Cyclone Nivar, which made landfall in the early hours of November 26, seemed far less destructive compared to earlier cyclones that swept the coast of Tamil Nadu.
Some reasons why the impact was minimised could be a distributed rainfall pattern, reduced inflow of water into the Chembarambakkam reservoir and better management of the outflow compared to the 2015 floods.
Battered coast
The coloured dots in the map correspond to the landing points of recent cyclones that hit Tamil Nadu’s coastline. The bigger the dots and the deeper the red, the higher the cyclone's speed at the time of landfall.
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1. Cyclone name : Thane | Date of landfall : Dec. 30, 2011 | Approximate speed during/just before landfall (knots) : 75 | Location : Close to north of Cuddalore.
2. Gaja | Nov. 15, 2018 | 70 knots | Between Nagapattinam and Vedaranyam.
3. Nivar | Nov. 25, 2020 | 70 knots | Between Mamallapuram and Karaikal.
4. Unnamed | Nov. 29, 2000 | 60 knots | Near Cuddalore.
5. Vardah | Dec. 12, 2016 | 60 knots | Near Chennai port.
6. Nisha | Nov. 27, 2008 | 45 knots | North of Karaikal.
7. Nilam | Oct. 31, 2012 | 45 knots | Near Mamallapuram, south of Chennai.
8. Fanoos | Dec. 10 2005 | 35 knots | Close to Vedaranyam.
9. Jal | Nov. 7, 2010 | 30 knots | Close to North Chennai.
10. Madi | Dec. 12, 2013 | 25 knots | Near Vedaranyam.
11. Nada | Dec. 2, 2016 | 25 knots | Near Nagapattinam.
Rainfall pattern: Less intense and more spread out
One possible reason why Nivar seemed far less destructive than many previous cyclones was the fact that the rainfall was distributed over three days and did not pour down in short and intense spells.
A comparison with the rainfall over Mumbai during the September 2019 floods helps understand the difference. The second graph shows rainfall recorded in Nungambakkam station, every 15 minutes, starting from 9.15 p.m. on Nov. 23, 2020 to 1.30 p.m. on Nov. 26, 2020. The first graph shows rainfall recorded in Mumbai’s Santa Cruz station, every 15 minutes, starting from 9.30 a.m. on Sep. 2, 2019 to 1.45 a.m. on Sep. 5, 2019.
Both periods amount to 64 hours and 15 minutes in total. In the considered period, Nungambakkam recorded 320.5 mm of rainfall, lower than that of Santa Cruz’s 476 mm. Thus, the overall intensity was lower.
Both Mumbai and Chennai had two spells of very heavy rainfall in the considered period. However, during the second spell, Mumbai received a more intense rainfall in a short period of time, leading to massive floods. On the other hand, Chennai recorded no such intense spells. The rainfall was spread out across the period and the two heavy spells were similar and less intense.
In 2015, the sudden and excess outflow of water from Chembarambakkam reservoir was cited as the reason behind much of the flooding in Chennai. This time around the reservoir management was much better.
Chembarambakkam: What was done right in 2020
The charts compare the inflow (orange bar) and outflow (yellow bar) (both in cubic foot per second or cusec) of water, the actual storage level of water in mcft (blue line), and the amount of rainfall received at the Chembarambakkam reservoir (depicted inside clouds) (in mm) at specific periods in 2015 and 2020. (in graph k = 1,000).
Less rainfall in 2020 : In 2015, 475 mm of rainfall fell on the reservoir on a single day (ending at 6 a.m. on Dec. 2). In 2020, the highest rainfall received by the reservoir on a single day was 132.4 mm (ending at 6 a.m. on Nov. 26). Also, the rainfall received was spread out across many hours, unlike in 2015.
Less inflow in 2020 : In 2015, the inflow of water suddenly swelled to 26,000 cusecs on a single day. Whereas in 2020, the inflow was much more spread out as the rainfall was intermittent and less intense. The inflow was less despite the fact that the upstream weather station in Tambaram received 310 mm in a single day ending at 8.30 a.m. on Nov. 26.
Less outflow in 2020 : In 2015, the outflow suddenly increased to 29,000 cusecs on a single day . Whereas in 2020, due to less rainfall and fewer inflows, there was no sudden spurt in the outflows.
Maintaining storage levels : In 2015, due to a sudden spurt in rainfall and inflows, the actual storage level jumped to 3,396 mcft (93.2%) from 86.2% on a single day. In 2020, the storage increased gradually.
