(Bloomberg) -- Mandatory evacuations have been ordered in parts of the Houston area after torrential rains overwhelmed rivers and bayous.

“There’s an imminent threat to life and property” in some areas and hundreds of houses and other structures probably will be inundated as floodwater rise, Harris County Judge Lina Hidalgo said in a briefing on Friday. A precise count of the number of people at risk wasn’t available, but she indicated it’s a significant portion of the metropolitan area’s 7.3 million residents.

A “flood wave” is expected to hit the Kingwood neighborhood and suburban areas north and east of downtown Houston that lie along the San Jacinto River within hours, said Hidalgo, the county executive. 

The danger extends at least a half mile (0.8 kilometer) outside the river’s banks, she said. As the flood moves downstream, high water levels and large debris probably will impact the Houston Ship Channel, one of North America’s most important industrial thoroughfares, Hidalgo added.

Residents who don’t evacuate threatened areas are “sitting ducks,” she said. Widespread flooding in recent days has shut highways, schools and a key river ferry after as much as a foot of rain drenched the region upstream of metropolitan Houston. 

(Adds threat to Houston Ship Channel in fourth paragraph.)

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