By Dr.V. Subramanyan
Among Nature’s deadly arsenal of Weapons of Mass Destruction, floods and landslides have been taking a heavy toll of human lives in Mumbai in recent years and, going by the sporadic minor tremors in the Mumbai region in the past decade or so, it would seem that earthquakes are just waiting in the wings, watching where the wind sits.
For all this, Mumbai is probably the only city in India that can boast of a very comprehensive Disaster Management Plan, prepared in the aftermath of the disastrous Bhuj earthquake of January 26, 2001, with significant technical inputs from outside experts too.
The Plan failed to take off during the 26/7 deluge because it was not clear who would press the button first with two lines of authority running parallel – the ministers, their secretaries, the collectors, etc., and the BMC Commissioner and his subordinates down to the Assistant Municipal Commissioner.
In this unfortunate administrative wrangling, the Mithi River had a field day rising nonchalantly to the first floor of some buildings at Kalina. The Plan per se cannot be faulted.
The Problem of Flooding
While Mumbaikars are used to the annual ritual of rainwater stagnating on the roads and submerging the railway tracks for a day or two, this sort of deluge with water flooding many residential areas and even the airport was unheard of.
It all happened quite clearly due to the non-availability of adequate natural waterways in the metropolis and its suburbs, with quite a large number of the streams having been eliminated by the indiscriminate urbanization that has been going on for years -- in simple terms, the streamlines have been systematically reclaimed for the construction of buildings and roads.
Consequently, the excess rainwater does not have many channelways to flow through safely to the sea and is left with no other option than to bring about waterlogging -- the ‘flooding’ as it is known in Mumbai.
That the natural streams have been the casualties of intense urbanization can be readily understood by comparing old topographical maps of Mumbai with more recent ones. For instance, the maps of 1970 show that since 1924, about half the number of the tiny streams between Kanjur and Mulund have disappeared. More recent maps are not available since they are not prepared frequently by the Survey of India but if a map of even 2000 is scrutinized, it will turn out in all probability that more streams are gone with the wind.
The Remedies
Encroachments on the areas drained by streams have to be stopped to save those streams that have survived urbanization. In addition, it is necessary to restore the surface drainage to its earlier self-sufficient position provided by nature by excavating a network of canals wherever possible to act as streams.
The Problem of Landslides
Landslides are usually fast movements of rock or soil mass over hill slopes due to their destabilization by natural or anthropogenic activity. Rock and debris slides are normally confined to hill-stations like Shimla, Mussoorie and Darjeeling where the elevations are high with attendant heavy rainfall and the hill-slopes are pretty steep.
However, they have been occurring in Mumbai very regularly in localities like Saki Naka, Andheri and Ghatkopar among several other places. The point of great concern is that they have been taking a heavy toll of human lives wherever they occur. For instance in the Chiragnagar, Ghatkopar slide of July 13, 2000, over 70 people died and during the Saki Naka slide of July 26, 2005, some 80 people lost their lives. It is sad that these figures are a lot more than the death toll in the Jabalpur Earthquake of May 22, 1997 of only 59!
The main reason for landslides occurring in Mumbai almost annually is that the natural hill-slopes that are stable if left undisturbed have been the victims of extensive quarrying for rock over the years. When a hill-slope that is in equilibrium at a certain angle is cut into a vertical face by opening up a quarry, the slanting rock-blocks lose their support at their bottom and the slope also gets steepened in the process. These two are among the most common ‘active’ causes of rockslides. The rainfall impact does the rest by loosening the blocks and lubricating the rock-planes.
The Remedies
It needs to be realized that any disaster management activity has to necessarily include disaster prediction and prevention wherever possible. The landslides of all categories can be predicted by picking up the signals of an impending slide in the form of cracks showing up and enlarging on the surface. They can also be prevented by adopting slope-stabilizing measures like driving rock-bolts, spraying a cement mixture, building retaining walls below, throwing a wire-net on the slope, etc. Though the BMC has an exhaustive inventory of over 100 places which are susceptible to landslides, sadly it does not go beyond sticking evacuation notices at these vulnerable spots due to administrative problems. Unless it makes it a point to stabilize the hill-slopes with its enormous resources, the landslides will continue to lead to the loss of human lives.
The Problem of Earthquakes
That Mumbai is located in seismic zone 3 and can be rocked by an earthquake of magnitude 6.0 to 6.5 has been known for more than three decades. The current Seismic Zone Map of India (IS:1893–2002) which shows four zones designated 2,3 4 and 5 with the entire West Coast falling in zone 3. That no major earthquake has visited Mumbai in the recent past is no consolation because the build-up time for a major earthquake is more than one hundred years. It is to be noted that a minor tremor of magnitude less than 4.0 occurred at Thane on 21st August, 2010 and on June 15th 2005, a similar shock hit Kalyan. The BMC must insist therefore on appropriate construction of buildings.
(The writer is former Professor of Geology, IIT Bombay and Member, BMC’s Expert Committee on its Disaster management plan for the city)