A mother, father and child were killed when a tree fell on their car during heavy rain and flooding in Tennessee, where submerged roads also led to dramatic rescues of people trapped in their cars, authorities said.

The three were killed when saturated ground caused a large tree to fall in the Chattanooga suburb of East Ridge just after midnight, Hamilton County Office of Emergency Management spokesperson Amy Maxwell said Wednesday.

Additionally, authorities found a body Wednesday while searching for a man who was swept away when he ran past firefighters and a barricade blocking a flooded road on Tuesday, according to the Chattanooga Fire Department. The local police and medical examiner will determine the cause of death.

The flooding prompted rescues of people stuck in homes and swamped vehicles.

At a news conference Wednesday, officials said they didn't expect so much rain and flooding to hit so quickly.

Chattanooga's airport recorded about 16 centimetres of rain Tuesday, marking the second-wettest day recorded for the city dating back to 1879, according to a social media post by the National Weather Service in Morristown.  

Chattanooga Fire crews rescued people trapped in vehicles and residents stuck in their homes, fire department officials said. Flooding closed parts of Interstate 24 in the area, but it reopened once the water receded.

Swift-water rescue teams rescued residents of three East Ridge homes trapped by rising floodwaters, according to the Hamilton County Sheriff's Office.

At one point, there were 60 vehicles on the flooded interstate, said Chris Adams, director of emergency management for Hamilton County. Some first responders were carrying people on their backs who couldn't move well through the water, placing them on the raised highway divider, Adams added.

"We all know to 'turn around, not drown,' but when you look at it, and it's two inches deep, and then next thing you know it's four feet deep, that's something you've never seen before," Adams said.

There were so many calls for help that 911 calls were "holding in every minute of every hour for about three hours straight," with more than 940 calls between 6 p.m. and midnight, said Barbara Loveless, director of operations for Hamilton County 911.

Dramatic rescue of motorist

Troy Plemons, a communications systems technician for EPB, Chattanooga's electricity and telecommunications utility, said he was stuck in traffic on an interstate in his bucket truck for two to three hours Tuesday evening.

Plemons said he saw the floodwaters lift an SUV, and when he and two Lawson Electric workers encouraged a woman inside to get out, she threw up her hands because she didn't know if she could. Plemons moved to the bed of a truck next to him to try to get closer, but the water was rising to her chest.

"I didn't think there was any time," he said. "I tried my best."

Plemons said the water was reaching neck level for the woman in the SUV when he used a boring bit offered by the Lawson Electric workers to break the window and help the woman get out.

"It was a rush, for sure. I felt like I was pretty calm until I broke the window," Plemons said. "I was doing everything I could to get her out because the water was rising pretty quick."

There were several rescues of people whose cars were overwhelmed by water in the area until the water receded about two to three hours later and traffic began to move again, Plemons said.

"I felt like I was there at the right time," he said. "I'm thankful I was there to help that lady."

Lawson Electric said its workers, Austin Camp and Brandon Shadwick, co-ordinated for hours with Plemons, as well as authorities to help move between 25 and 35 people.

"From babies to seniors, we just kept moving. We didn't talk to each other," Shadwick said in a news release. "We just worked as hard and as fast as we could to move people to safety."

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Anderson Stout watched it unfold from his truck.

"As soon as he pulled her out of that vehicle, I'm not joking, in maybe three minutes, her vehicle was almost completely submerged under the water," Stout said.