The flooding of Kochi airport in the past week is the latest example of the vulnerability of major Indian airports that have sprung up in the floodplains of rivers. That vulnerability could rise, say experts, as climate change is projected to increase extreme rainfall events.Cochin International Airport (CIAL) is expected to remain shut until August 28. The prolonged closure echoes what happened in Mumbai in 2005 and Chennai in 2015 when those cities were hit by floods due to unprecedented rainfall. Both cities saw their airport runways closed and operations affected for weeks following the flood. The airports were not only built near rivers but had expanded their runways into the nearby rivers.Kochi’s airport, built in the floodplains of the Periyar River, is relatively new. In the 1990s, “Kochi was at risk of falling off the aviation map entirely. Out of necessity, [the new airport] was founded on paddy fields and land that used to be part of the Chengal canal,” said a CIAL official.Locals and activists blame the development for aggravating flooding in the area. “Every river has a floodplain that is supposed to hold the water, and this is an integral part of the river. In Kerala, these floodplains usually manifest as canals and ponds,” says local environmentalist C R Neelakandan.CIAL managing director VJ Kurian, however, denied the airport had made the area flood-prone. Authorities did not build over the Chengal canal, he said, but diverted it to prevent flooding at the airport.As Mumbai has seen, however, altering the course of a waterway can be disastrous. Both runways of the Mumbai airport were extended over the time across the adjacent Mithi River, squeezing the width of the channel. The second runway, built in the 1970s, created a right angle-bend in the river.The airport’s expansion is widely held to have contributed to the 2005 floods, when the Mithi River overflowed after hours of intense rainfall. Airport authorities have since sought to mitigate the problem by doubling the width of the Mithi as well as dredging the riverbed to make it deeper.Despite the 2005 experience, the Maharashtra government has decided to locate the region’s second airport near two rivers. Workers are already leveling a hill and diverting the Ulwe river for the Navi Mumbai airport. Officials say the Ulwe River’s mouth will be widened and the airport located above flood level. But surrounding villages are already complaining of more flooding.In Chennai, too, the second runway was extended in 2011 across Adyar River on a bridge supported by closely-stacked pillars that block water flow. Although the airport has flood retaining walls and a raised runway, they were no proof against the water released from Chembaramkkam dam after the 2015 intense rainfall. Authorities have since widened part of the river and regularly desilt the channel near the bridge.Why would anyone place a critical and expensive piece of infrastructure near a river? Probably because of the flat topography, says Kapil Gupta, professor of civil engineering at IIT-Bombay, who was consulted by the Mumbai airport after the 2005 floods. He notes that at least 4km of flat land is needed for a runway, apart from what is needed for the adjacent facilities. Plus, older airports like Mumbai and Chennai were small when they were first built several decades ago and would not have been so close to the river. As air traffic increased, planes became bigger, and the city developed around them, these airports have expanded toward the river.Not every airport near a water body is vulnerable. Goa airport is a coastal one but is on a plateau with cliffs on two sides, deep storm water drains and the runways have water spilloff channels. Mysuru airport is near two lakes and a stream but it is upstream of the water bodies and is not so vulnerable.The National Disaster Management Authority guidelines highlight the importance of airports in emergencies and recommend the provision of ample drainage and holding ponds. “Such ponds should be kept empty to absorb flood overflow,” said urban drainage expert Gupta, who had also recommended this measure to the Mumbai airport. Such holding ponds are present in Dulles airport in Washington DC as well as Heathrow in London.With 100 new airports proposed, experts say it’s vital to plan them well. “Ideally, you should not have critical infrastructure in a floodplain, but if you do, you need to design it properly for drainage,” says Gupta. “You need to build infrastructure to survive at least a100-year-flood.”(Inputs from Vikram Vinod, Vaishnavi Chandrashekhar, Newton Sequeira, Ganesh Lakshman, V Ayyapan, Shrinivasa M)