Seven years after Hurricane Katrina devastated New Orleans, Hurricane Isaac crossed the Louisiana coast last week, testing defences built after the August 2005 disaster.

Isaac made landfall on Tuesday as a category one storm near Plaquemines Parish in Louisiana, south of New Orleans. It made a second landfall the following day further along the Louisiana coast.

Two levees in Plaquemines Parish were breached, flooding about 800 homes. Heavy rain continued as Isaac, which was subsequently downgraded to a tropical storm, moved slowly across Louisiana.

Catastrophe modeller AIR Worldwide says the hurricane “has put to the test the massive $US14.5 billion ($14.1 billion) flood control system built in the area”.

Another catastrophe modeller, Eqecat, puts insured losses from property damage caused by Isaac at $US500 million-$US1.5 billion ($485 million-$1.5 billion), plus $US500 million-$US1 billion ($485 million-$970 million) in losses to energy assets in the Gulf of Mexico.

AIR Worldwide estimates the insured damage bill to onshore property from Isaac at up to $US2 billion ($1.94 billion). It expects damage to offshore energy assets in the Gulf of Mexico to be minimal, with little damage to rigs and platforms.

Risk Management Solutions (RMS) says the storm surge from Isaac was larger than a typical category one hurricane and flooding caused the bulk of the damage.

“Reports of wind damage have been fairly limited,” Director of Model Solutions Michael Kistler said.

RMS says this year’s Atlantic hurricane season is showing signs of increased activity. Tropical storms and hurricanes are named alphabetically each season. Over the past 60 years the letter “I” has not usually been reached until mid-September.

Isaac is the fifth Atlantic hurricane of the season. It was followed by Hurricane Kirk, which has now been downgraded to a tropical storm. Another tropical storm, Leslie, has formed in the Atlantic but currently poses no threat to land.