Come monsoon and the media reports are full of floods in the metros of India. Mumbai, the film city of India was literally drowned in the great floods of 2005. The floods caused a direct loss of Rs. 450 crores or US $ 100 million. This was just an example to show the enormity of the problem. Each year the state governments come out with the flood 'protection measures' taken by them and also about the 'preparedness', yet with each passing year the situation appears to worsen. Why?
We will try to seek answers to this question along with other similar questions like why the nature's scourge has started to hit the urban centers more and what is the way out?
The experience of floods in Mumbai and Chennai in 2005 has been one of the worst in the recent years. Amongst the metros, Mumbai is one place which is cosmopolitan. People from all over the country try to seek jobs there and elk a living. Many of them succeed. A stage comes when they want to settle down there. The last floods have shaken all such would be settlers.
The ire of the Mithi 'nadi' in Mumbai was such that it caused imponderable loss of life, property, public facilities, interrupted all activities and above all worse were the daily wagers. The stoppage of work left them high and dry. In such circumstances people curse the government and vice versa. The government tries its best to provide relief to people and assuage the feelings, but that does not help much. Post flood rehabilitation, health safety and reconstruction is a big burden on the exchequer that comes all of a sudden and unexpected.
Flooding is not a new phenomenon. The recent experience has however, shown that sudden, incessant rains are the major factor behind deluges. Well one can not control the rains, but it is possible to tame and utilize the flood waters in urban areas.
Let us take example of Mumbai. The reasons which led to massive flooding as per the wikipedia include, antiquated drainage system. The 20th century drainage network of Mumbai is capable of carrying only 25 millimeter of rainwater per hour. With drains clogged at several places it proved inadequate for the 944mm rain which lashed Mumbai in one day alone. It seems that only three drains which drain into the sea have gates whereas, other 102 outlets have no such gates. Problem with coastal areas is lack of adequate gradient for water to flow into the sea. During high tides, the sea water incursion takes place through these drains. Drains without gates become vulnerable points and a salt water deluge engulfs upcountry. It goes without saying that the drainage system needs a thorough overhaul with gates to man the backflow of the sea water. During the last floods of Mithi River the residents had to live under the constant fear of a deluge even after rains, just because high tide water was difficult to control. Mithi River drains the Salsette Island on which the city of Mumbai is situated. Originating at Powai the river flows through densely populated areas and industrial complexes of Powai, Saki Naka, Kurla, Bandra-Kurla complex, Dharavi and Mahim where it meets the Arabian Sea. Dense clusters of slums right on the river bank, disposal of industrial waste, sludge and domestic waste all along have converted this natural drain to an open sewer. This river carries the excess water from Powai Lake. During the monsoon the river naturally swells. The river used to act as a storm water drain for Mumbai. Now choked to capacity this natural facility is of no help during the rains. Yes it does add to the problem by flooding the banks.
Since the British days the floods and their remedies have been the forte of the engineers. In the recent years some of the earth scientists have studied the problem of flooding in detail and have suggested remedies too. H.S. Saini of Geological Survey Of India (GSI) in one of the papers published in the prestigious science Journal, Current Science says that the mechanism of urban flooding is complex and site specific. Heavy rains, river overflowing the banks, sudden release of water from dams due to natural or anthropogenic reasons, coastal hurricanes and tsunamis and a combination of any of the above can create havoc in the urban areas. If we revert back to Mumbai we realize the gentle slope of the ground is towards the sea. Water flows down the slope. During rains if the path of water is obstructed by buildings and the drains have no capacity, the outcome is what the people of Mumbai experienced in 2005.
Floods in the rivers are dependent on topography, drainage, rainfall and the ground geology. For example, if the drainage is poor as in Mumbai even less rainfall will cause floods. If the channel of the river is made of clay at a particular site excess water will rather spillover as it will not be able to go down to the depths. All these criteria have to be considered before planning development of a particular area. Unfortunately it is not so, and as a consequence most of the cities are flood prone.
One of the most urbanized areas is the Indo-Gangetic plain. Initially towns came up on the higher terraces along the rivers at these places. Gradually the human activity including development of huge residential complexes descended down to the river bank or river lowlands. These areas are now vulnerable to floods. Yet another problem faced by these urban centers is the tremendous water logging during monsoons. Anthropogenically superimposed concrete topography influences the origin and augmentation of flooding/water-logging. The concrete pavements, buildings and road dividers produce a triple effect. They reduce the efficiency of the ground to permit seepage of water to depths, obstruct lateral or down slope movement of water to reach a sink, like a river or a natural depression. Pavements and apartments can reduce percolation of water up to 90% says Saini. The vertical structures become geographical barriers and divide the city into several sub-basins, which are dislodges and disconnected from the main natural outflow.
Lucknow, the Capital of Uttar Pradesh never had the problem of water logging. A posh colony, Gomtinagar, a dream of politicians and builders came up on the bank of Gomti River despite warnings by the earth scientists. In order to save the residential area thus 'developed' massive Bunds were constructed all along the route of the river throughout the city. It is common sense that the natural slope of the ground in a city on the banks of a river is towards the river. Rainwater just moves down the slope to reach the river. The approaches of water been closed by the Bunds the city becomes a huge pool every monsoon. People blame the government for not cleaning the drains and the government blames the people for choking them. Unfortunately no one realizes that water-logging has been actually 'invited'.
The coastal areas have mangroves as buffers between the land and the sea. In Mumbai more than 40% mangroves have been 'reclaimed' in-between 1995 to 2005 by the builders and slum dwellers and the land has been constructed upon. Destruction of Mangroves along Mithi river near Mahim creek is one of the root causes of sea water incursion during tides.
The urban areas have been constructed upon and now it is too late to plan and tackle the problem of flooding for such centers. Yet if the people want to save themselves from the wrath of the floods and the government wants to avoid paying recurrent compensations to the people one of the way out is to construct large tanks where rain water could be stored and also to puncture the ground at several places like it is done for rain water harvesting. This would augment the seepage capacity of the ground. In addition a holistic drainage system for every urban complex would save many a lives, economic losses and inconvenience due to floods. Seepage holes will prevent water-logging and the menace of the mosquitoes.