Bengaluru’s record rain brought largely man-made floods. Here is how, why

How much did it rain on Tuesday?

Between 3 am and 8 am on August 15, a thunderstorm accompanied by heavy rainfall struck the southern and eastern parts of the city.

The meteorological observatory at Bengaluru airport recorded rainfall of 128.7 mm during this time, the most in a single day in August in the last 127 years. On August 27, 1890, Bengaluru had recorded 162.1 mm of rainfall. The normal rainfall expected in Bengaluru on August 15 any year is only 5 mm. In fact, the city gets a total of between 130 mm and 140 mm of rain in the entire month of August. On Tuesday, therefore, the city got rain in a few hours what it usually gets in the entire month.

Is this what is called a ‘cloudburst’? What climatic/meteorological factors were in play over Bengaluru that caused this phenomenon?

Tuesday’s rain in Bengaluru, although a case of concentrated high rainfall, was not a ‘cloudburst’ event. A rainfall event is described as a case of cloudburst if a small area of about 10 km x 10 km receives rainfall in excess of 10 cm in an hour. Bengaluru received its 13 cm of rainfall over a five-hour period.

Weather scientists attributed the abnormal rainfall activity to an unusual simultaneous presence of four atmospheric systems. Three upper air cyclonic circulation systems were prevailing over different areas — one over the east central Arabian Sea and adjacent to the Maharashtra coast, the second over the coast of Tamil Nadu and neighbouring areas, and a third over southwest Bay of Bengal over north Tamil Nadu and south Andhra Pradesh. Besides, there was also a crucial east to west shear zone along the 0.8 degree North latitude at a height of 5.8 km above mean sea level. The sudden downpour on Tuesday can be attributed to their combined impact.

But does this mean the city can do nothing in the face of nature?

Even small amounts of rain can inundate low-lying areas because the lake system that lay at the core of Bengaluru’s drainage has been virtually destroyed by encroachments as the city has grown exponentially. Earlier, lakes in the higher areas of the city would spill into the lakes in the lower areas, creating a natural storm water drainage system. Bengaluru, which had as many as 272 lakes in and around the city four decades ago, has only about 70 now, the rest having been swallowed by rampant real estate construction.

The government itself has reclaimed dozens of lakes to create bus stands, sports stadiums, housing complexes and for institutions like ISRO to set up facilities in the city. Real estate firms have been allotted land for residential complexes along lakeshores.

In July 2016, civic authorities faced public anger for failing to prevent encroachments of stormwater drains and lakes. The criticism forced the authorities to identify 1,000-odd properties around Bengaluru that had encroached on stormwater drains, but the five-day demolition drive triggered more anger — for it was the civic authorities themselves who had authorised much of the construction on the raja kaluves (stormwater drains) and lake beds.

Was Bengaluru the only place in south interior Karnataka that received heavy rainfall on Tuesday?

No. The southern district of Mandya experienced heavy rainfall (133.3 mm), Srirangapatna on the outskirts of Mysore received 110.1 mm, Ramanagara received 75 mm, and Kanakapura on the southern outskirts of Bengaluru got 67 mm of rain on Tuesday. Some flooding was reported in Mandya; the other regions, which are not densely populated, did not report flooding.

Is there a parallel between the situation in Bengaluru and the one that Mumbai faced in 2005, Uttarakhand in 2013 and Chennai in 2015?

No. Although a record for Bengaluru, Tuesday’s rain in the city was nowhere close to these other events. On July 26, 2005, Mumbai received as much as 944 mm of rain in 24 hours, an unprecedented event that claimed several hundred lives and completely paralysed the city. In 2013, some areas in Uttarakhand received torrential rain over three days in June — the worst affected areas saw over 300 mm of rain in that period, and thousands were killed. Chennai city received more than 300 mm of rain in a single day. The Bengaluru event pales in comparison.