Published May 15 2010
Corps explains higher figure for 100-year flood
Will this affect how many homeowners have to buy high-risk flood insurance?
The short answer is “no,” said Megan Floyd, a spokeswoman for the Federal Emergency Management Agency, which is preparing to release a revised floodplain map for the metro area.
Floyd said FEMA’s mapping process uses the most recent data available at the time – in this case, data from September 2001 and January 2003 from the corps’ St. Paul district.
“If we tried to incorporate everything in as it came out, it would always be a moving target,” she said Friday.
That doesn’t mean FEMA won’t consider the corps’ new estimate in future floodplain revisions, she said, but considering how long it has taken FEMA to complete the current map revision, residents can rest easy for a while.
Walaker was pleased with the answer, given what the alternative could have meant.
“My understanding is that the 100-year flood was going to be 40 (feet),” he said. “Now, if we move that to 42 (feet), that changes the ballgame in a major way. That means where I live down by South High, I would have to have flood insurance.”
As it stands now, FEMA’s revised map will likely bring 7,500 to 7,600 Fargo properties into the 100-year floodplain, mostly south of Interstate 94 and east of 45th Street, Fargo City Engineer Mark Bittner has said. Homeowners with mortgages in those high-risk areas would be required to buy flood insurance when the map is finalized, which is expected to take more than a year.
Floyd said she doesn’t know the public release date for the floodplain map. A FEMA engineer who previously said the agency was shooting for a June release wasn’t available Friday.
While it won’t affect FEMA’s map, the corps’ new flood estimate – which made waves Thursday when it was announced during a meeting about a proposed Red River diversion channel – nonetheless raised eyebrows.
“I was surprised,” Bittner said. “That’s pretty much all of Fargo and probably a large part of West Fargo and Horace.”
Craig Evans, a project manager with the corps, explained Friday how the corps arrived at 42.6 feet.
It’s based on two factors: hydrology, which looks at the amount of water and frequency of flooding, and hydraulics, which is how high the water rises as it flows through the cities.
The hydraulics piece involved tweaking the corps’ model to account for the 2009 flood, when the Red River reached a record 40.84 feet, Evans said.
Updating the model increased the 100-year flood stage at the Fargo gauge “by about a foot,” he said.
The hydrology factor was a bit more complicated.
The Red River basin’s hydrologic records extend back to the late 1800s. Hydrologists use those records to determine the probability that a flood will occur in a given year.
However, that requires a “big assumption” that the chances are the same from year to year, Evans said.
The corps used that traditional model in its initial analysis and found that the 100-year flood level – meaning every year there’s a 1 percent chance of the Red hitting that mark – was 39.3 feet, which was consistent with previous corps studies and FEMA estimates.
But when the corps talked to local officials familiar with the region’s water issues, they learned of wet conditions that have plagued the area since the 1940s, Evans said.
In September, the corps assembled a panel of water experts, mostly from federal agencies, along with North Dakota’s state climatologist and a biology professor, for an “expert opinion elicitation.”
In a nutshell, their job was to give guidance on how to account for climate change and to identify steps the corps should take to account for the future probability and uncertainty in flood flows.
Their advice was to divide the period of record into two parts: a dry period from 1901 to 1941 and a wet period from 1942 to 2009.
Corps officials agreed and found that by using the wet period data to make assumptions for the next 50 years, there’s a 1 percent chance every year of a flood stage of 42.6 feet.
“It’s a reflection of the historic record and looking at that in a different way then we did before,” said Evans, who added it’s unusual for the corps to allow nonstandard assumptions.
Also, the assumptions don’t necessarily correlate to other Red River cities such as Grand Forks, Evans said.
“It’s something that seems to be occurring at Fargo and Moorhead that isn’t necessarily transferrable to any other place. It’s very unusual,” he said.
Readers can reach Forum reporter Mike Nowatzki at (701) 241-5528