Geologists find some truth in Ahmedabad's founding fable

AHMEDABAD: About 608 years ago, the founding sultan of the city, Ahmed Shah I, was facing a torrid time building Gujarat's capital, especially its fort walls and citadel on the east bank of the Sabarmati. New scientific research by geologists from MS University, by studying the sediments in the Sabarmati ravines, found that the city was hit by frequent high-magnitude floods in the river between 1400 and 1440 AD, which destroyed nearly everything in their path.

These floods can explain the origin of Ahmedabad's founding fable, about Baba Maneknath and Sultan Ahmed Shah. The fable says that in the day, when Ahmed Shah's men built the city walls, the Baba would weave a 'godari' (a quilt). At night, the Baba would unweave it and the walls would crumble. The sultan then turned to the Baba and his mentor, Sufi saint Sheikh Ahmed 'Ganj Baksh' Khattu, for spiritual help and to redesign the city's layout.

Geologists from MSU, Alpa Sridhar, L S Chamyal and Mansi Patel, studied slackwater palaeoflood deposits (SWD) preserved in the ravines of the Sabarmati. These deposits have recorded at least five or six major floods. One such flood had a peak discharge 15,680 cubic metres per second (cumecs), five times higher than the 2006 flood, when the Sabarmati was flowing at 3,050 cumecs.

The researchers studied SWD beds at Dedhrota, Derol and Juna Sangpur beside the river and found the highest SWD deposits at Juna Sarangpur. "In the last 100 years, the flood of 1973 is considered the highest in the Sabarmati basin, with a peak discharge of 16,000 cumecs, but there is no consensus," the research paper states. The researchers used accelerator mass spectrometry and radiocarbon (14C) dating to study the sediments.

The research further adds that the flood timing was synchronous with the Medieval Warm Period, which was a time of strong south-west monsoons. These flood events may thus be linked to this phase of strong monsoons. According to the historical record, high-magnitude floods are also known to have occurred in 1683, 1714, 1739, 1755, 1868, 1927, 1941, 1950 and 1992, suggesting a clustering of flood events.

The research also indicates that in nearby river basins, two phases of high-magnitude floods were documented in the SWD in the upper reaches of Mahi river basin. The first had a discharge of 7,300 cumecs, and the second, in the middle reaches of the Mahi river basin, was reported in late medieval times, or the 14th to early 15th century, based on potsherds found in the deposits.