What about those 100-year floods?

KELOLAND.com Original

SIOUX FALLS, S.D. (KELO) — Just because it may be called a 100-year flood, it doesn’t mean it can’t happen more than once every 100 years.

“I have worked trying to remove that term from our vernacular because it’s misleading,” Matt Buedie of the FEMA office in Denver, Colorado, said.

The term “100-year flood” applies to the magnitude of a flood and not the frequency, Buedie explained. “What it is, is really a chance of 1 in 100 that it will happen.

“…it can happen twice or three times in a year or back to back in multiple years,” Buedie said.

FEMA uses the 1% or 1 in 100-year chance of a flood as the standard for mapping flood risk, Buedie said.

A 1 in 100-year flood means there is 1 chance in 100 that a portion of land will be flooded in a given year, Buedie said.

FEMA also uses a 1,000-year flood event as the chance for a portion of land to be flooded in a given year. That’s a 1 in 1,000 chance, he said.

Defining the magnitude of a 1% or 1 in 100-year flood depends on the potential impacted area, Buedie said.

“Each individual watershed has its individual characteristics,” Buedie said.

It won’t take as much water to cause a lot of impact to a small stream, he said.

In contrast the watershed for the Missouri River or Colorado River is very large, Buedie said. Both have smaller bodies of water that feed into the larger river, he said.

Brian Hvinden with external affairs in the FEMA Denver office visited Sioux Falls when he was a kid. Back then Sioux Falls ended in the east close to Interstate 229, he said. The city has had considerable development since then.

The Red River will flood over more acres than other rivers because the Red River flows through a low elevation area, Hvinden said.

A flooded river in the mountains of Colorado will flow faster downstream because of elevation, Buedie said.

FEMA will use data from climate, past history of storms, topography, changes in land use and other information to create floodplain maps.

“FEMA does not develop maps in a vacuum,” said Hvinden. “We do this in partnership with our local communities.”

“We worked with Tea and Lincoln County (South Dakota) along Nine Mile creek…,” Buedie said. FEMA wanted to make sure that flood risks were accounted for and risk was mitigated so than any development would be safer, he said.

Whether they’d be classified as 100-year floods or not, South Dakota had a lot of flooding in 2019.

The National Oceanic and Atmospheric Administration said more than 500 flood events were recorded from Jan. 1 to Dec. 31 in South Dakota. Sixty-two zones or counties were affected. The state had 76 days with an event.

This video from the U.S. Geological Service compares flooding in 2019 compared to flooding in 1969.

The state has had more than 700 flood events in the past two years. NOAA said 202 flood events were recorded in 2018. KELOLAND News reported a record 39.17 inches of rain was recorded.

According to data from NOAA, extreme precipitation events are not new in South Dakota. An extreme precipitation event is a day with more than one inch of precipitation. The state has had an above average number of extreme events since 1990. From 1990 to 2014, the state had 4.3 days of extreme events per year.

The 500 recorded flood events in 2019 is nearly twice as many as the highest number in a year also known for its flooding.

NOAA said the state had 265 flood events in 2011 in 59 zones with 41 days of an event.

From 2012 to 2017, the state recorded no more than 45 flood events in a year.

NOAA predicts that annual precipitation in South Dakota is projected to increase, with the largest increases occurring during spring and winter. Heavy precipitation events are also projected to increase, leading to increased runoff and flooding.

The number of floods and rainfall are part of the data FEMA uses to develop floodplain maps, Buedie said.

There are three components to make a map. Those components are hydrology, which is how much what is coming through, what is the discharge and changes, Buedie said.

The second component is a hydraulic model and the third is topography.

“All of those components can change (with new information),” Buedie said. “Do we change (a floodplain map) based on hydrology? Absolutely.”

Yet, “The floodplain map is always going to be a snapshot of a moment in time,” Hvinden said.

As FEMA gets more climate information or more land use information, for example, it will work to update floodplain maps, Hvinden said.

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