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Hurricane Gustav crashed on to America's Gulf Coast today sending surging water over the levees protecting New Orleans.

Three boats have been ripped from their moorings by the powerful storm and cars have been seen floating in the floodwater.

The levee defence networks are still in the midst of multi-million dollar reinforcement works after the destruction wreaked by Katrina three years ago. So far the levees are holding back a 14ft storm surge but water is already lapping over a levee around a canal in the west of the city.

The water was also up to the lip of an earthen levee protecting the Upper Ninth Ward of New Orleans, which is virtually deserted after almost two million people to fled their homes in the city and surrounding areas.

The eye of the storm made landfall at Cocodrie, Louisiana, west of the city at around 1600 BST today, according to the National Hurricane Centre. It downgraded Gustav from a Category 3 to a Category 2 storm, but disaster experts warned that its destructive potential was still enormous.

Asked if Gustav could be as bad as Katrina, Jody James, a meteorologist for the National Weather Service said: “It’s a little early to say. Katrina was a Category 4 downgraded to three just before landfall. Gustav is a little bit weaker.

“The storm is weakening, but it is slowing. That is bad news,” he said, warning that the region faces “several more hours of rain” and “serious flooding” is possible.

Storms are rated category 2 when their wind speed is in the range 96-110 mph, causing a storm surge of 6ft-8 ft. Category 3 storms on the Saffir-Simpson Hurricane Scale register wind speeds of 111-130 mph, and a storm surge of 9ft-12ft.

Harvey E. Johnson, second-in-command at the US Government’s emergency agency, said this morning that Gustav's surge was expected to breech levees and at least partially to flood the city.

Dubbed the "mother of all storms", Gustav's arrival has prompted a massive evacuation operation, in a concerted effort to prevent a repeat of the disastrous response to Hurricane Katrina three years ago, which flooded 80 per cent of New Orleans and killed more than 1,600 people along the Gulf Coast.

Three critically ill patients are reported to have died in transit last night, as hospitals, care homes and prisons were emptied of their residents.

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