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By Michael Le Page
Parts of New Orleans flooded on Wednesday after 15 centimetres of rain fell in places, faster than pumps could drain the city. Now Tropical Storm Barry is moving ashore. There are fears the slow moving storm could cause major flooding as it moves over the region this weekend.
Didn’t New Orleans’ flood defences get upgraded after Hurricane Katrina devastated the city in 2005?
They did. But flood defences can’t stop rain falling from the sky, and because New Orleans is mostly below sea level it fills up like a bowl. Barry is predicted to dump up to 50 centimetres of rain in places. It looks as though New Orleans will not get nearly that much rain but there could still be “widespread and dangerous flash flooding” – and that’s not the only threat.
What’s the other threat?
The Mississippi river that meanders around the south side of the city is currently 5 metres above sea level. Barry could create a storm surge that raises the level close to 6 metres, the point at which the river could overtop the lowest levees. The current forecast is that the water level will not get that high, but it’s worryingly close.
Why aren’t the levees higher?
The Mississippi usually only gets this high briefly at the end of winter as snows melt inland. It then falls to as low as a metre above sea level during summer. The flood defences are built on the assumption the river will be around 2.5 metres above sea level during the hurricane season.
Why are river levels so high so late in year?
Because the US has just had its wettest year on record, with flooding in many places upriver.
An unfortunate coincidence?
Maybe not. This could well be a result of human-induced climate change – the atmosphere is becoming moister as it warms, meaning more rain can fall. This also means storms like Barry can grow stronger faster and dump far more rain.
Does that mean New Orleans won’t be safe even if Barry isn’t as bad as feared?
Yes. Although $14 billion has been spent on improving flood defences since 2005, these flood defences are only designed to protect against a “1-in-a-100 year event”. That’s a low standard – it means there’s a 1 per chance of the defences failing each year. For comparison, London’s Thames Barrier is designed to protect against a 1-in-a-1000-year flood.
And then there’s global warming?
Indeed. Global warming is drastically changing the odds of exceptional events – Houston had three “1-in-500-year” floods in three years from 2015 to 2017. What’s more, sea level rise and oil extraction has led to massive land loss south of New Orleans, meaning the city is more exposed to storms.
“I expect New Orleans will still be here on Monday. But big picture? It’s horrible,” Andy Horowitz, who writes about the threats facing New Orleans, tweeted on Thursday. “This is one of the world’s great places, and nobody is fully confident it will survive the weekend intact, let alone the summer.”
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