Tokyo can show Delhi how to prevent floods

Delhi has three major natural drains namely Trans-Yamuna, Barapullah and Najafgarh. These drains were intended to act as a stormwater network to carry excess water during rains and help in recharging groundwater. Most of the time now they carry sewage outfalls from various sewage treatment plants (STPs). Dumping of waste along drains, illegal discharge of sewage and effluents and choking by garbage also add to these woes, thereby reducing the drainage networks to a stinking ‘nala’ that carries wastewater.

Rapid unplanned urbanisation has led to encroachments along the drains, previously forested areas, and a reduction in numerous waterbodies, lakes and ponds. A recent water body census by the Jal Shakti ministry found that 216 of Delhi’s 893 waterbodies are encroached upon and 77 of them were not in use/dried up. A classic example is the case of the Seva Nagar bus terminal, which was built by encroaching upon a part of the Barapullah drain. Poor management and ill-planning led to encroachment, reduction in size and nature of drains and thus reduced their capacity to carry stormwater when in need.

Tokyo Plan

Delhi must learn from the drainage management and flood prevention strategy of Tokyo, Japan. Tokyo had commissioned the G-Cans project or Metropolitan Area Outer Underground Discharge Channel which consists of five huge silos, 6.5 km connecting tunnels, storage tanks and pumps. This huge flood protection infrastructure was commissioned in 1992 and completed by 2006 at a whopping cost of over $2 billion. It has saved the Japanese government about $1.4 billion dollar in disaster relief and rehabilitation. Flood waters from rivers and drains are channelled to these five silos which are interconnected, the water from these silos flows into a storage tank through these tunnels where it is stored and pumped out into the Edo river at a rate of 200 cubic metres per second.

The flood protection system is put to test almost seven times each year during the rain and typhoon season, almost 90 percent of flood damage to buildings, humans and livestock has been averted due to the G-Cans project. During the devastating rains of 2015, the system quickly drained almost 617 million cubic feet of water, preventing a catastrophic flood and reducing the damage.

With climate change increasing the frequency and intensity of rainfalls, Delhi must consider implementing Tokyo’s strategy with certain modifications. Inlet channels could be built along the Yamuna river and along important drains such as Najafgarh and Shahdara from where water can be diverted to underground silos. The water can be pumped back to Yamuna after the rains stop or the water level in the river falls. This will prevent damage to infrastructure, lives and property that has become a recurring sight. These tunnels also have the possibility of reducing flood risk by a minimum of 50 percent as it diverts the waters quickly to a storge. One added advantage of this design would be the reuse of flood waters for human needs by pumping them back into the river.

Lastly, the Water Sensitive Urban Design and Planning (WSUDP) approach must be followed which focuses on recycling and reuse of treated wastewater for non-potable purposes. This will reduce the amount of outflow from various STPs discharged into the drainage network thus enhancing their capacity to carry stormwaters.

Sonia Grover is Fellow and Area Convenor, Water Resources Division, TERI and Bharat Ramachandran is water student, TERI School of Advanced Studies. Views are personal, and do not represent the stand of this publication.