Monday, July 22, 2013

Wettest 24hr rainfall day in Tamil Nadu - Ketti, Nilgris in November 2009

The recent very heavy rainfall over Uttrakhand just reminded me of a severe rainfall event which media ignored completely. Note that the rainfall (3 day accumulated rainfall) was more severe than the one witnessed in Uttrakhand in June 2013-( It happened in Nilgris district in Tamil Nadu in November, 2009.

Ketti is a small town nestled in Ketti valley. It is located in Coonoor Taluk in the Nilgiris District of the state of Tamil Nadu in India. The valley is the last major valley en route to Ooty along the Ooty-Coonoor highway. Ketti, is located at a height of about 2100 m above mean sea level in Nilgiri (11° 22″ 01′ N & 76° 44″ 32′ E) is a valley surrounded by hills in three directions and opening is only in the ESE direction. It recorded the heaviest rainfall of 820.0 mm in 24 hrs ending 0830 hrs IST on 10 November 2009 which is the all time 24 hrs rainfall record for Tamil Nadu.

Extreme weather events like continuous very heavy rain and floods caused heavy loss of lives and property, damage of roads, banks of rivers in urban and rural areas. Extremely very heavy rainfall over a small areas in the hilly regions make misery in the life of hilly region people due to land slides, soil erosion across roads, rail lines, damage of crops and heavy loss of human lives. In this article the reasons of extreme record rainfall at Ketti in Nilgiri hills and its impacts have been analysed.  

Nilgiris, the hilly region of Tamil Nadu being the part of Western Ghats generally receives more rainfall during SW monsoon than during NE monsoon. On many occasions whenever a system cross Tamil Nadu coast and move inside the land area the interior Tamil Nadu including hilly areas of Nilgiris district receives copious rainfall with isolated extremely very heavy rain in the district. Before 10th November 2009 the heaviest rainfall in 24 hrs recorded in Nilgiri district was 448.3 mm at Kodanad on 15th Nov, 1992. Some of the Storms and Depressions which form in the Bay of Bengal during November and December affect the weather over the Nilgiris district causing heavy rain.

Nilgiris November 2009  - 3 day Rainfall amount in mm

Pre-Cyclone Phyaan
A low pressure formed over Comorin area on 07 November 2009 and became a well marked low on 08th. The Comorin area and adjoining Arabian Sea became Depression on 9th, moved towards north and intensified as deep depression on 10th November 2009. Under its influence, heavy to very heavy rainfall occurred in west interior and coastal Tamil Nadu. The Ketti valley which has the average elevation of 2000 m a.s.l. is surrounded by hills except east-southeast side situated in west interior parts of Tamil Nadu. It received extremely heavy rainfall on 10th Nov 2009. The valley faces windward side for NE monsoon and leeward side for SW monsoon. The presence of a depression in Arabian sea (Lat. 11.5° N, Long. 71.5° E at 1200 UTC) on 9th Nov 2009, about 450 km west of Mangalore, influenced the easterlies from Bay of Bengal. The moist air mass entered inland through the land mass of Tamil Nadu. As the depression moved initially in the northerly direction, the strong low level easterly wind mass between Lat. 11 and Lat. 12 deg N rushed towards the centre of the depression/deep depression in Arabian sea through the landmass of Tamil Nadu. The winds from the Bay of Bengal entered into Ketti Valley from south to southeasterly direction and were forced for ascend by the orography. This elevated ascent of moist air triggered the formation of mesoscale vortex over the valley and rained there heavily. Nilgiri Mountains trapped the moisture of easterly wind which condensed into raindrops. Lifting of the unstable moist air by the process of convection lead to cooling and condensation of moisture lead to heavy downpour over the Ketty valley during day and night on 9th as well as early morning hours on 10th. The state revenue officials recorded the rainfall at Ketti, near Ketti railway station, measured good number of rain spells throughout the day and night to avoid overflow of rain water from the rain gauge jar on 9th & 10th of November 2009. Totally 48 people lost their lives in the valley due to this heavy rains. Photos of the impact of heavy rains by Purnan -

Highest 24-hours rainfall in Tamil Nadu (Top 10)
  1. 820 mm at Ketti (Nilgirs dt) on November 10, 2009 (Cyclone Phyaan)
  2. 709 mm at Vandavasi (Thiruvannamalai dt) on August 5, 1965
  3. 656 mm at Orathanadu (Thanjavur dt) on November 27, 2008 (Cyclone Nisha)
  4. 528 mm at Thanjavur on November 27, 2008 (Cyclone Nisha)
  5. 572 mm at Cuddalore on May 18, 1943
  6. 562 mm at Avalanchi (Nilgiris dt) on June 23, 2007
  7. 540 mm at Mahabalipuram (Kanchipuram dt) on November 20, 1970 
  8. 516 mm at Aanamalai (Coimbatore dt) on November 20,1959
  9. 510 mm at Mandapam (Ramanathapuram dt) on April 4, 2005
  10. 503 mm at Upper Bhavani (Nilgiris dt) on June 17, 2007
Source: Mausam Docs 
Rainfall Data : Own Compilation

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