Why it is in news?

• Bengaluru and Vijaywada faced heavy rains
• Local weather patterns were responsible resulting in the abnormal situation
• The 65mm downpour caused tremendous inconvenience to the citizens

What is an urban flood?   

• The term urban flood consists of two parts - ‘urban’ and ‘flood’. 
•  According to the Census of India, 2011, an urban area is defined as

 (a) all statutory places with a municipality, a corporation, a cantonment board or a notified town area committee, etc.; or

 (b) all other places satisfying all three conditions:

 i) a minimum population of 5,000;

 ii) at least 75 per cent of male working population engaged in non-agricultural pursuits; and

iii) a density of population of at least 400 per sq. km.

• 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.

Impact of urban floods

• Cities are densely populated, and an urban flood affects a large number of people in a very small area.
• In addition, an urban flood results in inundation
• Damage to vital infrastructure, and disruption to roads and services
• Brings life to a standstill.
• Leads to major economic losses which have both local and global implications.
• Outbreak of diseases.

Comparison of urban and rural floods

• The extent of flooding in a rural flood may be quite large and there may be agricultural losses but the number of people affected is much lower in comparison to urban areas.
• Moreover, the flood peaks in urban areas are about two-eight times and flood volume is about six times when compared with the rural floods.

What causes urban flooding?

• Urban flooding is caused by three main factors:
1.  Meteorological: heavy rainfall, cyclonic storms and thunderstorms
2. Hydrological : presence or absence of overbank flow channel networks and occurrence of high tides impeding the drainage in coastal cities
3.  Human factors: 
1. land use changes
2. surface sealing due to urbanization (which increases run-off),
3. occupation of flood plains and obstruction of flood flows
4. urban heat island effect (which has increased the rainfall in and around urban areas)
5. sudden release of water from dams located upstream of citizen towns and the failure to release water from dams resulting in backwater effect
6. The indiscriminate disposal of solid waste into urban water drains and channels is a major impediment to water flow during the monsoon season.

Which parts of the country are more prone to urban flooding?

• All cities are vulnerable to flooding.
•  Most  have reached a saturation point in terms of population growth and accommodation
• The developmental activities have now shifted to low-lying areas and areas next to the riverbanks.
• Thunderstorms and heavy rainfall can occur anywhere.
• Consequently whenever a city experiences a large amount of rainfall within a short time, there are chances it gets flooded.
• Examples
• In 2012, Jaipur received 170 mm of rainfall in just two hours
• Chennai was severely affected due to heavy rainfall in December 2015,
• In 2016, Gurugram (Gurgaon), Bengaluru and Hyderabad have witnessed severe disruptions.  

what are the major infrastructural improvements needed?

• Several infrastructural improvements are required.
1.  existing drainage path should be well demarcated.
2. There should be no encroachments on the natural drainage channels of the city.
3.  A large number of bridges, flyovers and metro projects are being constructed with their supporting columns located in the existing drainage channels. This can be avoided using proper engineering designs, such as cantilever construction.
4.  Storage of rainwater in tanks can reduce the overflows and help in reducing urban flood volumes.
5. Storage or holding ponds should also be provided at judiciously selected locations to store water during heavy rainfall so that it does not cause downstream flooding.
6. Once the rain subsides, the water can be released gradually.
7. It has also been observed that roads are surfaced and resurfaced several times, thus increasing their level above the plinth-level.
8. Whenever a road is resurfaced, the existing layer be scraped first and then the new layer be laid. This shall ensure that the plinth level and the road level remain where they were prior to the resurfacing.
9.  Porous pavements.- These allow the water to gradually infiltrate into the underlying soil thereby maintaining the pre-development sub-soil water conditions. 

Will the instances of urban flooding increase in the future ?

• The Intergovernmental Panel on Climate Change (IPCC) has indicated that in future, there could be increase in instances of heavy rainfall in shorter spans of time.
• This means that our existing drainage systems have to be redesigned to accommodate the increased flow-levels
• This can be done either by resizing the drains or by judiciously integrating the best management practices into the drainage infrastructure. 

What steps must be taken at individual, community and authoritative levels?

• We should also insure our belongings against natural disasters and be prepared for such an eventuality.
•  Avoid living in the low-lying areas.
• Alternatively, future constructions in low-lying areas should be on stilts.
•  At the community level, people should spread awareness and be ready to respond to a flood as a community.
• Schools have a greater role to play - as children need to be sensitized not only about floods but other disasters as well.
•  At the city level, the authorities should ensure that the building by-laws are followed both in spirit and practice at the ground level. 

What is ‘Nowcasting’? 

• It refers to real-time weather updates. ‘Nowcasting’ is a more frequent issuing of alerts.
• IMD is presently issuing nowcasting alerts every three hours for the public and every 30 minutes for the aviation industry.
• The Municipal Corporation of Greater Mumbai issues nowcasting alerts for floods every 15 minutes on its urban flooding website during the rainy season.

What are the major takeaways from the recent urban floods?

• India experienced major urban flood in 2005 in Mumbai and then we heard of Srinagar, Kolkata, Surat and most recently, Chennai, Gurugram (Gurgaon), Bengaluru and Hyderabad during 2015-16. 
• The major takeaway is that no city is safe from flood disasters. Heavy rainfall can occur in any city any time.
• City authorities and residents should identify flood prone areas and be prepared to tackle flooding.
• They should take measures to ensure that when heavy rainfall occurs, adequate drainage systems are in place and these are unclogged so that flooding does not occur in the vulnerable areas.
• Through proper planning and retrofitting of best management practices, we can make our cities more flood resilient.