Mumbai has an average elevation of 14 m, with some coastal areas lying 6-8 m above sea level. Low-lying creek areas are more vulnerable, as some like Gorai are at sea level while others, like Mira Bhayandar and areas around Versova, are only 1.5-2.5 m above sea level. This feature of the landscape poses limitations on the drainage system. As the drainage’s eventual outlet cannot be below the sea level, the situation becomes worse during high tide, the latter blocking the drainage outlet and stopping stormwater being discharged in the sea.
In the 1950s-60s, low-lying streets in Tokyo would get flooded during rains. It was a geographical challenge to build an underground stormwater drainage system in Tokyo due to its soft alluvial soil and frequent earthquakes. But a tunnel and an underground stormwater reservoir at a depth of about 100 m, with a floor area equal to the area of two football fields, were built in Kasukabe at the outskirts of Tokyo. This massive concrete structure supported by huge pillars each weighing about 500 tons — the Metropolitan Area Outer Underground Discharge Channel (MAOUDC) — is an impressive engineering feat that was completed in 2006.
When any of the rivers overflow, the water falls into one of the five 70 m high underground cylindrical tanks. The entire stormwater drainage system is operated from a control room equipped with the latest technology.
Similarly, the low-lying Happy Valley area in Hong Kong, hemmed in by hilly terrain, used to face heavy flooding during rains. Authorities came up with an innovative flood protection scheme, and built an underground stormwater storage system. They drilled holes into the ground and on the hill slopes, leading stormwater to an underground concrete sump — a pit or hollow in which the water collects — thereby arresting the surface run-off rainwater before it can cause damage.
This system has movable weirs — low dams that can be lowered or raised to regulate the water entering those massive underground tanks. This, too, is managed from a control room equipped with Supervisory Control and Data Acquisition (SCADA), which tracks real-time water and tidal levels. Such a system prevents not only water-logging, but also associated business and human losses.
These giant concrete underground storages hold water temporarily. Once the surge is over, this water can either be diverted to a nearby water-deficient area through a network of underground tunnels (Tokyo), or can be used for irrigating urban parks, watering plants and flushing toilets after it passes through an onsite treatment unit (Hong Kong).
Mumbai’s drainage system is about 140 years old. The drains are designed to discharge water into the sea. They can drain up to 25 mm of rain water an hour during low tide. After the 2005 floods, stormwater drain capacity was upgraded in some parts of Mumbai under the Brihanmumbai Stormwater Disposal System (BRIMSTOWAD) from handling 25 mm to 50 mm of rainwater an hour.
According to the Indian Meteorological Department (IMD), the annual frequency of heavy rainfall, 64.5-115.5 mm, has seen a steady increase in recent decades. Increasing the drainage capacity to 50 mm an hour can only be a stopgap arrangement, that too during low tide. Besides, BRIMSTOWAD operates on the assumption that all rainwater drains out into the sea through a network of pumps, pipes and drains.
But rainwater is fresh water, a rare commodity with only 0.5% of Earth’s total water available in this form. GoI’s Jal Shakti Abhiyan captures this fact with its tagline, ‘Catch the rain, where it falls, when it falls.’ So, the stormwater drainage system designed in Hong Kong should be studied for feasibility for Mumbai, and many such local underground sumps should be made at places that have been vulnerable to floods historically.
Tokyo’s stormwater system cost about ₹15,000 crore in 2006. The system in Hong Kong cost around ₹1,000 crore in 2018. It is no secret that Brihanmumbai Municipal Corporation (BMC) holds ₹80,000 crore in fixed deposit (FD). Let this money sitting idly be put to concrete use, so as to deliver Mumbai and its adjoining areas from its annual monsoon woes.