The flood that rocked Louisiana last summer was likely stronger than a so-called 500-year event in some places, new research suggests.
However, scientists have argued against such simplistic language, which they call "misleading," and a paper published Monday paints a much more nuanced picture.
Following the August storm, the United States Geological Survey began taking high water marks and checking river gauges to measure the extent of the flooding. This week, USGS scientists published a 36-page report sharing their findings.
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Rivers constantly rise and fall but generally stick to a narrow, predictable range of depths. Occasionally they'll see high water, and scientists will use the data to estimate how badly a body of water can flood, and the likelihood of a flood happening. Those floods are often known as 100-, 500-, or 1,000-year events, but the USGS describes floods based on the chance they'll happen in a given year. So, what the Federal Emergency Management Agency describes as a 500-year storm is known as a 0.2 percent annual event by the USGS because it has a 1 in 500 chance of happening each year.
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FEMA defines high-risk areas as those in 100-year floodplains. Property owners in those areas have to meet elevation and, often, flood insurance requirements.
However, the government doesn't have 500, or even 100 years of data to base its estimates on, so it makes informed guesses, and their estimates are extremely broad. For example, the hypothetical "500-year" flood of the Amite River near Denham Springs happens when the water flows around 180,000 cubic feet per second. But in reality, the actual value could be as low as 126,000 feet per second or as high as 307,000, according to Monday's report. Its authors said the true value isn't known because scientists don't have millions of years of data, and even if they did, climates and river conditions constantly change.
In August, the Amite at Denham Springs actually flowed around 205,000 cubic feet per second, the USGS found, placing it within the range of a 500-year event and slightly above the estimated value.
Portions of the Tangipahoa, Tickfaw, Natalbany and Comite rivers also flowed at or above predicted 500-year levels during the worst of the storm. The Tchefuncta, part of the Tickfaw near Liverpool and the Amite near Darlington flowed closer to 100-year levels.
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The USGS did not analyze every body of water, such as the creeks of East Baton Rouge, Bayou Manchac or the Amite south of Denham Springs. In those areas, scientists either didn't have complete data or attributed the flooding to backwater, which happens when downstream areas don't drain, causing backups further upstream, explained hydrologist Kara Watson.
Unlike other agencies USGS scientists don't declare 1,000-year events, or those that have a 0.1 percent chance of happening in a given year, because it is so difficult to extrapolate existing data to account for such a rare event. Surface water specialist Brian Breaker said some professionals are uncomfortable even calling a storm the equivalent of a 500-year event.
Politicians have seized on terms like 1,000-year event to defend decisions to loosen floodplain regulations following the storm because of its perceived rarity. Other observers have said that continued development, as well as local and global changes to the climate, are making destructive storms more common, arguing that cities need to plan for bigger floods.
Now, the USGS scientists will hand over their data to agencies like FEMA, which must determine whether and how to use their findings to guide policy decisions. FEMA has previously indicated that it will not change its flood maps in response to the August flood. The agency did not return calls Monday seeking comment on the USGS's findings.