Guwahati: An official hydrological assessment has concluded that the devastating flash floods that inundated Upper Assam last week were triggered primarily by extreme rainfall over Nagaland’s hill catchments, lending scientific backing to the Assam government’s claim that the disaster originated upstream rather than from local rainfall alone.
The report, prepared after analysing rainfall between July 18 and 20, says Nagaland received 85 mm of rain against a normal of 30.3 mm — an excess of 181%.
The deluge generated massive runoff into river systems flowing into Assam, inundating Sivasagar, Jorhat and Charaideo, and causing widespread damage to homes, agriculture, roads and other infrastructure.
Among the worst-hit districts, Mokokchung recorded 238.9 mm of rainfall — 493% above normal — followed by Zunheboto with 181.7 mm (419% above normal) and Kiphire with 99.6 mm (427% above normal).
The report notes that the same weather system triggered multiple landslides and flash floods across Nagaland, particularly in Mon district, underscoring the severity of the event.
Upper Assam, too, witnessed exceptionally heavy rainfall during the period. Charaideo received 170 mm (436% above normal), Golaghat 147.9 mm (325% above normal), Jorhat 105.2 mm (143% above normal) and Sivasagar 93.2 mm (134% above normal).
However, the report concludes that local rainfall alone cannot explain the scale of the flooding.
Instead, it attributes the disaster to an extreme hydrometeorological event over Nagaland’s upper catchments, where heavy rain over steep hills generated rapid runoff that surged downstream into Assam, pushing rivers beyond their carrying capacity and inundating areas not usually prone to flash floods.
The findings broadly echo Assam Chief Minister Himanta Biswa Sarma’s assertion that the catastrophe warrants a deeper scientific investigation.
Referring to reports of a cloudburst in Nagaland, Sarma said the government would examine satellite imagery to determine exactly what transpired in the upper catchments and whether similar events could recur.
“They have said a cloudburst occurred, but what else happened along with it we will have to examine through satellite imagery. We also need to find out whether it was a one-time occurrence or something that could recur,” the Chief Minister said.
He said the government would undertake a comprehensive study after the floodwaters recede, with particular focus on satellite imagery of Mon district.
“After the floods recede, we will thoroughly study satellite maps to figure out what went wrong on the fateful day of the cloudburst in Nagaland and devise our strategies accordingly,” Sarma said in a post on X.
The proposed study is expected to help Assam develop long-term flood mitigation and disaster preparedness measures for downstream districts that remain vulnerable to sudden surges in rivers originating in Nagaland during extreme weather events.
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