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Serving: IN

There are Flood Plains, and then there are 100-Year Flood Plains!

There are Flood Plains, and then there are 100-Year Flood Plains!
Purdue specialist explains why the difference can be important to you.

Suppose you want to build a house, or maybe a livestock building. The printed soils map form your county soil and water conservation district says the area where you want to build is on a flood plain soil.

You check the Web Soil Survey Tool on the Internet, largely based on the same soil surveys, and confirm that the site contains flood plain soils. Does that mean you couldn't build there? Does that mean you shouldn't build there?

Soil judgers check soil: High school soil judging lets students determine if a soil is a flood plain or not. However, federal agencies decide what's a 100-year flood plain and what isn't a 100-year flood plain.

They're two different questions, says Gary Steinhardt, Purdue University Extension soils specialist. First, a soil scientist would need to confirm what type of soil was actually at the exact location where you want to build, especially if the project will include a septic system for waste disposal.

Related: Public-Private Collaboration Builds Soil Mapping Data to Explain Yield Variability

Soil surveys are reasonably accurate, but there can be pockets of other soils included in what's designated on a map as a certain soil type, Steinhardt says. Soil scientists walked many acres, but not every acre, in preparing maps that were turned into soils information. As a result, given the soil variation that can occur quickly from spot to spot in Indiana, the only sure way to tell is do an inspection.

The other issue is what's counted as a flood plain in soil judging contests is based on the characteristics of the soil, not how often the area floods, Steinhardt says. If the site has alluvium, or newly deposited material, it will classify as a flop plain, and a home site would not be recommended, in soil judging rules. "newly deposited" may still mean over thousands of years.

Related: Soil Judging Seemed Like a Good Idea at the Time

Federal agencies set 100-year flood plain liens around rivers and streams. "You're usually not allowed to build in these 100-year flood plains," he says. However, you might get by with building where soil judging rules say it's a flood plain, but federal agencies don't classify it as a 100-year flood plain."

Would it be smart? No, Steinhardt emphasizes. Would it be illegal? Likely not. What's smart and what's illegal aren't always the same thing.

TAGS: Extension
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