Harvey-like deluge would overwhelm Miami Valley’s flood system

1:00 p.m Sunday, Sept. 3, 2017 Community News
Ty Greenlees
In this file photo, the Great Miami River ran high on Jan. 25, 2017. Many stretches of the bike paths near downtown Dayton were covered with water. TY GREENLEES

The Miami Valley’s flood control systems would never be able to withstand the deluge that left Houston under water.

But, scientifically speaking, it shouldn’t have to.

Parts of Texas saw more than 50 inches of rainfall from Hurricane Harvey and its aftermath — as much as two feet of rain in 24 hours. While this is unprecedented, the situation was made worse by the lack of adequate control in Houston, which has been identified as one of the nation’s most flood-prone cities.

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An investigation by ProPublica and The Texas Tribune in December found that flooding is a recurring problem in Houston, made worse by rampant development and global warming. The media outlets questioned whether enough was being done to address it.

In one storm last year, parts of the Houston area received well over a foot of rain in 24 hours; a 2001 storm dropped 40 inches of rain on the city in five days, according to the news outlets.

Dayton, which has its own flood history, hasn’t had to handle that much water ever.

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The Miami Conservancy District was created after 8-11 inches of rain fell on saturated ground and Dayton flooded in 1913. The massive flood control system was built to withstand that event plus 40 percent — or 14 inches of rain in three days over the entire watershed.

That would be what’s called a 1,000-year storm, meaning it has a .001 percent chance of occurring in any given year, according to conservancy district officials.

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“Our system is designed to handle a 1,000-year event with storage to spare. Beyond that we can’t really say with confidence how much it can store,” said conservancy district spokeswoman Brenda Gibson.

In the century-plus since the 1913 flood, the worst storm the area has seen was in 1959 when up to 6 inches of rain fell over three days from Jan. 19-21.

In January 2005, two feet of snow melted and the area received 7 to 10 inches of rain over two weeks.

Both times, the rivers swelled but the system held.

“Geographically the Miami Valley is not in a location to experience the type of rainfall Texas accumulated with Harvey,” said Storm Center 7 Meteorologist McCall Vrydaghs.

“Due to the close proximity to the Gulf of Mexico, Harvey had a non-stop source of water to draw from. This helped to fuel the storm and continually add to the rain totals,” she said. “Although southwest Ohio can see tropical moisture stream from the Gulf, it has to travel a long distance, limiting the amount of water that can make the trip.”

A catastrophic storm is still possible, the conservancy district’s Gibson said.

The National Weather Service estimates the “possible maximum precipitation” for this area to be 22-24 inches over three days over the entire 4,000-square-mile watershed — which would overwhelm the system, said Gibson.

“That would be the worst we would ever expect in this area,” she said.

So what are the odds that could happen?

“A level of magnitude beyond what statistics can accurately predict,” she said. “That’s the most extreme scientifically possible event for our area.”

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