People launch boats from an overpass into floodwaters in the aftermath of Tropical Storm Harvey in Kountze, Texas. Photo / AP
People launch boats from an overpass into floodwaters in the aftermath of Tropical Storm Harvey in Kountze, Texas. Photo / AP
By Jason Samenow
As Harvey's rains unfolded, the intensity and scope of the disaster were so enormous that weather forecasters, first responders, the victims - everyone really - couldn't believe their eyes. Now the data are bearing out what everyone suspected: this flood is on an entirely different scale than what the United States has seen before.
A new analysis from the University of Wisconsin's Space Science and Engineering Centre has determined that Harvey is a 1-in-1000-year flood that has overwhelmed an enormous section of Southeast Texas, an area larger than the Auckland and Northland regions combined.Nothing in the historical record rivals this, according to Shane Hubbard, the Wisconsin researcher who made and mapped the calculation. "In looking at many of these events [in the United States], I've never seen anything of this magnitude or size," he said. "This is something that hasn't happened in our modern era of observations."
Flooding from Tropical Storm Harvey surrounds buildings in Sabine Pass, Texas. Photo / AP
Hubbard made additional calculations that accentuate the massive scale of the disaster:
At least 50cm of rain fell over 75,000sq km, an larger than Northland, Auckland, Waikato, Bay of Plenty and Manawatu combined. At least 76cm of rain fell over 28,400sq km, an area bigger than Waikato. A 1000-year flood is exceptionally rare. It signifies just a 0.1 per cent chance of one happening in any given year. "Or, a better way to think about it is that 99.9 per cent of the time, such an event will never happen," Hubbard said.
Apart from Harvey, there's no record of a 1000-year event occupying so much real estate.
Floodwater from Hurricane Harvey begin to recede in shopping center in Kingwood, Texas. Photo / AP
No one questions the exceptional nature of Harvey's rainfall, but some academics and flood planners have criticised the concept of a 1000-year flood. For one, rainfall and flood data generally go back only 100 years or so, so statistical tricks must be applied to determine what 500-year and 1000-year events actually represent. Furthermore, the climate is changing and precipitation events has become more intense in recent decades, so what constitutes different return frequencies (100-year, 500-year, 1000-year and so forth) is probably changing.
Climate change studies have found that what's considered a 500-year flood today may become much more frequent in coming decades.
But Hubbard, who analyses geographic information to help decision-makers plan for floods, stands by the use of these return interval metrics despite their shortcomings. "For a community, they help put these events into perspective and understand the impact of a flood," he said.
Floodwaters from Tropical Storm Harvey surround homes and businesses in Port Arthur, Texas. Photo / AP
He added that they have "tremendous" value for flood planning and designing infrastructure to be able to withstand events up to a certain intensity. "Decision-makers have to be able to pick a number and say this is the number we need to be prepared for," he said. "If we debate and belabor the accuracy of these estimates, the community will not have a value to plan for."
Hubbard agrees that the climate is changing and precipitation is becoming more intense in some areas, but he said it would be complicated to adapt the flood return frequencies. "The challenge is trying to separate when you have these 500-year events happening all the time, what part is a changing climate, what part is changes in urbanisation and agriculture and what part is the lack of understanding of what's happened in the past," he said.
In any event, Harvey puts an exclamation mark on the pattern of disastrous rains in recent years and may be a harbinger of more in the coming decades.
