12 Urban Flooding

Dr. Rajnish Ranjan

Objectives

• To comprehend urban flooding – its nature, causes and
  impacts.
• To understand the risk of urban flooding in India
• To understand the process of urban flood management and the urban flood management framework existing in India
• To comprehend urban flooding – its nature, causes and impacts.
• To understand the risk of urban flooding in India
• To understand the process of urban flood management and the urban flood management framework existing in India

    Keywords

Floods, Urban Flood, management

Rationale

This chapter provides an overview of urban flooding and elaborates on the nature, causes and impacts of urban floods. Utilizing different case studies from India, it reflects on the multifarious aspects of urban flooding. The chapter also provides an overview of the existing urban flood management mechanism, the processes involved and the direction it is headed.

    Unit 1 – Urban flooding

Definition–Urban Flooding may be defined as an unusual accumulation of water in an urban area above the ground (or any area utilized for anthropogenic activities). It may be caused by overflowing rivers, high tides, heavy rainfall, melting snow, rapid runoff from paved areas, breached water structures (created for storage, flow or drainage of water or to prevent water from entering certain areas) etc. (modified from EEA Environmental Glossary, 2007)

How urban flood is different from riverine flood

Urban flooding has root causes, characteristics and impacts differing from normal flooding events in the sense that it is created mainly due to the impact of urbanization and other associated localized phenomena . Urban flooding is characterized by localized inundation and water logging The Urban flooding creates inconvenience and sometimes they are life threatening too .. This also causes significant economic loss (as urban areas are the hub of economic activities), social chaos and environmental damage. It also causes severe disruption in the delivery of services and damages of the critical infrastructure. Mostly, the flooding events in urban areas are of short duration lasting from few hours to several days . Due to the significant role played by anthropogenic factors associated in urban flooding, they are difficult to predict.

Primarily changes in the natural drainage network, distortion of the catchment areas , land cover, deforestation, condition of built environment, poor condition of drainage, sewerage and solid waste disposal networks and anthropogenic activities cause urban flooding . The anthropogenic changes in urban areas increase the flood peaks from 1.8 to 8 times and flood volumes by up to 6 times. (NDMA Guidelines , 2010) The flood effects are pronounced and faster.

Causes of Urban Flooding – Physical and social components

1. Weather system–

 a. Rainfall – India receives maximum rainfall from the south west monsoon for a period of four months from June to September. The southern region of Tamil Nadu and Kerala receives rainfall in post monsoon and winter period (North East Monsoon ) as well. Though the rainfall pattern and duration is similar in almost all cities, the amount varies widely. Sudden peaks in the rainfall or sustained rainfall over a long duration of time often result in the increased water accumulation.

b. Storms – Cyclonic storms or localized storms associated with heavy rainfall also causes inundation.

c. Snowfall/Snowmelt – Sudden rise in water flow due to increased snowfall or snow melting in upper regions

d. Storm surges and tidal inundations –Storm surge is a phenomenon of rising water caused by low pressure weather events. Tidal variations resulting in inundation of nearby areas.

2. Natural System

a. Topographical features –Most of the urban areas have been developed along the flood plains due to having fertility for farming but even sometimes many of them have been developed in the flood plain itself. During high discharge these flood plains are inundated with surplus water. In many cases urban areas have a vessel like structure which makes it harder for the deposited water to flow out. Many cities are located in coastal areas which make them prone to coastal flooding, tidal inundation and cyclonic storms.

b. Hydrological factors – Factors like soil moisture level, ground water level and absorption capacity, surface infiltration rate, presence or absence of overflow channels etc.

3.  Sea level rise

Cities located primarily in coastal regions face higher susceptibility of flooding due to global rise in the sea level. e.g. Dhaka, Bangladesh is facing significant risk of flooding due to rising sea levels.

4. Urban heat island effect

Due to various kinds of anthropogenic activities in urban localities, the surface and atmospheric temperatures get higher than the normal . This is due to the high energy consumption, land and surface coverage by materials which have high heat capacities and conductivities than natural materials, low vegetation and water absorbing surfaces and less open spaces. This results in erratic pattern of rainfall and precipitation.

5. Urbanization and population growth

Anthropogenic events contribute significantly in the rising number of urban flooding.

a. Land cover change – The key issues arising out of land cover change which induce greater intensity of flood include:

1. Surface sealing due to tarmacked and paved surfaces (e.g. roads, roofs, built up area) reducing water infiltration and increasing surface water run-off and its speed

2. Deforestation resulting in the increased soil erosion and higher water run off

    3. Unplanned construction in catchment areas and flood plains

4. Rapid urbanization and redevelopment to higher density land use

    5. Loss of open surfaces and ground which can absorb water

6. Lack of rain water harvesting structures which can help absorb water and results in reduced water run off

b. Loss of aquatic ecosystem– The key issues are:

1. Alteration of natural drainage routes

2. Decline in the number of water bodies or wetlands

3. Declining ground water levels

4. Increased intake and outflow of water in the region but no watershed development for the same

5. Encroachment in river banks, drains and canals – The economically weaker sections of the society, the migrants etc. often stay in the river banks or canals which are supposed to function as drainage channels. Illegal construction and encroachment in the natural water outlets often restrict the flow of water out of the urban area.

c. Urban drainage, sewerage and solid waste disposal –The key areas of concern are as follows:

1. The storm water drainage network in the past was designed for a rainfall intensity of 12 – 20 mm (NDMA, 2010) Often, due to siltation and poor maintenance, these networks failed to function properly.

2. Improper solid waste disposal – Indiscriminate disposal of domestic, commercial and industrial waste in drains resulting in reduced capacity or choking of drains. Disposal of polythene bags in storm water outlets, open drains etc. resulted in choking water flow. Religious/social events resulting in the disposal of large quantity of impermeable materials along drainage networks.

3. Very efficient drainage in the upstream area resulting in higher inflow in a downstream area having undercapacity drainage networks

4. Growing population putting higher pressure on the existing drainage system.

d. Poor institutional/administrative mechanism – To address the concern of urban flooding, coordinated tasks by multiple authorities need to be carried out. Lack of ownership of responsibility by authorities is a major reason.

Lack of flood control measures and general awareness

Other anthropogenic events –Unforeseen events like sudden release of water from upstream areas, dam bursts etc. which result in urban flooding. e.g. Pune Khadakwasla dam burst.

In addition, often urban flooding occurs when two or more of the above factors coexist.

Case study – Different parts of Kolkata got affected in August 2015 when high tide and heavy rainfall induced by Cyclonic storm Komen coincided. Kolkata is located in the flood plain of the distributaries of the Ganges, the Hooghly River. The water outlets in the city drains in the Ganges, the eastern wetlands or through other channels leading out of the city. During the 2015 flooding, the Hooghly was experiencing the tidal bore which filled the capacity of the Hooghly itself and the drainage of excess water to Hooghly did not function. The capacity of the eastern catchment has also reduced significantly due to the unplanned development and real estate boom near the Eastern Wetlands and salt marshes in the past decade. Increased siltation, poor solid waste disposal (mainly plastic packets) and poor maintenance of the pipelines have decreased the efficiency of the water outlets. The scenario is further complicated by unauthorized constructions/encroachment near and on the canals, wetlands and free spaces across the city which act as a sponge soaking water in case of excessive water accumulation. With no outlet or space to drain or get absorbed in, the waterlogging and subsequent flooding occurred affecting the city for 2 – 3 days.

Source – zee news

Categories of Urban Flooding

Urban flood can be broadly categorized in to the following classes based on the causative element:

• Fluvial flood
• Caused by river overflow, breaching or overtopping flood defences o Occurs primarily in urban areas located in the flood plains
• Coastal flood
• Caused by tidal or storm surge in cities located in coastal areas or in deltas o Pluvial floo
• Caused by intense or prolonged rainfall exceeding the capacity of the drainage system
• Often affects urban areas which has witnessed rapid unplanned expansion o Flash flood
• Caused by rapid response of ephemeral streams to heavy rainfall, related, inter alia to steep slopes.
• Others Causes like structural failure of dams, sudden release of water from dams, increased snowmelt etc.

Impacts

• Economic Loss: The urban areas are considered as hub of the economic and allied activities. As a result of flooding in the urban areas, the following losses may occur –
• damage to public buildings, public utility works, housing and household asset
• Loss of industry and trade
• Daily earning of wage earners
• Loss of revenue from roadways, railways and other services
• Private organization and corporates loosing revenue of several days or compelled to shift operations to other location incurring huge loss
• Economic loss of the state in reconstruction activities

For instance, Aon Benfield report estimated the economic loss of Chennai floods (2015) at $3 billion and considered the flood as one of the eight most expensive natural disasters in the world in the year 2015.

o Complex issues – High levels of vulnerability as a significant number of people reside in hazard prone areas and have negligible social protections. Additionally the density of people in urban areas increase the complexity multiple times.

o  Secondary Hazards –Risk of secondary hazards like epidemics, tree falls etc.

o  Ecological impact – Damage to urban ecology and significant impact on parks, zoos, open areas etc.

o Public inconvenience – Water logging, foul stench, traffic congestion for several days resulting from collapsed trees, bridges, buildings etc. cause public inconvenience

Urban flood risk in India

The trends of urban flooding in India has increased significantly over the past few decades. Majority of the Indian cities have become hazard prone due to their location in the flood plains or coastal areas. Almost every Indian metros and cities, where a developmental onslaught is in process, have been affected by floods in the recent past, e.g. Hyderabad (2000), Ahmedabad (2001), Delhi (2002 and 2003), Chennai (2004), Mumbai (2005), Surat (2006), Kolkata (2007), Jamshedpur (2008), Delhi (2009) and Guwahati and Delhi (2010). Lately, the Chennai flood of 2015, a decade after the Mumbai floods serves as a harsh reminder of the deplorable situation in urban flood management.

Current status – A recent report by World Bank and OCED ranked 10 coastal cities across the world based on expected overall cost of damage in the coming years due to urban flooding events. Mumbai ranked 4th in the list. When measured in terms of percentage of GDP loss, Mumbai comes at 7th position. Another study published in the Natural Hazards journal conducted by scientists from UK and Netherland, measures vulnerability to flood covering 19 indicators which include parameters like weather pattern, environmental elements, preparedness mechanisms in place, resilience of the people etc. Of the 9 cities of India analyzed, Kolkata falls midway in the list and its vulnerability is attributed to huge population and high degree of exposure to storm. According to an online data analyzer of flood risk, Aqueduct Flood Analyzer, developed by four Dutch research agencies and WRI, India tops the list of countries considering the no. of people affected by flood, with 4.8 million affected per year which would rise to 19 million a year by 2030. India has $14.3 billion exposure to riverine flooding which would increase to about $154 billion by 2030. This report mentioned that reducing flood risk in fast expanding urban areas is pivotal to address the issue.

Source – DNA India

    Unit 2 – Urban flood risk management

Background

Urban areas have been affected by flood from time immemorial. Kolkata flood of 1737,1864, Pune flood of 1961, Delhi flood 1978 etc. indicates the flood vulnerability of urban areas. But the traditional flood management processes in India, have focused primarily on the management of floods which affect vast stretches of land, mostly in a rural setting.

Urban flooding became a matter of concern post Mumbai floods of 2005. The difference in the character of urban floods due to the evolving nature of urban areas was acknowledged. NDMA issued guidelines for the “Management of Urban Flooding” in 2010 which ushered an era of change in the way urban floods are managed in India.

Process

Urban areas are administered by a conglomeration of public welfare departments. To manage urban flood all these bodies require functioning together as a single unit. It requires a multidisciplinary holistic approach.

The process and activities of urban flood management spreading across various aspects of disaster management are as under :

Institutional and legislative frameworks – There is no single unified guidebook on urban flood management at the global and national level. Different agencies have come up with their own set of guidelines and handbooks. National Disaster Management Authority issued Urban Flood Management Guidelines in 2010 which calls for proactive, participatory, well-structured, failsafe, multi-disciplinary and multi-sector approach at various levels.

The Ministry of Urban Development has been declared as the nodal ministry for dealing with urban flooding.

The document calls for the development of Urban Flooding Cell in Ministry of Urban Development (MoUD), State Nodal Departments and urban local bodies.

JNNURM, Urban infrastructure development scheme for small and medium towns, North Eastern regional Urban Development Programme, National Urban Information System, Municipal Solid Waste Management agencies etc. have been entrusted to help ministry of urban development in developing structures and capacities to address urban floods.

Ministry of Environment & Forests (MoEF) may support MoUD in combating urban floods through implementation of environmental and forestry programmes through following the Municipal Solid Waste Rules, 2000, National Lake conservation plan etc.

Ministry of Water resources, Ministry of Earth Sciences, Ministry of Science and Technology and Ministry of Defence will also be supporting MoUD in the urban flood risk management .

In addition, organizations like NDMA, NEC, SDMAs, State Governments, Urban Local bodies etc, will also be functional .

Prevention, Preparedness and Mitigation mechanisms –The following areas of interventions fall under this category:

Early Warning Systems – Rainfall Forecasts are issueed by India Meteorological Department whereas flood forecast information is generated and issued by Central Water Commission in India. To accelerate the efforts, different urban local bodies are coming up with their flood early warning systems. There are few important action points suggested in NDMA Urban Flood Management Guideline for early warning-

• Establishing a Technical Umbrella for Urban Flood Forecasting and Warning both at the National Level and State/UT levels
• Establishing Urban Flood Early Warning System
• Establishment of National Hydro Meteorological Network
• India Meteorological Department (IMD) will develop a Protocol for Sub-Division of Urban Areas on the basis of Watershed and issue Rainfall Forecast on the Watershed-basis
• IMD will establish a ‘Local Network Cell’
• Establishment of Local Network of Automatic Rainfall Gauges (ARGs) for Real- time Monitoring with a density of 1 in every 4 sq km in all 2325 Class I, II and III cities and towns
• Strategic Expansion of Doppler Weather Radar Network in the country to cover all Urban Areas for enhanced Local-Scale Forecasting Capabilities with maximum possible Lead-time

Example – Surat Climate Change Trust and Surat Municipal Corporation have partnered to develop flood early warning system for Surat city. It creates a model scenarios of heavy release from Ukai dam into the Tapi River and provides 48 to 72 hour time window for preparedness.

Mitigation – Sructural and Non structural mitigation measures needs to be undertaken to address the issue of urban flooding in the country. Structural measures include establishment of rainwater harvesting systems in buildings, separate pipelines for different types of wastes, development of channel for excess water outflow, establishment of higher capacity pumping stations, flood proofing structures etc. Non-structural measures include provisions for land use and zoning plans, regulations mandating establishment of rainwater harvesting facilities, proper solid waste recycling and disposal mechanism, better maintenance of drainage networks, canals and pipelines etc. It also includes risk identification and assessment.

There are certain points of mitigation suggested in the action plan of NDMA Urban Flood Management Guidelines –

• Catchment will be the basis for Design of Stormwater Drainage System
• Watershed will be the basis for all Urban Flooding Disaster Management Actions
• About 2325 Class I, II and III cities and towns to be mapped on the GIS platform o Inventory of the existing storm water drainage system will be prepared on a GIS platform
• Contour Mapping will be prepared at 0.2 – 0.5 m contour interval
• Future Stormwater Drainage Systems to be designed with proper Runoff Coefficient Method taking into account the Approved Land-use Pattern
• Pre-Monsoon De-silting of Drains will be completed before March 31 every year o Every building shall have Rainwater Harvesting as an integral component of the building utility
• Encroachments on Drains and in Floodplains will be removed by providing alternative accommodation to the poor people
• Better Compliance of the Techno-legal Regime will be ensured
• Establish the Incident Response System for Coordinated Response Actions o Insurance and risk transfer mechanisms to be set in place

During and Post Disasters – The key actions to be undertaken during and post urban floods include:

• Communication
• Disbursal of information and communicating with the public through radio and other communication mediums
• Communicating and functioning in coordination with the different departments associated.
• Response – Activation of incident response plans and business continuity plans o Recovery
• Post disaster recovery process have to be people centric and community driven in order to be sustainable
• The reconstruction process should not create further vulnerabilities

    Summary

1. Urban Flooding may be defined as an unusual accumulation of water in an urban area above the ground (or any area utilized for anthropogenic activities).
2. Urban flooding has root causes, characteristics and impacts which differ from normal flood events.
3. Significant role played by anthropogenic factors associated in causing urban flooding.
4. Causes include :

Weather system – Rainfall, Storms, Snowfall/Snowmelt, Storm surges and tidal inundations

Natural system – Topographical features, Hydrological factors, Sea level rise

Urban heat island effect

Urbanization and population growth – Land cover change, Loss of aquatic ecosystem, Urban drainage, sewerage and solid waste disposal, Poor institutional /administrative mechanisms, Lack of flood control measures and general awareness and other anthropogenic factors.

5. Urban flooding may also occur when two or more of the above factors coexist
6. Types – Fluvial flood , Coastal flood, Pluvial flood, Flash flood and Others
7. Impacts – Economic loss, Environmental damage, Social chaos, Public inconvenience, Risk of secondary hazards.
8. High risk of urban flood in India; most Indian cities have experienced flood in the past few decades
9. Kolkata, Mumbai highly flood prone
10. The NDMA issued guidelines for the “Management of Urban Flooding” in 2010
11. Nodal Ministry for Urban Flooding is Ministry of Urban Development
12. Steps for strengthening of early warning systems, establishing mitigation measures and resilience building is encouraged in the NDMA guidelines.