DIVERSION DISCUSSION: FEMA, corps 100-year flood definitions differ

FARGO – At what Red River level is a 100-year flood in Fargo-Moorhead?

Scientifically speaking, the answer depends on the source of information: the Federal Emergency Management Agency or the U.S. Army Corps of Engineers.

The two federal agencies use different methods to define a 100-year flood, so there are technically two answers.

FEMA defines a 100-year flood level as 38.5 feet on the Red River in Fargo-Moorhead.

With revised maps in the works, that definition will be raised to 39.5 feet, likely this summer.

The Army Corps estimates a 100-year flood is 42.4 feet, nearly 3 feet higher than FEMA’s definition.

Why the significant difference?

The Diversion Authority offers an explanation in its monthly e-newsletter this month:

Essentially, the difference lies in how the figure is calculated, because a 100-year flood level is determined based on past events.

Each flood event over time can influence the definition, so it’s an evolving number and the base of reference can make a difference.

FEMA and the corps also use separate techniques to evaluate factors that can help define a 100-year flood.

The agencies pull from different base flood flows and different time periods, among other data, to derive their 100-year flood definitions for Fargo-Moorhead.

FEMA’s hydrology uses information from 1971 that states the discharge of water during a 100-year flood is 29,300 feet per second near downtown Fargo.

The corps’ modeling uses streamflow data through 2009, so its definition is one with a discharge of 34,700 cubic feet per second.

FEMA uses the 1997 flood as its benchmark, whereas the corps uses years of data in its modeling.

Because the corps’ hydrograph includes all recorded years through 2009, it accounts for the climactic wet cycle the Red River Valley has experienced since 1992.

In contrast, FEMA’s modeling doesn’t include historic-level events Fargo-Moorhead experienced in 2006, 2009 and 2010.

“Both the FEMA and corps’ analyses utilized the best data available at the time they were developed,” the Diversion Authority says. “However, improvements in modeling technology and a longer dataset make the corps’ data the most current. There is a high likelihood that future FEMA updates will more closely match the corps numbers.”

That explains why the Diversion Authority and Fargo-Moorhead leaders are erring on the side of caution and aiming for flood protection to the higher level of 42.5 feet.

Projects built to that height would protect against FEMA’s 100-year flood while offering 3 feet of buffer, a safeguard that allows projects to be certified by FEMA.

Metrowide certification means thousands of Fargo-Moorhead residents will no longer be required to buy flood insurance.

The cities of Fargo and Moorhead have spent millions to shore up their flood protection projects so they can be certified. Fargo’s five-year plan for all of its projects is expected to cost about $240 million.

The Red River diversion project will protect to the corps’ 100-year flood level, while giving the metro area the ability to fight even worse floods.

Emergency measures similar to those taken during the record 2009 event could work in tandem with the diversion to protect Fargo-Moorhead to a 500-year flood event, which the corps defines as 46.5 feet.

Have a question about the proposed Red River diversion, or want to recommend a topic you’d like to see addressed in an upcoming column?
Send an e-mail to Forum reporter Kristen Daum at kdaum@forumcomm.com (Subject: Diversion Discussion) or write to Kristen Daum c/o The Forum, P.O. Box 2020, Fargo, ND 58107.
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