Urban Flooding: Reasons and Suggestive Measures

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Context: Urban flooding has wreaked havoc on major Indian cities in the recent past. In Hyderabad recently, floods caused unprecedented damage to lives and infrastructure. Barely five years ago, it was Chennai that saw a massive flood costing much damage and lives; Gurugram over the past few years comes to a complete standstill during the monsoon months, and for Mumbai, the monsoon has become synonymous with flooding and enormous damages. As the incidence of climate variability and extreme weather events increases, urban flooding becomes more and more common. While the untimely heavy rains can be attributed to climate variability, the urban flooding is largely due to unplanned urbanisation. 

In this article, we will look at major reasons behind urban flooding and measures that can be taken for effective urban flood management. 

Relevance:
Mains: GS III- Conservation, environmental pollution and degradation, environmental impact assessment. Disaster and disaster management and Essay.

 

The term urban flood consists of two parts – ‘urban’ and ‘flood’. 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 a large number of people due to the high population density in urban areas.

Why is Urban Flooding Different? 

  • Developed Catchments: 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.
  • Faster Flow times: Consequently, flooding occurs very quickly due to faster flow times, sometimes in a matter of minutes.
  • Vital Infrastructure: Urban areas are centres of economic activities with vital infrastructure which needs to be protected 24×7. In most of the cities, damage to vital infrastructure has a bearing not only locally but could even have global implications.
  • Epidemics: Secondary effects of possible epidemics and exposure to infection take a further toll in terms of loss of livelihood, human suffering, and, in extreme cases, loss of life.

India has a long coastline of 7,500 km and an estimated 14% of India’s total population lives alongside it that traverses nine states, 77 towns and cities including some of the fastest-growing such as Mumbai, Chennai, and Kochi making them more prone to flood due to adjacent sea and erratic monsoonal behaviour.

Aside from the present danger posed by extreme weather events such as heavy rainfall and cyclonic storms, more slow-onset impacts brought about by climate change also threaten the coastline. A 2016 UN report estimated that 40 million people in India will be at risk from sea-level rise by 2050.

Causes of Urban Flooding

  1. Meteorological Factors: Heavy rainfall, cyclonic storms, and thunderstorms.
  2. Hydrological Factors: Overbank flow channel networks, the occurrence of high tides impeding the drainage in coastal cities.
  3. 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 the risk of floods.
      • Reduced infiltration due to 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.
    • Storm-water Drainage systems: These were designed for a rainfall intensity of 12-20 mm. Indian urban centres receive much heavier rainfalls and these outdated designs are not relevant now. The old and ill-maintained drainage system is another factor making cities in India vulnerable to flooding.
    • Encroachments: Encroachments are also a major problem in many cities and towns. Habitations started growing into towns and cities alongside rivers and watercourses. As a result of this, the flow of water has increased in proportion to the urbanization of the watersheds. The capacity of the natural drains has decreased, resulting in flooding. The number of wetlands has reduced to 123 in 2018 from 644 in 1956.
    • Climate Change: Climate change due to various anthropogenic events has led to extreme weather events.
    • Poor Solid Waste Management System: Domestic, commercial and industrial waste and dumping of construction debris into the drains also contribute significantly to reducing their capacities. Improper waste management system, clogging of storm-water drains because of silting, accumulation of non-biodegradable wastes and construction debris are major concerns.
    • Reducing Seepage: Indian cities are becoming increasingly impervious to water, not just because of increasing built up but also because of the nature of materials used (hard, a non-porous construction material that makes the soil impervious).
    • Lax Implementation: Even with provisions of rainwater harvesting, sustainable urban drainage systems, etc, in regulatory mechanisms like the Environmental Impact Assessment (EIA), adoption at user end as well as enforcement agencies remains weak.
    • No Community Participation: Flood control measures planned without participation of the affected community are unsustainable as they do not meet the needs of relevant stakeholders.
Meteorological Factors Hydrological Factors Anthropogenic Factors
  • Rainfall
  • Cyclonic storms
  • Small-scale storms
  • Temperature
  • Snowfall
  • Soil moisture level
  • Groundwater level prior to a storm
  • Natural surface infiltration rate
  • Presence of impervious cover
  • Channel cross-sectional shape and roughness
  • Presence or absence of overbank flow channel network
  • Synchronization of runoffs from various parts of the watershed
  • High tide impending drainage
  • Land-use changes (e.g. surface sealing due to urbanization, deforestation) increase runoff and sedimentation.
  • Occupation of the flood plain and thereby obstructing flows.
  • Inefficiency or non-maintenance of infrastructure.
  • Too efficient drainage of upstream areas increases flood peaks
  • Climate change effects,  magnitude, and frequency of precipitation and floods
  • Urban micro-climate (Urban Heat Island) may enforce precipitation events
  • A sudden release of water from dams located upstream of cities/towns
  • Failure to release water from dams resulting in backwater effect
  • Indiscriminate disposal of solid waste.


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
  • Chennai in 2018
  • Pune in 2019
  • Mumbai in 2020
  • Hyderabad in 2020


Impacts of Urban Flooding

  • Impact on Human Life:
    • Loss of life & physical injury
    • Increased stress; psychological trauma
    • Contamination of water supplies leading to diseases
    • A 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 the financial burden of the government
  • Impact on Transport and Communication:
    • Increased traffic congestion, disruption in rail services
    • Disruption in communication- impact on the telephone lines, internet cables
  • Impact on the environment:
    • Loss of tree cover, loss of habitat
    • Impact on animals in the zoo, the prevalence of stray animals

Measures to prevent Urban Flooding

  1. Need for Climate Resilient Infrastructure
    • Given the unprecedented rate of climate change-related severe disaster in recent times, urban areas must have a climate-resilient infrastructure. 
    • According to UN projections, by 2050 more than 68% of the world’s population could be concentrated in urban areas.
    • Additionally, daily life in urban areas is highly dependent on certain critical services and products provided by critical infrastructures (CIs).
    • Therefore, it is especially relevant to understand how Climate Change affects urban CIs in order to develop mechanisms to improve their capacity to handle crises derived from CC.
    • In this context, resilience-based strategies provide a holistic approach, considering both predictable and unpredictable threats.
    • Innovative approaches like Sponge Cities- wetland restoration, flushing systems using collected rooftop water, bioswales, and public spaces as flexible water retention facilities can be applied to Indian urban areas. 
      • Other such methods include permeable material for roads and pavement, green roofs and harvesting systems in buildings.
  2. Early Warning Systems and Communication
    • Dissemination of flood warnings must be carried out, using a wide range of latest technologies.
    • This would help in giving real-time data where traditional systems fail.
    • Tools such as predictive precipitation modelling can help do that and are also able to link it with the adaptive capacity of urban land use.
  3. Design and Management of Urban Drainage System
    • Watershed management and emergency drainage plan should be clearly enunciated in policy and law.
    • Proper management of the drainage system is necessary to ensure that the water does not get stored in one place.
    • Solid waste increases hydraulic roughness, causes blockage and generally reduces flow capacity.
    • These drains need to be cleaned on a regular basis to permit the free flow of water.
    • Vulnerability analyses and risk assessments should form part and parcel of city master plans.
  4. Rainwater Harvesting
    • Due to urbanization, groundwater recharge has decreased and the peak runoff from rainfall and consequent flooding has increased.
    • It will serve the twin purposes of lowering the peak runoff and raising the groundwater table.
    • Many municipal corporations in India have already made rainwater harvesting compulsory. 
  5. Conservation of Water Bodies
    • Urban water bodies like lakes, tanks, and ponds also play a very important role in the management of urban flooding by reducing the stormwater run-off by capturing it.
  6. Role of Science and Technology 
    • The management of urban flooding has to be treated holistically in a multi-disciplinary manner.
    • Science and technology can play a significant role in improved monitoring, modelling/ forecasting, and decision-support systems.
    • One method for improving the preparedness for urban flooding is by setting up a vulnerability-based geospatial framework to generate and analyze different scenarios.
    • It helps in identifying and planning for the most effective/ appropriate actions in a dynamic way to incorporate day-to-day changes that take place in urban areas, having the potential to alter the prevailing vulnerability profile.

The Concept of Sponge Cities: A Case Study
  • Asian cities are struggling to accommodate rapid urban migration, and development is encroaching on flood-prone areas.
  • Hastily built urban areas are being affected by flooding across India, Nepal, and Bangladesh.
  • This is not a trend only in developing countries; floods in Houston, USA, highlighted the risks of development in environmentally sensitive and low-lying areas.
  • In 2012, a severe flood in Beijing wreaked havoc on the city’s transportation systems, and in 2016 floods overwhelmed drainage systems in Wuhan, Nanjing, and Tianjin. The challenges are clear.
  • Groundwater over-extraction, waterway degradation, and urban flooding are forcing China’s cities to address a vicious cycle.
  • Sprawling urban development and use of impervious material prevent soil from absorbing rainwater, prompting further investment in infrastructures that typically impede natural processes and worsen flood impacts.
  • China’s “sponge city initiative” aims to arrest this cycle through the use of permeable surfaces and green infrastructures. 

The concept

  • Launched in 2015 in 16 cities, the initiative seeks to reduce the intensity of rainwater runoff by enhancing and distributing absorption capacities more evenly across targeted areas.
  • The resulting groundwater replenishment increases the availability of water for various uses.
  • This approach not only reduces flooding but also enhances water supply security.
  • The initiative is similar to the North American concept of low-impact development (LID), which according to the United States Environmental Protection Agency (EPA) mimics natural processes in order to protect water quality.
  • The case of Lingang, a planned city in Shanghai’s Pudong district, illustrates typical sponge city measures.
  • These include rooftops covered by plants, scenic wetlands for rainwater storage, and permeable pavements that store excess runoff water and allow evaporation for temperature moderation.
  • Chinese cities are making noteworthy efforts. In a pledge to expand coverage of urban greenery, Shanghai announced in early 2016 the construction of 400,000 square meters of rooftop gardens.
  • The project is a collaborative effort among city regulators, property owners, and engineers.
  • Sponge city projects in Xiamen and Wuhan have performed effectively during heavy rainfall.
  • Innovative water initiatives have been adopted worldwide, including wetland restoration in the American Midwest, flushing systems using collected rooftop water in Oregon USA, bioswales in Singapore, and public spaces as flexible water retention facilities in the Netherlands.


Issues in Urban Flood Disaster Risk Management:

  • Improper Risk Assessment: Risk assessment has not been done properly by the concerned departments. This can be achieved by improving institutional capacities and operational abilities based on local scale vulnerability analysis and risk assessment.
  • Improper designing of drainages: Stormwater drainage systems in the past were designed for a rainfall intensity of 12 – 20 mm. These capacities have been getting very easily overwhelmed whenever rainfall of higher intensity has been experienced.
  • Encroachments: Encroachments have led to a decrease in natural capacities of natural drains. This has led to the disturbance in the natural flow of the rivers, lakes, etc.
  • No Community Participation: Flood control measures planned without participation of the affected community are unsustainable as they do not meet the needs of relevant stakeholders.

NDMA guidelines on the management of urban flooding: The National Disaster Management Authority (NDMA) has issued guidelines on the management of urban flooding in 2010.

The key guideline was to create a National Hydro-meteorological Network.

The guidelines provide, that for providing early warning, the Central Water Commission (CWC) should maximize the real-time hydro-meteorological network to cover all the urban centres in dealing with urban flooding.

The requirement should consider all cities/ towns which are particularly located on river banks, upstream and downstream of major and medium dams and island cities. Based on that assessment, CWC will initiate the process to prepare a plan and implementation strategy.

Other recommendations:

  1. Use of Doppler Weather Radars to be expanded to cover all urban areas in the country.
  2. Coordination mechanism to be established among all agencies for deriving maximum benefit from the efforts of each individual organization.
  3. A dedicated high bandwidth communication channel is to be built, for ensuring smooth underlying sensor web flow of all available information and products.
  4. State-of-the-art automatic water level recorders must be installed throughout the drainage network of the watershed, which may sometimes extend beyond the administrative boundary of the ULB.
  5. Technical Umbrella for urban Flood Forecasting and Warning to be established at the national and state level.
  6. An inventory of the existing stormwater drainage system to be prepared. The inventory will be both watershed-based and ward-based.
  7. Catchment to be the basis for planning and designing the stormwater drainage systems in all ULBs.
  8. Contour mapping of urban areas to be prepared at 0.2 to 0.5 m contour interval for detailed delineation of the watershed/ catchment for planning drainage systems.
  9. Pre-monsoon desilting of all major drains to be completed by March 31 each year.
  10. Suitable interventions in the drainage system like traps, communicators, trash racks can be provided to reduce the amount of solid waste going into the storm sewers.
  11. All future road and rail bridges in cities crossing drain to be designed such that they do not block the flows resulting in backwater effect.
  12. Inlets to be provided on the roads to drain water to the roadside drains and these have to be designed based on current national and international practices.
  13. Every building in an urban area must have rainwater harvesting as an integral component of the building utility.
  14. Concept of Rain Gardens to be incorporated in planning for public parks and on-site stormwater management for larger colonies and sites that are to be developed.
  15. Low-lying areas in cities have to be reserved for parks and other low-impact human activities.
  16. Encroachments on the drain should attract penal action.
  17. Flood hazard assessment has to ascertain the level of acceptable risk of flooding on the basis of projected future scenarios of rainfall intensities and duration and land-use changes.
  18. Flood damage has to be according to the physical characteristics of the area such as land use, topography, drainage area, outfall system and the capacity of the existing stormwater drainage system.
  19. Ward level Information System has to be developed using high-resolution satellite images/aerial photos, integrated with socio-economic data covering natural resources and infrastructure facilities on an appropriate scale (1:1000) at the community level.
  20. States/UTs have to build partnerships with public/ private insurance companies and civil society to sensitive communities about available schemes and also develop appropriate micro-insurance schemes targeted at low-income groups.
  21. The database of the National Urban Information System (NUIS) will be expanded to cover infrastructure facilities at the community level integrated with socio-economic data.
  22. Urban Flooding has to be dealt as a separate disaster, de-linking it from riverine floods which affect the rural areas.
  23. Stormwater drainage concerns will be made a part of all EIA norms.
  24. Buildings have to be designated as Flood Shelters and all necessary arrangements have to be ensured ahead of the flood season. Children, women, the aged and the differently-abled persons have to be given special attention.
  25. Post-floods, restoration of power, telecommunications, road, and railway transport will get top priority.
  26. Media, corporate, NGOs has to be involved in awareness generation.


Way Forward

  • Better forecasting of rainfall events
  • 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.
  • Installation of a proper, functional drainage system. Maintenance of existing drainage channels, providing an alternative drainage path
  • Develop ‘sponge’ cities in line with cities in China.
    Launched in 2015, the Sponge City Initiative invests in projects that aim to soak up floodwater. The projects are being built in 30 cities, including Shanghai, Wuhan, and Xiamen. By 2020, China hopes that 80% of its urban areas will absorb and re-use at least 70% of rainwater.
  • Proper solid waste management system– control of solid waste entering the drainage systems
  • Restrict encroachments in natural drainage areas
  • Each city should have its Flood mitigation plans strongly embedded within the master plan of the city.
  • There should be a prompt, well-coordinated and effective response in case of urban floods to minimize casualties and loss of property and also facilitate early recovery.

Urban Flood management will not just help control recurring floods but also respond to other fault lines, provide for water security, more green spaces, and will make the city resilient and sustainable.

 



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