Rain-Drain 8212; Why the trickle turns into a flood

When the British reclaimed land, the process was not uniform. While merging the islands of Mazgaon and Walkeshwar, for instance, some portions remained below sea level. The flooding at Bombay Central and Tardeo today is the outcome of that blunder.

The island city has storm water drains SWDs which were designed to handle only 25 mm of rainfall per hour, that too, during low tide. But with reclamation and the resultant growth of thecity, the load on the SWDs has increased manifold. Moreover, there are no pumping stations to flush out the water, which flows into the sea by sheer force of gravity. The slopes which guide the drains are so gradual that the water is not drained effectively. Besides, in low-lying areas, the underground drains have been laid below sea level. Thus, when heavy rain synchronises with high tide, flooding is inevitable as water flows back into the city.

Also, most of the old drains are occupied with utilities like cables, water services, pipes, electric cables, gas mains, etc. These obstructions further restrict the flow of rain water and cause floods.

In the good old days, Mumbai had a large number of open areas like ponds or talaos, which used to act as a buffer. Large quantities of rain water would collect in them, reducing the pressure on the drains. Now, most of these have disappeared due to reclamation. Hence, every drop of water falling on the ground goes into the SWDs. The only outlet left is thesea.

In stark contrast, 90 per cent of the drains in the suburbs are open with the result that huge quantities of garbage and debris are dumped into them. Besides, hutment-dwellers have encroached upon these nullahs, on both sides, disposing sewage and other waste into them. This is a major cause of flooding. Often, they reduce the size of the nallahs to gain additional space to extend their huts. Such bottlenecks pose severe problems during the monsoon. It is cumbersome to take drain-cleaning machinery to such sites.

Areas like Milan subway, Andheri, Jogeshwari and Goregaon house huge stables, from where dung is released daily into the nallahs. Thus even though we desilt the nallahs prior to the monsoon, they get choked with garbage and dung in no time.

Every year, we contract the work for desilting between March and May. Seventy per cent of desilting is done before the onset of the monsoon, 20 per cent during and 10 per cent after. Then there are culverts across the Eastern and Western Expresshighways. These are maintained by highway authorities under the state government. Before the monsoon, trenches and excavations on roads and pavements are stopped. The trenches are filled to prevent loose earth flowing back into the drains.

While the archaic drainage system is the target of much abuse, few pay attention to a ready solution that is waiting to be implemented 8212; the Brihanmumbai Storm Water Drain BRIMSTOWAD project, which was formulated in 1993. The BMC appointed consultants who recommended among others, replacement of old drains, a five-metre wide road alongside major nallahs to desilting and maintenance with machines and removal of obstructions such as waterpipes, cables etc from all the drains to increase the area for rainwater flow.

They also suggested redesigning and reconstruction of the entire SWD system with a capacity to handle 48 mm of rainfall per hour. Another extremely important recommendation by the consultants is rehabilitating slum-dwellers. These suggestions also involveaugmentation of railway culverts across the Central, Western and Harbour line tracks.

The estimates of the BRIMSTOWAD worked out to Rs 650 crore based on the 1993 price index. In the last five years, the BMC has implemented works worth Rs 130 crore depending on availability of funds. The BMC now needs to spend at least Rs 70 crore every year for the next 10 years if the project is to be implemented in its entirety.

As far as the BRIMSTOWAD suggestions are concerned, we have procurred some drain cleansing equipment such as two amphibous dredgers for desilting major nallahs and two vehicular-mounted suction and jetting units. Implementing BRIMSTOWAD will to a large extent mitigate Mumbai8217;s monsoon woes.

N B Achrekar is deputy municipal commissioner engineering and is an expert on the subject

As told to Rosy Sequeira