Urban Floods: Causes, Impacts and Remedies – Explained, pointwise

Introduction

The city of Bengaluru witnessed several spells of heavy rainfall in the last week. The city received > 130 mm of rainfall on September 05, 2022 which led to inundation of most parts of the city. Over the last few years, similar incidents of urban floods have occurred in Mumbai, Chennai, Patna and several other major cities. In fact, urban floods seem to have become an annual phenomena in most urban centres in India. While heavy rainfalls are a significant factor in urban floods, a major cause is poor planning, inadequate infrastructure, unauthorized construction and encroachment of river and lake beds.

What are Urban Floods?

Floods are a result of both meteorological and hydrological factors. Meteorological factors include rainfall frequency and intensity, storms, temperature etc. and hydrological factors are associated with groundwater levels, extent of impervious surface etc. When combined with anthropogenic factors in urban centres, events of inundation of large areas by water are classified as urban floods. Anthropogenic factors like land-use changes, exploitation of floodplains by construction and similar activities, poor solid waste management, and destruction of drainage complicate the system.

According to NDMA, Urban flooding is significantly different from rural flooding as urbanization leads to developed catchments, which increases the flood peaks from 1.8 to 8 times and flood volumes by up to 6 times. As a result, flooding occurs very quickly due to faster flow times (in a matter of minutes). Urban areas are densely populated and thus the economic impact can be much higher than floods in rural areas.

What are the reasons behind Urban Floods?

Urban floods are caused by a wide variety of factors, but they typically result from a combination of natural and human influences.

Meteorological Factors: Most Indian cities receive heavy rainfall during monsoons. The extreme weather events with concentrated heavy rainfall are becoming more frequent due to climate change (increased cyclonic activity in Arabian Sea and Bay of Bengal due to warming). Most urban floods in recent times were consequent to episodes of heavy rainfall e.g., Bengaluru witnessed > 130 mm rainfall in one day, against average rainfall of ~4.5 mm/day in this time of the year. Similarly Mumbai recorded ~950 mm of rainfall on July 26, 2005.

Hydrological Factors: Flood risk arises when the surface runoff is greater than the infiltration rate during The infiltration rates depend upon the type of soil, their respective water retention capacities, vegetation cover (reduces speed of water and increases infiltration) etc. Urbanization reduces rate of infiltration due to paved roads and lack of vegetation cover. Lower infiltration leads to higher run-off and flooding.

Anthropogenic/Human Factors: The risk of urban flooding has increased as a result of human interference in the natural environment. Urbanization has led to encroachments on water bodies and construction on beds of rivulets/rivers and lakes. This obstructs natural flow of water. Consequently, such areas are inundated with water during events of heavy rainfall e.g., in Bengaluru, the Dakshina Pinakini river had been dry for three decades and was considered extinct. With heavy rainfall it was flooded and damaged buildings in its path. In 2021 , a Comptroller and Auditor General report had highlighted that the Bruhat Bengaluru Mahanagara Palike (BBMP), had not removed 714 encroachments out of the 2,626 identified near water bodies.

Source: Down to Earth. Number of Urban Flood Events in major cities since 2000 and the loss (in %) of water bodies due to urbanization. Bengaluru has witnessed an unprecedented 79% loss of water bodies.

Similarly, the number of water bodies in Gurugram has decreased from 644 in 1956 to 123 in 2018 and the green cover is only 9%. Delhi’s Commonwealth Games Village (CWG) has been built right next to Yamuna’s floodplain. The secondary runway of Chennai International Airport has been built right over the Adyar river. Many projects in Andhra Pradesh’s Amaravati Capital City Project, have been proposed to be built on the floodplains of the Krishna river.

In addition, most Indian cities have undergone haphazard urbanization. The infrastructure has not been commensurately upgraded with rising population e.g., storm water drainage lacks capacity to adjust for loss of natural run-off due to construction. The Karnataka State Action Plan on Climate Change (2013) had noted that drain infrastructure of Bengaluru is not enough to handle even moderate rainfall event. Yet, the infrastructure has not been upgraded. Also, lack of regular cleaning and upkeep of drains leads to clogging and flooding.

In many cases waste water from cities is drained in rivers and lakes. This causes siltation and reduces the capacity of natural drains, causing urban floods.

Source: Deccan Herald

What are the impacts of Urban Floods?

Urban regions are hubs of economic activity and contain critical infrastructure that require round-the-clock security. Damage to critical infrastructure in major urban centres has an impact at both the State and National level e.g., 2005 flood in Mumbai led to disruption of operations of  Stock Exchanges (BSE and NSE) and shut down financial services, resulting in losses exceeding US$ 100 million.

Disruptions in power and transportation impact day-to-day life of residents. Extreme events cause loss of life and property.

Additionally, flood waters from industrial facilities or storage locations may disperse dangerous chemicals and fuels into water. Toxic chemicals and untreated sewage can both pose serious risks to the general public’s health and the water supply.

Spread of infection post flood event can cause spread of epidemics.

For lower income groups, the major challenge is the ability to earn a livelihood, which is directly affected by urban flooding. The urban poor are at more risk, because in most cases the slums in which they live are most vulnerable to urban floods.

How can the issue of Urban Floods be addressed?

The most critical need is to increase the resilience of the communities and adaptive capacity of the infrastructure.

First, Water-sensitive urban design and planning techniques are most critical. These methods take into consideration the topography, types of surfaces (pervious or impervious), natural drainage, and have very little impact on the environment. Vulnerability analyses and risk assessments should be mandatory in city master plans.

Second, a comprehensive database of all wetlands and water bodies in urban areas should be established. Protected areas for river, lake, and other water channel catchments must be identified and freed from encroachments. Flood plains and natural water bodies must be restored.

Third, there is a need to adopt scientific management measures to deal with urban floods. Risk reduction should start with a mapping of flood vulnerability. Hydraulic and hydrological models must be used to analyse the flood pattern. The findings must be in urban master plans. Land use in urban areas should be strictly regulated.

Fourth, the urban infrastructure must be upgraded, including construction of new storm water drains and expansion of existing sewerage infrastructure. Moreover, the pavement cover must be reduced and vegetation/green cover should be expanded to reduce storm water run-off.

Fifth, there is a need for improved coordination between various institutions especially between water resources and disaster management authorities for integrated approach towards flood management. Engagement with grassroots organisations is necessary to promote awareness on holistic flood risk management. Participatory approach for early action coordination among stakeholders can help mitigate flood risks.

Sixth, as recommended by the draft New National Water Policy there is a need for renewed thrust on protection and revival of traditional local water bodies in both urban and rural areas. These water bodies would form part of urban blue-green infrastructure for improved water levels and quality, as also flood mitigation, through specifically curated infrastructure such as rain gardens and bioswales, urban parks, green roofs and green walls etc.

Source: Deccan Herald

Seventh, some countries are experimenting with concept of Sponge Cities, and initial experience has shown promise. If the solution is successful and scalable, same can be adopted in India in the new upcoming urban expansions. Sponge cities can solve the problem of urban flooding as well as scarcity of water during the summer season.

Conclusion

A Climate Change Assessment Report (2020) of the Ministry of Earth Sciences has noted that the increased frequencies of heavy rainfall has enhanced flood risk all over India, particularly in the urban areas. This requires an urgent fix. Addressing the challenge of urban floods requires a concerted effort of Union, State and Local Governments along with the citizens.

Syllabus: GS III, Disaster and disaster management.

Source: Indian Express, Indian Express, Down to Earth, Mint, The Times of India