Meteorologists have forecast that the historic rainfall will dump up to 50 inches by Wednesday.
Heavy rain bands were expected to move in over the Houston area overnight, which will continue the catastrophic and life-threatening flash-flood emergency in the area.
"A flood of this magnitude is an 800-year event, and it exceeds the design specification of our levees," Fort Bend County Judge Robert Hebert said in a statement today.
Rescuers received more than 1,000 distress calls from people forced on their rooftops in the submerged Houston area.
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Officials warned the danger was far from over. They said the flooding in Texas was unlikely to recede quickly and that the storm willforce more than 30,000 people from their homes.
Federal officials have widened the emergency zone to Louisiana, the southeastern US state on the Gulf of Mexico.
US President Donald Trump will travel to Texas tomorrow to get a first-hand information on the flooding.
According to the Indian Consulate in Houston, at least 200 Indian students at University of Houston have been stranded due to the flooding. Houston is home to a sizable Indian population.
The hurricane has now turned into a tropical storm and is dumping rain on southern Texas.
The National Weather Service (NWS) called the flooding in Texas "unprecedented". "We are seeing catastrophic flooding, and thiswill likely expand and it will likely persist as it's slow to recede," its director Louis W Uccellini said today.
Parts of Harris County, which encompasses Houston, were pelted with 30 inches of rain in 72 hours, the NWS said.
Harvey has been hammering Texas since Friday, when it first made landfall as a Category 4 hurricane. At least five people have died and the death toll is expected to rise as the storm has triggering record floods and tidal surges.
Officials say they have conducted more than 250 water rescues and rescued more than 1,000 people.
Major refineries were closing down, cutting at least one million barrels per day out of production, and the Houston Ship Channel, the busiest in the nation, is shut. Officials said Houston, a major centre for the nation's energy industry, suffered billions of dollars in damage and would take years to fully recover.
Authorities have closed Houston schools for the week and shuttered William P Hobby Airport until at least the middle of the week.
"It's scary. I've been through many storms in my city, but it has never been this terrible. This city is completely underwater," Guatzin, a local resident, told CNN.
"We have nowhere to go. I have a 2016 Chevy Silverado and the water is up to the door panels. The water keeps rising," said Jake Lewis, who was in Houston on business and woke up to ankle-deep water in his Houston hotel room.
Janet Castillo, who was trapped at home with children, said: "The water keeps rising. We have called several numbers but no luck."
The Brazos River, which runs southwest of Houston, is expected to reach record heights in the coming days.
Over the next few days, Harvey is forecast to head back into the Gulf of Mexico, where it will pick up moisture before moving back over Galveston and into Houston again, CNN meteorologist Karen Maginnis said, meaning at least four more days of rainfall.