the national capital, which has its own unique drainage pattern and dense unplanned settlements on its periphery, is not mentioned in the master plan.

Case study of Sangam Vihar

Sangam Vihar is a million-plus settlement located in the southeast of Delhi, spread across 5 sqkm. No stormwater management planning or infrastructure is currently in place for the region, other than a large stormwater drainage channel on the main trunk road. 

A 15-minute episode of high rainfall can lead to more than 11 million litres of stormwater spilling out of Sangam Vihar, as assessed by CSE in their report Water and Wastewater Visioning for Large, Dense Unplanned Urban Settlements in an Era of Climate Risk.

There are no open spaces because nearly 90 per cent of the area is paved, leaving very few or no areas for rainwater to recharge groundwater, resulting in high in-situ runoff during rains. The street drains that carry greywater also serve as stormwater drains, conveying all such water to the MB road main sewer and causing flooding.

To recap, 15-minute moderate rainfall leads to flooding of the MB road. This is due to the Sangam Vihar settlement’s one-directional slope, which runs from south to north, with the south being a forest area.

Total stormwater runoff generated in a 15-minute spell:

• Normal intensity rainfall: 52 million litres

• High-intensity rainfall: 117 million litres

The entire volume of this stormwater flows currently onto the MB Road.

There is a dictum of bureaucratic stormwater management ’those who own the road, manage the stormwater drains’. It does not address the watershed or catchment from which the stormwater originates, or what needs to be done first in terms of water conservation measures before considering discharge and drainage to prevent flooding.

The Delhi Metro Rail Corporation has been tasked with overseeing MB Road, where the new metro rail line will be constructed. However, the DMRC is not a suitable agency to plan for stormwater management because it lacks the locus standi to do so.

Stormwater management is possible in Sangam Vihar by strategically channelling and redirecting both internal sewage (black and greywater) and stormwater to its periphery.  Treated wastewater from decentralised sewage treatment plants (STP) and short-duration stormwater will provide an assured source of water supply to recharge ground water near Sangam Vihar, directly benefiting the water supply in this unplanned settlement.

This will not only ensure stormwater conservation and flood prevention in Delhi, but it will also provide much-needed green and blue infrastructure to combat heat stress and climate change.

Large cities like Delhi, therefore, require a reimagining of water supply, sanitation systems and stormwater management. In the absence of this, retrofitting solutions to all three challenges will never work. 

This approach would alleviate pressure on MB road infrastructure and effectively address the management of total wastewater and stormwater in Sangam Vihar.

Climate change is real and it will exacerbate the current issue of poor stormwater management. Building more drainage channels on the one hand and discussing rainwater recharge on the other, require a nuanced understanding of where to conserve rainwater, how and with what additional infrastructure. Following that, determine what cannot be conserved and should be allowed to flow into the drainage system and the Yamuna, while also creating appropriate drainage. This will necessitate coordinated spatial planning for water conservation and drainage in each city.

CSE’s analysis of the recent water crisis in the summer month of June 2024 revealed that water supply shifts from northwest, north and northeast Delhi (where the majority of water supply plants are located) to south, southeast and southwest Delhi. Water supply pipelines are dry by the time water reaches Sangam Vihar.

When it comes to flooding, the opposite happens: All of the stormwater from Sangam Vihar and other upscale south Delhi colonies floods the lower parts of the city. This is because of the topography and contour slope of the national capital.

Hence, the solution for water supply augmentation — decentralised water supply using lake recharge with treated STP water — is also similar to the decentralised stormwater management we have proposed using the case study of Sangam Vihar. 

For a Global South water-sensitive cities framework, we will need to look at all related aspects of decentralised sanitation systems, decentralised water supply systems and decentralised stormwater management. Meaning, a reimagining of water and cities, as well as the associated policy, institutional and governance framework for urban water. This represents a significant paradigm shift; simply retrofitting solutions that address one problem at a time will not suffice.