Urban Flooding- An Overview

Context:

Recent flood like situation in Mumbai leading to disruption in civic life

What is urban flooding?

• Flood is defined as “an overflow of a large body of water over areas not usually inundated”. Thus, flooding in urban areas is caused by intense and/or prolonged rainfall, which overwhelms the capacity of the drainage system.
• Urban flooding is significantly different from rural flooding- urbanization increases flood risk by up to 3 times, increased peak flow result in flooding very quickly. Further, it affects large number of people due to high population density in urban areas

Causes of urban flooding

Meteorological Factors:

• Heavy rainfall, cyclonic storms and thunderstorms

Hydrological Factors:

• Overbank flow channel networks, occurrence of high tides impeding the drainage in coastal cities.

Anthropogenic Factors:

Unplanned Urbanization:

Unplanned Urbanization is the key cause of urban flooding. A major concern is blocking of natural drainage pathways through construction activity and encroachment on catchment areas, riverbeds and lakebeds.

Some of the major hydrological effects of urbanization are:

• Increased water demand, often exceeding the available natural resources;
• Increased wastewater, polluting rivers and lakes and endangering the ecology-
• Destruction of lakes is a major issue in India cities. Lakes can store the excess water and regulate the flow of water. However, pollution of natural urban water bodies and converting them for development purposes has increased risk of floods
• Reduced infiltration due paving of surfaces which decreases ground absorption and increases the speed and amount of surface flow
• Reduced groundwater recharge, increased use of groundwater, and diminishing base flow of streams
• Increased peak flow

Climate Change:

• Climate change due to various anthropogenic events has led to extreme weather events

Poor Solid Waste Management System:

• Improper waste management system
• Clogging of storm-water drains because of silting, accumulation of non-biodegradable wastes and construction debris.

Drainage System:

• Old and ill maintained drainage system is another factor making cities in India vulnerable to flooding.

Impact of Urban Flooding:

Impact on Human:

• Loss of life & physical injury
• Increased stress; psychological trauma

Disease outbreak:

• Contamination of water supplies leading to diseases
• Rise in mosquito borne diseases

Impact on Economy:

• Damage to buildings, roads and other infrastructures
• Disruptions to industrial production
• Disruptions to utility supplies
• Impact on heritage or archaeological site
• Post-disaster rescue and rehabilitation adds to financial burden of the government

Impact on Transport and Communication:

• Increased traffic congestion, disruption in rail services
• Disruption in communication- on telephone, internet cables

Impact on environment:

• Loss of tree cover, loss of habitat
• Impact on animals in zoo, stray animals

Notable Urban Flooding Events in India since 2000:

• Hyderabad in 2000
• Ahmadabad in 2001
• Delhi in 2002 and 2003, 2009, 2010
• Chennai in 2004, 2015
• Mumbai in 2005, 2017
• Surat in 2006
• Kolkata in 2007
• Jamshedpur in 2008
• Guwahati in 2010
• Bengaluru in 2017

National Disaster Management (NDMA) Guidelines on Urban Flood Management:

• In 2010, NDMA had issued guidelines on Urban Flood Management in India

1. To create a National Hydro-meteorological Network.

• The guidelines say that for providing early warning, the Central Water Commission (CWC) CWC should maximize the real-time hydro-meteorological network to cover all urban centres to effectively deal with the problem of urban flooding

2. Use of Doppler Weather Radarsto be expanded to cover all urban areas in the country

1. An inventory of the existing storm water drainage system to be prepared. The inventory will be both watershed based and ward based.

4. Catchment to be the basis for planning and designing the storm water drainage systems in all ULBs
5. All future road and rail bridges in cities crossing drains to be designed such that they do not block the flows resulting in backwater effect
6. Every building in an urban area must have rainwater harvesting as an integral component of the building utility.
7. Low-lying areas in cities have to be reserved for parks and other low-impact human activities.
8. Encroachments on the drain should attract penal action.

1. Pre-monsoon desilting of all major drains to be completed by March 31 each year.

10. Urban Flooding has to be dealt as a separate disaster, de-linking it from riverine floods which affect the rural areas.

1. Suitable interventions in the drainage system like traps, trash racks can be provided to reduce the amount of solid waste going into the storm sewers.
2. Inlets to be provided on the roads to drain water to the roadside drains and these has to be designed based on current national and international practices.
3. Every building in an urban area must have rainwater harvesting as an integral component of the building utility.
4. Concept of Rain Gardens to be incorporated in planning for public parks and on-site storm water management for larger colonies and sites those are to be developed.

15. Flood hazard assessments should be done on the basis of projected future scenarios of intensities and duration of rainfall and land use changes.

Way Forward:

1. Better forecasting of rainfall events; timely dissemination of information to the mass- ‘Nowcasting’ alerts or real-time weather updates

• Example: The Municipal Corporation of Greater Mumbai has installed 60 automatic weather stations for recording the rainfall every 15 minutes. This information is disseminated through the GMDMA (Greater Mumbai Disaster Management Authority) web portal in real-time during the monsoon.

2. Installation of proper, functional drainage system. Maintenance of existing drainage channels, providing alternative drainage path
3. Develop ‘sponge’ cities in line with cities in China

4. Proper solid waste management system- control of solid waste entering the drainage systems
5. Restrict encroachments in natural drainage areas; clearance of river beds, proper implementation of Coastal Regulation Zone rules.
6. Each city should have their Flood mitigation plans strongly embedded within the master plan of the city.
7. There should be prompt, well-coordinated and effective response in case of urban floods to minimize casualties and loss of property and also facilitate early recovery.