What is an urban flood? How is it different from rural flood or any other 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. Our 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 and damage to vital infrastructure, and disruption to roads and services, thereby affecting all walks of life. It often leads to major economic losses which have both local and global implications. Outbreak of diseases is yet another hazard after a major urban flood.

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

There are multiple agencies which claim authority over waste disposal systems. Is there a system in place currently where different agencies work together and mitigate the risk?

In Mumbai, the Municipal Corporation of Greater Mumbai has designated a Chief Engineer exclusively in charge of solid waste management. The solid waste department ensures that solid waste is collected and transported to the solid waste disposal site. Other cities may think of adopting a similar administrative structure so that their solid waste is swiftly and effectively transferred to the solid waste disposal site.

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

All our cities are vulnerable to flooding. Most of them have now reached a saturation point in terms of population growth and accommodation, and the developmental activities have now shifted to low-lying areas and areas next to the riverbanks. Thunderstorms and heavy rainfall can occur anywhere. So, whenever a city experiences a large amount of rainfall within a short time, there are chances it gets flooded. For example, in 2012, Jaipur received 170 mm of rainfall in just two hours; similarly, Chennai was severely affected due to heavy rainfall in December 2015, and in 2016, Gurugram (Gurgaon), Bengaluru and Hyderabad have witnessed severe disruptions. So, in my opinion, in today’s times, all cities are equally vulnerable to flooding – be it coastal cities, inland cities, hilly cities, cities on the banks of major rivers or near dams/reservoirs.

Talking about infrastructural improvement for mitigating the risk, what are the major infrastructural improvements that should be brought about in urban planning at various levels?

To avoid urban flooding, several infrastructural improvements are required. Firstly, the existing drainage path should be well demarcated. There should be no encroachments on the natural drainage channels of the city. Secondly, 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. Storage of rainwater in tanks at the rooftop, intermediate, ground or underground levels can reduce the overflows and help in reducing urban flood volumes.

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. Once the rain subsides, the water can be released gradually.

It has also been observed that roads are surfaced and resurfaced several times, thus increasing their level above the plinth-level. The Indian Roads Congress has recommended that 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. Also, various cities, across the world, have constructed 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 if the current trends continue?

If we take appropriate measures, we can ensure that the flood incidences remain within tolerable limits. 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 is that first basic issue that needs to be addressed at the individual, community and authoritative levels?

Each one of us should realize that disasters like floods and earthquakes can happen anytime. Just like we take life insurance and motor vehicle insurance, we should also insure our belongings against natural disasters and be prepared for such an eventuality. Obviously, if we know the flood risk map of our city, then we can 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. People should also cooperate with municipal authorities.

Talking about the extent of damage, what is the ratio between the investment needed to avoid an urban flood and the actual cost of damage incurred because there was no investment?

The amount of investment is generally a fraction of the total damages. International studies have shown that the investment needed, in terms of following building by-laws, constructing proper infrastructure, establishing and enforcing mitigation measures, is only about seven per cent of the total cost of damage that would have occurred if the above measures were not put in place. In that sense, it is prudent to invest in preventive and mitigation measures rather than incurring huge damages afterwards.

What should affected people do immediately after they receive a flood warning?

Affected people should immediately evacuate to an identified evacuation centre. Or if such a centre does not exist, they should go to their neighbours staying in higher levels. They should take their important things such as documents and valuables in a water-proof bag, which they should have packed beforehand.

What important preparations can people living in risk-prone urban centres make in advance to tackle an urban flood?

People living on ground floors should take simple measures such as installing the gas connection, water heater, and electric panels high above the ground to brace themselves against flooding incidents. They should also put check valves in sewer traps to prevent floodwater from backing up into the drain. They should also seal the walls in their basements to avoid seepage. 

What precautions should one take after the flood water recedes?

After the flood water recedes, the threat of epidemics such as malaria, chikungunya and dengue spread by mosquitoes, and leptospirosis caused due to mixing of rat’s urine with water looms large. Water should not be allowed to stagnate to prevent breeding of mosquitoes. People should consume packaged drinking water or boiled water, if possible, to prevent water-borne diseases and gastroenteritis.

Is it possible to forecast an imminent urban flood? Please explain the equipment and methodology involved.

With the currently available instrumentation, technology and information, it is quite easy to forecast an urban flood and issue early warnings over the internet. For 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. Water level measurements can be made on the receiving river or drain using ultrasonic water level sensors. The methodology is to look at the Indian Meteorological Department’s ‘Nowcasting’ forecasts and simultaneously monitor the satellite maps for movement of weather systems. Looking at the satellite animations, one can estimate when heavy rainfall is likely to take place over the city. Once the rainfall takes place, the automatic weather station records the rainfall and simultaneously, the flow gauges record the water level. From the moment the water falls on the rain gauge to the time it reaches the water body of the affected area, there is a delay of some time. Hence, we have some lead time to inform the people living in the downstream area that the river levels are likely to rise. For example, during one such heavy rainfall event in Mumbai in July 2013, people had 30 minutes warning to evacuate with their belongings.

What response mechanism should be put in place for help to reach affected people on time?

The local community is the first responder in case of most disasters. The second line of responders is the city municipal corporation or the urban local bodies. In Mumbai, the fire and rescue services are within the purview of the Municipal Corporation of Greater Mumbai and they can reach the affected areas within 15-30 minutes. Other cities can adopt a similar model for help to reach the affected people within a reasonable time.

What is ‘Nowcasting’? How can it be useful in managing urban floods?

It refers to real-time weather updates. Earlier, the Indian Meteorological Department (IMD) used to issue warnings twice a day, at 8:30 am in the morning and 5:30 pm in the evening. ‘Nowcasting’ is a more frequent issuing of alerts. Currently,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.

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. What is the major takeaway from these instances?

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.