Monday, July 27, 2009


About 100 years ago, if Mumbai city were to receive a rainfall, as heavy as the one witnessed in the monsoon of 2005, its outcome would not have been as catastrophic. This is because the population of the city has grown to ten times of what it was a century ago. To accommodate this population, the city has risen vertically, open spaces have shrunken, the arterial roads cannot be widened any further and the drainage systems fail to keep pace with the ever-increasing requirements of the metropolis. Thus, this natural catastrophe that shook the Mumbai region in 2005 can be ascribed to three main factors as stated by Dr. Kelkar: (1) Unusually heavy rains and rise in sea-levels, (2) Antiquated building regulations and (3) The inefficiency of the existing drainage system, necessarily in that order. These causes are discussed in detail in the following section. What added to this disaster was the lack of a precise preliminary warning from the weather department. Moreover, the absence of post-disaster management plans on the part of the city’s governing authorities further compounded the situation.

Since the discharge of all the storm water and treated sewage is released into the Arabian Sea, tidal variation also constitutes a major component in the system of the storm water drainage (SWD). It results in excessive flooding and water retention in the event of heavy rains and the water recedes only during the low tide. This phenomenon was largely observed during the floods of 26th July 2005, when high tide forced all the drained storm water from the Arabian Sea back into the city.

1. Unusually Heavy Rains and Rising Sea-levels:
1.1. Climate Change and Heavy Rains

Heavy rains to a magnitude of more than 240mm are almost of a regular occurrence in Mumbai at the onset of the monsoons.
However, after the monsoon sets in and moves into its active phase, the situation is conducive to the occurrence of very heavy rains over Mumbai, when they are collectively a result of the following factors: “(1) Development of a low pressure belt over the sea, (2) Intensification of the monsoon trough and the development of embedded convective vortices over central India, (3) Amplification of the Arabian Sea current of the monsoon and (4) Super-positioning of a meso-scale ‘Off-shore Vortex’ over the northeast of Arabian Sea and its northward movement. All these conditions build up a synoptic situation that is conducive to the occurrence of a heavy rainfall in the area.” This phenomenon was largely observed in 2005.

Offshore Vortex: It is a rare meteorological phenomenon, characterized by a heavy but extremely localized rainfall that spreads over an area of, as little as, 30 square kilometers. Scientists and experts claim that in the case of the Mumbai floods, the phenomenon started with high velocity air currents in the Arabian Sea, which turned at 360 degrees, giving rise to a trough. The turn gave rise to a vortex, which resulted in a low pressure. In the meanwhile, powerful winds rose up in the atmosphere, leading to a heavy downpour.

However, there is a scientific explanation for the occurrence of this rare phenomenon. The reason, as Dr. R. K. Pachauri explains, is global climate change, in addition to the city’s geographic proximity to the Sahyadri Hills, that has played a significant role in the intensity of the downpour of July, 2005. Even as this research is in progress, current climatic changes worldwide have been exhibiting unexpected behavior and have been a constant cause of concern for all.

All scientific studies confirm that this “climate change is significantly anthropogenic (i.e. human induced).” One of the most predicted effects of the global climate change is extreme variations in tropical climates, including irregular rainfall patterns. An example of this phenomenon was witnessed in the form of heavy rains in Mumbai on 26th July 2005. The immense loss of life and property caused by this event are testimony to the fact that the impacts of climate change can add significantly to the vulnerability of the locations and communities that face the danger of natural disasters anyway. The human influence on climate change is caused by a rise in the concentration of the so-called greenhouse gases (GHGs) in the earth’s atmosphere. The most prominent among these is carbon-dioxide, which has been emitted, in increasing quantities, from the burning of fossil fuels, since the beginning of industrialization, in the mid-19th century. As a result, by the end of this century, temperatures would go up by anywhere between 1.4 and 5.8 degrees Celsius. This increase would have impacts in the form of erosion of the coastal areas, threats to the existing ecosystems and biodiversity and problems with the supply of water. Sea levels would rise and create further risks from storm surges for populations that inhabit small island states, low-lying coastal areas and floodplains. On the whole, the changing climate would unevenly impact the developing nations and the impoverished within all the countries with a similar geographical background as that of Mumbai. Floods are also likely to get more frequent and severe in the rest of India. All of this means that we would have to create mechanisms and infrastructure by which we would have early warning about the impeding changes in the climate. For this purpose, a modern and responsive meteorological system should be set up in the most vulnerable areas. The cost to be incurred in establishing such facilities would be “justified by the substantial reduction in the damage to property and the threat to human life.”

However, as mentioned earlier, the adverse consequences of the inundation of Mumbai were further aggravated when the rains were followed by floods. These floods were mainly a result of a rise in the sea level and an inefficient drainage system.

1.2. Rising Sea-level

A gradual rise in the city’s sea-level and a high tide were highly instrumental in the intensity of the Mumbai floods. Floods in the coastal regions usually occur when there is a rise in the water levels, as a result of heavy storms. When storm tides exhibit a continuous increase over the normal high tide, it is termed as a storm surge. “The maximum intensity of a storm surge is accompanied by a high tide; therefore, storms that persist through several high tides are most severe and these lead to severe flooding.” Mumbai, being a coastal area also experienced a similar storm surge. As Dr. Kelkar puts it, in the case of Mumbai’s inundation, “the term flooding is a misnomer to a certain extent, as it is not a result of the water spilling over from a flooded river. It was an inundation caused by the accumulation of heavy local rainfall and the inability of the drainage process to match the rainfall rate.” Additionally, as stated by Dr. R. K. Pachauri, a rise in the sea-level, which forced the storm water back onto the land, was also equally responsible in bringing about this event.

This rise in the sea level is primarily a result of the global climate change that “is unequivocal. This fact is now evident from an observed increase in the global average air and ocean temperatures and the resulting widespread melting of snow and ice” at the polar caps. In addition to the global climate change, the extensive reclamation of land from the sea has also been a cause of the rise in sea-levels. event of a heavy downpour and a high tide, residential, port facilities and various business generating sectors, located in the city would face immense flooding. Tourism would also be adversely affected, if the beaches and “tourist infrastructure like hotels and lodges” were to suffer flooding. Mumbai has a large population of fisher-folk living along the coast. Therefore, shoreline fishing would also be affected if “fish habitats in the reefs and estuaries were to be disturbed due to a rise” in the sea-level.

But again, there is another significant cause to such an alarming rise in the sea levels and this cause is primarily man-made. The explanation for this phenomenon could be traced in the rampant reclamation (Adjoining picture) that has been carried out along the coast of Mumbai as well as along the banks of the city’s natural drains. A detailed description of this development and its consequences are discussed in the following section.

1.3. Extensive Reclamation and Faulty Zoning Regulations

It is evident from the city’s planning history, discussed in Chapter 2, that in the process of housing construction and setting up industries, the waterways that allowed the accumulated rain water to drain out, have been drastically reduced. The reclamation that was carried out originally only to link the seven islands of Mumbai was eventually performed to a greater extent to accommodate the ever burgeoning population of the city Large slum colonies as well as planned constructions are being developed on the land reclaimed from the sea. However, some of this development has extended further to encroach upon the city’s existing storm water drains, in order to meet the housing demand of the city’s growing population.

According to certain officials from the city’s municipal corporation, the government has justified the rampant reclamation in the city by citing a “faulty’’ report prepared of Trivandrum-based Centre of Earth Sciences (CES). The CES in its report has obliterated many areas that were earlier in the jurisdiction of the Coastal Regulation Zone (CRZ) and shown them outside the prohibitory area. Even the big patches of mangroves in this zone have gradually disappeared as a result of this plot. The report thereby made way for the rash developments, which were otherwise not permissible.

“Mangroves are known to be a vital link in the ecological chain, serving as a buffer between the land and ocean. Thus, their destruction disturbs the ecological balance.” However, developers pay little heed to this fact and ignore the coastal zone regulations to continue with their reclamation. As explained earlier in the city’s planning history in Chapter 2, these developers lobby with the local authorities to reclaim and concretize the city’s coastal areas. They end up destroying the mangroves in the process, to make way for further development. The rapid and extensive concretization of these patches of land has now led to the reduction of: (1) Infiltration rate of rainwater and (2) Availability of depression storage on the earth’s natural surface. This breach of trust with the nature has eventually led to the nature paying back in the form of disastrous floods.

However, it is the city’s inefficient drainage system that claims a lion’s share in contributing to the magnitude of the Mumbai floods. Similarly, the outdated building regulations, that have been applied to develop the city, have also been instrumental in a substantial loss of human lives and property.

2. Outdated building Regulations

Although Global climate change has been observed almost since the 1970’s, their unpleasant effects were not alarming enough for the governments and planning authorities of cities around the world, to sit and take notice of. Thus, Mumbai’s authorities were never prompted to draft new planning policies, considering the new climatic pattern. This has also been true in the case of the city authorities of Mumbai. Prior to 26th July 2005, the city’s existing zoning and building regulations, that were drafted almost three decades ago, were used to scrutinize and regulate the new developments. They regulations failed to consider the factor of the rapidly changing local climate. Mumbai, which is known to receive an annual rainfall of around 240mm, restricted only to the months of June-September, now bears a downpour of almost eight times the average expected rainfall, in addition to the untimely winter showers. However, none of these have been considered to draft new planning policies for the city that can prevent the inundation caused by these rains and the rising sea levels. Most of the new developments permit the construction of basements, underground pedestrian bypasses and habitable space at ground level. Also, numerous old and abandoned buildings are being revitalized and remodeled to be used for a different purpose. However, the change of use of buildings from “ordinary to critical functions is carried out without strengthening the building" and without considering the climatic changes in the region. In the event of the floods, these areas get water-logged, causing destruction of life and property at large. These woes are added to by an antiquated drainage system that has been serving the city since the past century. Moreover, there has also been a blatant ignorance on the government’s and planning authority’s part to promote sustainable building construction.

3. Faltering Drainage Systems:
3.1. Mumbai’s planned Storm Water Drainage (SWD) System

Mumbai’s existing storm water drainage system has largely contributed in the inundation of the city. The city’s storm water drainage system is basically a complicated system of simple drains and rivers, creeks, drains and ponds. “The network comprises of a hierarchical system of roadside surface drains (about 2,000 km mainly in the suburbs), underground drains and laterals (about 440 km in the island city area), major and minor canals (200 km and 87 km respectively) and over 180 outfalls, which discharge all the surface runoff into the rivers and the Arabian Sea (Adjoining figure).”Of these outfalls, some drain directly into the Arabian Sea, while others empty into the Mahim creek, Mahul creek or the Thane Creek. Additionally, some out-falls that drain out storm water from the western suburbs empty directly into sea while the water from the remaining ones is discharged into the Mithi River which ultimately joins the Mahim creek.

3.2. Mumbai’s Natural Drain: Mithi River
The Mithi River (Adjoining figures) constitutes a major component of the city’s SWD system. The location of the river is important from the point of view of the city as it serves as a dividing line between the city and its suburbs. Thus, its flooding has direct or indirect repercussions on the disruption of the traffic on the five transport corridors viz. Central Railways, Western Railways, Western Express Highway, Eastern Express Highway & the Harbor Railway Line. The storm water drainage of the river is encroached upon by a large number of hutments, storages, processing industries, workshops and scrap yards situated along its banks. These settlements make it difficult even to define the path of the river. Direct discharges of the untreated sewage, wastewater, trash from the unauthorized settlements and industrial effluents flow into the river’s course and choke it up, thus raising the level of water during heavy rains. Similarly, most of the other storm water drains carry sewage and dry garbage in summers which clog them and disrupt the natural flow of rain water. Moreover, illegal settlements and reclamation for future development further reduce the existing width of the river they are built on. These result in the “reduction of the river’s natural storage and aquifer recharge.”

3.3 Major Drawbacks in the Existing Drainage network

The storm water drainage system of Mumbai was built largely in the days of the British Rule in 1860, when the population of Mumbai was merely one-tenth of what it is at present. After the initial development, improving the drainage has never been a priority for the government. The system comprises of about 400 km of underground drains and laterals, built on the basis of the population and weather conditions of the times it was constructed in. This antiquated storm water drainage system is capable of handling rain intensity of 25 mm per hour at low tide. If the rain intensity exceeds 25 mm per hour and a high tide occurs, there is always a possibility of inundation. The city’s existing drainage system is designed to tackle a rainfall of normal intensity, with the assumption that there are no significant solids deposits in the drains. This is because the slope of the drains is supposedly designed to generate a self-cleaning velocity in the flow and is thus built to keep flooding relatively rare. But the fact is other wise. Most of the drains throughout the city have been found to be occupied by a substantial amount of garbage and other solid deposits.

The resulting decrease in the capacity of the city’s storm water drainage system has been proved by the disastrous effects of the inundation that hit the city of Mumbai on 26th July 2005. The city was caught unawares and un-prepared to deal with the crisis that followed the floods. It was not in the capacity of the city’s drains to let out the excess water.

3.4. Current Efforts in Maintaining the Drainage System

At present, the city authorities of Mumbai are working towards maintaining the existing drainage network with the help of certain mechanical equipment. The municipal corporation of Mumbai has a total of 63 such devices (Adjoining figure). These include amphibious dredgers, jetting and suction machines and other customized vehicle mounted equipment, among others for the de-silting and de-choking of drains. However, of the 63 devices, about 24 are over 8 years old and are rendered non-usable as per the ruling of Honorable Supreme Court/Regional Transport Office Norms. Moreover, poor workmanship and un-trained labor also result in a non-satisfactory job of cleaning the drains. In addition, there are several stretches along these drains that are lined and encroached by slums that restrict the use of such equipment. Also, the width of numerous drains is poached upon to accommodate other utilities. This further reduces the capacity of the drains to carry the storm water and the approachability for the drainage maintenance equipment.

Since the discharge of all the storm water and treated sewage is released into the Arabian Sea, tidal variation also constitutes a major component in the system of the storm water drainage (SWD). It results in excessive flooding and water retention in the event of heavy rains and the water recedes only during the low tide. This phenomenon was largely observed during the floods of 26th July 2005, when high tide forced all the drained storm water from the Arabian Sea back into the city.

In addition to the above-mentioned causes, the blame for the immense losses caused by the floods in Mumbai can be credited to the lack of proper equipment in the city’s weather bureau. The absence of modern technical tools prevented the city’s weather department from imparting precise warnings about the impending heavy rains to the citizens. Moreover, the lack of proper co-ordination amongst the various disaster management authorities in the city also delayed the relief operations, causing further damage.

Thus, a study of the most probable causes for the inundation of Mumbai city and its aftermath provides assistance to formulate the prefect strategies to mitigate each of the micro-issues originating from these causes.

However, the next chapter first scrutinizes the flood-management strategies currently adopted by Mumbai. A review of these strategies helps comprehend their drawbacks that prevented them from controlling the floods in the city.


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  4. Great Blog...
    One Question : I read a long back that the Mithi Mouth has been encroached in construction of Bandra-Kurla Complex / Bandra reclaimation. I searched on net, but couldn't get any information on this. So can you please clarify and elaborate on this...
    It will be great help for me.

  5. thank-you for this! it's really helped me out with understanding the devastating causes and effects of the Mumbai floods. :D

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