Prime Minister Narendra Modi has grand designs to build 100 futuristic 'smart' cities in India, but as this week's devastating flooding in Chennai shows, fixing today's accident-prone metropolises appears to be the more pressing task.

Locals are in no doubt as to why a mix of flood waters and sewage is swilling neck-deep around parts of the southern city of 6 million people after weeks of monsoon rains culminated in a 345 mm (14-inch) cloudburst in 24 hours.

"There is nowhere for the water to go because of unchecked building activity," said V.R. Devika, who fled her ground-floor apartment to seek refuge with her upstairs neighbours.

"All the lake areas have been converted into housing colonies," added the 61-year-old, who runs an arts charity.

What started out as a natural disaster in the state capital of Tamil Nadu soon became a more serious man-made one, with experts blaming bad urban planning and rampant "encroachment" by fly-by-night property developers for making the situation worse.

Similar floods struck Mumbai in 2005 and Kashmir's winter capital, Srinagar, last year. There, as in Chennai, construction blocked storm water channels and reduced the capacity of reservoirs designed to soak up unseasonal rains.

"As cities grow, with a nod and a wink, unauthorised construction along these channels and lakes chokes water flow," said Shailesh Pathak, executive director at Bhartiya Group, which has its own smart city project in Bengaluru, runs a special economic zone and is expanding into low-cost housing.

RUNWAY RIVER

The sight of three dozen airliners with their undercarriages covered by flood waters might lead stranded passengers to wonder whether Chennai's low-lying international airport was built in the wrong place.

Experts say they'd be right: One runway traverses the Adyar river, which burst its banks after some of the heaviest cloudbursts in the area in over a century swamped Chennai.

Some 270 people have died as a result of recent flooding and nearly 1,000 have been seriously injured.

"They have not made any provisions for preventing flood water from entering," said Mohan Ranganathan, a Chennai aviation expert and former air safety adviser to the national government.

"The authorities and the airlines just have commercial and political interests in mind. Safety is the last avenue."

While Chennai is not unique, experts say it was worse hit than the rest of India's southeastern seaboard because of a development boom that has gone unchecked in the last 15 years.

"Chennai needs to relearn its water management," said Sushmita Sengupta at the Centre for Science and Environment in New Delhi, adding that it was critical for the city to restore what had once been a good natural drainage system.

The Chennai Metropolitan Development Authority has counted more than 150,000 illegal buildings in the city. Yet, despite several demolition orders issued by the Madras High Court, they still stand.

Tamil Nadu Chief Minister Jayalalithaa Jayaram has described the flood damage as "unavoidable", according to TV reports.

It was not possible to reach the state government for comment, with mobile and landline networks knocked out by the floods in much of Chennai.

MAKING CITIES SMARTER

In July, 2005, 944 mm of rain fell on Mumbai in a single day, leading to the deaths of 500 people. Heavy development had destroyed green spaces and mangrove forests, its natural flood protection, leaving the financial metropolis of 18 million reliant on an inadequate drainage system.

The city set itself an ambitious plan then to install pumping stations, widen drains and clean waterways. But that has missed deadlines, seen significant cost overruns and is running nearly a decade behind schedule.

Bhartiya's Pathak said that under India's weak federal framework, Modi alone lacked the power to realise his smart cities initiative that promises a hygienic, networked life for a rising generation of urban Indians.

Better would be to empower major cities to determine their own fate, he said, as with the most successful urban projects in China.

"The top Chinese megacities, where most growth has happened, are city states," he said. "So would India consider city-state status, or sub-state status, for Chennai, Hyderabad, Bengaluru, Mumbai, Kolkata, Ahmedabad and Pune?"