A Meteorological Analysis of Flood in Eastern India During Monsoon 2014
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6 Meteorological features associated with flood over eastern India during southwest monsoon 2019 S Bandyopadhyay, H R Biswas, Anand Shankar, Umasankar Das, S D Kotal, Debapriya Roy, Sourish Bondyopadhyay and G K Das IMD, Regional Meteorological Center, Kolkata This chapter focuses on the meteorological analysis of two instances of flood –one urban flooding over Odisha during 12-14 August 2019 and another in Bihar during 27-30 September 2019. 6.1. Introduction Floods are very common in eastern India during southwest monsoon season. It brings a lot of misery to the people of this region. Monson season 2019 witness massive flood during August and September in eastern India. First instances of urban flood observed over Bolangir district of Western Odisha during 12-14August and second spells of massive flood in Bihar during 26-30September 2019.Patna, the capital city of Bihar has been inundated for several days for this severe flood. A lot of damage to property and few loss of life have been reported. 6.1.1 Physiography of Eastern India Eastern India consists of the six meteorological subdivisions, comprising of five states of eastern India namely West Bengal, Sikkim, Bihar, Jharkhand and Odisha. Sub Himalayan West Bengal (SHWB) and Sikkim form a single subdivision. West Bengal state consists of SHWB and the meteorological subdivision, Gangetic West Bengal (GWB).SHWB & Sikkim is dominated by orography mainly associated with Eastern Himalayas. GWB and Odisha is mainly coastal subdivision near Bay of Bengal whereas Bihar and Jharkhand is landlocked state/subdivision (Fig-6.1(a)).Major river basins in eastern India are Ganga river basin in Bihar and West Bengal area, Odisha has three river basins namely Mahanadi, 75 Subarnarekha, Brahmani and Baitarani (Fig-6.1(b)).Majority of tributary rivers of Ganga passing through Bihar and West Bengal. Fig-6.1(a): Physiogarphy of Eastern India Fig-6.1(b): Major River Basin of Eastern India Most of the portion of coastal State Odisha is falls very close to southern location of normal position to the monsoon trough during southwest monsoon season. Also many districts and important city of the state lies on the south west sector of monsoon lows/depression moving northwestwards to main land area from Bay of Bengal. Thus Odisha experiences maximum rainfall during south west monsoon season and in addition copious rainfall due to synoptic scale monsoon disturbances, sometimes causes flood in different parts of the state Odisha. Large spatial and temporal variability are exhibited in Indian summer monsoon rainfall in sub-regional scale over Odisha and is very large due to its varied physiographic features. Bolangir city is an important city of Western Odisha and head quarter of Bolangir District. Bolangir district can be characterized as two broad 76 physiographic areas viz., (i) undulating plains with residual hills and (ii) scattered hills and areas with high relief. The undulating plains covers the central and eastern parts of the district the average elevation being 200m to 300m above mean sea level with a general slope towards east. Fig-6.1(c): Physiogarphy of Odisha Fig-6.1(d): River basin of Bihar The areas with high relief and high hills are situated in the north western, western and south eastern parts of the district. Bolangir city is located in the northeast parts of the district. A physiographic map of Odisha along with location of Bolangir District/city is depicted in Fig.6.1(c). Orography plays vital role in spatial and temporal rainfall variability within a season. Weatherwise, two important cities of Bolangir district namely Bolangir and Titlagarh are well-known for very high temperature zone of Odisha during summer season. Ever recorded rainfall data(upto 2018) about 70 years reveals that Bolangir was received highest rainfall in 24 hours period, 401.6mm on 30th August, 1982 and this record breaks in 77 monsoon 2019 season. Bolangir has recorded 555.0mm at 0830 hours IST of 14th August, 2019 in past 24 hours. Also, extremely heavy rainfall 263.0 mm was recorded at Bolangir at 0830 hours IST of 13th August, 2019 in past 24 hours. So, within 48 hour period (0830 hours IST of 12th August to 0830 hours IST of 14th August,2019) total 818 mm rainfall which causes floods in different parts of the city and neighbourhood area. Characteristic features of this event have been described in this chapter. There are eight important river basins in Bihar (Figure-6.1(d). These are 1. Gandak, 2. Burhigandak, 3. Bagmati, 4. Adhwara group,5. Kamla Balan, 6. Kosi, 7. Mahananda 8. Ganga Stem in Bihar. The important rivers of North Bihar are Ghaghra, Gandak, BurhiGandak, Kosi and Mahananda etc. which directly flow into river Ganga, whereas Bagmati Adhwara, Kamla- Balan etc. drain into the Ganga through the Kosi. And in South Bihar the important rivers are Punpun, Kiul, Badua, Chandan etc. which also fall ultimately into river Ganga. 6.2. Analysis of exceptionally very heavy rainfall in Bolangir city of western Odisha causing urban floods during 12-14 August 2019 6.2.1 Rainfall Analysis Odisha state is a meteorological subdivision of India has received 7% above normal rainfall 1233mm against normal value of 1155 mm during southwest monsoon season 2019, though in first two months, June and July the rainfall was deficient by 21%. Similarly, Bolangir district has received 16% above normal rainfall 1372mm against normal value of 1180 mm during southwest monsoon season 2019 and upto 31st July 2019(in first two months of the season) rainfall was deficient by 33%( 406 mm rainfall occurred against normal value 602 mm) . Climatologically, annual normal rainfall of Bolangir city is 1418.2 mm and southwest monsoon season (June to September) normal rainfall value is 1215.1mm which is 86% of annual normal value. It is great concern that within 48 hour period (0830 hours IST of 12th August to 0830 hours IST of 14th August,2019) Bolangir city has received total 818 mm rainfall which is 58% and 67% of its annual and monsoon season normal rainfall value respectively. Further out of the 818 mm rainfall, 718 mm rainfall occurred in 24hours period from 12th August Evening (1200 UTC) to 13th August Evening (1200 UTC) which might have cause more vulnerable situation in Bolangir city. The highest rainfall in a day recorded during last 10 years over Bolangir city is shown in Table 6.1. It is observed that during last 10years there is no extreme rainfall (≥20cm) event over Bolangir city. The overall distribution of rainfall including state raingauge station occurred in past 24 hours recorded at 0830 hours IST of 14th August,2019 is also depicted in Fig. 6.2. It shows that heavy rainfall zone mostly confined in central parts of western Odisha with maximum core in Bolangir. 78 Fig. 6.2 : Blockwise rainfall distribution in past 24 hours recorded at 0830 hrs IST of 14th August,2019 (The rainfall map obtained from Odisha State Disaster Management Authority (OSDMA), Govt. of Odisha). Table 6.1 Highest rainfall recorded in 24 hours period in different year over Bolangir city during last 10 years. Year Highest Rainfall in Date 24 hours (in mm) 2010 70.0 25-07-2010 2011 128.3 26-08-2011 2012 111.0 21-06-2012 2013 197.8 23-07-2013 2014 148.0 29-07-2014 2015 86.0 09-01-2015 2016 170.4 07-02-2016 2017 107.0 17-07-2017 2018 92.8 27-08-2018 2019 555.0 14/08/2019 79 6.2.2 Synoptic Features Under Influence of a cyclonic circulation over northeast Bay of Bengal and neighbourhood, a low pressure area has formed over northwest Bay of Bengal adjoining West Bengal and Bangladesh coasts with the associated cyclonic circulation extending upto 7.6 km above mean sea level tilting southwest wards with height on 12th (0300 UTC) August,2019. It lay over southern parts of Gangetic West Bengal adjoining north Odisha with the associated cyclonic circulation extending upto 7.6 km above mean sea level tilting southwest wards with height at 0300 UTC of 13th August,2019. It moved west northwestwards and lay over northwest parts of Chhattisgarh with the associated cyclonic circulation extending upto 7.6 km above mean sea level tilting southwest wards with height at 0830 hours IST of 14th August,2019. Mean sea level chart of13th August,2019 evening have been depicted in Fig.6.3 as representative and ECMWF model reanalysis wind charts of 850hPa level overlaid on toposheet for 12th ,13th and 14th August (00UTC) have been depicted in Fig.6.4. Fig. 6.3 : Mean sea level pressure chart of 1200 UTC of 13th August,2019 80 Fig. 6.4 : ECMWF model reanalysis wind charts of 850hPa level overlaid on toposheet for 12th ,13th and 14th (00UTC) August 2019 It is observed that central parts of western Odisha falls on south west and left forward sector of the low pressure system. Moreover, moist air striking towards hill slope located to the southeast side of Bolangir district. Thus Bolangir city and its neighbourhood area fall in the strong moisture convergence zone and southwest sector of a low pressure system tilting south-westwards with height. It may cause exceptionally very heavy rainfall over this area. ECMWF model 850 hPa vorticity analysis reflected highest rainfall area (Bolangir and its neighbourhood) was in the edge points of higher values of low level vorticity which may be due to tiling of cyclonic circulation associated with the low pressure system. Insat 3D Satellite images of different times (12th August 1200 UTC to 13th August 0900 UTC) are depicted in Fig.