Harvey flood damage 'may match Katrina'

Flood damage in Texas from Hurricane Harvey might equal that from 2005's Hurricane Katrina, the costliest natural disaster in US history, an insurance research group says.

As heavy rain pounded Houston and Texas's coastal counties, the Insurance Information Institute said on Sunday it was still too soon to estimate the damage to homes and businesses.

"It could be a flood loss like Katrina because of the amount of water that's coming in ... not as much wind as it will be water," institute spokeswoman Loretta Worters said.

Hurricane Katrina resulted in more than $US15 billion in flood insurance losses in Louisiana and Mississippi that were paid by the National Flood Insurance Program (NFIP), a federal program that is the only source of flood insurance for most Americans.

The NFIP is already deeply in debt and likely will have to be bailed out again by US taxpayers, as it was after Katrina, to cover the bill for flood damage claims from Harvey.