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Days of heavy rain have caused historic flooding in the US state of Louisiana, bringing as much as 31in (79cm) across a third of the state.

The downpour continued for more than 72 hours, leading to widespread and dramatic flooding in the affected area and downstream along the Amite river.

Louisiana has seen four times the average amount of rainfall for August.

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"What made this so disastrous is the large area," says Ken Graham, meteorologist-in-charge at the National Weather Service's New Orleans/Baton Rouge office.

Graham says much of the eastern portion of the state, from the northern Mississippi border all the way to the Gulf of Mexico saw rainfall of at least 15in over 48 hours. That's about 11,000 sq miles, about twice the size of Connecticut.

"Normally you would see some small pockets with these kinds of rainfall totals, but this is the largest area of this kind of rainfall I've seen in my career."

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The result has been devastating.

At least 13 people have died and more than 40,000 homes have been damaged.

Towns along the Amite river have experienced some of the worst flooding.

In Denham Springs, the river topped at more than 46ft, some 4ft higher than its previous record in 1983.

In one parish east of Baton Rouge, officials believe 75% of the homes had been damaged, according to the Los Angeles Times.

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The Federal Emergency Management Agency (Fema) has made disaster declaration in 20 of the state's 64 parishes.

More than 86,500 people have registered for assistance from the agency.

Dramatic video footage has emerged from the region, picturing people being rescued from houses and sinking cars. State officials estimate about 30,000 people have been rescued.

Others have not been so lucky. According to the New York Times, a number of people have died after being swept away by surging rivers.

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For the state of Louisiana, the storm is among the worst in the state's history.

Graham said there had been major flooding from storms in 1983 and 1995. "We talk about these as benchmarks - but this one beat them all."

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Thousands of members of the US National Guard have been deployed in the region to assist with search-and-rescue operations.

The Red Cross has said it is launching its largest relief effort in America since Hurricane Sandy in 2012. In the four years since Sandy, other major disasters in the US have included, among others:

the 2014 Washington state mudslide, which killed 43

the Moore, Oklahoma tornado, which killed 24 and involved $2bn (£1.5bn) in damage

major flooding and tornado outbreaks in Texas and Oklahoma in 2015, which killed 31

the 2016 East Coast blizzard, which killed at least 45

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A local Baton Rouge paper has criticised President Obama for not visiting the affected area, but he said on Friday he would visit the following week.

On Tuesday Governor Bel Edwards made a comment that while federal agencies have been responsive, the flooding had not received as much attention as some of the major hurricanes that have hit the area.

"When you have a storm that is unnamed - it wasn't a tropical storm, it wasn't a hurricane - a lot of times people underestimate the impact that it would have," Mr Edwards said, calling the flooding "historic" and "unprecedented".

Additional reporting and production by Charlie Northcott