India, Aug. 5 -- "It was terrifying. no one really understood what was coming," said Sudhanshu Semwal, 20, a resident of Mukhba village, still shaken after watching the floodwater and debris crash into Dharali, a bustling market town barely five kilometres from Harsil on the Gangotri highway in Uttarkashi district.

The water came "with a roar" and swept through Dharali with terrifying speed, leaving nothing behind. "I heard a deafening sound like boulders grinding. And then we saw the Kheer Ganga river turn into a monster. It reminded me of the 2013 disaster - the panic, the helplessness, the destruction. It's happening again," said Sunita Devi, another resident of Mukhba.

The flash flood, triggered by a suspected cloudburst over the up...