By Ritambhara
The Mahalanobis flood statistics of the 1920s and 1930s tested what engineers believed about the rivers of Bengal and Orissa against long runs of rainfall and gauge records. Prasanta Chandra Mahalanobis is best known for founding the Indian Statistical Institute in Calcutta in 1931. His first large public problems were floods, and his evidence was the record of the rivers themselves.
Mahalanobis was born in Calcutta in 1893 and studied physics at Presidency College and at Cambridge. Sir Gilbert Walker, the meteorologist, persuaded him to take up statistical problems of weather. He worked at the Alipore Observatory in Calcutta from 1922 to 1926.
Fifty-three years of North Bengal rain
A major flood struck northern Bengal in 1922, and the government formed a flood committee. Mahalanobis answered with a report on rainfall and floods in North Bengal from 1870 to 1922. The Bengal Irrigation Department published it in 1927, with 90 pages of discussion and a separate volume of maps.
The report placed the 1922 flood inside 53 years of rainfall and flood records. A single disaster could then be weighed against half a century of the region’s own weather. The Quarterly Journal of the Royal Meteorological Society reviewed the work in 1928.
Gauge readings on the Brahmani and Mahanadi
A severe flood on the Brahmani in 1926 brought him to Orissa. Its delta is built by three rivers, the Mahanadi, Brahmani and Baitarani, and it flooded often. He was asked to suggest a remedy.
His papers to the Indian Science Congress show the method. In 1932 he published a statistical analysis of the height of the Brahmani at Jenapore. In 1933, with K C Basak, he measured the intensity of Mahanadi floods from 1868 to 1929. With Nistaran Chakravarti he studied rainfall and the share of it that ran off into the Mahanadi.
Run-off is the fraction of rain that reaches a river, and it decides how much water a reservoir must hold. Mahalanobis concluded that dams on the upper reaches would control the floods best. His calculations became one basis for the Hirakud dam on the Mahanadi, completed in 1957.
Records against assumptions
J K Ghosh, writing on his life, notes that the government accepted his flood recommendations although they often conflicted with engineering opinion of the time. His case rested on river and rainfall series that ran back to the 1860s.
The rain those series describe has since changed. A study led by M K Roxy of the Indian Institute of Tropical Meteorology, published in Nature Communications in 2017, found a threefold increase in widespread extreme rain events over central India between 1950 and 2015. The authors linked the rise to more variable monsoon winds over the Arabian Sea. The upper Mahanadi basin lies in that central belt.
Mahalanobis showed that a flood policy should rest on the full record of a river and its rain. His work now asks that the record be read again as it lengthens, and that flood defences planned on older data be checked against the monsoon India has today.
