Louisiana faces triple threat of storm surge, river and rain

The massive rainfall from Tropical Storm Barry will have to be pumped out, taxing the city’s ancient and historically underfunded drainage system. And the Mississippi River, which drains most of the water that falls in a vast section of the United States and even parts of Canada, is held in check by tall levees.

Here’s a look at the defences that protect the New Orleans area and risks that remain:

Post-Katrina improvements

After Hurricane Katrina, billions of dollars were spent to improve the system of levees, pumps and other infrastructure that protects the city from storms coming up from the Gulf. The US Army Corps of Engineers worked to raise levees several feet, install new stronger floodwalls at critical places and strengthen almost every section of the 210-kilometre perimeter that protects the greater New Orleans area. The system is built to hold out storm surge of about 9 metres where the city’s boundaries meet the swamps and lakes near the Gulf of Mexico.

The post-2005 improvements include several massive floodgates that are shut when a storm approaches. In particular, a new surge barrier and gate that closes off the Inner Harbor Navigation Canal near the Lower 9th Ward has reduced the risk of flooding in an area long viewed as the city’s Achilles heel.

But experts note that the system was built to protect against what experts once estimated would be a 100-year level of storm surge — a surge that has a 1 per cent chance of happening any given year. With rising seas from climate change and subsidence in Louisiana’s coast, there’s concern that these killer surges will hit more frequently.

The rain

Tropical Storm Barry’s rainfall is a particular problem for the low-lying city. Rainwater is pumped out through a century-old system of canals, drainage pipes and pumps — all suffering from decades of neglect, laid bare during a particularly bad August 2017 deluge that dropped as much as 23 centimetres in three hours.

Since then, the city’s Sewerage and Water Board, one of the key agencies responsible for drainage, has poured tens of millions of dollars into generating power to make sure there’s enough electricity to keep pumps working. And they’ve improved systems to allow operators to see in real-time how the pumps and turbines are operating. The city also has moved to clean out catch basins to make sure water flow isn’t impeded.

Ahead of Barry, officials said 118 of the city’s 120 drainage and constant-duty pumps are available. That includes major drainage pumps able to move 29 cubic metres of water per second.

But officials also have cautioned that the system can’t prevent all flooding. An intense rainstorm or a slow-moving hurricane that sits over the city could overpower it. They’ve also emphasised the need to do more things such as ripping up concrete, and building water retention ponds and underground cisterns. These so-called “green” infrastructure projects are designed to let rainwater seep slowly underground instead of pumping it out.

The river

The New Orleans area is protected from the mighty Mississippi River by levees that started going up right around the time of the city’s first settlers three centuries ago. Reinforced and strengthened over the years, they are now about 6 to 7 metres high, and are regularly inspected for seepage or other indications that the structural integrity is compromised.

The levees have never been overtopped in the city’s modern-day history, but the river remains swollen by months of rain across the Midwest, and its flows will push against a storm surge flowing upstream from the Gulf of Mexico.

Governor John Bel Edwards said this would be the first time a hurricane made landfall in Louisiana when the Mississippi River was already at flood stage.

The river had been expected to crest at the same height as the lowest parts of the levees, but a positive forecast Friday night showed the river cresting slightly lower, raising hopes that the water will be contained.