What explains massive flooding in Bengaluru?

The city received more rain in the 24 hours to 8.30 AM Monday than it did in 99.9% of the days since the begining of the 20th century (the average for the city was 58.5 mm, the 44th highest in any 24-hour period since January 1, 1901).

The easy answer to this question is unprecedented rainfall -- but that hides more than it reveals.

Members of a rescue team row their boat past submerged vehicles following torrential rains in Bengaluru, India, September 5, 2022. (REUTERS/Samuel Rajkumar)

The city received more rain in the 24 hours to 8.30 AM Monday than it did in 99.9% of the days since the begining of the 20th century (the average for the city was 58.5 mm, the 44th highest in any 24-hour period since January 1, 1901).

This year’s monsoon has also been the rainiest, and also the most extreme since 1901. Overall, the Bengaluru and Bengaluru Rural districts taken together received 747.9 mm rain from June 1 to 8:30 AM on September 4 according to India Meteorological Department’s (IMD) gridded dataset. This is the highest rain the metro has received in this interval since 1901 and 2.8 times the average for 1961-2010 period (265.1 mm). After adding the September 5 data (up to 8:30 AM), this becomes three times the 1961-2010 average . The fact that the average rainfall for the entire monsoon period of June-September in 1961-2010 is just 408.2 mm, shows how much of an outlier the current monsoon season has been for the city.

Even the unprecedented rain this monsoon season may not have led to floods if the rain was spread equally over the 97 days from June 1 . That has not been the case. On 17 of the 79 days it has rained this monsoon season, it rained more than 35.5 mm at least in parts of Bengaluru (this volume is classified as heavy rain by the Indian weather office). Half (50.5%) of the rain this monsoon has been of this heavy kind, when in the 1961-2010 period such rain was only 21%. In absolute terms, the 407.6 mm heavy rain received this season is over seven times the average of 56.8 mm of heavy rain in the 1961-2010 period. The 24 hours ending at 8:30 AM on September 4, the day before vehicles floated on Bengaluru’s roads, was also a period of heavy rain in parts of the city, and is bound to have increased the impact of the rain in the 24 hours ending 830 AM September 5, when, again, there was heavy rain all across the city.

Clearly, events of the past few days, and also during this monsoon in the city are part of a trend, which is likely the result of the climate crisis. (HT Illustration)

But the interesting bit is that this was an event foretold by numbers. The average rain in the 2011-2020 decade was second only to that in the 1901-10 decade. Even the volume of heavy intensity rain in the 2011-20 decade is the second highest for all decades since 1921-30.

Clearly, events of the past few days, and also during this monsoon in the city are part of a trend, which is likely the result of the climate crisis. That means the city should have been prepared.

Adapting for more rain and more heavy rain is also necessary for Bengaluru because it is not the city it was three decades ago. The urban area in the Bengaluru has increased three-fold between 1992 and 2015, according to data from the European Space Agency’s (ESA) Climate Change Initiative (CCI). This urbanisation has come at the cost of agricultural land (decreased 9% since 1992 in the two districts combined), forests and shrubs (decreased 2.4%), and even some water bodies (decreased 0.2%). To be sure, ESA-CCI data is likely to miss changes at a smaller scale because of its coarse 300 metre resolution.

These numbers for Bengaluru should be a warning for other metros in the country, as they have all undergone similar changes. Delhi, 30% urban in 1992, is now 49% urban. Mumbai’s two already heavily urbanised districts in 1992 (57.5%) are now 61.1% urban. With barely any soil on the ground, even small amounts of rain in these cities is likely to accumulate and cause floods.