Ranjan Kumar Padmapati
(The writer can be reached at rkpadmapati@yahoo.co.in)
Pre-monsoon floods a week before caused havoc among the people of Assam, communications were cut off in different parts of Assam, bridges washed away, railway tracks and National Highways disrupted. In the city of Guwahati, Rukminigaon, Nabin Nagar, Zoo Road and many more areas were inundated with flood water. The Housefed Residential complex that we live in, was waterlogged and became an island, surrounded by hip-deep flood water. People were ferried by boats to the GS Road by Disaster Management team. When we first came to this area10 years before, flood water entered the premises occasionally, 3-4 times a year, and the water drained out in a short time to the rivulet Bahini. But as the years passed by, it has become very frequent and severe now, and little drizzles make the premises waterlogged.
The following rivulets, namely the Basistha, Bahini, Bharalu, Mora Bharalu, Lakhminijan channel, Bondajan channel are flowing and covering most of the areas of modern Guwahati and serving as natural drains of the city. The Bharalu, Bahini and Basistha are flowing through thickly-populated residential and commercial areas. Originally, these rivulets were capable of draining the rain water together with household waste waters either to the water bodies or the Brahmaputra. As the population has increased, which stands roughly at 15 lakh now, human activities have also increased. People are building houses on hilltops of the nearby hills. Hill -cutting is rampant for earth filling or dwelling purposes. Grass and vegetation covers of the hills are lost, red soil is visible even from a distance. As a result the top soil of the hills is exposed to torrential rain and the top soil is carried away as sediments. The sediments are deposited in the rivulet beds and wetlands. Not only are the rivulets beds becoming shallow, but the Brahmaputra bed has come up after the great earthquakes. When the Brahmaputra is in spate, the gravity flow of the rivulets to the Brahmaputra is hampered and sometimes the flow becomes almost stationary. As the velocity of water falls below the threshold velocity (for not silting), siltation takes place heavily in river beds. On the other hand, less turbulent velocity sediments easily settle down, making river beds shallow, channel cross section becomes less and less year after year, responsible for low flow and gradient loss. For the same reason , household drains, different roadside drains connected to rivulets and rain waters generated in the premises are not capable of draining out of its own easily to the rivulets because of low gravity gradients in case of a heavy downpour, leading to waterlogging in the premises. In the worst cases, when the rivulet water level reaches above the surroundings, back flow of water takes place through the same household drain openings and water overflows the rivulets banks, flooding the low-lying areas. All these are due to heavy siltation of stream beds.
The digging of the beds of those shallow streams is not carried out in proper perspective. Amphibian dredger seems to be ineffective in the present circumstances. Specialists in the line may think of an innovative idea to install Recharge Shafts in all along the rivulets beds at distances to be fixed by expert groups with concrete pipe of a diameter of one meter or so to a depths of semi-pervasive strata . This will serve duel purposes - recharging the aquifer that serves as a source of ground water will improve the ground water table. An added advantage would be reduced rivulet run-off, as a part of the water will percolate below the ground. Another way of reducing run-off of the rivulets is to construct check dams across the flow at upper reaches in the hills to retain water to percolate below in rivulet beds. These systems are being followed in south Indian riverbeds to rejuvenate the rivers and charging the aquifer during monsoon period.
Guwahati is fortunate to have the following water bodies which serve as temporary reservoir of rainwater and receive rain water through their channels. These are the Bharalu Basin, Silsaku Basin, Deepor Beel, Kalmoni Basin and two or three more. These water bodies collect surface run-off and minimize flood. But these water bodies are also silting and their water holding capacity is being reduced. The water bodies are to be de-silted in a planned, phased manner. The situation further complicated by rampant encroachment by the public, reducing the basin holding capacity. The low-lying land areas, where once paddy was grown and previously allowed to percolate water below, has been now filled in for construction of buildings. Rainwater harvesting must be done in the upcoming buildings. Shrinkage of wetlands is so significant that the Ramsar site Deepar Beel, which was 40 square km in area 50 years ago, is now reduced to just 4 square km in winter and 10 square km during monsoons, according to a belated report.
The solid household wastes disposing habit of the people is also contributing to the floods. Of the solid household wastes thrown to the streams, the heavier materials settle in the river bed and lighter materials float and are carried by the water, obstructing its free flow. The Basistha, Bahini and Bharalu carry these wastes to a greater extent. The floating materials are plastic bottles, polythene bags and thermocol that choke the drains. Commercial establishments that sprang up on the river banks are also contributing in a similar way by dumping garbage into the rivulets. The de-watering stations' mouths are jammed by these materials and de-watering is hampered. The solid waste collection system needs to be strengthened, augmented and geared up all along the lengths of the rivulets banks so that household solids are not dumped into the rivulets. By 2025, the solid waste burden would be 0.8kKg/per capita/per day and hospital waste would be 0.5kg/per capita/per day. At present biodegradable waste accounts for 57%, tin-iron 28%, glass and bottles14%, plastic 1% and the city generates 550 tonne of waste per day.
Report is on circulation that Assam will be badly affected by climate change. As much as 60% excess rain has been reported as of now. Guwahati receives 162cm of annual rainfall and 110 average rainy days in a year. Monthly total average rainfall varies between 200-400mm, 22% precipitation takes place between April-May and 67% precipitation takes place during June, July August and September. Torrential rain intensity has a direct bearing on urban flood. Once Guwahati was a flood-free town but undesirable human interferences with wetlands, natural streams etc., have damaged the ecosystem. Though climate change is linked to floods, human-induced causes are also equally responsible for floods. Let us conserve and rejuvenate naturally gifted rivulets, water bodies, ponds, streams etc.