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Adventure #4:
Why do some places flood more than others?
The Capital Building in Boise
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Buildings
Buildings
When there are lots of buildings there is usually lots of pavement around them. Pavement prevents water from infiltrating into the ground.
Snow Packed in someone's front yard
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Snowpack
Snowpack
Excessive snowfall in the winter leads to snowpack which will often stick around all year long high up in the mountains. However, if it warms too quickly, the snow can rapidly melt and cause flooding.
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Rain Patterns
Rain Patterns
Sometimes there is just too much rain too quickly. Sudden downpours, even in places with excellent soil for infiltration, can cause floods if the soil pores become overfilled with water.
What is in cities?
Our Capital of Boise with all the Buildings and streets
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Buildings
Buildings
Cities are made up of buildings and pavement. Buildings and their foundations sit on top of the soil, preventing it from absorbing water efficiently. The rain that lands on buildings eventually makes its way all the way down to the ground where it is likely to meet yet another barrier. All of the water will pool up and follow the pull of gravity until it can find a surface that is can begin to infiltrate or until it evaporates.
Downtown Pocatello across from Idaho State University
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Pavement
Pavement
This is the true enemy of infiltration. In cities, there are usually lots of roads and sidewalks made out of pavement. Now all of this pavement is great for people to get around, but it's not so great for water to infiltrate into the ground. All of the buildings along with the roads and sidewalks block the water off from the soil, creating the perfect environment for flooding. All the rainwater will begin to collect on the surface and create large channels of moving water as the water follows the pull of gravity.
Cities are almost entirely made of impermeable surfaces, which means the water can't infiltrate! This is what puts cities at such a high risk for floods. Cities are at the highest risk of catastrophic flooding events, especially if there is a lot of water in a short period of time.
Cities are almost entirely made of impermeable surfaces, which means the water can't infiltrate! This is what puts cities at such a high risk for floods. Cities are at the highest risk of catastrophic flooding events, especially if there is a lot of water in a short period of time.
What does snow have to do with flooding?
Snow itself doesn't have a lot to do with flooding. It is a lot of fun to play in but floods don't typically occur when it's snowing outside.
Real flooding occurs when the snow starts to melt. When snowpack (the snow that is frozen really hard on the ground) starts to melt at a fast rate this can lead to flooding even when it's not raining.
Think about your lawn on that first really hot day of spring when the snow begins to melt. As the snow melts it pools onto the grass right and then runs down the hill off of your lawn and into the streets and gutters in some places.
What about when there is a lot of snow melting all at once?
As you can imagine, the more snow that melts the more water comes rushing down the mountain, filling up the rivers and watersheds.
Think about Yellowstone. Yellowstone had a lot of snow this winter! During the warm summer, the snowpack began melting. At that same time, there was also heavy rainfall in Yellowstone and all of this water combined led to a tragic disaster. A large part of the park was flooded and destroyed.
To the right is a picture of one of the places in the park that got flooded. All of that wreckage and debris used to be a bridge! Floods are a very powerful and often terrifying force of nature.
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Rain causes floods, right?
Boise City During a slight Rain storm
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Rain is one of the leading factors of floods.
But how much rain is needed to cause a flood?
This question all relies upon the infiltration rate of the soil underneath it, the potential of runoff, and the snowpack from the winter before. All of these variables depend upon your specific location and environment.
In short, rain will only lead to flooding if all of the conditions are right.
Activity:
Below there are two places with specific conditions paired together. Using your knowledge of infiltration, porosity, runoff, soil properties, and of location decide between the two which one is more likely to flood.
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A BIG City
A BIG City
There is lots of pavement here.
OR
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Small Farm Town
Small Farm Town
Here there is a lot of open ground and good soil.
Which do you think?
Which do you think?
In this pair, the big city would probably be more likely to flood. The city has a lot of pavement so it will flood more easily than a farm that has good soil for infiltration.
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A canyon with a river at the bottom.
A canyon with a river at the bottom.
Much like Hells canyon on the border of Idaho and Oregon.
OR
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A flat field with big sized particles.
A flat field with big sized particles.
Think about Craters of the Moon in Arco. All of the rocks there are very big.
What one do you think?
What one do you think?
If you said Craters of the Moon, you were just thinking in the wrong direction. There have never been any floods there due to the higher porosity of the rocks and the big gaping holes. It is not likely to flood here because of these holes, however that does not mean that it will never flood there.
The river up in Coure de'Alene is likely to flood because the water just runs off the side of the mountain and into the river. This causes the river to flood more likely and therefor the flooding continues along the riverbed.
Go back into your Field Journal and think about those first questions you answered.
How does water accumulate?
When does the amount of water available become concerning?
Why do we create barriers for floods?
Does my area of the world have a high flood risk?
How does water change the landscape around you?
Now that you have learned about these topics; infiltration, runoff, porosity, soil properties, and location-- see what is different about your answer compared to the beginning of the project. Why do you think your answer might have changed?
Nice! You have earned another Adventure Puzzle, the Water Hazard Piece!
Just below this text, you can left-click on the puzzle piece, print it out, cut it out, and add it to your puzzle. If you need a new puzzle, feel free to print out a new one on the Home Page.
Ready for the next Adventure? Click the link below to learn about how water impacts ecosystems!
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