Dibrugarh: The devastating floods that swept through Upper Assam in July were caused by a combination of factors rather than heavy rainfall alone, experts said at a brainstorming session at Dibrugarh University on Friday.
They pointed to extreme rainfall in the upstream hills, changes in the Dikhow river basin, river morphology, sedimentation, mining, deforestation, altered drainage and gaps in disaster preparedness.
The discussion was organised by the Centre for Atmospheric Studies, Dibrugarh University, under the theme “Need of an integrated understanding through trans-disciplinary approach for effective risk reduction and adaptation”.
It examined how an extreme rainfall event developed into a disaster of such magnitude and what measures could reduce the risk of similar events in the future.
The session was moderated by Kalyan Bhuyan, while Partha Jyoti Das, Professor of Practice at Dibrugarh University, delivered the keynote address and set the framework for the discussion.
Das placed before the panel several issues requiring closer examination. These included the role of pre-monsoon rainfall and water accumulation in coal-mining excavation sites, stone quarrying and riverbed mining, deforestation, shifting cultivation, hillslope degradation and destabilisation, barriers created by dams, sluice gates, roads and bridges, rising riverbeds in floodplains, degradation of natural drainage channels, Brahmaputra drainage congestion and backflow, and shortcomings in disaster management, forecasting, early warning, coping and adaptation.
Taking the discussion upstream, Jogendra Nath Sarma said the July floods needed to be examined in the context of exceptionally heavy rainfall in Nagaland, particularly in Mokokchung, Mon, Tuensang and Longleng, which feed into the wider river systems of Upper Assam.
Post-event assessments have confirmed the unusually heavy rainfall during the period, with more than 400 mm of cumulative rainfall reported at Mokokchung from July 18 to 20. Very heavy rainfall was also recorded at other stations in the region.
However, Sarma said rainfall was only one part of the explanation and drew attention to what he described as changes in the basin architecture of the Dikhow river.
He said the behaviour of the river during an extreme flood needed to be understood in relation to its geomorphology, channel configuration and sediment load, rather than by looking only at rainfall and discharge figures.
Sarma cited the example of a sharp meander near Chantak, where, according to his observation, the river’s meandering course extends for about 10 km, while the actual distance across the bend is only around 3.5 km.
During normal flow, the river follows its established course. However, when a large volume of water and sediment is suddenly forced through the system, the river may seek a shorter route, cutting across the meander and carrying floodwaters towards areas that would otherwise remain outside its immediate course.