Weather in relation to the floods in Bihar, Bengal and ...metnet.imd.gov.in/mausamdocs/5544.pdf · Curiously enough, while north Bihar, north Bengal and AI"~am were writhing under

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Weather in relation to the floods in Bihar, Bengal andAssam during July and August 1954

K. PART HASAHATHY

.11cleoTological Office, Ncw Delhi

(Recei"',d 28 (tctober If/54)

The recent floods in t he rivers Gaudnk,K"osi and Brah maputra and t heir tributariesduring July and Augu st lH5·1 hav e receivedserious con eiderntion in Govem ment circlesas well 118 from t he pu blic. According to t hestatements in the press, t hese Hoods in Bihar ,Bengal lind Assam had taken II toll of ~.j ilives. ,I A total area of 25,650 square milesan d nearly !)!} lakhs of people ha ve hceuaffected and n large number of ca tt le-hea dshuv e peri shed. Ser ious damage to crops ,roads. rai lways and bridges bas al80 occurredon a sca le IH'Y('f experienced before " .According to newspaper report s th e floods oft his yea r arc unparall eled within Jivingmemory, TIl(' 1Il 0~t severe flood ill the Kosiwit hin li\' ing nU'lIlc ry oc eurred in 1927.The ma ximu m disdlfirge during t hat floodhns heeu ostimuted het wr-en (;00,000 nndi OO,OOO cusecs (C"'INC , 1950), Bllt. t hisyear, t he est imat ed dischnrgs of Kosi on26 July wa s (i25,OOO l'lHH'C R, nearly the sumoas in t he 19~7 flood ; ill the last week ofAugust also a d ischarge of the same orde rwas recorded.

Curiously enough, while north Bihar, nort hBengal and A I"~am were writ hing und er t heimpact of Hoot Is, th e southern district s ofD.P., south ltihar and Orissa were having apartial drought. These floods have , th ere­fore, been en t iroly due to tho ra in:'! ill toeH imalayan catchments of t.lIf' rivers concern­ed. One is led to WOII(]('I' what has beent he character of t he rains ill th e Himalavanregion which were responsible for t he ffdod~of such st upen dous magn itude and whatspecial meteorological features were prevalentduring July and August of th is year whichcaused t he intense rains in t he EasternHi malayas.

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Regarding the first quest ion, 1:;: ., the ac tua lintensity am] distribution of min in t hevarious JiiJI catchments in the Hi malayas ,unfortunately, definit e answers cannot befound, as most of these catchments do nothave An)'" rain recording stations in th em.The catchments concerned are those of th eGand ak, t he Kosi and th e B rahmaputra su dt heir tribu taries. Of th ese, only in the Kosicatchment in Nepal; rain recording stat ionshave heen funerioning for t he last few years ;for th e others, IlO rainfall data are availab le.•Fig. 1 shows the net work of observatories inthe Kosi catchment.

The analysis of t he data from the stat ion.in th e Kosi ca tchm ent for the yea r. 1948-52,together with t he associated synopt ic situa­tion has 8110wn, among ot her things, t hathca vv ra infa ll in t he cat chment is almostulwavs associated with ' break ' cond it ions inth e "monsoon, during the period June toSoptemher (ThID, 1954). This conclusioncan also be taken to apply to t he ot her .catchments in t he Eastern Himalayas,

Southwest monsoon seasen and • breaks ' In themonsoon

The norma l isoba ric and wind conditions0\'('1' India du ring t he southwest monsoon,t hough well known, are briefly descr ibedhere. A elceed • low ' pressure area lies overth e region Baluchistan, Sind and neigh­hourhood ami an elonga ted trough extendsfr.nu there to the northwest angle of th e Bayof Bengal , The axis of t he trough on a dayof normal monsoon run s approximately fromGangnnagar to Sandheads through Agra andDalt onganj . Sout h of the axi s, the wind.are weste rly and north of it, ill the Ganget icbasin t hey are easterlies. In the hjgher

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