MANDALAY, Myanmar — U Kyaw Swa, an environmental activist, received an urgent government warning on Saturday to evacuate his home immediately because of the danger of heavy flooding in eastern Myanmar.

But it was too late.

The water had arrived nearly 24 hours earlier and flooded his home so quickly that he and his family had to be plucked by boat from a second-floor window.

“The flow of water was so fast, our house was flooded in 30 minutes,” he said. “We have nothing, and now we have to start our life from the beginning.”

This year’s monsoon season has brought crippling floods to many parts of Southeast Asia. In some cases, poor dam construction, deforestation and a lack of emergency preparations have worsened the effects.