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Flooding Basics: Your Questions Answered

In Minnesota, our four seasons really boil down to two — winter snow and spring flooding. Luckily, we have great, on-site expertise in both. To help you get through the flooding months and to give a basic overview of what it all means, Meteorologist Mike Augustyniak is here to answer all our flooding questions.

What causes flooding?

Flooding is too much water in any given river or stream, at any one time. So typically, the way we see flooding around here is spring flooding. Most areas of the country experience that, especially areas that get snow. It’s the combination of melting and melting too quickly, so that produces too much runoff, combined with — what we often get in the spring — rain storms. That can aggravate it. So it’s all that water filtering into the rivers and streams.

Does a snowier winter generate more flooding?

It usually would. A lot of it depends on how saturated the ground is to begin with and what determines that, by and large, is what happens in the fall. Usually what happens is, say in the summer you’ll get big thunderstorms. There will be time for that water to evaporate out of the ground or work through the groundwater system into the rivers and streams. What happened this past year was we had a heavy, heavy rain storm and street and neighborhood flooding in the fall and there wasn’t enough time between when that happened and when the ground froze, for us to get that water out of the ground, to make room for the snow melt that’s coming this spring. So this year is kind of a three-fold threat — it’s the abnormally high amount of water in the ground from the fall, it’s the abnormally high amount of snowfall that we got this year that ultimately will melt and on top of that, any rain storms we get this spring.

So this could be the worst year for flooding?

Yes, it could be historical.

What are the different kinds of flood warnings and what do they mean?

Flash flood warnings and flood warnings are two different things. Flood warnings are typically issued when they’re going to last a long time and when there’s enough lead time to properly forecast the flooding. A flash flood warning is kind of more spot and immediate — more what they call a short-fuse warning. Where we know flooding is going to happen somewhere, we don’t know exactly where, so we issue a flash flood warning as soon as we see it starting to happen.

What areas are most at risk for floods?

This year, it’s everywhere. It seems like typically, it takes more to make the larger rivers flood than the smaller creeks and streams. But this year, really, the threat is everywhere. Everywhere that’s near a body of running water is at threat for flooding.

In Minnesota, what are the primary flooding months?

Depending on how quickly we melt during the spring, I would say March and April are probably the two main months but it can extend into May and this year, it may even extend into early June.

Do lakes flood?

They can. But because of the way that they’re fed, they generally don’t. Lakes are kind of more independent bodies of water, streams and rivers are all interconnected in some way. The smaller creeks and streams feed into the rivers, which all flows downstream. There are more variables controlling what happens to any given river or stream, to control flooding, because water could be coming from many different areas, unlike a lake. It’s harder to create flooding in lakes, but it’s possible.

How are you able to forecast flooding? What do you look for?

Ground water. During the spring, ice jams can be a problem, so we look to see where ice jams might be forming and that’s just big blocks of essentially ice cubes floating down any river or stream that gets stopped by some geographic barrier and that can act like a short-term dam, to dam up the water, so we’ll watch for those. The U.S. Geological Survey operates as a network of hydrologic gauges, which essentially are remote ways to sense how deep the water is at any given point. There’s also flow measurements where people actually go out and stick a turbine in the stream and see how fast it’s flowing.

What is the 100-year flood level? What does that mean?

It means nothing in the real world. In the same way that when we say that today’s normal high is 38 degrees — it’s completely normal for the weather to be abnormal. So the 100-year flood is essentially just a statistical mark. It would be if you averaged 1,000 years worth of flood events, it’s just essentially a numerical average as to how often you’d get a flood like that. It doesn’t mean anything to you and me.

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