Making sense of the 2023 Chennai floods: A comparison with 2020, 2021 and 2015 | Data

Chennai received heavy rainfall in two days, with Perungudi recording the highest average rainfall of 293.4mm. All parts of the city and its surroundings were battered. In 2021, intense spells caused floods despite much less rain than 2020. In 2023, numerous bursts of rain over a longer period with brief intervals caused floods. Controlled outflow of water from Chembarambakkam reservoir helped preserve lives. #Chennai received heavy rainfall in two days, with Perungudi recording the highest avg. of 293.4mm. All parts of the city & its surroundings were battered. 2021 saw intense spells causing floods despite much less rain than 2020. 2023 saw numerous bursts of rain over a longer period, with brief intervals, causing floods. Controlled outflow of water from Chembarambakkam reservoir helped preserve lives.

The Chennai district received close to 400 mm of average rainfall in two days — between the 24-hour period ending at 8.30 am on December 4 and the 24-hour period ending at 8.30 am on December 5 — according to TNSDMA. In the district, ward 169 in Perungudi registered the highest average rainfall of 293.4mm, followed by CD Hospital (ward 43) in Tondiarpet (233.6 mm) and ward 111 in Nungambakkam (223 mm). Of the 43 stations in the city, 17 recorded over 200 mm of average rainfall in the two days. In 22 stations 150 mm was recorded and only four stations received less than 150 mm rainfall. 

If stations around the Chennai district were also considered, Avadi (278.2 mm) and Poonamalle (241.3 mm) followed Perungudi. These figures show that all parts of the city and its surroundings were battered.

Map 1 | The map shows the average rainfall received over two days in the 51 stations in and around Chennai in the two days mentioned above.

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This year, the rainfall pattern was different. In 2020, Cyclone Nivar-hit Chennai escaped widespread flooding despite receiving copious rainfall as it was distributed over three days with many gaps in spells. In 2021, many parts were inundated despite relatively less rainfall, as rain came down in short and intense spells. In 2023, the city neither received heavy downpours in a short time like 2021, nor did it have many gaps like in 2020. Instead, it recorded numerous bursts of rain over a longer period, with only brief intervals in between as explained in Chart 2 and Table 3.

Chart 2 | The charts show rainfall recorded in Nungambakkam in three periods — every 15 minutes from 9.15 pm on November 23, 2020, to 11.30 am on November 26 2020, and every 15 minutes from 12 am on November 5, 2021, to 2.45 pm on November 7, 2021 and every 15 minutes from 12.15 pm on December 2, 2023 to 3.15 pm am on December 5, 2023 — all amounting to about 64 hours each.

2020: In the 64 hours in November 2020, 320.5mm of rainfall was recorded. None of the 15-minute spells crossed the 20mm mark.