New Delhi, Sept. 23 -- The flood arrived, as it almost always does in Cox's Bazar, before the warning reached the camps. In a settlement of a million people built on degraded hillsides with no legal right to move elsewhere and no engineering margin for the water that comes every monsoon, warning systems can measure what is coming but cannot change where a million people sleep.

Dirai, in Sunamganj district, recorded 402 millimetres within 24 hours in July, a figure the department called exceptional even for a region climatologists have long identified as one of the wettest inhabited landscapes in the world.

The flooding has arrived in successive pulses since early July, each one hitting communities still clearing debris and counting loss...