Hurricane Harvey: It's big. Really big. And some say it could be as bad as Hurricane Katrina

Hurricane Harvey: It's big. Really big. And some say it could be as bad as Hurricane Katrina

Hurricane Harvey: It's big. Really big. And some say it could be as bad as Hurricane Katrina

Updated 29 August 2017, 8:25 AEST

The tropical storm currently smashing Texas is not your typical storm.

Tropical Storm Harvey is not your typical storm.

The US National Weather Service has described it as "unprecedented" and warned its impacts will be "beyond anything experienced".

Those aren't words that are used lightly.

And by now you've probably seen how big it is, from this photo taken from the International Space Station:

But if that's not enough, let's try adding some more perspective.

It's affecting millions of people

Harvey is smashing into the east coast of Texas at the moment and is making its way west.

The state of Texas has a population of 27.8 million people, more than the entire population of Australia.

Texas Governor Greg Abbott said on Sunday that 50 counties had been declared state disaster areas.

It's not clear how many people have been rescued from floodwaters. Up to 1,200 people had to be rescued in Galveston County alone, said county judge Mark Henry, the county's top official.

More than 80,000 people in Houston, a city of around 2 million, have been left without power.

At least three people have been killed, and that's expected to rise.

The water reached chest height in some areas

Harvey made landfall as the most powerful hurricane to hit Texas in more than 50 years.

Already it's dumped about 80 centimetres of water on Houston, and that's expected to double over the next few days.

That amount of water is so unprecedented that meteorologists had to update the colour charts on the weather service's official rainfall maps.

In some areas, people were forced to wade through chest-deep water as rivers and channels overflowed their banks.

In others, floodwaters reached the rooflines of single-storey homes.

Every major highway in the area has been flooded to some extent.

Some coastal parts of Texas are expected to receive 127cm of rain by the end of the week, which is the same amount of rainfall those areas usually receive in an entire year.

To compare that to another storm, Tropical Storm Allison dumped 102cm of rain on Texas in 2011. That storm flooded 70,000 homes and caused $9 billion in damage.

The rescue effort is immense

The Federal Emergency Management Agency says it has deployed more than 1,800 staff across Texas in response to the disaster.

And according to ABC America, Texas Governor Greg Abbott has activated the entire Texas National Guard, or roughly 12,000 guardsmen, to help with the recovery effort.

In Houston alone, the mayor said there were at least 22 aircrafts, 35 boats and 93 rubbish trucks working to help locate and rescue flood-affected residents.

Many of those rescues were of people trapped on their roofs or in their attics.

The city's 911 system has so far received 75,000 calls. It normally averages 8,000 to 9,000 calls per day.

Further north in Dallas, the city's convention centre has been turned into a "mega shelter" to house 5,000 evacuees.

The US economy will take a hit

A US insurance research group says the flood damage in Texas could equal that from Hurricane Katrina — the costliest natural disaster in US history.

Katrina devastated New Orleans in 2005, killing 1,800 people and resulting in more than $15 billion in flood insurance losses.

That was paid for by the federal National Flood Insurance Program (NFIP), which is the only source of flood insurance for most Americans.

But the NFIP is now deeply in debt and likely will have to be bailed out again by US taxpayers.

John Dickson, the president of the NFS Edge Insurance Agency, said about 15 per cent of the Houston area was insured for flood.

Meanwhile, the storm has also struck at the heart of the country's oil and gas industry.

Several refineries have been closed and some offshore platforms have been evacuated.

So far that's knocked out 11 per cent of US refining capacity and a quarter of oil production from the US Gulf of Mexico.

The Gulf is home to nearly half of the nation's refining capacity, and the reduced supply could affect gasoline supplies across the US.

Analysts say the outages will also limit the availability of US crude, gasoline and other refined products for global consumers and could push up prices.

ABC/wires