Floodwaters have submerged more than 400 villages across the delta, forcing families onto embankments and school rooftops as rivers continue to rise for a fourth consecutive day.
District administrators say the scale of the inundation outpaced evacuation plans drawn up after the last major flood. Early warning messages reached fewer than half of the affected unions, according to two officials who spoke on condition of anonymity because they were not authorised to discuss internal assessments.
Hydrologists point to a combination of unusually intense upstream rainfall, silted riverbeds and ageing embankments. "The engineering assumptions underpinning this flood defence system belong to a different climate," said one river scientist who has studied the basin for two decades.
Relief committees have opened more than 1,200 shelters, but supplies of drinking water and oral rehydration salts are running short. Aid groups say the recovery bill will run into the hundreds of millions of dollars, most of which remains unfunded.
The crisis has reopened a long-running argument about who pays for adaptation in countries that contributed least to global emissions. Delegates from the region are expected to press the question again at the next round of climate finance talks.