The 100-Year Flood

People in riverside communities often hear the term 100-year flood, especially as snows melt and forecasters predict rainy days. This confusing term simultaneously evokes a sense of imminent large-scale disaster and feelings of safety—like we are safe from this kind of disaster for 100 years.

And yet, a 100-year flood doesn’t just happen once every 100 years. In a particularly wet period, there could be a 100-year flood every year. How is that possible and why do we call it a 100-year flood?

One way the term, 100-year flood, is used is as a category of risk. In terms of probability, it’s like saying we have a 1 in 100 (1%) chance of experiencing a flood of that size in any given year. However, this does not guarantee a flood once a century.

It’s hard to conceptualize what a 1% probability actually looks like. Imagine a bag full of 100 marbles. Ninety-nine of the marbles are green and one marble is blue. When you reach into the bag there is a 1% chance of you pulling out the blue marble. While it seems unlikely, you might pull out the blue marble two times in a row!

However the probability calculations to determine a 100-year flood are much more complicated. It’s not just random chance like pulling out the blue marble; there are several factors that contribute to flooding like land cover and weather conditions. The people who make these probability estimates keep tabs on lots of climate data. These scientists and policy makers also regularly measure the water that passes through a river or stream. By monitoring the stream flow in specific areas of a river or in tributaries, we can make predictions about flooding conditions down river. For example, a large snowmelt in southern Minnesota may raise the stream flow of the Mississippi River. While locally the water level of the river rises, this weather event doesn’t bring anything too disastrous.

But let’s say a region of southwest Wisconsin along the Wisconsin River also experiences a large snowmelt as well as several days of heavy rain. By monitoring the elevated streamflow at these two different locations, we can make a much better estimate of the possible flood conditions where they meet in Iowa.

Yet water moves fast and catching floods before they happen can be challenging. This has become more challenging especially as climate change makes abnormal and extreme weather events occur more frequently.

Climate scientists don’t only look at rain and snow. They also take into account on the ground conditions. Some landscapes are more prepared to handle increased streamflow. Urban areas typically have fewer surfaces that allow precipitation to soak into the ground. In urban areas rain and snowmelt will run off asphalt, cement, and other such surfaces into storm drains that go directly to the river. When precipitation has the opportunity to soak into the ground, or infiltrate, it takes less time for the water to get to the river which is good for reducing flood conditions and for taking pollution out of water.

In summary, flooding certainly gives us a lot to think about! And in this blog I just briefly touched on what a 100-year flood is and the conditions that can lead to flood events. There’s much more to learn and discuss!

In the meantime, River Citizens, you can help by creating a rain garden in your yard using native plant species.

Native grasses and wildflowers usually have long roots that are good at helping the infiltration process. The Nature Conservancy and Lake Simcoe Region Conservation Authority provide useful information about how rain gardens work and instructions on how to grow one.

Stay tuned to the 1 Mississippi blog for future posts on the social and political consequences of flooding and more!

And don’t forget to follow us on Facebook and Twitter.

Thank you,

Maria Lee

Minnestota 1 Mississippi Outreach Assistant