04 Dec 2015 18:20 IST

Lessons in water management from Tokyo and Melbourne

In 20 years from the 1980s to the 1990s, six major floods ravaged the city, destroying thousands of houses

An extensive storm-water drain system and measures to counter flooding can go a long way in avoiding disasters

Last week I wrote about how greed-induced construction in Chennai has killed over 600 of its water bodies making it vulnerable to flooding during the heavy rains that lashed the city in November. It was the wettest November in 20 years and was close to a 100-year record.

As luck would have it, the worst was yet to come. On the first day of December, the city got 345 mm of rains, the heaviest in more than a century. Already under a deluge after the wet November, the rains this time have submerged most of the city. Major transport was shut down, roads caved in, bridges disappeared and thousands of people lost their homes.

Chennai’s drainage system is nothing to mention of. Only half of the city’s 2,847-km road network have storm water drains. And these drains can carry only 3cm (or 30mm) rainfall per day.

It will be sometime before the city comes back to its feet. But as the city’s administration hopefully starts rebuilding its roads and drainage systems, it can take cues from experience of other cities across the world.

There are two cases that I want to mention here. One is Tokyo’s $2-billion underground waterway called G-Cans. The second is Melbourne’s 1,700-km-long network of drains.

Tokyo’s wonder world

Tokyo’s urbanisation has some similarity to Chennai’s. While the southern Indian metro expanded over its lakes, in Japan’s capital ‘development’ ate up its rice fields. Earlier, agricultural land would soak in almost all of the water from typhoons that frequently lashed Tokyo. Not anymore. In 20 years from the 1980s to the 1990s, six major floods ravaged the city, destroying thousands of houses.

To prevent similar damage in the future, the city built an underground drain on the outskirts of Tokyo that has become a man-made wonder and a tourist spot. Built over 17 years, the G-Cans is a series of five giant sink holes (each 74m in height and 32m in diameter). A giant pipeline transports the water from the sinkholes over 6 km away to the ‘underground temple.’ The temple is a reservoir that is 180m long, 80m wide and 18m high.

The water collected here is then pumped out by modified jet engines, which otherwise fly large aircraft, into the Edo River nearby. The system is said to have reduced the flood impact by 80 per cent. Check out the pictures here .

Melbourne’s cave

If Tokyo’s solution to its flood woes was a giant drain hole, Melbourne has a web of tunnels, waterfalls, and reservoirs. This is how the system works .

From the houses and streets, the water runs through gutters to the drains. These ‘residential’ drains are connected to those under the streets and roads. This network then joins the regional drains that direct the water into the nearest river and creeks that empty the water into the Port Phillip Bay.

It is a complex network. Apart from the 1,700 km of drains, the system stretches to 8,400 km of rivers and creeks that flow into Port Phillip Bay. “A further 25,000 km of local drains are managed by councils,” says the website of Melbourne Water, the state body that controls the water system in the city.

The organisation has identified over a 100,000 properties that are vulnerable to flooding. Identifying flood risks, and measures to counter them, are an integral part of planning a new suburb or area. Melbourne’s flood management also includes 200 retarding basins, which are low-lying areas set aside to temporarily store storm water.

Both the systems are complex, and take time and money. Chennai’s local administration hasn’t yet shown gumption for similar work. Recently, a senior IAS officer was transferred after he levied a fine on road contractors for shoddy work. The officer had warned the administration of the havoc a flood could have on the infrastructure. He was not heeded. Chennai will hope that its public servants will now pay attention to Nature’s fury.