Heavy, short-duration rainfall in the Nepalese catchment area of the Gandak River basin has triggered a severe flash flood, placing downstream districts in Bihar and Uttar Pradesh on high alert.

The flash flood, triggered by the recent Nepal flood, caused breach of Gandak embankment at Bagaha on Monday, threatening floods in adjoining areas.

According to the Water Resources Department in Bihar, a massive discharge of 5,43,500 cusecs was recorded at the Valmikinagar Gandak Barrage at 6 am on Tuesday.

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The river maintained a discharge exceeding 500,000 cusecs for 13 hours and over 400,000 cusecs for 32 hours, sending floodwaters surging through downstream districts including West Champaran, East Champaran, Gopalganj, Saran, Muzaffarpur, and Vaishali.

Heavy rainfall across border districts such as West Champaran, East Champaran, and Gopalganj between September 26 and 27 had already elevated water levels in the lower reaches prior to the surge.

“Although the peak discharge from the barrage was slightly lower than the 2024 record of 562,500 cusecs, the river established a new historical record at the Bagaha gauge site. Water levels reached a new Highest Flood Level (HFL) of 91.25 meters, surpassing the previous record of 90.90 meters set in 2024 by 0.35 meters (35 cm),” according to a statement by the water resources department.

The immense volume of water exerted intense pressure on river embankments, causing widespread seepage, piping, and overtopping. Field teams managed to secure 169 seepage and piping points through immediate emergency repairs. However, during the midnight hours between September 28 and 29, critical breaches occurred at three sites, the statement noted.

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In West Champaran, Bihar, two breaches were recorded on the left bank of the Champaran embankment. The first was a minor breach of approximately 10 meters caused by sudden subsidence at the 4 km mark in Khairatwa village under Bagaha-1 block.

Spilling water is spreading through nearby fields and is expected to re-enter the river around the 9.50 km mark, though low-lying areas in Agastya, Khairatwa, and Chandrapur villages face potential inundation and disrupted connectivity.

A second breach occurred at Phuliyakhar village in Bairia block. Another embankment breach was reported on the right bank of the river in Kushinagar district, Uttar Pradesh.

Receding trend

As of 8 am on Tuesday, water levels at the Valmikinagar Barrage showed a clear receding trend, with downstream discharge reducing to 191,200 cusecs. Upstream and downstream gauge levels at the barrage were recorded at 356.70 feet and 354.10 feet, respectively, while the Devghat gauge station in Nepal reported a discharge of 219,677 cusecs (6.14 meters) at 6:25 qm with a falling trend.

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While water levels upstream continue to drop, downstream reaches are experiencing a temporary rise as the surge progresses toward lower areas.

The Water Resources Department and West Champaran district administration have mobilised teams on the ground to undertake breach repairs and emergency protection works.

Given the severity of the situation, headquarters dispatched four executive-engineer-led response teams overnight to Bagaha, Bettiah, Motihari, and Gopalganj to oversee anti-erosion measures and structural interventions alongside previously deployed engineering staff. Senior department officials continue to monitor the flood response round the clock.