Flooding Rains, Failing Drains

As the global temperature rises, the frequency of extreme weather events is also increasing, and so is their unpredictability. One such extreme weather event—a menace for coastal cities—is cyclones. Cities by the sea need to quickly adapt to the occurrence of frequent, high-intensity cyclones. 

It is noteworthy that nearly 11 per cent of the global populace, constituting approximately 896 million individuals, currently reside within the Low Elevation Coastal Zone, which is expected to exceed one billion by the year 2050. India’s situation is not all that different. With a coastline of over 7500 km, the peninsular part of India touches three great oceans—the Arabian Sea, the Bay of Bengal, and the Indian Ocean—making its coastal cities like Mumbai, Chennai, and Visakhapatnam, vulnerable to sea-level rise, cyclones, and storm surges. The cyclonic storm brings along high-intensity, short-duration rainfall that causes severe flooding. Our drainage system is not at all prepared for this. This vulnerability has been exposed multiple times. The latest of such events has been cyclone Michaung, which ravaged Chennai in 2023. When the cyclone reached the coastal city, it brought more than 400mm of rainfall in just 48 hours. As reports suggest, the frequency of such high-intensity cyclones will only increase. Therefore, the coastal cities need to invest more in flood mitigation as well as adaptation efforts.

In that regard, the island nation of Japan, frequently battered by typhoon-induced rainfall, could serve as a significant source of inspiration. Few countries have experienced the forces of nature quite like Japan, making it a compelling example. Based on the statistics for 1981–2010, on average, 25.6 typhoons develop in the western North Pacific basin annually; about 2.7–11.4 of such typhoons make landfall every year. The country has been able to develop robust flood control mechanisms.

Advent of floodwater cathedral

Japan has had a long history with typhoons, which result in the discharge of significantly large volumes of water in a very short time, causing massive floods. Typhoon Kathleen is one such example that ravaged Japan in 1947. It destroyed around 31,000 homes and claimed the lives of 1100 people. Another massive typhoon named Kanogawa, or Ida, hit the city in 1958, causing around 400mm of rain in a week.

As the years passed, the situation only deteriorated. From 1976 to 2000, Japan recorded over 5500 instances of rain when it poured at least 50mm in an hour, with an annual average of 230 such incidents. Between 2001 and 2020, more than 6000 incidents of more than 50mm of rainfall in an hour have been recorded, meaning an average of 320 such incidents each year, indicating an increased frequency of such events.

The topography of Japan further adds to its worries, as most of its vital cities are just beside the ocean, not only in terms of distance but elevation too. Japan has a national average elevation of only 438 meters above sea level. The city of Tokyo, which sits only 20 meters above sea level, is highly susceptible to flooding. Not to forget that about 60 million people, or half of Japan’s population, live on the 10 per cent of land that would sink below river level in the event of a flood. Also, 75 per cent of the country’s assets are concentrated in these areas.

One such exemplary project to make the cities flood-resistant is the Metropolitan Area Outer Underground Discharge Channel (MAOUDC). MAOUDC, also known as the G-Cans project, with its $2 billion ‘Floodwater Cathedral’ is a testimony to the country’s grit to withstand the force of nature. This engineering marvel, completed in the early decades of the 21st century, is a 6.3-kilometre-long network of underground tunnels connecting five huge 65-metre silos to a tank that is almost 67,000 cubic metres in volume. This system can clear up to 200 cubic metres of water every second, the equivalent of draining an Olympic swimming pool in 12 seconds. In the event of a flood, four 14,000-horsepower turbines push the water out through the Edo River and into Tokyo Bay.

Current scenario

The system swung into action during Typhoon Hagibis in October 2019, when it poured heavily in the Nakagawa River Basin, too, at a rate of 8.5 inches over 48 hours. The system diverted some 12.18 million cubic metres of flood water—equivalent to over 4800 Olympic-sized swimming pools. This new system has cut the number of flooded homes in the river basin by 90 per cent and prevented 26.4 billion yen ($1.76 billion) in related damages. Such a system focused on adapting to the effects of extreme weather events could provide valuable insights for cities not only in India but also across the world.

The experts and reports suggest that the Indian subcontinent is also witnessing an increased frequency of high-intensity cyclones. In past years, India’s coastal provinces have been struck by several cyclones, including Cyclone Amphan (2020), Cyclone Tauktae (2021), Very Severe Cyclone Yaas (2021), and Cyclone Michaung (2023). Therefore, we need to learn not just the technological prowess of the land of the rising Sun but also the dedication and willingness of its political leadership and people to protect their cities.