Hurricane Barry: Thousands without power as Louisiana braces for hurricane in first test of post-Katrina flood defences

'There is no system in the world that can handle that amount of rainfall in such a short period,' warns New Orleans mayor

US soldiers add sandbags to levees by a refining plant in Chalmette, Louisiana, before Tropical Storm Barry hits (AP)

Heavy rains and strong winds have knocked out power on the US Gulf Coast as a category one hurricane churns towards land in a first test of flood defences installed after Hurricane Katrina devastated New Orleans 14 years ago.

Hurricane Barry was expected to make landfall on Saturday near Morgan City, west of New Orleans, where authorities have urged people to secure property, stock up on supplies, and shelter indoors.

Some nervous residents opted instead to flee, while mandatory evacuations were ordered in outlying coastal areas beyond the protection of levees.

As dawn approached on Saturday, more than 45,000 people in the south of Louisiana had already lost power. Flights into and out of New Orleans Airport have been cancelled, while the National Weather Service issued a tornado warning.

Barry is poised to make landfall as the first Atlantic hurricane of 2019. Although relatively weak, with winds just over the 74 mph (119 kph) hurricane classification threshold, the storm threatens disastrous flooding across a swath of the Gulf Coast.

The worst of the damage is expected in Louisiana and parts of Mississippi, where wind and rain could affect more than three million people.

Downpours lashed the two states, as well as neighbouring Alabama, leaving some roads underwater overnight on Friday.

Meteorologists have warned that torrential rain – as much as 2ft (60 cm) in some places – could unleash severe flooding as the storm moves inland from the Gulf of Mexico, where oil and gas operators have already cut production by nearly 60 per cent.

"It's powerful. It's strengthening. And water is going to be a big issue," warned Ken Graham, director of the National Hurricane Centre.

New Orleans mayor LaToya Cantrell said 48 hours of heavy downpours could overwhelm pumps designed to purge streets and storm drains of excess water.

"There is no system in the world that can handle that amount of rainfall in such a short period," she tweeted.

US president Donald Trump declared a state of emergency for Louisiana on Friday, freeing up federal disaster assistance if needed.

Rescue crews and about 3,000 National Guard troops have been posted around the state with boats, high-water vehicles and helicopters.

Governors have also declared emergencies in Louisiana and Mississippi, while authorities took unprecedented precautions in closing floodgates and raising the barriers around New Orleans.

The impending storm could test beefed-up flood defences installed since Katrina, which left much of the city underwater and killed more than 1,800 people in 2005.

All the levees in the system built to contain the lower Mississippi river, which winds through the heart of New Orleans, have been closed for the first time since Katrina.

A coastal storm surge into the mouth of the Mississippi was expected to push its crest to 19ft (5.8m) in New Orleans on Saturday, the highest level since 1950 and dangerously close to the top of the city's floodgates.

But while street flooding seemed probable, the US Army Corps of Engineers insisted that significant breaching of the 20ft-tall levees was unlikely.

About 10,000 people in exposed, low-lying areas along the Gulf Coast have been told to leave, but no evacuations have been ordered in New Orleans, where officials urged residents to "shelter in place."

Before taking refuge, shoppers packed stores to stock up on bottled water, food, and other essentials.

"My concerns are just hoping it's not going to be another Katrina," said Donald Wells, a restaurant chef.

Authorities asked residents to remain indoors after 8pm on Friday (1am Saturday GMT) but some, in keeping with New Orleans's party spirit, decided to hunker down in more festive surroundings.

"The rain's not going to shut us down, we're going to shut this place down," said Brett Tidball, 33, who gathered with eight friends at Bourbon Street bar for his stag do and asked the in-house pianist for back-to-back songs about bad weather.

Prince's "Purple Rain" echoed down the street.

Additional reporting by agencies