Following a change of course in the river, a 10-metre stretch of the main embankment had breached at Simaria village on the night of September 15 following which a new ring bund was erected.
The water was gushing through the newly-created ring bund since Saturday and the new embankment caved in a stretch of 30 metres early on Sunday morning following which flood waters entered several villages of Barauli block in the district, the sources said.
The officials had sounded an alert on September 15 about the river changing its course and putting pressure on the embankment and asked to people to shift to safer places.
Disaster management department had already rushed five teams of National Disaster Response Force (NDRF) and adequate relief materials to Goaplganj as a precautionary measure.
Principal secretary, Disaster management, Vyasji said in Patna that if flood water spread to three blocks in Gopalganj, two blocks in Siwan and as many blocks in Saran district would be affected.
Gopalganj district administration had been provided Rs 14 crore and Rs 5 crore to remain in readiness for any eventuality.