We All Know Delhi Needs to Tackle Flooding. But HOW Should We Go About It?

The national capital's drainage system has been failing. In this piece, I list seven ways to go about fixing it.

When rain falls on any landmass, its absorption by the soil depends on the land surface's character. Natural surfaces absorb a portion of the rainwater, whereas impervious surfaces like pavements and concrete reduce absorption, increasing stormwater runoff.

For instance, a century ago, Delhi would have absorbed about half of its average rainfall. Today, due to increased impervious surfaces, only 20 percent is absorbed, leaving 80 percent as stormwater, which may be seen as one of the reasons for flooding. 

Stormwater Movement and Urban Drainage Systems

Stormwater moves over the land, driven by gravity, from higher to lower elevations. Each drainage system, whether a river or a small drain, follows this principle. The size of a drain is determined by the rate of stormwater it needs to carry. Larger contributing areas require larger drains. This natural drainage system has evolved over millions of years. The changing hydro-meteorological conditions can give rise to morphological changes in the drainage systems. 

In urban areas, artificial drains are designed to carry stormwater from rooftops, streets, and roads to natural drains. When gravity-based drainage is not possible, such as in underpasses, pumps are used. The size and slope of these drains are determined using hydraulic design principles based on the desired return period of a storm (for example, one in five years or one in two years). Designing for higher return periods requires larger, more expensive drains, and so a balance between financial capacity and protection level is often sought.

For example, if drains are designed for a storm with a one in five-year return period, they are sized to handle the volume of water from such a storm. In contrast, designing for a one in two-year return period would result in smaller drains. This means that any storm larger than a storm for which the drains have been designed will cause flooding.

While it is technically possible to design drains for very high return periods, such as one in 20 years, the sizes of the required drains would be very large, demanding significant space and financial resources. Therefore, cities typically adopt a return period between two to five years based on their financial capacity. 

The Issues With Delhi's Drainage

Delhi's drainage system has been failing due to several reasons: 

Poor Maintenance: Drains are often not properly cleaned, leading to blockages. Large stretches of drains are permanently covered without access, yet are claimed to be desilted. If a small patch is left uncleaned, the entire system's functionality is compromised. 

Mixing of Wastewater: Many areas without sewer systems discharge wastewater into storm drains, reducing their capacity during rains. About 40 percent of Delhi lacks a sewerage system, and all wastewater from these areas goes into stormwater drains. Even in areas with a sewerage network, significant quantities of wastewater end up in storm drains due to malfunctioning or punctured sewers.

Additionally, many households connect their rooftop drains to sewer lines, which are not designed to handle stormwater. During rains, stormwater enters the sewer lines, causing them to surcharge, resulting in sewage oozing out of manholes and flooding areas with sewage and stormwater, also damaging the sewer lines. 

Climate Change: Increasing rainfall intensity due to climate change makes existing infrastructure inadequate. The magnitude of rainstorms is increasing, which existing drainage systems cannot handle, leading to more frequent and severe flooding. 

Some Solutions

Improved Maintenance: Ensuring drains are clean and free of blockages is crucial. The city needs to adopt effective desilting procedures with proper validation to ensure drains function as intended. 

Proper Sewerage: Expanding sewer systems to cover all areas and preventing illegal connections of stormwater to sewer lines is essential. Implementing separate sewerage systems effectively diverts all wastewater generated by users to sewage treatment plants and prevents it from encroaching on the capacity of storm drains. 

Advanced Planning: Using scientific tools, like the GIS-based simulation framework developed by IIT Delhi, and provided to the Delhi Government to evaluate and enhance the stormwater network's efficacy. This framework provides a detailed evaluation of the existing stormwater network and identifies areas prone to flooding, helping to develop targeted solutions. 

Reducing Runoff: Encouraging groundwater recharge, creating underground storage, and using rooftop gardens to retain rainwater can significantly reduce stormwater runoff. Reducing impervious surfaces and promoting permeable pavements and parks can also help in increasing groundwater recharge. 

Innovative Infrastructure: Implementing solutions like stormwater tunnels to carry excess water to rivers or water bodies, though expensive, can be effective. Such infrastructure can handle large volumes of stormwater and prevent urban flooding. Developed countries use large stormwater tunnels to transport stormwater from cities to water bodies, but this solution still requires good street-level stormwater infrastructure. 

Community Participation: Engaging the community in stormwater management practices, such as helping maintain local drains, reporting blockages, and adopting rainwater harvesting techniques, can enhance the effectiveness of drainage systems. 

Policy and Regulation: Strengthening policies and regulations to control urban development, limit impervious surfaces, and ensure proper drainage infrastructure in new developments is essential. Enforcing strict building codes and drainage standards can prevent future problems. 

Climate Change Adaptation

Given the increasing impact of climate change on rainfall patterns, Delhi needs to adopt adaptive measures to enhance its resilience. These measures include: 

• Rainwater Harvesting: Promoting rainwater harvesting to reduce runoff and increase groundwater recharge. 

• Green Infrastructure: Incorporating green roofs, green walls, and urban green spaces to absorb rainwater and reduce runoff. 

• Early Warning Systems: Developing early warning systems for heavy rainfall and flooding to alert residents and enable timely response. 

In conclusion, addressing Delhi's flooding requires the intent and willingness of the administration, combined with active stakeholder participation. By adopting these strategies, the severity of flooding can be significantly reduced.

Implementing effective maintenance, expanding sewerage systems, using advanced planning tools, reducing runoff, and investing in innovative infrastructure are crucial steps. With dedicated and continuous attention, Delhi can mitigate the impacts of flooding. 

(The author is Former Professor, Indian Institute of Technology Delhi, Hauz Khas, New Delhi. This is an opinion piece. The views expressed above are the author’s own. The Quint neither endorses nor is responsible for them.)