Even though Chennai received lower rainfall 2021, compared to 2020, it still led to waterlogging as large amounts of rainfall came down in short time periods.
Short and intense spells of rain in the past two days have left several areas of Chennai inundated. This was in direct contrast to the rainfall pattern observed in November 2020 when Cyclone Nivar made landfall. At that time, though the amount of rainfall was more, it was distributed over three days and did not pour down in brief and intense spells, restricting the floods to only vulnerable localities.
More rainfall in 2020, but in longer spells
The graph below shows rainfall recorded in Nungambakkam, every 15 minutes starting from 9.15 pm on Nov. 23, 2020 to 1.30 pm on Nov. 26, 2020. In the 64 hours, the station received 320.5 mm of rainfall.
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Less rainfall in 2021, but in short & intense spells
The graph below shows rainfall recorded in Nungambakkam, every 15 minutes starting from 12 am on Nov. 5, 2021 till 2.45 pm on Nov. 7, 2021. In the 63 hours, the station recorded 252.6 mm of rain, much less than 2020. Nearly 21cm rainfall was received in Nungambakkam in the 24 hours ending at 8.30 a.m. on Nov. 7. About 23cm rainfall was received in the DGP office station in the 24 hours ending at 8.30 a.m. on Nov. 7. While the total rainfall received was lower in 2021, it still led to waterlogging as large amounts of rainfall came down in short time periods, unlike 2020.
The charts show inflow (blue column), outflow (red column) in 1,000 cusecs and storage level in % (yellow line) in reservoirs at 6 a.m. on Nov 6, 6 a.m. and 4 p.m. on Nov. 7. In Poondi, Red Hills and Chembarambakkam inflows swelled to more than 2,000 cusecs by 4 p.m. on Nov. 7.
In Poondi and Red Hills, the storage levels were close to 90% while in the other two, it was near 85%. Extremely heavy rainfall (>204 mm) will likely occur in Chennai on Nov. 10 and 11.