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 <200 mm but >150 mm was recorded and only four stations received less than 150 mm rainfall.
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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.
2021: In the 64 hours in November 2021, 252.6 mm of rainfall was recorded. Three 15-minute spells crossed the 20mm mark.
2023: In the 64 hours in December 2023, 501.5 mm of rainfall was recorded. None of the 15-minute spells crossed even the 15mm mark.
Table 3 | The table summarises the three charts. For instance, in the 246 15-minute spells considered each year, only 107 spells recorded no rainfall (0 mm) in 2023, whereas 2021 saw 178 such spells and 2020 saw 140 spells. This shows in 2023, breaks between spells were rare.
In 2023, though there were no >15mm spell, there were many more spells in the 3-4mm, 4-5mm, 5-6mm, 6-7mm, 7-8mm, 9-10mm and 10-11mm ranges than 2020 and 2021. This shows that consistent rainfall of medium intensity with very few breaks caused the floods this year. Also the total rainfall of over 500 mm was way higher than 2020 and 2021.
In 2015, the sudden and excess flow of water from Chembarambakkam reservoir was cited as the reason behind floods claiming hundreds of lives in Chennai. In 2023, the management was much better, leading to the controlled outflow of water and preservation of lives as shown in Chart 4.
Chart 4 | The charts compare the inflow (red bars) and outflow (blue bars) of water to and from the Chembarambakkam reservoir in 2015 and 2023. Both inflow and outflow are measured in cubic foot per second or cusec and plotted on the left axis of the graphs. The actual storage in the Chembarambakkam reservoir as a share of its full capacity in % is plotted on the right axis.
In 2023, after a 90mm rainfall on November 30, the outflows were increased in the subsequent days in a controlled manner and the storage was reduced from 89% to 76%. When two spells of heavy rains — 162mm on 24 hours ending December 4 and 204.8 mm on 24 hours ending December 5 — fell on the catchment areas, there was no need to increase outflows too much as a counter. In contrast, in 2015, the storage levels were maintained at the same level after initial rainfall of over 100mm on November 24, leading to uncontrolled outflows on December 2 after heavy rainfall.
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