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Floods in St. Paul raise the issue “What’s the ‘floodplain?'”

March 31, 2010Patrick NunnallyRiversComments Off on Floods in St. Paul raise the issue “What’s the ‘floodplain?'”

The annual spring river watch has largely passed, here in Minnesota, with the Red River scheduled to go below flood stage in Fargo in the next day or so, and the Mississippi predicted to drop below flood stage in St. Paul by Friday. The floods didn’t turn out to be as large as predicted/feared, but still, there were impressive water levels running in Minneapolis and St. Paul last week–I’ll write more on that in a day or so.

For now, the question has come up: what do we mean by “flood stage,” and the related term “floodplain”? “Flood stage” basically refers to that elevation at which lives or property are threatened. If a city is built right up to the banks of a river, flood stage is reached any time the river rises even a little bit. Here in the Twin Cities, much of the riverfront is in park land, so parks and trails are closed regularly for annual spring rises.

In times of flooding, the term “floodplain” usually is mentioned as part of a phrase such as “the 100 year floodplain.” The 100 year floodplain is that area which has a 1% chance of flooding every year. In St. Paul, recent development along the Upper Landing was placed on thousands of truckloads of fill dirt to get the construction out of the historic 100 year floodplain.

Here’s where things get interesting, though. As a recent Minneapolis Star Tribune story reports FEMA, the Federal Emergency Management Agency, recently recalculated the 100 year floodplain in St. Paul, and established the new line as significantly elevated over the old one. This change could be due to any number of factors, but the most obvious one might be the increased development and river channeling upstream throughout the watershed. More roads, streets, and houses means rain and snowmelt run off into streams and rivers rather than soak into the ground. When that water hits the river, if the river is constrained by levees and floodwalls–such as the new floodwall at Holman Field airport in downtown St. Paul–then the water can’t spread out, but is forced higher, raising the level at which it may rise on any given year.

The article goes into more detail on the consequences of altered floodplain levels, in terms of insurance costs and the like, and I encourage you to take a look at it. The fact remains, though: as long as we build as much as we do in the watershed, and pay as little attention as we do to how water is managed, then more, and higher, floods are inevitable.


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