Urban flooding has acquired extraordinary dimensions in our times. Earlier, when cities received excessive rainfall, it would usually result in waterlogging in low-lying areas. Today, such is the pace at which cities are growing – making demands on natural systems such as marshlands, wetlands, lakes and rivers – that a high rainfall event leads to massive urban flooding, crippling urban life and the economy. The frequency of urban floods has increased with not a year passing without some city or the other getting submerged.
It is important to recognise that urban floods begin with an unanticipated high rainfall event which seriously disrupts public transport, electricity and communications and also plays havoc with the urban economy. The complex administrative set-up of cities, where multiple authorities and agencies function with competing jurisdiction, does not make things any easier. That is why steering disaster management in urban areas through various stages – from early warning and response to relief and recovery – will always present a huge challenge. There is an urgent need to analyse these interrelated issues as the 2015 Chennai floods and the 2005 Mumbai floods demonstrate.
Emerging pattern of heavy localised downpours
The northeast monsoon, which has its onset in Tamil Nadu around October 20 every year, brings an average total rainfall varying between 430 mm to 490 mm. However, Chennai experienced a rainfall of 340 mm in the course of just one day – on December 1, 2015. Even in 2005 and 2006, heavy rains from the months of October to December had flooded Chennai.
For that matter, the phenomenon of metropolitan cities receiving excessive rainfall, leading to sudden flooding, has been seen in cities such as Mumbai and Kolkata also. Mumbai experienced massive floods on July 26, 2005 when it received an unprecedented 944 mm of rain within a span of 18 hours, one of the eight heaviest rainfalls ever recorded. More than 500 people lost their lives in the floods.
Kolkata, considered among the most flood-prone of Indian cities, suffered last year at the hands of Cyclone Komen which brought heavy rainfall upon it. The city was flooded in 2013 too when it received rains that exceeded the average by almost 38 %, while all adjoining districts received normal rainfall.
A heavy rainfall event is always a complex event. The Chennai rains have generally been explained in terms of the El Niño effect. Here it is important to understand that the El Niño effect has to be seen in terms of an entire season; it cannot really explain excessive individual rainfall events. For that matter El Niño’s impact on the northeast monsoon generally leads to higher than normal precipitation, whereas it impacts the southwest monsoon in the opposite manner, usually resulting in deficient rainfall and drought.
Explaining the Chennai rainfall event, B. Mukhopadhyay, Additional Director-general of Meteorology (Research) at the Indian Meteorological Department (IMD) in Pune, stated that “an individual episode like that of December 1 is a combination of several factors and in every such episode, the combination changes. On December 1, the lower-level moisture supply was high and upper air evacuation of the moisture was also strong. We call this phenomenon upper air divergence, and the effect is that the cloud becomes very intense. Both coincide very rarely.” The upper air divergence led to formation of clouds over the Bay of Bengal, and the easterly winds drove these clouds towards Chennai where heavy precipitation took place.In the case of Mumbai in 2005, the unprecedented rainfall on July 26, 2005 was attributed to a combination of four factors: the development of a low pressure area over the northwest Bay of Bengal, intensification of the monsoon, strengthening of the Arabian Sea current of the monsoon, and the super positioning of a meso-scale offshore vortex over northeast Arabian Sea (localised heavy clouds over the sea).
These meteorological factors come together to form intense clouds over urban sprawls largely due to increased surface temperature. The relationship between larger cities and increased rainfall is explained by the Urban Heat Island effect. It has been observed that the temperature over urban areas is higher than in the surrounding areas. Whenever rain-bearing clouds pass over these areas, the hot air pushes the clouds up, resulting in highly localised rainfall which may sometimes be of high intensity. According to an Indian Institute of Tropical Meteorology (IITM) study which analysed heavy rainfall events during the period 1970-2006, there is an increasing trend of very heavy rainfall (≥125mm) in Mumbai, Delhi and Chennai.
The importance of early warning
An effective early warning system is the only way to alert people to the intensity of rainfall and its likely consequences. One would imagine that providing early warning would be much easier in cities, what with a large number of media channels and a dense communication network. That is easier said than done due to a coming together of several local issues.
Chennai has a network of Automatic Weather Stations in selected micro watersheds and a functioning Doppler radar, which did provide accurate information about the rainfall being moderate, but since the moderate rain continued for more than 24 hours, it resulted in excessive rainfall on a cumulative basis. Here the issue was that while an early warning system can alert people to the amount of rainfall, it does not provide much information about the aggregate impact of continued moderate rainfall, in this case inundation. In urban areas, due to lack of open spaces and impervious surfaces, the permeation levels are low with the result that flooding occurs very quickly within a matter of hours, as opposed to flooding in rural areas.
In Mumbai, for instance, the rainfall event was concentrated in suburbs, while the island city did not experience much rain. As soon as the rainfall began on 26 July 2005, the IMD autographic rainfall recorder in Santacruz was submerged in rainwater. Though an IMD observer working in Mumbai during the rainstorm had taken great pains to record hourly observations, they were never conveyed to the relevant government agencies — namely the state government and Municipal Corporation of Greater Mumbai – because the telephone lines were not working. Since the telephone and mobile services had stopped working by evening, government authorities in Mumbai city did not have any authentic information about the severity of rainfall till the next day.
An effective early warning system for metro cities would consist of measuring rainfall and water levels in different rivers and lakes on a real-time basis and providing information to the people on the possible extent of inundation. So a city must have a network of automatic rain gauges, water-level recorders and automatic weather stations to provide information on the volume and flow of water in different areas of the city. As the network collects data and calibrates it to indicate the level of inundation, the agencies dealing with floods can be notified of its spread and magnitude. Appropriate flood warnings can then be issued to urban residents through announcements on television and radio, and through SMS.
Reviving natural flood protection systems
While there is a need to shore up early warning systems, there is equally an urgent need to realise that wetlands and urban rivers provide natural drainage for floodwaters and hence should be preserved. The reality in Chennai and Mumbai is just the opposite, for wetlands have been encroached upon and reclaimed for construction.
In Chennai, the marshlands, spread over more than 5,000 hectares, provided a natural system for absorbing excess rainwaters; today they have been reduced to one-tenth their original size. Mumbai’s wetlands near Sewri and other areas in New Mumbai are also shrinking continuously on account of being treated as massive dumping sites for solid waste. Salt pans which hold a huge amount of tidal waters are exposed to the risk of being used for housing construction, which increases the city’s flood risks considerably.
The degradation of rivers across cities has also exacerbated the situation. In Chennai, the floodwaters are carried by Cooum and Adyar rivers, and the Buckingham Canal all of which do not have enough water flow in normal times. They are silted and choked and often encroached upon, which reduces their carrying capacities. River Cooum, which was once a source of fresh water, has become a drainage course running through 40 km of peri-urban and urban areas in Chennai. Once used for fishing and boat racing, the river has suffered heavily on account of solid waste dumping and encroachments. In a dry season, the river has been found to be contaminated with polluted water discharged by Chennai Water Board and by a large number of surrounding industries. About 30 per cent of the untreated sewage gets into the Cooum.
The story of Mumbai’s 18 km long Mithi River, which originates from the overflow of Powai and Vihardams, is somewhat similar. The river, which can carry surplus water into the sea,has atrophied because it is being treated as a dumping ground for debris and its banks have been encroached upon for the expansion of buildings, slums and industries. The Airports Authority too has encroached upon the river bed to expand the runway at the Santacruz airport.Urban reservoirs too are a significant part of the city landscape. As is public knowledge, the release of water from the Chembarambakkam reservoir into Adyar River in Chennai worsened the flood situation and caught people by surprise. Reservoir management, always a difficult task in urban areas, becomes far more tricky when faced with a high rainfall event. Urban reservoirs supply drinking water to the city and their water levels are monitored on a regular basis. For most part these reservoirs do not have sufficient water and although there are guidelines for managing them, there is generally a tendency on the part of civic authorities to store as much as water as possible, releasing it only when the stored quantity poses a risk to the reservoir. As the water is released at the last minute, there isn’t enough time to evacuate people. The release of water has the impact of flash floods. In Mumbai, three important lakes –Tulsi, Vihar and Powai — have ungated spillways, and when these lakes overflow, the surplus water is discharged into Dahisar River in the north and Mithi River in the south. However, when rivers such as Mithi are completely choked, the result is massive inundation of low-lying areas across the banks.
As per the Bureau of Indian Standards’ guidelines, control of floods is better achieved if the reservoir level is kept low in the early stages of the monsoon season. However, the increased variability of intra-seasonal rainfall has made it difficult to observe these guidelines. Furthermore, as the number of reservoirs has increased, a simultaneous release of water from various reservoirs increases the flow of water immediately, which leads to inundation over a vast area. There is a growing realisation of the need to change the very concept of reservoir operation by ceasing to see each reservoir as a single entity, instead developing the concept of integrated operation of reservoirs. The guidelines for management of reservoirs need to be revised on the basis of current data relating to intra-seasonal variation in rainfall and the likely impact of release of water on ever-increasing urban areas.
The role of stormwater drainage
While stormwater drainage is yet another important means of discharging floodwaters into the sea, as per a report in the Down to Earth (15 December 2015), Chennai has only 855 km of stormwater drains as against 2,847 km of urban roads; consequently, even a marginally heavy rain could cause floods. Stormwater drains are an essential part of urban infrastructure, which have not received adequate attention or investment.
The design of stormwater drains is equally important. As witnessed in Mumbai, they simply did not have the right diameter to carry the heavy amount of rainfall. The stormwater drains in Mumbai were originally designed for 25 mm / hour with a coefficient of run-off of 0.5. The Brinhanmumbai Storm Water Disposal System (Brimstowad) report commissioned in 1993 by the Municipal Corporation of Greater Mumbai recommended an enhanced design parameter of 50 mm / hour, the cost of which was very high. It is important to keep in mind that while the increased diameter improves drainage greatly, even an upgraded system would be of limited use when faced with a rainfall intensity of 130 mm/ hour as was witnessed on 26 July 2005.
It was only after the 2005 Mumbai floods that the report was taken up for implementation. The Government of India supported the Brimstowad project to get off ground in 2006. The project, which was to have been completed in 2007, involved the installation of eight pumping stations and 58 different works for an upgrade and improvement of the city’s century-old drainage network. Only four have been made operational so far. The project has missed two more deadlines – in 2011 and 2015. There have been significant cost overruns too, with the initial cost of Rs. 1,200 crore now having escalated to Rs. 4,000 crore. A new deadline of 2019 has been set for the project’s completion.
A co-efficient of run-off of 0.5 implies that 50% of the rainfall infiltrates into the ground through various open spaces, or is held temporarily in low-lying areas and ponds. However, due to increased paving and impervious surfaces, the permeation level reduces, thus allowing the water to rise very fast. In most cities, municipal authorities are expected to increase the carrying capacity of stormwater drains through regular desilting and cleaning before the rains. In most cities this exercise is fraught with corruption, with frequent allegations that contractors do not clean the stormwater adequately.
There is one more thing: Cities need to develop separate systems for disposal of sewage and stormwater. But in the absence of a proper sewerage system in many cities, urban residents discharge their sewage into the existing stormwater drains. In Mumbai, storm drains receive possibly 40 % of sewage from thecity either by direct discharge/overflow from sewers or by drainage across the ground. The problem is compounded when industries discharge effluent directly into the drains. The use of stormwater drains for discharge of sewage seriously reduces the capacity for draining the rainwater, besides polluting the outfall (the place where the stormwater drain discharges water into the sea).
Strengthening city-level disaster management
In the ultimate analysis, as more and more cities get affected by floods there is a need for coordination between various agencies dealing with different parts of the urban system for a prompt and efficient response.
With little time to prepare, agencies find it difficult to deploy their resources and alleviate the situation. Besides, most agencies do not have much experience in managing urban floods, nor is there a well-functioning coordination structure at the state or municipal level to bring together different elements of the urban system to deal with the exigencies of the situation. Facilities such as state or city-level Emergency Operations Centres have either not been set up or function with untrained staff. Information management and communications often emerge as weak aspects of flood management, and there is always an impression of government agencies not being able to rise to the occasion effectively. The Government of India follows up all major urban flood events with announcements of Central assistance to the tune of Rs. 1,000 crore for relief as a matter of course.
One of the most important aspects in flood management is that of first response and prompt evacuation. In Chennai, the armed forces and the National Disaster Response Force were deployed as first responders. It took about 24 hours to deploy the first responders, which delayed evacuation and provision of essential supplies such as food, drinking water and medicines. Moreover, the first responders were not familiar with the city, which made it more difficult for them to come to the aid of urban residents. At such times the city is generally in a gridlock with airline and train services suspended, and this makes the movement of boats and other life-saving equipment difficult. In Mumbai, the army and navy could not be deployed till the third day due to these logistical difficulties. There is no viable alternative to setting up well-equipped first response teams at the state and district levels if the first response has to be quick and immediate.
In the end, the city gets divided. A part of the city which is planned, serviced and well-fortified emerges relatively unscathed. The other part of the city, which is low-lying and irregular, and consists of slums and squatter settlements, gets badly affected. While floods often lead to a tragic loss of many lives, there are certain deaths which show us in poor light, such as patients dying in the intensive care units of hospitals in Chennai, or a large number of young men dying of leptospirosis in Mumbai. A number of people were smothered to death in Mumbai when they could not get out of their cars in the midst of rising waters.
The impact of urban floods can be severe, lingering for a long time. Chennai is a major centre of auto manufacturing, with global brands such as BMW, Ford, Hyundai and Renault-Nissan having set up their manufacturing facilities around the city. It is estimated that auto manufacturers may suffer a production loss of 15 percent. This is reminiscent of the 2011 floods in Bangkok when leading car manufacturers such as Toyota and Honda, which support their global supply chains through their auto part manufacturing facilities in the Bangkok metropolitan area, had to reduce production in their factories in many other countries. Despite this, we in India do not see much merit in conducting a detailed assessment of damages and losses, nor do we carry out risk assessment exercises and plan for better flood management in future.
The time has come to become proactive for urban floods are a certainty now, occurring on an annual basis. This phenomenon is not limited to India either. Urban floods are a global risk, an outcome of demographic transition and environmental changes which follow a familiar script everywhere: a convectional cloud bringing unusual amount of rainfall, release of vast quantities of water from dams, clogged drains, shrinking wetlands, overflowing lakes, a dysfunctional city government and a serious breakdown of civic services.
If these concerns are not taken care of, the consequences could be damaging such as witnessed in cities across the world where the floods continue for a long time. For instance, starting from July 2011, Bangkok remained flooded for more than five months. There is an urgent need to view urban flooding as a wider governance and natural resource management issue with a focus on more equitable planning. Each of the contributory factors needs to be addressed discretely and together so that cities are better prepared to deal with the next deluge.
The writer works as Recovery Advisor as part of the Climate Change and Disaster Risk Reduction team in UNDP. The views expressed here are his own.